Event Handlers

Serf's true power and flexibility comes in the form of event handlers: scripts that are executed in response to various events that can occur related to the Serf cluster. Serf invokes events related to membership changes (when a node comes online or goes offline) as well as custom events or queries.

Event handlers can be any executable, including piped executables (such as awk '{print $2}' | grep foo), since event handlers are invoked within the context of a shell. The event handler is executed anytime an event occurs and is expected to exit within a reasonable amount of time.

Inputs and Parameters

Every time an event handler is invoked, Serf sets some environmental variables:

  • SERF_EVENT is the event type that is occurring. This will be one of member-join, member-leave, member-failed, member-update, member-reap, user, or query.

  • SERF_SELF_NAME is the name of the node that is executing the event handler.

  • SERF_SELF_ROLE is the role of the node that is executing the event handler.

  • SERF_TAG_${TAG} is set for each tag the agent has. The tag name is upper-cased.

  • SERF_USER_EVENT is the name of the user event if SERF_EVENT is "user".

  • SERF_USER_LTIME is the LamportTime of the user event if SERF_EVENT is "user".

  • SERF_QUERY_NAME is the name of the query if SERF_EVENT is "query".

  • SERF_QUERY_LTIME is the LamportTime of the query if SERF_EVENT is "query".

In addition to these environmental variables, the data for an event is passed in via stdin. The format of the data is dependent on the event type.

If the SERF_EVENT is "query", then the handler is also used to generate a query response. If the handler exits with a 0 status code, then any output on stdout and stderr is used as the response. The response should be of a limited size or the response will fail to send due to size restrictions.

Membership Event Data

For membership related events (member-join, member-leave, member-failed, member-update, and member-reap), stdin is the list of members that participated in that event. Each member is separated by a newline and each field about the member is separated by a single tab (\t). The fields of a membership event are name, address, role, then tags. For example:

mitchellh.local    web    role=web,datacenter=east

User Event Data

For user events, stdin is the payload (if any) of the user event.

Query Data

For queries, stdin is the payload (if any) of the query.

Specifying Event Handlers

Event handlers are specified using the -event-handler flag for serf agent. This flag can be specified multiple times for multiple event handlers, in which case each event handler will be executed.

Event handlers can also be filtered by event type. By default, the event handler will be invoked for any event which may occur. But you can restrict the events the event handler is invoked for by using a simple syntax of type=script. Below are all the available ways this syntax can be used to filter an event handler:

  • foo.sh - The script "foo.sh" will be invoked for any/every event.

  • member-join=foo.sh - The script "foo.sh" will only be invoked for the "member-join" event.

  • member-join,member-leave=foo.sh - The script "foo.sh" will be invoked for either member-join or member-leave events. Any combination of events may be specified in this way.

  • user=foo.sh - The script "foo.sh" will be invoked for all user events.

  • user:deploy=foo.sh - The script "foo.sh" will be invoked only for "deploy" user events.

  • query=foo.sh - The script "foo.sh" will be invoked for all queries.

  • query:load=uptime - The uptime command will be invoked only for "load" queries.

Forking event handlers

There are some cases where it may be desirable to fork a background process when an event handler fires. This is mainly useful for invoking scripts which take a minute or two to execute, and where the process output is not important. By default, Serf's execution subsystem will block, waiting for output and a return code. It is possible, however, to "detach" a process by forking and replacing the file descriptors for both stdout and stderr.

In shell, this would look something like:

sleep 5 &>/dev/null &

In the above example, sleep 5 is the lengthy process. Notice the first ampersand, which copies the file descriptor instead of just redirecting output.

Similarly, in Python this might look like:

out_log = file('/dev/null', 'a+')
os.dup2(out_log.fileno(), sys.stdout.fileno())
os.dup2(out_log.fileno(), sys.stderr.fileno())

Or in ruby:

$stdout.reopen('/dev/null', 'w')
$stderr.reopen('/dev/null', 'w')

Note: This method is really only useful for event handlers, and is mostly useless for queries.