README

README.md

@@ -1,17 +1,137 @@
 litestream
 ==========
 
-Streaming replication for SQLite.
+Litestream is a standalone streaming replication tool for SQLite. It runs as a
+background process and safely replicates changes incrementally from one or more
+SQLite databases. Litestream only communicates with SQLite through the SQLite
+API so it will not corrupt your database.
 
 
-## Questions
+## Usage
 
-- How to avoid WAL checkpointing on close?
+### Installation & configuration
 
+You can download the binary from the [Releases page](https://github.com/benbjohnson/litestream/releases).
 
-## Notes
+Once installed locally, you'll need to create a config file. By default, the
+config file lives at `/etc/litestream.yml` but you can pass in a different
+path to any `litestream` command using the `-config PATH` flag.
 
-```sql
--- Disable autocheckpointing.
-PRAGMA wal_autocheckpoint = 0
-```
+You can copy this configuration below and update the source path (`/path/to/db`)
+and the destination path (`/path/to/replica`) to where you want to replicate
+from and to.
+
+```yaml
+databases:
+  - path: "/path/to/db"
+    replicas:
+      - type: file
+        path: /path/to/replica
+```
+
+
+### Replication
+
+Once your configuration is saved, run the `litestream replicate` command:
+
+```sh
+# Replicate using the /etc/litestream.yml configuration.
+$ litestream replicate
+
+# Replicate using a different configuration path.
+$ litestream replicate -config /path/to/litestream.yml
+```
+
+The `litestream` command will initialize and then wait indefinitely for changes.
+You should see your destination replica path is now populated with a
+`generations` directory. Inside there should be a 16-character hex generation
+directory and inside there should be snapshots & wal files. As you make changes
+to your source database, changes will be copied over to your replica incrementally.
+
+
+### Restoring a backup
+
+Litestream can restore a previous snapshot and replay all replicated WAL files.
+By default, it will restore up to the latest WAL file but you can also perform
+point-in-time restores.
+
+A database can only be restored to a path that does not exist so you don't need
+to worry about accidentally overwriting your current database.
+
+```sh
+# Restore database to original path.
+$ litestream restore /path/to/db
+
+# Restore database to a new location.
+$ litestream restore -o /tmp/mynewdb /path/to/db
+
+# Restore database to a specific point-in-time.
+$ litestream restore -timestamp 2020-01-01T00:00:00Z /path/to/db
+```
+
+Point-in-time restores only have the resolution of the timestamp of the WAL file
+itself. By default, litestream will start a new WAL file every minute so
+point-in-time restores are only accurate to the minute.
+
+
+### Validating a backup
+
+Litestream can perform a consistency check of backups. It does this by computing
+a checksum of the current database file and then computing a checksum of
+a restored a backup. Litestream performs physical replication so backed up
+databases should be the same byte-for-byte. Be aware that this can incur some
+cost if you are validating from an external replica such as S3.
+
+```sh
+$ litestream validate /path/to/db
+```
+
+Note that computing the checksum of your original database does obtain a read
+lock to prevent checkpointing but this will not affect your read/write access
+to the database.
+
+
+## How it works
+
+SQLite provides a WAL (write-ahead log) journaling mode which writes pages to
+a `-wal` file before eventually being copied over to the original database file.
+This copying process is known as checkpointing. The WAL file works as a circular
+buffer so when the WAL reaches a certain size then it restarts from the beginning.
+
+Litestream works by taking over the checkpointing process and controlling when
+it is restarted to ensure that it copies every new page. Checkpointing is only
+allowed when there are no read transactions so Litestream maintains a
+long-running read transaction against each database until it is ready to
+checkpoint.
+
+The SQLite WAL file is copied to a separate location called the shadow WAL which
+ensures that it will not be overwritten by SQLite. This shadow WAL acts as a
+temporary buffer so that replicas can replicate to their destination (e.g.
+another file path or to S3). The shadow WAL files are removed once they have
+been fully replicated.
+
+Litestream groups a snapshot and all subsequent WAL changes into "generations".
+A generation is started on initial replication of a database and a new
+generation will be started if litestream detects that the WAL replication is
+no longer contiguous. This can occur if the `litestream` process is stopped and
+another process is allowed to checkpoint the WAL.
+
+
+
+## Open-Source, not Open-Contribution
+
+[Similar to SQLite](https://www.sqlite.org/copyright.html), litestream is open
+source but closed to contributions. This keeps the code base free of proprietary
+or licensed code but it also helps me continue to maintain and build litestream.
+
+As the author of [BoltDB](https://github.com/boltdb/bolt), I found that
+accepting and maintaining third party patches contributed to my burn out and
+eventual archival of the project. Writing databases & low-level replication
+tools involves nuance and simple one line changes can have profound and
+unexpected changes in correctness and performance. Even small contributions
+typically required hours of my time to properly test and validate them.
+
+I am grateful for community involvement and when folks report bugs or suggest
+features. I do not wish to come off as anything but welcoming, however, I've
+made the decision to keep this project closed to contribution for my own
+mental health and long term viability of the project.