effect package

Motivation

When processing an operation, Temporal's persistence accumulates changes to the mutable state and only commits them to the database at the end of the operation. For a Workflow Update, the caller of the UpdateWorkflowExecution API can only be unblocked after the database write has successfully completed, ensuring that the update is durable.

Unblocking the caller immediately after processing the worker's Update result in RespondWorkflowTaskCompleted would introduce a poor design by tightly coupling these two separate concerns.

To address this, the effect package was introduced.

How it works

effect.Buffer allows to accumulate callbacks - in the same way persistence accumulates changes

• and exposes methods to Apply() or Cancel() them.

For each operation, two callbacks should be registered: one for the success case, and one for when the operation needs to be canceled. Then, the Apply() method should be called right after the operation succeeded - or Cancel() when it failed (usually in the defer block). The order the callbacks are executed in is the same order they were registered when using Apply(), and reversed for Cancel().

NOTE

It is important to note that the effect package does not provide any transactional guarantees. For example, the callbacks in effect.Buffer can be partially applied (or not applied at all) after the persistence write completed successfully (which is acceptable for the Workflow Update case).

Usage

This package was created specifically for the Workflow Update feature, and is currently used only there. It is not coupled with Workflow Update, though, and can be used anywhere where delayed execution is required.

TODO

workflowTaskResponseMutation is one of the candidates to migrate to effect package.