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<erlref>
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    <title>net_kernel</title>
    <prepared>Claes Wikstrom</prepared>
    <docno>1</docno>
    <date>96-09-10</date>
    <rev>A</rev>
  </header>
  <module>net_kernel</module>
  <modulesummary>Erlang Networking Kernel</modulesummary>
  <description>
    <p>The net kernel is a system process, registered as
      <c>net_kernel</c>, which must be running for distributed Erlang
      to work. The purpose of this process is to implement parts of
      the BIFs <c>spawn/4</c> and <c>spawn_link/4</c>, and to provide 
      monitoring of the network.</p>
    <p>An Erlang node is started using the command line flag
      <c>-name</c> or <c>-sname</c>:</p>
<pre>$ <input>erl -sname foobar</input></pre>
    <p>It is also possible to call <c>net_kernel:start([foobar])</c>
      directly from the normal Erlang shell prompt:</p>
<pre>1> <input>net_kernel:start([foobar, shortnames]).</input>
{ok,&lt;0.64.0>}
(foobar@gringotts)2></pre>
    <p>If the node is started with the command line flag <c>-sname</c>,
      the node name will be <c>foobar@Host</c>, where <c>Host</c> is
      the short name of the host (not the fully qualified domain name).
      If started with the <c>-name</c> flag, <c>Host</c> is the fully
      qualified domain name. See <c>erl(1)</c>.</p>
    <p>Normally, connections are established automatically when
      another node is referenced. This functionality can be disabled
      by setting the Kernel configuration parameter
      <c>dist_auto_connect</c> to <c>false</c>, see
      <seealso marker="kernel_app">kernel(6)</seealso>. In this case,
      connections must be established explicitly by calling
      <c>net_kernel:connect_node/1</c>.</p>
    <p>Which nodes are allowed to communicate with each other is handled
      by the magic cookie system, see
      <seealso marker="doc/reference_manual:distributed">Distributed Erlang</seealso> in the Erlang Reference Manual.</p>
  </description>
  <funcs>
    <func>
      <name name="allow" arity="1"/>
      <fsummary>Limit access to a specified set of nodes</fsummary>
      <desc>
        <p>Limits access to the specified set of nodes. Any access
          attempts made from (or to) nodes not in <c><anno>Nodes</anno></c> will be
          rejected.</p>
        <p>Returns <c>error</c> if any element in <c><anno>Nodes</anno></c> is not
          an atom.</p>
      </desc>
    </func>
    <func>
      <name name="connect_node" arity="1"/>
      <fsummary>Establish a connection to a node</fsummary>
      <desc>
        <p>Establishes a connection to <c><anno>Node</anno></c>. Returns <c>true</c>
          if successful, <c>false</c> if not, and <c>ignored</c> if
          the local node is not alive.</p>
      </desc>
    </func>
    <func>
      <name name="monitor_nodes" arity="1"/>
      <name name="monitor_nodes" arity="2"/>
      <fsummary>Subscribe to node status change messages</fsummary>
      <desc>
        <p>The calling process subscribes or unsubscribes to node
          status change messages. A <c>nodeup</c> message is delivered
          to all subscribing process when a new node is connected, and
          a <c>nodedown</c> message is delivered when a node is
          disconnected.</p>
        <p>If <c><anno>Flag</anno></c> is <c>true</c>, a new subscription is started.
