| # Switchover (SWO) | |
| Switchover (SWO) is a feature that allows a live system to switch from one database to another safely and with little to no user impact. | |
| ## Development | |
| To start the dev instance in switchover mode, run `make start-swo` | |
| ## Theory of Operation | |
| Switchover mode is initiated by starting GoAlert with an additional DB URL `--db-url-next`. The database referenced by `--db-url` is referred to as the "old" DB and the `--db-url-next` is the "new" DB. | |
| All new application DB connections first acquire a shared advisory lock, then check the `use_next_db` pointer. If the pointer is set, all new connections will be made to the "new" DB (without the checking overhead), and the connection to the "old" DB will be terminated. | |
| The switch is performed by first replicating a complete snapshot of the "old" DB to the "new" DB. After the initial sync, subsequent synchronization is an incremental "diff" of snapshots -- more info on how this works is available in the `swosync` package. | |
| After repeated logical sync operations (to keep the next-sync time low), a stop-the-world lock (i.e., an exclusive lock that conflicts with the shared advisory locks) is acquired, followed by the final logical sync. During the same transaction, the `use_next_db` pointer is set. After the lock is released, the connector will send all new queries to the "new" DB. | |