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The <pre/> section is rendered as-is on the page. That is, if all the lines are prefixed with 4 spaces the rendered page will also have them. Problem is if we put a box around such <pre/> because the content might not fix into it. Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
921 lines
36 KiB
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921 lines
36 KiB
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>libvirt RPC infrastructure</h1>
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<ul id="toc"></ul>
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<p>
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libvirt includes a basic protocol and code to implement
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an extensible, secure client/server RPC service. This was
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originally designed for communication between the libvirt
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client library and the libvirtd daemon, but the code is
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now isolated to allow reuse in other areas of libvirt code.
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This document provides an overview of the protocol and
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structure / operation of the internal RPC library APIs.
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</p>
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<h2><a name="protocol">RPC protocol</a></h2>
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<p>
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libvirt uses a simple, variable length, packet based RPC protocol.
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All structured data within packets is encoded using the
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<a href="http://en.wikipedia.org/wiki/External_Data_Representation">XDR standard</a>
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as currently defined by <a href="https://tools.ietf.org/html/rfc4506">RFC 4506</a>.
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On any connection running the RPC protocol, there can be multiple
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programs active, each supporting one or more versions. A program
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defines a set of procedures that it supports. The procedures can
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support call+reply method invocation, asynchronous events,
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and generic data streams. Method invocations can be overlapped,
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so waiting for a reply to one will not block the receipt of the
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reply to another outstanding method. The protocol was loosely
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inspired by the design of SunRPC. The definition of the RPC
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protocol is in the file <code>src/rpc/virnetprotocol.x</code>
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in the libvirt source tree.
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</p>
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<h3><a href="protocolframing">Packet framing</a></h3>
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<p>
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On the wire, there is no explicit packet framing marker. Instead
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each packet is preceded by an unsigned 32-bit integer giving
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the total length of the packet in bytes. This length includes
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the 4-bytes of the length word itself. Conceptually the framing
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looks like this:
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</p>
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<pre>
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|~~~ Packet 1 ~~~|~~~ Packet 2 ~~~|~~~ Packet 3 ~~~|~~~
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+-------+------------+-------+------------+-------+------------+...
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| n=U32 | (n-4) * U8 | n=U32 | (n-4) * U8 | n=U32 | (n-4) * U8 |
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+-------+------------+-------+------------+-------+------------+...
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|~ Len ~|~ Data ~|~ Len ~|~ Data ~|~ Len ~|~ Data ~|~
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</pre>
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<h3><a href="protocoldata">Packet data</a></h3>
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<p>
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The data in each packet is split into two parts, a short
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fixed length header, followed by a variable length payload.
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So a packet from the illustration above is more correctly
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shown as
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</p>
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<pre>
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+-------+-------------+---------------....---+
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| n=U32 | 6*U32 | (n-(7*4))*U8 |
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+-------+-------------+---------------....---+
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|~ Len ~|~ Header ~|~ Payload .... ~|
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</pre>
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<h3><a href="protocolheader">Packet header</a></h3>
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<p>
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The header contains 6 fields, encoded as signed/unsigned 32-bit
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integers.
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</p>
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<pre>
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+---------------+
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| program=U32 |
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+---------------+
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| version=U32 |
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+---------------+
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| procedure=S32 |
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+---------------+
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| type=S32 |
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+---------------+
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| serial=U32 |
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+---------------+
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| status=S32 |
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+---------------+
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</pre>
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<dl>
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<dt><code>program</code></dt>
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<dd>
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This is an arbitrarily chosen number that will uniquely
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identify the "service" running over the stream.
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</dd>
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<dt><code>version</code></dt>
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<dd>
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This is the version number of the program, by convention
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starting from '1'. When an incompatible change is made
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to a program, the version number is incremented. Ideally
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both versions will then be supported on the wire in
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parallel for backwards compatibility.
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</dd>
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<dt><code>procedure</code></dt>
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<dd>
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This is an arbitrarily chosen number that will uniquely
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identify the method call, or event associated with the
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packet. By convention, procedure numbers start from 1
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and are assigned monotonically thereafter.
