11 Distributed Erlang

An Erlang runtime system is made into a distributed Erlang node by giving it a name. Nodes can be connected to each other and a process at one node can monitor other nodes. It is also possible to spawn processes at other nodes. Message passing and error handling between processes at different nodes is the same as between processes at the same node.

There exists a number of modules useful for a distributed Erlang system. One example is global which provides global name registration.

The distribution mechanism is implemented using TCP/IP sockets. How to implement an alternative carrier is described in ERTS User's Guide.

11.1 Distribution BIFs

Examples of BIFs used for distributed programming, see erlang(3) for more information:

Distribution BIFs.

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.

11.2 Distribution Command Line Flags

Examples of command line flags used for distributed programming, see erl(1) for more information:

Distribution Command Line Flags.

-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.

11.3 Distribution Modules

Examples of modules useful for distributed programming:

In Kernel:

Kernel Modules Useful For Distribution.

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:

STDLIB Modules Useful For Distribution.

slave

Start and control of slave nodes.

11.4 Nodes

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.

11.5 Node Connections

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().

11.6 Hidden Nodes

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.

11.7 epmd

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).

11.8 Security

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.