1.5 KiB
Design
Persistent storage
Persistent storage must implement the Storage trait that the MVCC module uses to essentially store a write-ahead log (WAL) of mutations.
Figure 1 shows an example of write-ahead log across three transactions.
The first transaction T0 executes a INSERT (id) VALUES (1) statement, which results in a mutation with id set to 1, begin timestamp to 0 (which is the transaction ID) and end timestamp as infinity (meaning the row version is still visible).
The second transaction T1 executes another INSERT statement, which adds another mutation to the WAL with id set to 2, begin timesstamp to 1 and end timestamp as infinity, similar to what T0 did.
Finally, a third transaction T2 executes two statements: DELETE WHERE id = 1 and INSERT (id) VALUES (3). The first one results in a mutation with id set to 1 and begin timestamp set to 0 (which is the transaction that created the entry). However, the end timestamp is now set to 2 (the current transaction), which means the entry is now deleted.
The second statement results in an entry in the WAL similar to the INSERT statements in T0 and T1.
Figure 1. Write-ahead log of mutations across three transactions.
When MVCC bootstraps or recovers, it simply reads the write-ahead log into the in-memory index, and it's good to go. If the WAL grows big, we can compact it by dropping all entries that are no longer visible after the the latest transaction.