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</dd>
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<dt><code>type</code></dt>
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<dd>
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<p>
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This can be one of the following enumeration values
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</p>
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<ol>
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<li>call: invocation of a method call</li>
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<li>reply: completion of a method call</li>
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<li>event: an asynchronous event</li>
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<li>stream: control info or data from a stream</li>
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</ol>
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</dd>
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<dt><code>serial</code></dt>
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<dd>
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This is an number that starts from 1 and increases
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each time a method call packet is sent. A reply or
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stream packet will have a serial number matching the
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original method call packet serial. Events always
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have the serial number set to 0.
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</dd>
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<dt><code>status</code></dt>
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<dd>
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<p>
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This can one of the following enumeration values
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</p>
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<ol>
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<li>ok: a normal packet. this is always set for method calls or events.
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For replies it indicates successful completion of the method. For
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streams it indicates confirmation of the end of file on the stream.</li>
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<li>error: for replies this indicates that the method call failed
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and error information is being returned. For streams this indicates
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that not all data was sent and the stream has aborted</li>
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<li>continue: for streams this indicates that further data packets
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will be following</li>
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</ol>
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</dd>
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</dl>
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<h3><a href="protocolpayload">Packet payload</a></h3>
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<p>
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The payload of a packet will vary depending on the <code>type</code>
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and <code>status</code> fields from the header.
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</p>
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<ul>
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<li>type=call: the in parameters for the method call, XDR encoded</li>
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<li>type=call-with-fds: number of file handles, then the in parameters for the method call, XDR encoded, followed by the file handles</li>
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<li>type=reply+status=ok: the return value and/or out parameters for the method call, XDR encoded</li>
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<li>type=reply+status=error: the error information for the method, a virErrorPtr XDR encoded</li>
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<li>type=reply-with-fds+status=ok: number of file handles, the return value and/or out parameters for the method call, XDR encoded, followed by the file handles</li>
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<li>type=reply-with-fds+status=error: number of file handles, the error information for the method, a virErrorPtr XDR encoded, followed by the file handles</li>
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<li>type=event: the parameters for the event, XDR encoded</li>
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<li>type=stream+status=ok: no payload</li>
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<li>type=stream+status=error: the error information for the method, a virErrorPtr XDR encoded</li>
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<li>type=stream+status=continue: the raw bytes of data for the stream. No XDR encoding</li>
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</ul>
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<p>
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With the two packet types that support passing file descriptors, in
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between the header and the payload there will be a 4-byte integer
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specifying the number of file descriptors which are being sent.
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The actual file handles are sent after the payload has been sent.
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Each file handle has a single dummy byte transmitted as a carrier
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for the out of band file descriptor. While the sender should always
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send '\0' as the dummy byte value, the receiver ought to ignore the
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value for the sake of robustness.
