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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>1996</year><year>2018</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
</legalnotice>
<title>net_kernel</title>
<prepared>Claes Wikstrom</prepared>
<docno>1</docno>
<date>1996-09-10</date>
<rev>A</rev>
</header>
<module since="">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 operational 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 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,<0.64.0>}
(foobar@gringotts)2></pre>
<p>If the node is started with command-line flag <c>-sname</c>,
the node name is <c>foobar@Host</c>, where <c>Host</c> is
the short name of the host (not the fully qualified domain name).
If started with flag <c>-name</c>, the node name is <c>foobar@Host</c>,
where <c>Host</c> is the fully qualified domain name.
For more information, see
<seecom marker="erts:erl"><c>erl</c></seecom>.</p>
<p>Normally, connections are established automatically when
another node is referenced. This functionality can be disabled
by setting Kernel configuration parameter
<c>dist_auto_connect</c> to <c>never</c>, see
<seeapp marker="kernel_app"><c>kernel(6)</c></seeapp>. In this case,
connections must be established explicitly by calling
<seemfa marker="#connect_node/1"><c>connect_node/1</c></seemfa>.</p>
<p>Which nodes that are allowed to communicate with each other is handled
by the magic cookie system, see section
<seeguide marker="system/reference_manual:distributed">Distributed Erlang</seeguide>
in the Erlang Reference Manual.</p>
<warning>
<p>
Starting a distributed node without also specifying
<seecom marker="erts:erl#proto_dist"><c>-proto_dist inet_tls</c></seecom>
will expose the node to attacks that may give the attacker
complete access to the node and in extension the cluster.
When using un-secure distributed nodes, make sure that the
network is configured to keep potential attackers out.
See the <seeguide marker="ssl:ssl_distribution">
Using SSL for Erlang Distribution</seeguide> User's Guide
for details on how to setup a secure distributed node.
</p>
</warning>
</description>
<funcs>
<func>
<name name="allow" arity="1" since=""/>
<fsummary>Permit access to a specified set of nodes</fsummary>
<desc>
<p>Permits access to the specified set of nodes.</p>
<p>Before the first call to <c>allow/1</c>, any node with the correct
cookie can be connected. When <c>allow/1</c> is called, a list
of allowed nodes is established. Any access attempts made from (or to)
nodes not in that list will be rejected.</p>
<p>Subsequent calls to <c>allow/1</c> will add the specified nodes
to the list of allowed nodes. It is not possible to remove nodes
from the list.</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" since=""/>
<fsummary>Establish a connection to a node.</fsummary>
<desc>
<p>Establishes a connection to <c><anno>Node</anno></c>. Returns
<c>true</c> if a connection was established or was already
established or if <c><anno>Node</anno></c> is the local node
itself. Returns <c>false</c> if the connection attempt failed, and
<c>ignored</c> if the local node is not alive.</p>
</desc>
</func>
<func>
<name name="get_net_ticktime" arity="0" since=""/>
<fsummary>Get <c>net_ticktime</c>.</fsummary>
<desc>
<p>Gets <c>net_ticktime</c> (see
<seeapp marker="kernel_app"><c>kernel(6)</c></seeapp>).</p>
<p>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="getopts" arity="2" since="OTP 19.1"/>
<fsummary>Get distribution socket options.</fsummary>
<desc>
<p>Get one or more options for the distribution socket
connected to <c><anno>Node</anno></c>.</p>
<p>If <c><anno>Node</anno></c> is a connected node
the return value is the same as from
<seemfa marker="inet#getopts/2"><c>inet:getopts(Sock, Options)</c></seemfa>
where <c>Sock</c> is the distribution socket for <c><anno>Node</anno></c>.</p>
<p>Returns <c>ignored</c> if the local node is not alive or
<c>{error, noconnection}</c> if <c><anno>Node</anno></c> is not connected.</p>
</desc>
</func>
<func>
<name name="monitor_nodes" arity="1" since=""/>
<name name="monitor_nodes" arity="2" since=""/>
<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 processes 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 from Kernel version 2.11.4, and ERTS version
5.5.4, the following is guaranteed:</p>
<list type="bulleted">
<item><p><c>nodeup</c> messages are delivered before delivery
of any message from the remote node passed through the
newly established connection.</p></item>
<item><p><c>nodedown</c> messages are not delivered until all
messages from the remote node that have been passed
through the connection have been delivered.</p></item>
</list>
<p>Notice that this is <em>not</em> guaranteed for Kernel
versions before 2.11.4.</p>
<p>As from Kernel version 2.11.4, subscriptions can also be
made before the <c>net_kernel</c> server is started, that is,
<c>net_kernel:monitor_nodes/[1,2]</c> does not return
<c>ignored</c>.</p>
<p>As from Kernel version 2.13, and ERTS version
5.7, the following is guaranteed:</p>
<list type="bulleted">
<item><p><c>nodeup</c> messages are delivered after the
corresponding node appears in results from
<c>erlang:nodes/X</c>.</p></item>
<item><p><c>nodedown</c> messages are delivered after the
corresponding node has disappeared in results from
<c>erlang:nodes/X</c>.</p></item>
</list>
<p>Notice that this is <em>not</em> guaranteed for Kernel
