without-durability-postgres

without-durability's two interfaces over one Postgres: three tables, and no mechanism of its own.

from psycopg_pool import AsyncConnectionPool
from without_durability_postgres import PostgresCheckpointer, PostgresDurable, PostgresScheduler, migrate

pool = AsyncConnectionPool(dsn, open=False)
await pool.open(wait=True)
await migrate(pool)
durable = PostgresDurable(PostgresCheckpointer(pool=pool), PostgresScheduler(pool=pool))

What is worth reading this package for is how little of it is mechanism. Every write the Redis store needs a Lua script for is one statement here, or one transaction, and neither is something this package supplies. Redis needs scripts because it has no way to say "check this, then write that, and let nobody in between"; SQL says it by default. The claim is an upsert whose DO UPDATE carries a WHERE on the lease; the fenced record is one statement whose FOR UPDATE CTE serializes it against a claim in flight; the queue takes with FOR UPDATE SKIP LOCKED, so several workers polling one table fan out instead of queueing on its head.

Three things that are live questions over Redis do not arise. A workflow id is a query parameter rather than key structure, so it carries no constraints at all. Nothing expires, so the TTL that can lose a suspended workflow is gone, and the fencing token can be an ordinary counter. And a default Postgres commits synchronously, so record returning means what the durable runner assumes it means.

SqlEffect is a callback handed a cursor inside the open transaction, so a step's own business write and its checkpoint commit together: exactly-once for that step, over the application's own tables. PostgresDurable extends that to the interface above, committing a value's arrival and the workflow's wakeup at once, which is what makes "you need no second system" a claim this can actually make. It is also where "one datastore" stops meaning "Postgres", since a transaction is local only on a single node.

See the without-durability-postgres guide (with the API reference) for the statements, the schema, what co-location a sharded deployment owes, and what a deployment still owes anyway (a sweep, and a real migration tool).