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</p>
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<p>
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For the exact payload information for each procedure, consult the XDR protocol
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definition for the program+version in question
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</p>
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<h3><a name="wireexamples">Wire examples</a></h3>
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<p>
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The following diagrams illustrate some example packet exchanges
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between a client and server
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</p>
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<h4><a name="wireexamplescall">Method call</a></h4>
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<p>
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A single method call and successful
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reply, for a program=8, version=1, procedure=3, which 10 bytes worth
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of input args, and 4 bytes worth of return values. The overall input
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packet length is 4 + 24 + 10 == 38, and output packet length 32
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</p>
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<pre>
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+--+-----------------------+-----------+
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C --> |38| 8 | 1 | 3 | 0 | 1 | 0 | .o.oOo.o. | --> S (call)
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+--+-----------------------+-----------+
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+--+-----------------------+--------+
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C <-- |32| 8 | 1 | 3 | 1 | 1 | 0 | .o.oOo | <-- S (reply)
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+--+-----------------------+--------+
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</pre>
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<h4><a name="wireexamplescallerr">Method call with error</a></h4>
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<p>
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An unsuccessful method call will instead return an error object
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</p>
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<pre>
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+--+-----------------------+-----------+
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C --> |38| 8 | 1 | 3 | 0 | 1 | 0 | .o.oOo.o. | --> S (call)
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+--+-----------------------+-----------+
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+--+-----------------------+--------------------------+
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C <-- |48| 8 | 1 | 3 | 2 | 1 | 0 | .o.oOo.o.oOo.o.oOo.o.oOo | <-- S (error)
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+--+-----------------------+--------------------------+
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</pre>
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<h4><a name="wireexamplescallup">Method call with upload stream</a></h4>
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<p>
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A method call which also involves uploading some data over
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a stream will result in
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</p>
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<pre>
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+--+-----------------------+-----------+
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C --> |38| 8 | 1 | 3 | 0 | 1 | 0 | .o.oOo.o. | --> S (call)
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+--+-----------------------+-----------+
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+--+-----------------------+--------+
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C <-- |32| 8 | 1 | 3 | 1 | 1 | 0 | .o.oOo | <-- S (reply)
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+--+-----------------------+--------+
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+--+-----------------------+-------------....-------+
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C --> |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | --> S (stream data up)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+-------------....-------+
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C --> |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | --> S (stream data up)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+-------------....-------+
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C --> |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | --> S (stream data up)
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+--+-----------------------+-------------....-------+
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...
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+--+-----------------------+-------------....-------+
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C --> |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | --> S (stream data up)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+
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C --> |24| 8 | 1 | 3 | 3 | 1 | 0 | --> S (stream finish)
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+--+-----------------------+
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+--+-----------------------+
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C <-- |24| 8 | 1 | 3 | 3 | 1 | 0 | <-- S (stream finish)
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+--+-----------------------+
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</pre>
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<h4><a name="wireexamplescallbi">Method call bidirectional stream</a></h4>
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<p>
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A method call which also involves a bi-directional stream will
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result in
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</p>
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<pre>
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+--+-----------------------+-----------+
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C --> |38| 8 | 1 | 3 | 0 | 1 | 0 | .o.oOo.o. | --> S (call)
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+--+-----------------------+-----------+
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+--+-----------------------+--------+
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C <-- |32| 8 | 1 | 3 | 1 | 1 | 0 | .o.oOo | <-- S (reply)
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+--+-----------------------+--------+
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+--+-----------------------+-------------....-------+
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C --> |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | --> S (stream data up)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+-------------....-------+
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C --> |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | --> S (stream data up)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+-------------....-------+
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C <-- |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | <-- S (stream data down)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+-------------....-------+
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C --> |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | --> S (stream data up)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+-------------....-------+
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C --> |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | --> S (stream data up)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+-------------....-------+
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C <-- |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | <-- S (stream data down)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+-------------....-------+
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C <-- |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | <-- S (stream data down)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+-------------....-------+
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C <-- |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | <-- S (stream data down)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+-------------....-------+
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C --> |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | --> S (stream data up)
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+--+-----------------------+-------------....-------+
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..
