Control the way synchronous calls to defer are handled

When your app is synchronous (see Asynchronous operations & concurrency), you may want to defer tasks synchronously. In most cases, if you set up an asynchronous connector, Procrastinate will automatically select the right connector for you. However, you can also explicitly set up a synchronous connector.

SyncPsycopgConnector

By setting your App’s connector to an instance of SyncPsycopgConnector (or any other synchronous connector), you will get “classic” synchronous I/O. Note that in this case, the only thing you’ll be able to do is defer tasks. Other operations trigger an error with a synchronous connector.

import procrastinate

app = procrastinate.App(
    connector=procrastinate.SyncPsycopgConnector(
        host="somehost",
    ),
)

SyncPsycopgConnector uses a psycopg_pool.ConnectionPool (see psycopg documentation).

Psycopg2Connector

The Psycopg2Connector is a connector that uses a psycopg2_pool.ThreadedConnectionPool. It used to be the default connector, but SyncPsycopgConnector is now the preferred option. There is no plan to deprecate Psycopg2Connector.

SQLAlchemyPsycopg2Connector

If you use SQLAlchemy in your synchronous application, you may want to use an SQLAlchemyPsycopg2Connector from the contrib.sqlalchemy module instead. The advantage over using a Psycopg2Connector is that Procrastinate can use the same SQLAchemy engine (and connection pool) as the rest of your application, thereby minimizing the number of database connections.

from sqlalchemy import create_engine

from procrastinate import App
from procrastinate.contrib.sqlalchemy import SQLAlchemyPsycopg2Connector

engine = create_engine("postgresql+psycopg2://", echo=True)

app = App(connector=SQLAlchemyPsycopg2Connector())
app.open(engine)

Having multiple apps

If you need to have multiple connectors interact with the tasks, you can create multiple synchronized apps with App.with_connector():

import procrastinate


app = procrastinate.App(
    connector=procrastinate.PsycopgConnector(...),
)

sync_app = app.with_connector(
    connector=procrastinate.SyncPsycopgConnector(...),
)

Procrastinate’s automatic connector selection

Async connectors are able to summon their synchronous counterpart when needed (using BaseConnector.get_sync_connector).

All sync operations in Procrastinate (so mainly deferring tasks synchronously) will request the synchronous connector from the async connector under the hood.

Note

If you’re relying on this mechanism, note the following mechanism:

If you request the synchronous connector before opening the app, you will be using a synchronous connector.

If you request the synchronous connector after opening the app, you will get the asynchronous connector, with a compatibility layer to make synchronous operations. This will only work if you call it inside a function decorated with asgiref.sync.sync_to_async (such as inside a sync job). Otherwise, you will likely get a RuntimeError.