The net_ticktime changes have to be initiated on all nodes in the network (with the same NetTicktime) before the end of any transition period on any node; otherwise, connections may erroneously be disconnected.
The net kernel is a system process, registered as net_kernel, which must be running for distributed Erlang to work. The purpose of this process is to implement parts of the BIFs spawn/4 and spawn_link/4, and to provide monitoring of the network.
An Erlang node is started using the command line flag -name or -sname:
$ erl -sname foobar
It is also possible to call net_kernel:start([foobar]) directly from the normal Erlang shell prompt:
1> net_kernel:start([foobar, shortnames]). {ok,<0.64.0>} (foobar@gringotts)2>
If the node is started with the command line flag -sname, the node name will be foobar@Host, where Host is the short name of the host (not the fully qualified domain name). If started with the -name flag, Host is the fully qualified domain name. See erl(1).
Normally, connections are established automatically when another node is referenced. This functionality can be disabled by setting the Kernel configuration parameter dist_auto_connect to false, see kernel(6). In this case, connections must be established explicitly by calling net_kernel:connect_node/1.
Which nodes are allowed to communicate with each other is handled by the magic cookie system, see Distributed Erlang in the Erlang Reference Manual.
Types:
Nodes = [node()]
Limits access to the specified set of nodes. Any access attempts made from (or to) nodes not in Nodes will be rejected.
Returns error if any element in Nodes is not an atom.
connect_node(Node) -> true | false | ignored
Types:
Node = node()
Establishes a connection to Node. Returns true if successful, false if not, and ignored if the local node is not alive.
monitor_nodes(Flag) -> ok | Error
monitor_nodes(Flag, Options) -> ok | Error
Types:
Flag = true | false
Options = [Option]
Option -- see below
Error = error | {error, term()}
The calling process subscribes or unsubcscribes to node status change messages. A nodeup message is delivered to all subscribing process when a new node is connected, and a nodedown message is delivered when a node is disconnected.
If Flag is true, a new subscription is started. If Flag is false, all previous subscriptions -- started with the same Options -- are stopped. Two option lists are considered the same if they contain the same set of options.
As of kernel version 2.11.4, and erts version 5.5.4, the following is guaranteed:
Note, that this is not guaranteed for kernel versions before 2.11.4.
As of kernel version 2.11.4 subscribtions can also be made before the net_kernel server has been started, i.e., net_kernel:monitor_nodes/[1,2] does not return ignored.
As of kernel version 2.13, and erts version 5.7, the following is guaranteed:
Note, that this is not guaranteed for kernel versions before 2.13.
The format of the node status change messages depends on Options. If Options is [], which is the default, the format is:
{nodeup, Node} | {nodedown, Node} Node = node()
If Options /= [], the format is:
{nodeup, Node, InfoList} | {nodedown, Node, InfoList} Node = node() InfoList = [{Tag, Val}]
InfoList is a list of tuples. Its contents depends on Options, see below.
Also, when OptionList == [] only visible nodes, that is, nodes that appear in the result of nodes/0, are monitored.
Option can be any of the following:
Types:
Res = NetTicktime | {ongoing_change_to, NetTicktime}
NetTicktime = int()
Gets net_ticktime (see kernel(6)).
Currently defined return values (Res):
set_net_ticktime(NetTicktime) -> Res
set_net_ticktime(NetTicktime, TransitionPeriod) -> Res
Types:
NetTicktime = int() > 0
TransitionPeriod = int() >= 0
Res = unchanged | change_initiated | {ongoing_change_to, NewNetTicktime}
NewNetTicktime = int() > 0
Sets net_ticktime (see kernel(6)) to NetTicktime seconds. TransitionPeriod defaults to 60.
Some definitions:
If NetTicktime < PreviousNetTicktime, the actual net_ticktime change will be done at the end of the transition period; otherwise, at the beginning. During the transition period, net_kernel will ensure that there will be outgoing traffic on all connections at least every MTTI millisecond.
The net_ticktime changes have to be initiated on all nodes in the network (with the same NetTicktime) before the end of any transition period on any node; otherwise, connections may erroneously be disconnected.
Returns one of the following:
start([Name]) -> {ok, pid()} | {error, Reason}
start([Name, NameType]) -> {ok, pid()} | {error, Reason}
start([Name, NameType, Ticktime]) -> {ok, pid()} | {error, Reason}
Types:
Name = atom()
NameType = shortnames | longnames
Reason = {already_started, pid()} | term()
Note that the argument is a list with exactly one, two or three arguments. NameType defaults to longnames and Ticktime to 15000.
Turns a non-distributed node into a distributed node by starting net_kernel and other necessary processes.
stop() -> ok | {error, not_allowed | not_found}
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 start/1, otherwise returns {error, not_allowed}. Returns {error, not_found} if the local node is not alive.