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+--+-----------------------+-------------....-------+
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C --> |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. | --> S (stream data up)
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+--+-----------------------+-------------....-------+
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+--+-----------------------+
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C --> |24| 8 | 1 | 3 | 3 | 1 | 0 | --> S (stream finish)
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+--+-----------------------+
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+--+-----------------------+
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C <-- |24| 8 | 1 | 3 | 3 | 1 | 0 | <-- S (stream finish)
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+--+-----------------------+
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</pre>
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<h4><a name="wireexamplescallmany">Method calls overlapping</a></h4>
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<pre>
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+--+-----------------------+-----------+
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C --> |38| 8 | 1 | 3 | 0 | 1 | 0 | .o.oOo.o. | --> S (call 1)
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+--+-----------------------+-----------+
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+--+-----------------------+-----------+
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C --> |38| 8 | 1 | 3 | 0 | 2 | 0 | .o.oOo.o. | --> S (call 2)
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+--+-----------------------+-----------+
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+--+-----------------------+--------+
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C <-- |32| 8 | 1 | 3 | 1 | 2 | 0 | .o.oOo | <-- S (reply 2)
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+--+-----------------------+--------+
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+--+-----------------------+-----------+
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C --> |38| 8 | 1 | 3 | 0 | 3 | 0 | .o.oOo.o. | --> S (call 3)
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+--+-----------------------+-----------+
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+--+-----------------------+--------+
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C <-- |32| 8 | 1 | 3 | 1 | 3 | 0 | .o.oOo | <-- S (reply 3)
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+--+-----------------------+--------+
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+--+-----------------------+-----------+
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C --> |38| 8 | 1 | 3 | 0 | 4 | 0 | .o.oOo.o. | --> S (call 4)
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+--+-----------------------+-----------+
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+--+-----------------------+--------+
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C <-- |32| 8 | 1 | 3 | 1 | 1 | 0 | .o.oOo | <-- S (reply 1)
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+--+-----------------------+--------+
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+--+-----------------------+--------+
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C <-- |32| 8 | 1 | 3 | 1 | 4 | 0 | .o.oOo | <-- S (reply 4)
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+--+-----------------------+--------+
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</pre>
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<h4><a name="wireexamplescallfd">Method call with passed FD</a></h4>
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<p>
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A single method call with 2 passed file descriptors and successful
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reply, for a program=8, version=1, procedure=3, which 10 bytes worth
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of input args, and 4 bytes worth of return values. The number of
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file descriptors is encoded as a 32-bit int. Each file descriptor
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then has a 1 byte dummy payload. The overall input
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packet length is 4 + 24 + 4 + 2 + 10 == 44, and output packet length 32.
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</p>
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|
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<pre>
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+--+-----------------------+---------------+-------+
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C --> |44| 8 | 1 | 3 | 0 | 1 | 0 | 2 | .o.oOo.o. | 0 | 0 | --> S (call)
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+--+-----------------------+---------------+-------+
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+--+-----------------------+--------+
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C <-- |32| 8 | 1 | 3 | 1 | 1 | 0 | .o.oOo | <-- S (reply)
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+--+-----------------------+--------+
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</pre>
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|
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<h2><a name="security">RPC security</a></h2>
|
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|
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<p>
|
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There are various things to consider to ensure an implementation
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of the RPC protocol can be satisfactorily secured
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</p>
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<h3><a name="securitytls">Authentication/encryption</a></h3>
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|
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<p>
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The basic RPC protocol does not define or require any specific
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authentication/encryption capabilities. A generic solution to
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providing encryption for the protocol is to run the protocol
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over a TLS encrypted data stream. x509 certificate checks can
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be done to form a crude authentication mechanism. It is also
|
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possible for an RPC program to negotiate an encryption /
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authentication capability, such as SASL, which may then also
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provide per-packet data encryption. Finally the protocol data
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stream can of course be tunnelled over transports such as SSH.
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</p>
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<h3><a name="securitylimits">Data limits</a></h3>
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<p>
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Although the protocol itself defines many arbitrary sized data values in the
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payloads, to avoid denial of service attack there are a number of size limit
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checks prior to encoding or decoding data. There is a limit on the maximum
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size of a single RPC message, limit on the maximum string length, and limits
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on any other parameter which uses a variable length array. These limits can
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be raised, subject to agreement between client/server, without otherwise
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breaking compatibility of the RPC data on the wire.
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</p>
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|
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<h3><a name="securityvalidate">Data validation</a></h3>
|
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|
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<p>
|
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It is important that all data be fully validated before performing
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any actions based on the data. When reading an RPC packet, the
|
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first four bytes must be read and the max packet size limit validated,
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before any attempt is made to read the variable length packet data.
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After a complete packet has been read, the header must be decoded
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and all 6 fields fully validated, before attempting to dispatch
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the payload. Once dispatched, the payload can be decoded and passed
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on to the appropriate API for execution. The RPC code must not take
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any action based on the payload, since it has no way to validate
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the semantics of the payload data. It must delegate this to the
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execution API (e.g. corresponding libvirt public API).
