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Unit of work

The unit of work is used to group multiple service operations into a single atomic unit. The Unit of Work maintains a list of operations and coordinates the commit, indexing and task execution.

The main purpose of the Unit of Work in Invenio is to coordinate when the database transaction commit is called, and ensure tasks that have to run after the transaction are executed (such as indexing and running celery tasks).

This ensures that we can group multiple service calls into a single database transaction and perform the necessary indexing/task execution afterwards.

When to use?

You should use the unit of work instead of running an explicit db.session.commit() or db.session.rollback() in the code. Basically any function where you would normally have called a db.session.commit() should be changed to something like:

from import \
    RecordCommitOp, unit_of_work,

def create(self, ... , uow=None):
    # ...
    uow.register(RecordCommitOp(record, indexer=self.indexer))
    # ...
    # Do not use `db.session.commit()` in service.

Any private method that need to run operations after the database transaction commit should take the unit of work as input:

def _reindex_something(uow, ...):
    # Index after transaction (no record commit)

def _send_a_task(uow, ...):
    # Run a celery task after the database transaction commit.
    uow.register(TaskOp(my_celery_task, myarg, ... ))

When not to use?

If you're not changing the database state there's no need to use the unit of work. Examples include:

  • Reading a record
  • Search for records
  • Reindex all records - because there's no database transaction involved, and the method is also not intended to be grouped together with multiple other state changing service calls there's no need to use the unit of work.

How to group multiple service calls?

In order to group multiple service calls into one atomic operation you can use the following pattern:

from import UnitOfWork

with UnitOfWork() as uow:
    # Be careful to always inject "uow" to the service. If not, the
    # service will create its own unit of work and commit.
    service.communities.add(..., uow=uow)
    service.publish(... , uow=uow)

If you're not grouping multiple service calls, then simply just call the service method (and it will commit automatically):


Writing your own operation?

You can write your own unit of work operation by subclassing the operation class and implementing the desired methods:

from import Operation

class BulkIndexOp(Operation):
    def on_commit(self, uow):
        # ... executed after the database transaction commit ...