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<h1>12 Distributed Erlang</h1>
<h3><a name="id83013">12.1
Distributed Erlang System</a></h3>
<p>A <strong>distributed Erlang system</strong> consists of a number of
Erlang runtime systems communicating with each other. Each such
runtime system is called a <strong>node</strong>. Message passing between
processes at different nodes, as well as links and monitors, are
transparent when pids are used. Registered names, however, are
local to each node. This means the node must be specified as well
when sending messages etc. using registered names.</p>
<p>The distribution mechanism is implemented using TCP/IP sockets.
How to implement an alternative carrier is described in <strong>ERTS User's Guide</strong>.</p>
<h3><a name="id83044">12.2
Nodes</a></h3>
<p>A <strong>node</strong> is an executing Erlang runtime system which has
been given a name, using the command line flag <span class="code">-name</span>
(long names) or <span class="code">-sname</span> (short names).</p>
<p>The format of the node name is an atom <span class="code">name@host</span> where
<span class="code">name</span> is the name given by the user and <span class="code">host</span> is
the full host name if long names are used, or the first part of
the host name if short names are used. <span class="code">node()</span> returns
the name of the node. Example:</p>
<div class="example"><pre>
% <span class="bold_code">erl -name dilbert</span>
(dilbert@uab.ericsson.se)1> <span class="bold_code">node().</span>
'dilbert@uab.ericsson.se'
% <span class="bold_code">erl -sname dilbert</span>
(dilbert@uab)1> <span class="bold_code">node().</span>
dilbert@uab</pre></div>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>A node with a long node name cannot communicate with a node
with a short node name.</p>
</p></div>
</div>
<h3><a name="id83114">12.3
Node Connections</a></h3>
<p>The nodes in a distributed Erlang system are loosely connected.
The first time the name of another node is used, for example if
<span class="code">spawn(Node,M,F,A)</span> or <span class="code">net_adm:ping(Node)</span> is called,
a connection attempt to that node will be made.</p>
<p>Connections are by default transitive. If a node A connects to
node B, and node B has a connection to node C, then node A will
also try to connect to node C. This feature can be turned off by
using the command line flag <span class="code">-connect_all false</span>, see
<span class="code">erl(1)</span>.</p>
<p>If a node goes down, all connections to that node are removed.
Calling <span class="code">erlang:disconnect_node(Node)</span> will force disconnection
of a node.</p>
<p>The list of (visible) nodes currently connected to is returned by
<span class="code">nodes()</span>.</p>
<h3><a name="id83164">12.4
epmd</a></h3>
<p>The Erlang Port Mapper Daemon <strong>epmd</strong> is automatically
started at every host where an Erlang node is started. It is
responsible for mapping the symbolic node names to machine
addresses. See <span class="code">epmd(1)</span>.</p>
<h3><a name="id83184">12.5
Hidden Nodes</a></h3>
<p>In a distributed Erlang system, it is sometimes useful to
connect to a node without also connecting to all other nodes.
An example could be some kind of O&M functionality used to
inspect the status of a system without disturbing it. For this
purpose, a <strong>hidden node</strong> may be used.</p>
<p>A hidden node is a node started with the command line flag
<span class="code">-hidden</span>. Connections between hidden nodes and other nodes
are not transitive, they must be set up explicitly. Also, hidden
nodes does not show up in the list of nodes returned by
<span class="code">nodes()</span>. Instead, <span class="code">nodes(hidden)</span> or
<span class="code">nodes(connected)</span> must be used. This means, for example,
that the hidden node will not be added to the set of nodes that
<span class="code">global</span> is keeping track of.</p>
<p>This feature was added in Erlang 5.0/OTP R7.</p>
<h3><a name="id83230">12.6
C Nodes</a></h3>
<p>A <strong>C node</strong> is a C program written to act as a hidden node
in a distributed Erlang system. The library <strong>Erl_Interface</strong>
contains functions for this purpose. Refer to the documentation
for Erl_Interface and <strong>Interoperability Tutorial</strong> for more
information about C nodes.</p>
<h3><a name="id83254">12.7
Security</a></h3>
<p>Authentication determines which nodes are allowed to communicate
with each other. In a network of different Erlang nodes, it is
built into the system at the lowest possible level. Each node has
its own <strong>magic cookie</strong>, which is an Erlang atom.</p>
<p>When a nodes tries to connect to another node, the magic cookies
are compared. If they do not match, the connected node rejects
the connection.</p>
<p>At start-up, a node has a random atom assigned as its magic
cookie and the cookie of other nodes is assumed to be
<span class="code">nocookie</span>. The first action of the Erlang network
authentication server (<span class="code">auth</span>) is then to read a file named
<span class="code">$HOME/.erlang.cookie</span>. If the file does not exist, it is
created. The UNIX permissions mode of the file is set to octal
400 (read-only by user) and its contents are a random string. An
atom <span class="code">Cookie</span> is created from the contents of the file and
the cookie of the local node is set to this using
<span class="code">erlang:set_cookie(node(), Cookie)</span>. This also makes
the local node assume that all other nodes have the same cookie
<span class="code">Cookie</span>.</p>
<p>Thus, groups of users with identical cookie files get Erlang
nodes which can communicate freely and without interference from
the magic cookie system. Users who want run nodes on separate
file systems must make certain that their cookie files are
identical on the different file systems.</p>
<p>For a node <span class="code">Node1</span> with magic cookie <span class="code">Cookie</span> to be