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</p>
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<h2><a name="internals">RPC internal APIs</a></h2>
|
|
|
|
<p>
|
|
The generic internal RPC library code lives in the <code>src/rpc/</code>
|
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directory of the libvirt source tree. Unless otherwise noted, the
|
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objects are all threadsafe. The core object types and their
|
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purposes are:
|
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</p>
|
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|
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<h3><a name="apioverview">Overview of RPC objects</a></h3>
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<p>
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The following is a high level overview of the role of each
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of the main RPC objects
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</p>
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<dl>
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<dt><code>virNetSASLContextPtr</code> (virnetsaslcontext.h)</dt>
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<dd>The virNetSASLContext APIs maintain SASL state for a network
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service (server or client). This is primarily used on the server
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to provide a whitelist of allowed SASL usernames for clients.
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</dd>
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<dt><code>virNetSASLSessionPtr</code> (virnetsaslcontext.h)</dt>
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<dd>The virNetSASLSession APIs maintain SASL state for a single
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network connection (socket). This is used to perform the multi-step
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SASL handshake and perform encryption/decryption of data once
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authenticated, via integration with virNetSocket.
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</dd>
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<dt><code>virNetTLSContextPtr</code> (virnettlscontext.h)</dt>
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<dd>The virNetTLSContext APIs maintain TLS state for a network
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service (server or client). This is primarily used on the server
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to provide a whitelist of allowed x509 distinguished names, as
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well as diffie-hellman keys. It can also do validation of
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x509 certificates prior to initiating a connection, in order
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to improve detection of configuration errors.
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</dd>
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<dt><code>virNetTLSSessionPtr</code> (virnettlscontext.h)</dt>
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<dd>The virNetTLSSession APIs maintain TLS state for a single
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network connection (socket). This is used to perform the multi-step
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TLS handshake and perform encryption/decryption of data once
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authenticated, via integration with virNetSocket.
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</dd>
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<dt><code>virNetSocketPtr</code> (virnetsocket.h)</dt>
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<dd>The virNetSocket APIs provide a higher level wrapper around
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the raw BSD sockets and getaddrinfo APIs. They allow for creation
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of both server and client sockets. Data transports supported are
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TCP, UNIX, SSH tunnel or external command tunnel. Internally the
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TCP socket impl uses the getaddrinfo info APIs to ensure correct
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protocol-independent behaviour, thus supporting both IPv4 and IPv6.
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The socket APIs can be associated with a virNetSASLSessionPtr or
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virNetTLSSessionPtr object to allow seamless encryption/decryption
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of all writes and reads. For UNIX sockets it is possible to obtain
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the remote client user ID and process ID. Integration with the
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libvirt event loop also allows use of callbacks for notification
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of various I/O conditions
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</dd>
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<dt><code>virNetMessagePtr</code> (virnetmessage.h)</dt>
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<dd>The virNetMessage APIs provide a wrapper around the libxdr
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API calls, to facilitate processing and creation of RPC
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packets. There are convenience APIs for encoding/encoding the
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packet headers, encoding/decoding the payload using an XDR
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filter, encoding/decoding a raw payload (for streams), and
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encoding a virErrorPtr object. There is also a means to
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add to/serve from a linked-list queue of messages.</dd>
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<dt><code>virNetClientPtr</code> (virnetclient.h)</dt>
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<dd>The virNetClient APIs provide a way to connect to a
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remote server and run one or more RPC protocols over
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the connection. Connections can be made over TCP, UNIX
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sockets, SSH tunnels, or external command tunnels. There
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is support for both TLS and SASL session encryption.
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The client also supports management of multiple data streams
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over each connection. Each client object can be used from
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multiple threads concurrently, with method calls/replies
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being interleaved on the wire as required.
