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Background processing

For executing heavier requests and performing background tasks Waldur is using Celery. Celery is a task queue that supports multiple backends for storing the tasks and results. Currently Waldur is relying on Redis backend - Redis server must be running for requests triggering background scheduling to succeed.

If you are developing on OS X and have brew installed:

brew install redis-server

Please see Redis docs for installation on other platforms.

Finite state machines

Some of the models in Waldur have a state field representing their current condition. The state field is implemented as a finite state machine. Both user requests and background tasks can trigger state transition. A REST client can observe changes to the model instance through polling the state field of the object.

Let's take VM instance in 'offline' state. A user can request the instance to start by issuing a corresponding request over REST. This will schedule a task in Celery and transition instance status to 'starting_scheduled'. Further user requests for starting an instance will get state transition validation error. Once the background worker starts processing the queued task, it updates the Instance status to the 'starting'. On task successful completion, the state is transitioned to 'online' by the background task.

Error state of background tasks

If a background task has failed to achieve it's goal, it should transit into an error state. To propagate more information to the user each model with an FSM field should include a field for error message information - error_message. The field should be exposed via REST. Background task should update this field before transiting into an erred state.

Cleaning of the error state of the model instance should clean up also error_message field.

Last update: 2021-09-14