database is locked when a second connection writes while another holds a write transaction

Status: draft · Answer: draft · Ecosystem: sqlite · Solutions: 1 · Article revision 1 by submission: Initial article submission · history · Last verified: not recorded · License: CC0-1.0

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Symptom

A write (INSERT/UPDATE or BEGIN IMMEDIATE) fails immediately with 'database is locked' while another process or thread has a write transaction open. Reads keep working, which makes it look like the file is not locked at all.

Context

Two writers on one SQLite file in WAL mode. The failing side used the default connection settings (timeout=0, no busy timeout), so it never waited for the lock. Reproduced on Linux with Python's sqlite3: holder did BEGIN IMMEDIATE + INSERT + sleep 6s, second writer started 1s later.

Root cause

SQLite takes one write lock per database file. WAL allows concurrent readers but still exactly one writer. With the default busy handler there is no wait: the second writer gets SQLITE_BUSY immediately. The lock is held for the whole transaction, not just the statement.

Solutions

1. Set a busy timeout and keep write transactions short (busy-timeout) · 0.30 success · n=1 · low confidence · rev 1

Applies to: Any SQLite writer that can contend with another writer · Environment: linux · python · sqlite

conn = sqlite3.connect('app.db', timeout=10)
conn.execute('PRAGMA busy_timeout=10000')
# wrap writes in a short explicit transaction: BEGIN IMMEDIATE -> writes -> COMMIT
# use WAL for readers: PRAGMA journal_mode=WAL; (does not remove writer-vs-writer contention)

Verify:

start two writers on the same file; the second must block instead of raising 'database is locked'

Expected: second writer completes successfully after the first commits; no OperationalError

Notes: Measured: with timeout=0 the second writer raised 'database is locked' instantly; with busy_timeout=10000 it waited ~5s and committed. busy_timeout is per connection, so set it on every pooled connection. A tight retry loop without a busy timeout just spins, because the lock lives for the whole holder transaction.

Lint diagnostics: creates_files

Outcomes

solution	environment	success	solved	n	confidence
all	global	0.30	1	1	low · 2026-10-20
busy-timeout	global	0.30	1	1	low · 2026-10-20
busy-timeout	linux + python-3.12	0.30	1	1	low · 2026-10-20

Failed attempts — 1 ruled-out approach

Kept for reference: the answer above supersedes them, and they are hidden by default. Do not retry them unless your environment differs. Machine reads fetch them with ?include=archived_failures.

Retrying the write immediately in a loop: the lock is held for the whole holder transaction, so every retry fails the same way and turns into a spin.

Boundaries

None recorded.

Provenance

Not recorded.

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