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</dd>
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<dt><code>virNetClientProgramPtr</code> (virnetclientprogram.h)</dt>
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<dd>The virNetClientProgram APIs are used to register a
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program+version with the connection. This then enables
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invocation of method calls, receipt of asynchronous
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events and use of data streams, within that program+version.
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When created a set of callbacks must be supplied to take
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care of dispatching any incoming asynchronous events.
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</dd>
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<dt><code>virNetClientStreamPtr</code> (virnetclientstream.h)</dt>
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<dd>The virNetClientStream APIs are used to control transmission and
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receipt of data over a stream active on a client. Streams provide
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a low latency, unlimited length, bi-directional raw data exchange
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mechanism layered over the RPC connection
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</dd>
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<dt><code>virNetServerPtr</code> (virnetserver.h)</dt>
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<dd>The virNetServer APIs are used to manage a network server. A
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server exposed one or more programs, over one or more services.
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It manages multiple client connections invoking multiple RPC
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calls in parallel, with dispatch across multiple worker threads.
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</dd>
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<dt><code>virNetDaemonPtr</code> (virnetdaemon.h)</dt>
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<dd>The virNetDaemon APIs are used to manage a daemon process. A
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deamon is a process that might expose one or more servers. It
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handles most process-related details, network-related should
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be part of the underlying server.
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</dd>
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<dt><code>virNetServerMDNSPtr</code> (virnetservermdns.h)</dt>
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<dd>The virNetServerMDNS APIs are used to advertise a server
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across the local network, enabling clients to automatically
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detect the existence of remote services. This is done by
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interfacing with the Avahi mDNS advertisement service.
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</dd>
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<dt><code>virNetServerClientPtr</code> (virnetserverclient.h)</dt>
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<dd>The virNetServerClient APIs are used to manage I/O related
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to a single client network connection. It handles initial
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validation and routing of incoming RPC packets, and transmission
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of outgoing packets.
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</dd>
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<dt><code>virNetServerProgramPtr</code> (virnetserverprogram.h)</dt>
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<dd>The virNetServerProgram APIs are used to provide the implementation
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of a single program/version set. Primarily this includes a set of
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callbacks used to actually invoke the APIs corresponding to
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program procedure numbers. It is responsible for all the serialization
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of payloads to/from XDR.</dd>
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<dt><code>virNetServerServicePtr</code> (virnetserverservice.h)</dt>
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<dd>The virNetServerService APIs are used to connect the server to
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one or more network protocols. A single service may involve multiple
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sockets (ie both IPv4 and IPv6). A service also has an associated
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authentication policy for incoming clients.
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</dd>
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</dl>
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<h3><a name="apiclientdispatch">Client RPC dispatch</a></h3>
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<p>
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The client RPC code must allow for multiple overlapping RPC method
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calls to be invoked, transmission and receipt of data for multiple
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streams and receipt of asynchronous events. Understandably this
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involves coordination of multiple threads.
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</p>
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<p>
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The core requirement in the client dispatch code is that only
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one thread is allowed to be performing I/O on the socket at
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any time. This thread is said to be "holding the buck". When
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any other thread comes along and needs to do I/O it must place
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its packets on a queue and delegate processing of them to the
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thread that has the buck. This thread will send out the method
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call, and if it sees a reply will pass it back to the waiting
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thread. If the other thread's reply hasn't arrived, by the time
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the main thread has got its own reply, then it will transfer
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responsibility for I/O to the thread that has been waiting the
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longest. It is said to be "passing the buck" for I/O.
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</p>
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<p>
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When no thread is performing any RPC method call, or sending
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stream data there is still a need to monitor the socket for
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incoming I/O related to asynchronous events, or stream data
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receipt. For this task, a watch is registered with the event
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loop which triggers whenever the socket is readable. This
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watch is automatically disabled whenever any other thread
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grabs the buck, and re-enabled when the buck is released.
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</p>
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<h4><a name="apiclientdispatchex1">Example with buck passing</a></h4>
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<p>
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In the first example, a second thread issues an API call
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while the first thread holds the buck. The reply to the
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first call arrives first, so the buck is passed to the
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second thread.