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
<c>[]</c>, which is the default, the format is as follows:</p>
<code type="none">
{nodeup, Node} | {nodedown, Node}
Node = node()</code>
<p>If <c><anno>Options</anno></c> is not <c>[]</c>, the format is
as follows:</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
<seemfa marker="erts:erlang#nodes/0"><c>erlang:nodes/0</c></seemfa>,
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>Valid values for <c>NodeType</c>:</p>
<taglist>
<tag><c>visible</c></tag>
<item><p>Subscribe to node status change messages for visible
nodes only. The tuple <c>{node_type, visible}</c> is
included in <c>InfoList</c>.</p></item>
<tag><c>hidden</c></tag>
<item><p>Subscribe to node status change messages for hidden
nodes only. The tuple <c>{node_type, hidden}</c> is
included in <c>InfoList</c>.</p></item>
<tag><c>all</c></tag>
<item><p>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>.</p></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.</p>
<p>
<c>Reason</c> can, depending on which
distribution module or process that is used be any term,
but for the standard TCP distribution module it is
any of the following:
</p>
<taglist>
<tag><c>connection_setup_failed</c></tag>
<item><p>The connection setup failed (after <c>nodeup</c>
messages were sent).</p></item>
<tag><c>no_network</c></tag>
<item><p>No network is available.</p></item>
<tag><c>net_kernel_terminated</c></tag>
<item><p>The <c>net_kernel</c> process terminated.</p></item>
<tag><c>shutdown</c></tag>
<item><p>Unspecified connection shutdown.</p></item>
<tag><c>connection_closed</c></tag>
<item><p>The connection was closed.</p></item>
<tag><c>disconnect</c></tag>
<item><p>The connection was disconnected (forced from the
current node).</p></item>
<tag><c>net_tick_timeout</c></tag>
<item><p>Net tick time-out.</p></item>
<tag><c>send_net_tick_failed</c></tag>
<item><p>Failed to send net tick over the connection.</p></item>
<tag><c>get_status_failed</c></tag>
<item><p>Status information retrieval from the <c>Port</c>
holding the connection failed.</p></item>
</taglist>
</item>
</taglist>
</desc>
</func>
<func>
<name name="set_net_ticktime" arity="1" since=""/>
<name name="set_net_ticktime" arity="2" since=""/>
<fsummary>Set <c>net_ticktime</c>.</fsummary>
<desc>
<p>Sets <c>net_ticktime</c> (see
<seeapp marker="kernel_app"><c>kernel(6)</c></seeapp>) to
<c><anno>NetTicktime</anno></c> seconds.
<c><anno>TransitionPeriod</anno></c> defaults to <c>60</c>.</p>
<p>Some definitions:</p>
<taglist>
<tag>Minimum transition traffic interval (<c>MTTI</c>)</tag>
<item><p><c>minimum(<anno>NetTicktime</anno>,
PreviousNetTicktime)*1000 div 4</c> milliseconds.</p></item>
<tag>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> (that is,
((<c><anno>TransitionPeriod</anno>*1000 - 1) div MTTI + 1)*MTTI</c>
milliseconds).</p></item>
</taglist>
<p>If
<c><![CDATA[NetTicktime < PreviousNetTicktime]]></c>,
the <c>net_ticktime</c> change is done at the end of
the transition period; otherwise at the beginning. During
the transition period, <c>net_kernel</c> ensures that
there is outgoing traffic on all connections at least
every <c>MTTI</c> millisecond.</p>
<note>
<p>The <c>net_ticktime</c> changes must 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 can 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 has the value of
<c><anno>NetTicktime</anno></c> and is left unchanged.</p>
</item>
<tag><c>change_initiated</c></tag>
<item>
<p><c>net_kernel</c> 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 is <em>ignored</em> because
<c>net_kernel</c> is busy changing <c>net_ticktime</c> to
<c><anno>NewNetTicktime</anno></c> seconds.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name name="setopts" arity="2" since="OTP 19.1"/>
<fsummary>Set distribution socket options.</fsummary>
<desc>
<p>Set one or more options for distribution sockets.
Argument <c><anno>Node</anno></c> can be either one node name
or the atom <c>new</c> to affect the distribution sockets of all
future connected nodes.</p>
<p>The return value is the same as from
<seemfa marker="inet#setopts/2"><c>inet:setopts/2</c></seemfa>
or <c>{error, noconnection}</c> if <c><anno>Node</anno></c> is not
a connected node or <c>new</c>.</p>
<p>If <c><anno>Node</anno></c> is <c>new</c> the <c><anno>Options</anno></c>
will then also be added to kernel configration parameters
<seeapp marker="kernel:kernel_app#inet_dist_listen_options">inet_dist_listen_options</seeapp>
and
<seeapp marker="kernel:kernel_app#inet_dist_connect_options">inet_dist_connect_options</seeapp>.</p>
<p>Returns <c>ignored</c> if the local node is not alive.</p>
</desc>
</func>
<func>
<name since="">start([Name]) -> {ok, pid()} | {error, Reason}</name>
<name since="">start([Name, NameType]) -> {ok, pid()} | {error, Reason}</name>
<name since="">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>Turns a non-distributed node into a distributed node by
starting <c>net_kernel</c> and other necessary processes.</p>
<p>Notice 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 <c>15000</c>.</p>
</desc>
</func>
<func>
<name name="stop" arity="0" since=""/>
<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
<seemfa marker="#start/1"><c>start/1</c></seemfa>,
otherwise <c>{error, not_allowed}</c> is returned. Returns
<c>{error, not_found}</c> if the local node is not alive.</p>
</desc>
</func>
</funcs>
</erlref>
|