SQLite WAL Mode Can Lock Short-Lived Readers
We write extremely rarely to the database (in some cases less than once per month) but read tens or hundreds of times per second without connection pooling, because they’re independent processes. In other words: we open the database in read-only mode (SQLITE_OPEN_READONLY), run one SELECT, close the database.But since we didn’t want writers to block readers – as rare as they may be – we took the ostensibly safe route and put our databases into WAL mode anyway. For example, Python’s sqlite3 standard library module comes with a default busy timeout of 5 seconds. ↩︎This post was made possible by the donations from people and corporations who appreciate my public work.
- ▪We write extremely rarely to the database (in some cases less than once per month) but read tens or hundreds of times per second without connection pooling, because they’re independent processes.
- ▪In other words: we open the database in read-only mode (SQLITE_OPEN_READONLY), run one SELECT, close the database.But since we didn’t want writers to block readers – as rare as they may be – we took the ostensibly safe route and put our dat
- ▪For example, Python’s sqlite3 standard library module comes with a default busy timeout of 5 seconds. ↩︎This post was made possible by the donations from people and corporations who appreciate my public work.
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| Original publisher | Lobsters |
| Canonical URL | https://hynek.me/til/sqlite-read-only-wal-locked/ |
| Publication time | Sun, 26 Jul 2026 17:32:54 -0500 |
| Retrieval time | 2026-07-27T05:47:05.708Z |
| Last seen | 2026-07-27T05:47:05.708Z |
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TIL: SQLite WAL Mode Can Lock Short-Lived Readers 26 July 2026Locked SQLite database errors aren’t just for writers.A lot of online ink has been spilled over the infamous SQLite error:Cannot prepare SQLite statement: database is lockedThe usual advice goes something like this:Use WAL mode,have a non-zero busy timeout,use BEGIN IMMEDIATE for transactions that will write.And this advice is correct for the typical workload of a web application that has long-lived database connections1.However, Cthulhu has cursed me with an atypical workload once again. We write extremely rarely to the database (in some cases less than once per month) but read tens or hundreds of times per second without connection pooling, because they’re independent processes.
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Excerpt limited to ~120 words for fair-use compliance. The full article is at Lobsters.
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