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</p>
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<pre>
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Thread-1
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V
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Call API1()
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V
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Grab Buck
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| Thread-2
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V |
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Send method1 V
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| Call API2()
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V |
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Wait I/O V
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|<--------Queue method2
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V |
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Send method2 V
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| Wait for buck
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V |
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Wait I/O |
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| |
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V |
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Recv reply1 |
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| |
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V |
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Pass the buck----->|
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| V
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V Wait I/O
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Return API1() |
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V
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Recv reply2
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V
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Release the buck
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V
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Return API2()
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</pre>
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<h4><a name="apiclientdispatchex2">Example without buck passing</a></h4>
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<p>
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In this second example, a second thread issues an API call
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which is sent and replied to, before the first thread's
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API call has completed. The first thread thus notifies
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the second that its reply is ready, and there is no need
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to pass the buck
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</p>
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<pre>
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Thread-1
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V
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Call API1()
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V
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Grab Buck
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| Thread-2
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V |
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Send method1 V
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| Call API2()
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V |
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Wait I/O V
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|<--------Queue method2
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V |
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Send method2 V
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| Wait for buck
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V |
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Wait I/O |
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| |
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V |
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Recv reply2 |
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| |
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V |
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Notify reply2------>|
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| V
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V Return API2()
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Wait I/O
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V
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Recv reply1
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V
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Release the buck
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V
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Return API1()
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</pre>
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<h4><a name="apiclientdispatchex3">Example with async events</a></h4>
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<p>
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In this example, only one thread is present and it has to
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deal with some async events arriving. The events are actually
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dispatched to the application from the event loop thread
|
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</p>
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<pre>
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Thread-1
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V
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Call API1()
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V
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Grab Buck
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V
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Send method1
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V
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Wait I/O
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| Event thread
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V ...
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Recv event1 |
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| V
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V Wait for timer/fd
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Queue event1 |
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| V
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V Timer fires
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Wait I/O |
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| V
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V Emit event1
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Recv reply1 |
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| V
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V Wait for timer/fd
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Return API1() |
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...
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</pre>
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|
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<h3><a name="apiserverdispatch">Server RPC dispatch</a></h3>
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<p>
|
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The RPC server code must support receipt of incoming RPC requests from
|
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multiple client connections, and parallel processing of all RPC
|
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requests, even many from a single client. This goal is achieved through
|
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a combination of event driven I/O, and multiple processing threads.
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</p>
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|
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<p>
|
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The main libvirt event loop thread is responsible for performing all
|
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socket I/O. It will read incoming packets from clients and will
|
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transmit outgoing packets to clients. It will handle the I/O to/from
|
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streams associated with client API calls. When doing client I/O it
|
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will also pass the data through any applicable encryption layer
|
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(through use of the virNetSocket / virNetTLSSession and virNetSASLSession
|
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integration). What is paramount is that the event loop thread never
|
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do any task that can take a non-trivial amount of time.
|
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</p>
|
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|
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<p>
|
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When reading packets, the event loop will first read the 4 byte length
|
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word. This is validated to make sure it does not exceed the maximum
|
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permissible packet size, and the client is set to allow receipt of the
|
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rest of the packet data. Once a complete packet has been received, the
|
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next step is to decode the RPC header. The header is validated to
|
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ensure the request is sensible, ie the server should not receive a
|
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method reply from a client. If the client has not yet authenticated,
|
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a security check is also applied to make sure the procedure is on the
|
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whitelist of those allowed prior to auth. If the packet is a method
|
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call, it will be placed on a global processing queue. The event loop
|
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thread is now done with the packet for the time being.
