path: root/lib/kernel/doc/src/rpc.xml

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  <header>
    <copyright>
      <year>1996</year><year>2011</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
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      retrieved online at http://www.erlang.org/.
    
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      under the License.
    
    </legalnotice>

    <title>rpc</title>
    <prepared>Claes Wikstrom</prepared>
    <docno>1</docno>
    <date>96-09-10</date>
    <rev>A</rev>
  </header>
  <module>rpc</module>
  <modulesummary>Remote Procedure Call Services</modulesummary>
  <description>
    <p>This module contains services which are similar to remote
      procedure calls. It also contains broadcast facilities and 
      parallel evaluators. A remote procedure call is a method to call
      a function on a remote node and collect the answer. It is used
      for collecting information on a remote node, or for running a
      function with some specific side effects on the remote node.</p>
  </description>
  <datatypes>
    <datatype>
      <name name="key"/>
      <desc>
        <p>As returned by <seealso marker="#async_call/4">
          <c>async_call/4</c>.</seealso></p>
      </desc>
    </datatype>
  </datatypes>
  <funcs>
    <func>
      <name name="call" arity="4"/>
      <fsummary>Evaluate a function call on a node</fsummary>
      <desc>
        <p>Evaluates <c>apply(<anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>)</c> on the node
          <c><anno>Node</anno></c> and returns the corresponding value <c><anno>Res</anno></c>, or
          <c>{badrpc, <anno>Reason</anno>}</c> if the call fails.</p>
      </desc>
    </func>
    <func>
      <name name="call" arity="5"/>
      <fsummary>Evaluate a function call on a node</fsummary>
      <desc>
        <p>Evaluates <c>apply(<anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>)</c> on the node
          <c><anno>Node</anno></c> and returns the corresponding value <c><anno>Res</anno></c>, or
          <c>{badrpc, <anno>Reason</anno>}</c> if the call fails. <c><anno>Timeout</anno></c> is
          a timeout value in milliseconds. If the call times out,
          <c><anno>Reason</anno></c> is <c>timeout</c>.</p>
        <p>If the reply arrives after the call times out, no message
          will contaminate the caller's message queue, since this
          function spawns off a middleman process to act as (a void)
          destination for such an orphan reply. This feature also makes
          this function more expensive than <c>call/4</c> at
          the caller's end.</p>
      </desc>
    </func>
    <func>
      <name name="block_call" arity="4"/>
      <fsummary>Evaluate a function call on a node in the RPC server's context</fsummary>
      <desc>
        <p>Like <c>call/4</c>, but the RPC server at <c><anno>Node</anno></c> does
          not create a separate process to handle the call. Thus,
          this function can be used if the intention of the call is to
          block the RPC server from any other incoming requests until
          the request has been handled. The function can also be used 
          for efficiency reasons when very small fast functions are
          evaluated, for example BIFs that are guaranteed not to
          suspend.</p>
      </desc>
    </func>
    <func>
      <name name="block_call" arity="5"/>
      <fsummary>Evaluate a function call on a node in the RPC server's context</fsummary>
      <desc>
        <p>Like <c>block_call/4</c>, but with a timeout value in
          the same manner as <c>call/5</c>.</p>
      </desc>
    </func>
    <func>
      <name name="async_call" arity="4"/>
      <fsummary>Evaluate a function call on a node, asynchronous version</fsummary>
      <desc>
        <p>Implements <em>call streams with promises</em>, a type of
          RPC which does not suspend the caller until the result is
          finished. Instead, a key is returned which can be used at a
          later stage to collect the value. The key can be viewed as a
          promise to deliver the answer.</p>
        <p>In this case, the key <c><anno>Key</anno></c> is returned, which can be
          used in a subsequent call to <c>yield/1</c> or
          <c>nb_yield/1,2</c> to retrieve the value of evaluating
          <c>apply(<anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>)</c> on the node <c><anno>Node</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="yield" arity="1"/>
      <fsummary>Deliver the result of evaluating a function call on a node (blocking)</fsummary>
      <desc>
        <p>Returns the promised answer from a previous
          <c>async_call/4</c>. If the answer is available, it is
          returned immediately. Otherwise, the calling process is
          suspended until the answer arrives from <c>Node</c>.</p>
      </desc>
    </func>
    <func>
      <name name="nb_yield" arity="1"/>
      <fsummary>Deliver the result of evaluating a function call on a node (non-blocking)</fsummary>
      <desc>
        <p>Equivalent to <c>nb_yield(<anno>Key</anno>, 0)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="nb_yield" arity="2"/>
      <fsummary>Deliver the result of evaluating a function call on a node (non-blocking)</fsummary>
      <desc>
        <p>This is a non-blocking version of <c>yield/1</c>. It returns
          the tuple <c>{value, <anno>Val</anno>}</c> when the computation has
          finished, or <c>timeout</c> when <c><anno>Timeout</anno></c> milliseconds
          has elapsed.</p>
      </desc>
    </func>
    <func>
      <name name="multicall" arity="3"/>
      <fsummary>Evaluate a function call on a number of nodes</fsummary>
      <desc>
        <p>Equivalent to <c>multicall([node()|nodes()], <anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>, infinity)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="multicall" arity="4" clause_i="1"/>
      <fsummary>Evaluate a function call on a number of nodes</fsummary>
      <desc>
        <p>Equivalent to <c>multicall(<anno>Nodes</anno>, <anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>, infinity)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="multicall" arity="4" clause_i="2"/>
      <fsummary>Evaluate a function call on a number of nodes</fsummary>
      <desc>
        <p>Equivalent to <c>multicall([node()|nodes()], <anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>, <anno>Timeout</anno>)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="multicall" arity="5"/>
      <fsummary>Evaluate a function call on a number of nodes</fsummary>
      <desc>
        <p>In contrast to an RPC, a multicall is an RPC which is sent
          concurrently from one client to multiple servers. This is
          useful for collecting some information from a set of nodes,
          or for calling a function on a set of nodes to achieve some
          side effects. It is semantically the same as iteratively
          making a series of RPCs on all the nodes, but the multicall
          is faster as all the requests are sent at the same time
          and are collected one by one as they come back.</p>
        <p>The function evaluates <c>apply(<anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>)</c>
          on the specified nodes and collects the answers. It returns 
          <c>{<anno>ResL</anno>, <anno>BadNodes</anno>}</c>, where <c><anno>BadNodes</anno></c> is a list
          of the nodes that terminated or timed out during computation,
          and <c><anno>ResL</anno></c> is a list of the return values.
          <c><anno>Timeout</anno></c> is a time (integer) in milliseconds, or
          <c>infinity</c>.</p>
        <p>The following example is useful when new object code is to
          be loaded on all nodes in the network, and also indicates
          some side effects RPCs may produce:</p>
        <code type="none">
%% Find object code for module Mod 
{Mod, Bin, File} = code:get_object_code(Mod), 