          If <c><anno>Flag</anno></c> is <c>false</c>, all previous subscriptions --
          started with the same <c><anno>Options</anno></c> -- are stopped. Two
          option lists are considered the same if they contain the same
          set of options.</p>
        <p>As of <c>kernel</c> version 2.11.4, and <c>erts</c> version
          5.5.4, the following is guaranteed:</p>
        <list type="bulleted">
          <item><c>nodeup</c> messages will be delivered before delivery
           of any message from the remote node passed through the
           newly established connection.</item>
          <item><c>nodedown</c> messages will not be delivered until all
           messages from the remote node that have been passed
           through the connection have been delivered.</item>
        </list>
        <p>Note, that this is <em>not</em> guaranteed for <c>kernel</c>
          versions before 2.11.4.</p>
        <p>As of <c>kernel</c> version 2.11.4 subscriptions can also be
          made before the <c>net_kernel</c> server has been started,
          i.e., <c>net_kernel:monitor_nodes/[1,2]</c> does not return
          <c>ignored</c>.</p>
        <p>As of <c>kernel</c> version 2.13, and <c>erts</c> version
          5.7, the following is guaranteed:</p>
        <list type="bulleted">
          <item><c>nodeup</c> messages will be delivered after the
	  corresponding node appears in results from
	  <c>erlang:nodes/X</c>.</item>
          <item><c>nodedown</c> messages will be delivered after the
	  corresponding node has disappeared in results from
	  <c>erlang:nodes/X</c>.</item>
        </list>
        <p>Note, that this is <em>not</em> guaranteed for <c>kernel</c>
          versions before 2.13.</p>
        <p>The format of the node status change messages depends on
          <c><anno>Options</anno></c>. If <c><anno>Options</anno></c> is [], which is the default,
          the format is:</p>
        <code type="none">
{nodeup, Node} | {nodedown, Node}
  Node = node()</code>
        <p>If <c><anno>Options</anno> /= []</c>, the format is:</p>
        <code type="none">
{nodeup, Node, InfoList} | {nodedown, Node, InfoList}
  Node = node()
  InfoList = [{Tag, Val}]</code>
        <p><c>InfoList</c> is a list of tuples. Its contents depends on
          <c><anno>Options</anno></c>, see below.</p>
        <p>Also, when <c>OptionList == []</c> only visible nodes, that
          is, nodes that appear in the result of
          <seealso marker="erts:erlang#nodes/0">nodes/0</seealso>, are
          monitored.</p>
        <p><c><anno>Option</anno></c> can be any of the following:</p>
        <taglist>
          <tag><c>{node_type, NodeType}</c></tag>
          <item>
            <p>Currently valid values for <c>NodeType</c> are:</p>
            <taglist>
              <tag><c>visible</c></tag>
              <item>Subscribe to node status change messages for visible
               nodes only. The tuple <c>{node_type, visible}</c> is
               included in <c>InfoList</c>.</item>
              <tag><c>hidden</c></tag>
              <item>Subscribe to node status change messages for hidden
               nodes only. The tuple <c>{node_type, hidden}</c> is
               included in <c>InfoList</c>.</item>
              <tag><c>all</c></tag>
              <item>Subscribe to node status change messages for both
               visible and hidden nodes. The tuple
              <c>{node_type, visible | hidden}</c> is included in
              <c>InfoList</c>.</item>
            </taglist>
          </item>
          <tag><c>nodedown_reason</c></tag>
          <item>
            <p>The tuple <c>{nodedown_reason, Reason}</c> is included in
              <c>InfoList</c> in <c>nodedown</c> messages. <c>Reason</c>
              can be:</p>
            <taglist>
              <tag><c>connection_setup_failed</c></tag>
              <item>The connection setup failed (after <c>nodeup</c>
               messages had been sent).</item>
              <tag><c>no_network</c></tag>
              <item>No network available.</item>
              <tag><c>net_kernel_terminated</c></tag>
              <item>The <c>net_kernel</c> process terminated.</item>
              <tag><c>shutdown</c></tag>
              <item>Unspecified connection shutdown.</item>
              <tag><c>connection_closed</c></tag>
              <item>The connection was closed.</item>
              <tag><c>disconnect</c></tag>
              <item>The connection was disconnected (forced from the
               current node).</item>
              <tag><c>net_tick_timeout</c></tag>
              <item>Net tick timeout.</item>
              <tag><c>send_net_tick_failed</c></tag>
              <item>Failed to send net tick over the connection.</item>
              <tag><c>get_status_failed</c></tag>
              <item>Status information retrieval from the <c>Port</c>
               holding the connection failed.</item>
            </taglist>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="get_net_ticktime" arity="0"/>
      <fsummary>Get <c>net_ticktime</c></fsummary>
      <desc>
        <p>Gets <c>net_ticktime</c> (see
          <seealso marker="kernel_app">kernel(6)</seealso>).</p>
        <p>Currently defined return values (<c><anno>Res</anno></c>):</p>
        <taglist>
          <tag><c><anno>NetTicktime</anno></c></tag>
          <item>
            <p><c>net_ticktime</c> is <c><anno>NetTicktime</anno></c> seconds.</p>
          </item>
          <tag><c>{ongoing_change_to, <anno>NetTicktime</anno>}</c></tag>
          <item>
            <p><c>net_kernel</c> is currently changing
              <c>net_ticktime</c> to <c><anno>NetTicktime</anno></c> seconds.</p>
          </item>
          <tag><c>ignored</c></tag>
          <item>
            <p>The local node is not alive.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="set_net_ticktime" arity="1"/>
      <name name="set_net_ticktime" arity="2"/>
      <fsummary>Set <c>net_ticktime</c></fsummary>
      <desc>
        <p>Sets <c>net_ticktime</c> (see
          <seealso marker="kernel_app">kernel(6)</seealso>) to
          <c><anno>NetTicktime</anno></c> seconds. <c><anno>TransitionPeriod</anno></c> defaults
          to 60.</p>
        <p>Some definitions:</p>
        <taglist>
          <tag>The minimum transition traffic interval (<c>MTTI</c>)</tag>
          <item>
            <p><c>minimum(<anno>NetTicktime</anno>, PreviousNetTicktime)*1000 div 4</c> milliseconds.</p>
          </item>
          <tag>The transition period</tag>
          <item>
            <p>The time of the least number of consecutive <c>MTTI</c>s
              to cover <c><anno>TransitionPeriod</anno></c> seconds following
              the call to <c>set_net_ticktime/2</c> (i.e.
