How it works

Litestream is a streaming replication tool for SQLite databases. It runs as a separate background process and continuously copies write-ahead log pages from disk to one or more replicas. This asynchronous replication provides disaster recovery similar to what is available with database servers like Postgres or MySQL.

Understanding the WAL

SQLite has a journaling mode called “WAL” (write-ahead log) which writes database page changes to a separate -wal file first before later copying those pages back into the main database file. This lets SQLite provide safe, atomic transactions because it can simply delete WAL pages if a transaction gets rolled back. If pages were written directly to the database file then there would be no way to get back the original page data on rollback.

The WAL also allows read transactions have their own snapshot view of the database at the time the transaction started because there can be multiple instances of the same database page spread across the database file & WAL.

However, the WAL continually grows so eventually pages have to be moved back to the database file so the WAL can be restarted. This process is called checkpointing and can only be done when no transactions are active. That is the crux of what lets Litestream replicate SQLite.

The shadow WAL

Litestream works by effectively taking over the checkpointing process. It starts a long-running read transaction to prevent any other process from checkpointing and restarting the WAL file. Instead, it continually copies over new WAL pages to a staging area called the shadow WAL and manually calls out to SQLite to perform checkpoints as necessary.

The shadow WAL is a directory next to your SQLite database where WAL files are effectively recreated as a sequence. The first shadow WAL file starts with 00000000.wal and when a checkpoint occurs then it starts copying to 00000001.wal.

These WAL files contain the original WAL frames & checksums to ensure consistency. To restore a database, we can simply start from a snapshot of the database at some point in time and replay each WAL afterward to get it to the current state.

Snapshots & generations

In order to accurately restore a database, a snapshot and all subsequent WAL frames must be available. Any break in the WAL frames would result in a corrupted restored database file. This collection of snapshots & contiguous WAL files is called a generation in Litestream.

When Litestream first starts replicating a database, it creates a new generation. This is simply a 16-character random hex string. A snapshot is created by copying the current state of the database and then all WAL files created after are named as 8-character incrementing hex values starting with zero.

If Litestream detects that there is a break in the WAL frames then it will automatically start a new generation with a new random hex string and a snapshot. This ensures we always have a contiguous set of files to replay even if Litestream is stopped and misses WAL frames being written.

This approach also has the benefit that two servers that accidentally share the same replica destination will not overwrite each other’s data. However, note that it is not recommended to replicate two databases to the same exact replica path.