%% and load it on all nodes including this one 
{ResL, _} = rpc:multicall(code, load_binary, [Mod, Bin, File,]),

%% and then maybe check the ResL list.</code>
      </desc>
    </func>
    <func>
      <name name="cast" arity="4"/>
      <fsummary>Run a function on a node ignoring the result</fsummary>
      <desc>
        <p>Evaluates <c>apply(<anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>)</c> on the node
          <c><anno>Node</anno></c>. No response is delivered and the calling
          process is not suspended until the evaluation is complete, as
          is the case with <c>call/4,5</c>.</p>
      </desc>
    </func>
    <func>
      <name name="eval_everywhere" arity="3"/>
      <fsummary>Run a function on all nodes, ignoring the result</fsummary>
      <desc>
        <p>Equivalent to <c>eval_everywhere([node()|nodes()], <anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="eval_everywhere" arity="4"/>
      <fsummary>Run a function on specific nodes, ignoring the result</fsummary>
      <desc>
        <p>Evaluates <c>apply(<anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>)</c> on
          the specified nodes. No answers are collected.</p>
      </desc>
    </func>
    <func>
      <name name="abcast" arity="2"/>
      <fsummary>Broadcast a message asynchronously to a registered process on all nodes</fsummary>
      <desc>
        <p>Equivalent to <c>abcast([node()|nodes()], <anno>Name</anno>, <anno>Msg</anno>)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="abcast" arity="3"/>
      <fsummary>Broadcast a message asynchronously to a registered process on specific nodes</fsummary>
      <desc>
        <p>Broadcasts the message <c><anno>Msg</anno></c> asynchronously to
          the registered process <c><anno>Name</anno></c> on the specified nodes.</p>
      </desc>
    </func>
    <func>
      <name name="sbcast" arity="2"/>
      <fsummary>Broadcast a message synchronously to a registered process on all nodes</fsummary>
      <desc>
        <p>Equivalent to <c>sbcast([node()|nodes()], <anno>Name</anno>, <anno>Msg</anno>)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="sbcast" arity="3"/>
      <fsummary>Broadcast a message synchronously to a registered process on specific nodes</fsummary>
      <desc>
        <p>Broadcasts the message <c><anno>Msg</anno></c> synchronously to
          the registered process <c><anno>Name</anno></c> on the specified nodes.</p>
        <p>Returns <c>{<anno>GoodNodes</anno>, <anno>BadNodes</anno>}</c>, where <c><anno>GoodNodes</anno></c>
          is the list of nodes which have <c><anno>Name</anno></c> as a registered
          process.</p>
        <p>The function is synchronous in the sense that it is known
          that all servers have received the message when the call
          returns. It is not possible to know that the servers have
          actually processed the message.</p>
        <p>Any further messages sent to the servers, after this
          function has returned, will be received by all servers after
          this message.</p>
      </desc>
    </func>
    <func>
      <name name="server_call" arity="4"/>
      <fsummary>Interact with a server on a node</fsummary>
      <desc>
        <p>This function can be used when interacting with a server
          called <c><anno>Name</anno></c> at node <c><anno>Node</anno></c>. It is assumed that
          the server receives messages in the format
          <c>{From, <anno>Msg</anno>}</c> and replies using <c>From ! {<anno>ReplyWrapper</anno>, <anno>Node</anno>, <anno>Reply</anno>}</c>. This function makes such
          a server call and ensures that the entire call is packed into
          an atomic transaction which either succeeds or fails. It
          never hangs, unless the server itself hangs.</p>