              ((<c><anno>TransitionPeriod</anno>*1000 - 1) div MTTI + 1)*MTTI</c>
              milliseconds).</p>
          </item>
        </taglist>
        <p>If <c><![CDATA[<anno>NetTicktime</anno> < PreviousNetTicktime]]></c>, the actual
          <c>net_ticktime</c> change will be done at the end of
          the transition period; otherwise, at the beginning. During
          the transition period, <c>net_kernel</c> will ensure that
          there will be outgoing traffic on all connections at least
          every <c>MTTI</c> millisecond.</p>
        <note>
          <p>The <c>net_ticktime</c> changes have to be initiated on all
            nodes in the network (with the same <c><anno>NetTicktime</anno></c>)
            before the end of any transition period on any node;
            otherwise, connections may erroneously be disconnected.</p>
        </note>
        <p>Returns one of the following:</p>
        <taglist>
          <tag><c>unchanged</c></tag>
          <item>
            <p><c>net_ticktime</c> already had the value of
              <c><anno>NetTicktime</anno></c> and was left unchanged.</p>
          </item>
          <tag><c>change_initiated</c></tag>
          <item>
            <p><c>net_kernel</c> has initiated the change of
              <c>net_ticktime</c> to <c><anno>NetTicktime</anno></c> seconds.</p>
          </item>
          <tag><c>{ongoing_change_to, <anno>NewNetTicktime</anno>}</c></tag>
          <item>
            <p>The request was <em>ignored</em>; because,
              <c>net_kernel</c> was busy changing <c>net_ticktime</c> to
              <c><anno>NewNetTicktime</anno></c> seconds.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name>start([Name]) -> {ok, pid()} | {error, Reason}</name>
      <name>start([Name, NameType]) -> {ok, pid()} | {error, Reason}</name>
      <name>start([Name, NameType, Ticktime]) -> {ok, pid()} | {error, Reason}</name>
      <fsummary>Turn an Erlang runtime system into a distributed node</fsummary>
      <type>
        <v>Name = atom()</v>
        <v>NameType = shortnames | longnames</v>
        <v>Reason = {already_started, pid()} | term()</v>
      </type>
      <desc>
        <p>Note that the argument is a list with exactly one, two or
          three arguments. <c>NameType</c> defaults to <c>longnames</c>
          and <c>Ticktime</c> to 15000.</p>
        <p>Turns a non-distributed node into a distributed node by
          starting <c>net_kernel</c> and other necessary processes.</p>
      </desc>
    </func>
    <func>
      <name name="stop" arity="0"/>
      <fsummary>Turn a node into a non-distributed Erlang runtime system</fsummary>
      <desc>
        <p>Turns a distributed node into a non-distributed node. For
          other nodes in the network, this is the same as the node
          going down. Only possible when the net kernel was started
          using <c>start/1</c>, otherwise returns
          <c>{error, not_allowed}</c>. Returns <c>{error, not_found}</c>
          if the local node is not alive.</p>
      </desc>
    </func>
  </funcs>
</erlref>

