54 lines
2.2 KiB
Markdown
54 lines
2.2 KiB
Markdown
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# ADR-002: Write Mutex to Prevent SQLITE_BUSY Under Concurrent Load
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**Status:** Accepted
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**Date:** 2026-03-18
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## Context
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SQLite uses file-level locking. When multiple goroutines attempt write transactions
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concurrently, SQLite cannot acquire the write lock immediately and returns `SQLITE_BUSY`.
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Although the default Pragmas configure a 5-second busy timeout (`_timeout=5000`), this is a
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passive wait that still allows competing transactions to collide and fail under sustained
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concurrent write pressure.
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WAL mode (`_journal=WAL`) improves read concurrency but does not eliminate write contention:
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SQLite still allows only one writer at a time.
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## Decision
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The `sqliteComponent` holds a `writeMu sync.Mutex` field. `NewUnitOfWork` detects when its
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`Client` argument is the concrete `*sqliteComponent` type and extracts a pointer to that mutex.
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`unitOfWork.Do` acquires the mutex before beginning a transaction and releases it after
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commit or rollback:
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```go
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func (u *unitOfWork) Do(ctx context.Context, fn func(ctx context.Context) error) error {
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if u.writeMu != nil {
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u.writeMu.Lock()
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defer u.writeMu.Unlock()
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}
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tx, err := u.client.Begin(ctx)
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...
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}
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```
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This serialises all write transactions at the application level, guaranteeing that only one
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writer reaches SQLite at a time and eliminating `SQLITE_BUSY` errors entirely.
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The mutex is only applied when using `NewUnitOfWork`. Callers who manage transactions manually
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via `Begin`/`Commit`/`Rollback` are not protected and must handle contention themselves.
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## Consequences
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**Positive:**
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- `SQLITE_BUSY` is eliminated for all write workloads going through `UnitOfWork`.
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- Behaviour is deterministic and testable (see `TestUnitOfWork_WriteMutex`).
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- Reads are unaffected; the mutex only wraps writes.
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**Negative:**
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- Write throughput is bounded to one goroutine at a time. This is acceptable for SQLite's
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typical deployment profile (embedded, single-process, modest write rates).
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- The type assertion `client.(*sqliteComponent)` couples `NewUnitOfWork` to the concrete type.
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When a mock or alternative `Client` is supplied, `writeMu` is `nil` and serialisation is
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skipped silently. This is intentional for testing flexibility.
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