        <p>The function returns the answer <c><anno>Reply</anno></c> as produced by
          the server <c><anno>Name</anno></c>, or <c>{error, <anno>Reason</anno>}</c>.</p>
      </desc>
    </func>
    <func>
      <name name="multi_server_call" arity="2"/>
      <fsummary>Interact with the servers on a number of nodes</fsummary>
      <desc>
        <p>Equivalent to <c>multi_server_call([node()|nodes()], <anno>Name</anno>, <anno>Msg</anno>)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="multi_server_call" arity="3"/>
      <fsummary>Interact with the servers on a number of nodes</fsummary>
      <desc>
        <p>This function can be used when interacting with servers
          called <c><anno>Name</anno></c> on the specified nodes. It is assumed that
          the servers receive messages in the format <c>{From, <anno>Msg</anno>}</c>
          and reply using <c>From ! {<anno>Name</anno>, Node, <anno>Reply</anno>}</c>, where
          <c>Node</c> is the name of the node where the server is
          located. The function returns <c>{<anno>Replies</anno>, <anno>BadNodes</anno>}</c>,
          where <c><anno>Replies</anno></c> is a list of all <c><anno>Reply</anno></c> values and
          <c><anno>BadNodes</anno></c> is a list of the nodes which did not exist, or
          where the server did not exist, or where the server terminated
          before sending any reply.</p>
      </desc>
    </func>
    <func>
      <name name="safe_multi_server_call" arity="2"/>
      <name name="safe_multi_server_call" arity="3"/>
      <fsummary>Interact with the servers on a number of nodes (deprecated)</fsummary>
      <desc>
        <warning>
          <p>This function is deprecated. Use
            <c>multi_server_call/2,3</c> instead.</p>
        </warning>
        <p>In Erlang/OTP R6B and earlier releases,
          <c>multi_server_call/2,3</c> could not handle the case
          where the remote node exists, but there is no server called
          <c><anno>Name</anno></c>. Instead this function had to be used. In
          Erlang/OTP R7B and later releases, however, the functions are
          equivalent, except for this function being slightly slower.</p>
      </desc>
    </func>
    <func>
      <name name="parallel_eval" arity="1"/>
      <fsummary>Evaluate several function calls on all nodes in parallel</fsummary>
      <desc>
        <p>For every tuple in <c><anno>FuncCalls</anno></c>, evaluates
          <c>apply(<anno>Module</anno>, <anno>Function</anno>, <anno>Args</anno>)</c> on some node in
          the network. Returns the list of return values, in the same
          order as in <c><anno>FuncCalls</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="pmap" arity="3"/>
      <fsummary>Parallell evaluation of mapping a function over a list </fsummary>
      <desc>
        <p>Evaluates <c>apply(<anno>Module</anno>, <anno>Function</anno>, [<anno>Elem</anno>|<anno>ExtraArgs</anno>])</c>,
          for every element <c><anno>Elem</anno></c> in <c><anno>List1</anno></c>, in parallel.
          Returns the list of return values, in the same order as in
          <c><anno>List1</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="pinfo" arity="1"/>
      <fsummary>Information about a process</fsummary>
      <desc>
        <p>Location transparent version of the BIF
          <seealso marker="erts:erlang#process_info/1">
          <c>process_info/1</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="pinfo" arity="2"/>
      <fsummary>Information about a process</fsummary>
      <desc>
        <p>Location transparent version of the BIF
          <seealso marker="erts:erlang#process_info/2">
          <c>process_info/2</c></seealso>.</p>
      </desc>
    </func>
  </funcs>
</erlref>