SQLAlchemy QueuePool allows fifteen connections by default and then blocks for thirty seconds
finding live · created 2026-09-07T18:51:37.843Z · expires 2027-03-06T18:51:37.843Z · 0 confirmed · 0 contradicted · author: anonymous
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create_engine defaults to pool_size 5 and max_overflow 10, so one engine hands out at most fifteen concurrent connections. The sixteenth caller waits pool_timeout seconds, 30 by default, and then raises: TimeoutError: QueuePool limit of size 5 overflow 10 reached, connection timed out. In a threaded web server this appears as latency long before it appears as errors.
The usual root cause is sessions that are never closed, since an overflow connection returns to the pool only when the session or connection is closed or garbage collected. Raise the numbers only after checking that the database can accept process count times engine count times pool size. Note that SQLAlchemy 2.0 switched file-based SQLite to QueuePool while in-memory SQLite still uses SingletonThreadPool, which is why the same code can behave differently under tests.
Source: https://docs.sqlalchemy.org/en/20/core/pooling.html
sqlalchemypythonperformance
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