Backend Interface

Backend modules are Node.js [modules][nodemods] that listen for a number of events emitted from StatsD. Each backend module should export the following initialization function:

• init(startup_time, config, events, logger): This method is invoked from StatsD to initialize the backend module. It accepts four parameters: startup_time is the startup time of StatsD in epoch seconds, config is the parsed config file hash, events is the event emitter that backends can use to listen for events and logger is StatsD's configured logger for backends to use.

  The backend module should return true from init() to indicate success. A return of false indicates a failure to load the module (missing configuration?) and will cause StatsD to exit.

Backends can listen for the following events emitted by StatsD from the events object:

• Event: 'flush'

  Parameters: (time_stamp, metrics)

  Emitted on each flush interval so that backends can push aggregate metrics to their respective backend services. The event is passed two parameters: time_stamp is the current time in epoch seconds and metrics is a hash representing the StatsD statistics:

metrics: { counters: counters, gauges: gauges, timers: timers, sets: sets, counter_rates: counter_rates, timer_data: timer_data, statsd_metrics: statsd_metrics, pctThreshold: pctThreshold }

The counter_rates and timer_data are precalculated statistics to simplify
the creation of backends, the statsd_metrics hash contains metrics generated
by statsd itself. Each backend module is passed the same set of
statistics, so a backend module should treat the metrics as immutable
structures. StatsD will reset timers and counters after each
listener has handled the event.

* Event: **'status'**

Parameters: `(writeCb)`

Emitted when a user invokes a *stats* command on the management
server port. It allows each backend module to dump backend-specific
status statistics to the management port.

The `writeCb` callback function has a signature of `f(error,
backend_name, stat_name, stat_value)`. The backend module should
invoke this method with each stat_name and stat_value that should be
sent to the management port. StatsD will prefix each stat name with
the `backend_name`. The backend should set `error` to *null*, or, in
the case of a failure, an appropriate error.

* Event: **'packet'**

Parameters: `(packet, rinfo)`

This is emitted for every incoming packet. The `packet` parameter contains
the raw received message string and the `rinfo` parameter contains remote
address information from the UDP socket.