developer-notes.md

Developer notes

Streaming data types

The forwarder makes use of the following streaming-data-types schemas for encoding its data output and receiving configuration commands:

• Data schemas like f144, se00 and tdct to encode data from EPICS updates.
• ep01 to forward EPICS connection/disconnection events. Messages with this schema are automatically forwarded for all compatible data schemas (e.g. f144, se00, tdct).
• al00 to forward EPICS alarms. Messages with this schema are automatically forwarded for all compatible data schemas (e.g. f144, se00, tdct).
• x5f2 is used by the forwarder to push runtime status updates that can be used for monitoring.
• rf5k is a control message used to add/remove EPICS PVs to/from the forwarder's configuration. See the README section Configuring EPICS PVs to be forwarded for more information.

Periodic updates

The forwarder is designed to cache the last value received on all configured PVs and continuously forward this value to Kafka. This ensures that the last-known value is always available in the recent history of the Kafka topic, even if the last value change at the EPICS layer ocurred far in the past.

The period of these repeated updates is a global setting set via the --pv-update-period argument.

The cache and updates are managed by the SerialiserTracker class.

Class diagrams

UpdateHandlers and Serialisers