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</p>
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<p>
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The server has a pool of worker threads, which wait for method call
|
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packets to be queued. One of them will grab the new method call off
|
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the queue for processing. The first step is to decode the payload of
|
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the packet to extract the method call arguments. The worker does not
|
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attempt to do any semantic validation of the arguments, except to make
|
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sure the size of any variable length fields is below defined limits.
|
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</p>
|
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|
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<p>
|
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The worker now invokes the libvirt API call that corresponds to the
|
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procedure number in the packet header. The worker is thus kept busy
|
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until the API call completes. The implementation of the API call
|
|
is responsible for doing semantic validation of parameters and any
|
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MAC security checks on the objects affected.
|
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</p>
|
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|
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<p>
|
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Once the API call has completed, the worker thread will take the
|
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return value and output parameters, or error object and encode
|
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them into a reply packet. Again it does not attempt to do any
|
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semantic validation of output data, aside from variable length
|
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field limit checks. The worker thread puts the reply packet on
|
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the transmission queue for the client. The worker is now finished
|
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and goes back to wait for another incoming method call.
|
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</p>
|
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|
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<p>
|
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The main event loop is back in charge and when the client socket
|
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becomes writable, it will start sending the method reply packet
|
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back to the client.
|
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</p>
|
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<p>
|
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At any time the libvirt connection object can emit asynchronous
|
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events. These are handled by callbacks in the main event thread.
|
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The callback will simply encode the event parameters into a new
|
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data packet and place the packet on the client transmission
|
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queue.
|
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</p>
|
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|
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<p>
|
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Incoming and outgoing stream packets are also directly handled
|
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by the main event thread. When an incoming stream packet is
|
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received, instead of placing it in the global dispatch queue
|
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for the worker threads, it is sidetracked into a per-stream
|
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processing queue. When the stream becomes writable, queued
|
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incoming stream packets will be processed, passing their data
|
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payload on the stream. Conversely when the stream becomes
|
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readable, chunks of data will be read from it, encoded into
|
|
new outgoing packets, and placed on the client's transmit
|
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queue.
|
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</p>
|
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|
|
<h4><a name="apiserverdispatchex1">Example with overlapping methods</a></h4>
|
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|
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<p>
|
|
This example illustrates processing of two incoming methods with
|
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overlapping execution
|
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</p>
|
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|
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<pre>
|
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Event thread Worker 1 Worker 2
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| | |
|
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V V V
|
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Wait I/O Wait Job Wait Job
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| | |
|
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V | |
|
|
Recv method1 | |
|
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| | |
|
|
V | |
|
|
Queue method1 V |
|
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| Serve method1 |
|
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V | |
|
|
Wait I/O V |
|
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| Call API1() |
|
|
V | |
|
|
Recv method2 | |
|
|
| | |
|
|
V | |
|
|
Queue method2 | V
|
|
| | Serve method2
|
|
V V |
|
|
Wait I/O Return API1() V
|
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| | Call API2()
|
|
| V |
|
|
V Queue reply1 |
|
|
Send reply1 | |
|
|
| V V
|
|
V Wait Job Return API2()
|
|
Wait I/O | |
|
|
| ... V
|
|
V Queue reply2
|
|
Send reply2 |
|
|
| V
|
|
V Wait Job
|
|
Wait I/O |
|
|
| ...
|
|
...
|
|
</pre>
|
|
|
|
<h4><a name="apiserverdispatchex2">Example with stream data</a></h4>
|
|
|
|
<p>
|
|
This example illustrates processing of stream data
|
|
</p>
|
|
|
|
<pre>
|
|
Event thread
|
|
|
|
|
V
|
|
Wait I/O
|
|
|
|
|
V
|
|
Recv stream1
|
|
|
|
|
V
|
|
Queue stream1
|
|
|
|
|
V
|
|
Wait I/O
|
|
|
|
|
V
|
|
Recv stream2
|
|
|
|
|
V
|
|
Queue stream2
|
|
|
|
|
V
|
|
Wait I/O
|
|
|
|
|
V
|
|
Write stream1
|
|
|
|
|
V
|
|
Write stream2
|
|
|
|
|
V
|
|
Wait I/O
|
|
|
|
|
...
|
|
</pre>
|
|
|
|
</body>
|
|
</html>
|