hydra/doc/manual/src/notifications.md

hydra-notify and Hydra's Notifications

Hydra uses a notification-based subsystem to implement some features and support plugin development. Notifications are sent to hydra-notify, which is responsible for dispatching each notification to each plugin.

Notifications are passed from hydra-queue-runner to hydra-notify through Postgres's NOTIFY and LISTEN feature.

Notification Types

Note that the notification format is subject to change and should not be considered an API. Integrate with hydra-notify instead of listening directly.

build_started

• Payload: Exactly one value, the ID of the build.
• When: Issued directly before building happens, and only if the derivation's outputs cannot be subsituted.
• Delivery Semantics: Ephemeral. hydra-notify must be running to react to this event. No record of this event is stored.

step_finished

• Payload: Three values, tab separated: The ID of the build which the step is part of, the step number, and the path on disk to the log file.
• When: Issued directly after a step completes, regardless of success. Is not issued if the step's derivation's outputs can be substituted.
• Delivery Semantics: Ephemeral. hydra-notify must be running to react to this event. No record of this event is stored.

build_finished

• Payload: At least one value, tab separated: The ID of the build which finished, followed by IDs of all of the builds which also depended upon this build.
• When: Issued directly after a build completes, regardless of success and substitutability.
• Delivery Semantics: At least once.

hydra-notify will call buildFinished for each plugin in two ways:

• The builds table's notificationspendingsince column stores when the build finished. On startup, hydra-notify will query all builds with a non-null notificationspendingsince value and treat each row as a received build_finished event.

• Additionally, hydra-notify subscribes to build_finished events and processes them in real time.

After processing, the row's notificationspendingsince column is set to null.

It is possible for subsequent deliveries of the same build_finished data to imply different outcomes. For example, if the build fails, is restarted, and then succeeds. In this scenario the build_finished events will be delivered at least twice, once for the failure and then once for the success.

Development Notes

Re-sending a notification

Notifications can be experimentally re-sent on the command line with psql, with NOTIFY $notificationname '$payload'.