Features are implemented in specific libraries and can be enabled using CLI commands.

Features either write specific data or receive data.

Examples for writing data: DB IDO, Graphite, InfluxDB. GELF, etc. Examples for receiving data: REST API, etc.

The implementation of features makes use of existing libraries and functionality. This makes the code more abstract, but shorter and easier to read.

Features register callback functions on specific events they want to handle. For example the GraphiteWriter feature subscribes to new CheckResult events.

Each time Icinga 2 receives and processes a new check result, this event is triggered and forwarded to all subscribers.

The GraphiteWriter feature calls the registered function and processes the received data. Features which connect Icinga 2 to external interfaces normally parse and reformat the received data into an applicable format.

Since this check result signal is blocking, many of the features include a work queue with asynchronous task handling.

The GraphiteWriter uses a TCP socket to communicate with the carbon cache daemon of Graphite. The InfluxDBWriter is instead writing bulk metric messages to InfluxDB’s HTTP API, similar to Elasticsearch.