able to connect to, or accept a connection from, another node
<span class="code">Node2</span> with a different cookie <span class="code">DiffCookie</span>,
the function <span class="code">erlang:set_cookie(Node2, DiffCookie)</span> must
first be called at <span class="code">Node1</span>. Distributed systems with
multiple user IDs can be handled in this way.</p>
<p>The default when a connection is established between two nodes,
is to immediately connect all other visible nodes as well. This
way, there is always a fully connected network. If there are
nodes with different cookies, this method might be inappropriate
and the command line flag <span class="code">-connect_all false</span> must be set,
see <span class="bold_code"><a href="javascript:erlhref('../../','erts','erl.html');">erl(1)</a></span>.</p>
<p>The magic cookie of the local node is retrieved by calling
<span class="code">erlang:get_cookie()</span>.</p>
<h3><a name="id83364">12.8
Distribution BIFs</a></h3>
<p>Some useful BIFs for distributed programming, see
<span class="code">erlang(3)</span> for more information:</p>
<table border="1" cellpadding="2" cellspacing="0">
<tr>
<td align="left" valign="middle"><span class="code">erlang:disconnect_node(Node)</span></td>
<td align="left" valign="middle">Forces the disconnection of a node.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">erlang:get_cookie()</span></td>
<td align="left" valign="middle">Returns the magic cookie of the current node.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">is_alive()</span></td>
<td align="left" valign="middle">Returns <span class="code">true</span> if the runtime system is a node and can connect to other nodes, <span class="code">false</span> otherwise.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">monitor_node(Node, true|false)</span></td>
<td align="left" valign="middle">Monitor the status of <span class="code">Node</span>. A message<span class="code">{nodedown, Node}</span> is received if the connection to it is lost.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">node()</span></td>
<td align="left" valign="middle">Returns the name of the current node. Allowed in guards.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">node(Arg)</span></td>
<td align="left" valign="middle">Returns the node where <span class="code">Arg</span>, a pid, reference, or port, is located.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">nodes()</span></td>
<td align="left" valign="middle">Returns a list of all visible nodes this node is connected to.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">nodes(Arg)</span></td>
<td align="left" valign="middle">Depending on <span class="code">Arg</span>, this function can return a list not only of visible nodes, but also hidden nodes and previously known nodes, etc.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">set_cookie(Node, Cookie)</span></td>
<td align="left" valign="middle">Sets the magic cookie used when connecting to <span class="code">Node</span>. If <span class="code">Node</span> is the current node, <span class="code">Cookie</span> will be used when connecting to all new nodes.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">spawn[_link|_opt](Node, Fun)</span></td>
<td align="left" valign="middle">Creates a process at a remote node.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">spawn[_link|opt](Node, Module, FunctionName, Args)</span></td>
<td align="left" valign="middle">Creates a process at a remote node.</td>
</tr>
</table>
<em>Table
12.1:
Distribution BIFs.</em>
<h3><a name="id83662">12.9
Distribution Command Line Flags</a></h3>
<p>Examples of command line flags used for distributed programming,
see <span class="code">erl(1)</span> for more information:</p>
<table border="1" cellpadding="2" cellspacing="0">
<tr>
<td align="left" valign="middle"><span class="code">-connect_all false</span></td>
<td align="left" valign="middle">Only explicit connection set-ups will be used.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">-hidden</span></td>
<td align="left" valign="middle">Makes a node into a hidden node.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">-name Name</span></td>
<td align="left" valign="middle">Makes a runtime system into a node, using long node names.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">-setcookie Cookie</span></td>
<td align="left" valign="middle">Same as calling <span class="code">erlang:set_cookie(node(), Cookie)</span>.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">-sname Name</span></td>
<td align="left" valign="middle">Makes a runtime system into a node, using short node names.</td>
</tr>
</table>
<em>Table
12.2:
Distribution Command Line Flags.</em>
<h3><a name="id83799">12.10
Distribution Modules</a></h3>
<p>Examples of modules useful for distributed programming:</p>
<p>In Kernel:</p>
<table border="1" cellpadding="2" cellspacing="0">
<tr>
<td align="left" valign="middle"><span class="code">global</span></td>
<td align="left" valign="middle">A global name registration facility.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">global_group</span></td>
<td align="left" valign="middle">Grouping nodes to global name registration groups.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">net_adm</span></td>
<td align="left" valign="middle">Various Erlang net administration routines.</td>
</tr>
<tr>
<td align="left" valign="middle"><span class="code">net_kernel</span></td>
<td align="left" valign="middle">Erlang networking kernel.</td>
</tr>
</table>
<em>Table
12.3:
Kernel Modules Useful For Distribution.</em>
<p>In STDLIB:</p>
<table border="1" cellpadding="2" cellspacing="0"><tr>
<td align="left" valign="middle"><span class="code">slave</span></td>
<td align="left" valign="middle">Start and control of slave nodes.</td>
</tr></table>
<em>Table
12.4:
STDLIB Modules Useful For Distribution.</em>
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