A distributed Erlang system consists of a number of Erlang runtime systems communicating with each other. Each such runtime system is called a node. 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.
The distribution mechanism is implemented using TCP/IP sockets. How to implement an alternative carrier is described in ERTS User's Guide.
A node is an executing Erlang runtime system which has
been given a name, using the command line flag -name
(long names) or -sname
(short names).
The format of the node name is an atom name@host
where
name
is the name given by the user and host
is
the full host name if long names are used, or the first part of
the host name if short names are used. node()
returns
the name of the node. Example:
% erl -name dilbert (dilbert@uab.ericsson.se)1> node(). 'dilbert@uab.ericsson.se' % erl -sname dilbert (dilbert@uab)1> node(). dilbert@uab
A node with a long node name cannot communicate with a node with a short node name. |
The nodes in a distributed Erlang system are loosely connected.
The first time the name of another node is used, for example if
spawn(Node,M,F,A)
or net_adm:ping(Node)
is called,
a connection attempt to that node will be made.
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 -connect_all false
, see
erl(1)
.
If a node goes down, all connections to that node are removed.
Calling erlang:disconnect(Node)
will force disconnection
of a node.
The list of (visible) nodes currently connected to is returned by
nodes()
.
The Erlang Port Mapper Daemon epmd 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 epmd(1)
.
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 hidden node may be used.
A hidden node is a node started with the command line flag
-hidden
. 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
nodes()
. Instead, nodes(hidden)
or
nodes(connected)
must be used. This means, for example,
that the hidden node will not be added to the set of nodes that
global
is keeping track of.
This feature was added in Erlang 5.0/OTP R7.
A C node is a C program written to act as a hidden node in a distributed Erlang system. The library Erl_Interface contains functions for this purpose. Refer to the documentation for Erl_Interface and Interoperability Tutorial for more information about C nodes.
Nodes are protected by a magic cookie system. When a connection attempt is made from a node A to a node B, node A provide the cookie that has been set for node B. If the magic cookie is not correct, the connection attempt is rejected.
If the node is started with the command line flag
-setcookie Cookie
, then the cookie will be assumed to be
the atom Cookie
for all other nodes.
If the node is started without the command line flag, then
the node will create the cookie from the contents of the file
$HOME/.erlang.cookie
, where $HOME
is the user's
home directory. If the file does not exist, it will be created
and contain a random character sequence.
Which magic cookie to use when connecting to another node can
be changed by calling erlang:set_cookie/2
.
Some useful BIFs for distributed programming, see
erlang(3)
for more information:
erlang:disconnect_node(Node)
|
Forces the disconnection of a node. |
erlang:get_cookie()
|
Returns the magic cookie of the current node. |
is_alive()
|
Returns true if the runtime system is a node and
can connect to other nodes, false otherwise.
|
monitor_node(Node, true|false)
|
Monitor the status of Node . A message
{nodedown, Node} is received if the connection to it
is lost.
|
node()
|
Returns the name of the current node. Allowed in guards. |
node(Arg)
|
Returns the node where Arg , a pid, reference, or
port, is located.
|
nodes()
|
Returns a list of all visible nodes this node is connected to. |
nodes(Arg)
|
Depending on Arg , this function can return a list
not only of visible nodes, but also hidden nodes and
previously known nodes, etc.
|
set_cookie(Node, Cookie)
|
Sets the magic cookie used when connecting to Node .
If Node is the current node, Cookie will be used
when connecting to all new nodes.
|
spawn[_link|_opt](Node, Fun)
|
Creates a process at a remote node. |
spawn[_link|opt](Node, Module, FunctionName, Args)
|
Creates a process at a remote node. |
Examples of command line flags used for distributed programming,
see erl(1)
for more information:
-connect_all false
|
Only explicit connection set-ups will be used. |
-hidden
|
Makes a node into a hidden node. |
-name Name
|
Makes a runtime system into a node, using long node names. |
-setcookie Cookie
|
Same as calling erlang:set_cookie(node(), Cookie) .
|
-sname Name
|
Makes a runtime system into a node, using short node names. |
Examples of modules useful for distributed programming:
In Kernel:
global
|
A global name registration facility. |
global_group
|
Grouping nodes to global name registration groups. |
net_adm
|
Various Erlang net administration routines. |
net_kernel
|
Erlang networking kernel. |
In STDLIB:
slave
|
Start and control of slave nodes. |