fix: leave Granian backpressure at its default (#82)

Author: lesnik512

AGENTS.md

@@ -36,7 +36,7 @@ Python is 3.14, dependencies managed by `uv`. The API is exposed on `:8000`.
 
 `UseCases` group (request-scoped): one provider per use case in `app/use_cases/`. modern-di fills each dataclass field by its type, so a use case needs no `kwargs`.
 
-**Pool and server sizing**: `DB_POOL_SIZE` (positive) and `DB_MAX_OVERFLOW` (non-negative) size the pool, and `app/__main__.py` sets Granian's `backpressure` to their sum, so each worker accepts only as many requests at once as it has connections. Requests beyond that wait in Granian's socket backlog instead of queueing on the pool. `APP_WORKERS` sets the Granian worker count, and each worker has its own pool. `DB_POOL_TIMEOUT` (default 30 s) bounds the wait for a connection, and a timeout returns 503 through `exceptions.database_unavailable_handler`. advanced-alchemy wraps the timeout in a `RepositoryError`, so the handler is also registered for `RepositoryError` and answers anything not caused by a pool timeout the way Litestar's default handler would: the debug page when `SERVICE_DEBUG` is on, a plain 500 otherwise. `DB_POOL_RECYCLE` defaults to SQLAlchemy's `-1` (never). Set it below the idle timeout of any proxy or load balancer in front of Postgres, so a connection it dropped is not handed out and retried after a 1-2 s backoff.
+**Pool and server sizing**: `DB_POOL_SIZE` (positive) and `DB_MAX_OVERFLOW` (non-negative) size the pool, and a request that finds it empty waits for a connection. Granian's `backpressure` is left at its default: it limits open connections per worker, idle keep-alive ones included, so setting it to the pool size makes new connections wait while the app is idle. `APP_WORKERS` sets the Granian worker count, and each worker has its own pool. `DB_POOL_TIMEOUT` (default 30 s) bounds the wait for a connection, and a timeout returns 503 through `exceptions.database_unavailable_handler`. advanced-alchemy wraps the timeout in a `RepositoryError`, so the handler is also registered for `RepositoryError` and answers anything not caused by a pool timeout the way Litestar's default handler would: the debug page when `SERVICE_DEBUG` is on, a plain 500 otherwise. `DB_POOL_RECYCLE` defaults to SQLAlchemy's `-1` (never). Set it below the idle timeout of any proxy or load balancer in front of Postgres, so a connection it dropped is not handed out and retried after a 1-2 s backoff.
 
 **Use cases and lost connections**: `pool_pre_ping` is off by default (`DB_POOL_PRE_PING`). A pooled connection that died while idle is handled by `db_retry.postgres_retry` instead, which saves a round trip on every checkout. That only works if the retried function owns its whole transaction, so every use case has this shape:
 

app/__main__.py

@@ -11,7 +11,6 @@
         address=settings.app_host,
         port=settings.app_port,
         workers=settings.app_workers,
-        backpressure=settings.db_pool_size + settings.db_max_overflow,
         interface=Interfaces.ASGI,
         log_level=settings.log_level,
         loop=Loops.uvloop,

tests/test_main.py

@@ -17,13 +17,11 @@ def test_main(monkeypatch: pytest.MonkeyPatch) -> None:
     granian = mock.Mock()
     monkeypatch.setattr("granian.Granian", granian)
     monkeypatch.setattr(settings, "app_workers", 4)
-    monkeypatch.setattr(settings, "db_pool_size", 5)
-    monkeypatch.setattr(settings, "db_max_overflow", 2)
 
     runpy.run_module("app.__main__", run_name="__main__")
 
     assert granian.call_args.kwargs["workers"] == 4  # noqa: PLR2004
-    assert granian.call_args.kwargs["backpressure"] == 7  # noqa: PLR2004
+    assert "backpressure" not in granian.call_args.kwargs
 
 
 async def test_session() -> None: