WAL extensions

WAL extensions available in Tarantool Enterprise Edition allow you to add auxiliary information to each write-ahead log record. For example, you can enable storing an old and new tuple for each CRUD operation performed. This information might be helpful for implementing a CDC (Change Data Capture) utility that transforms a data replication stream.

Configuration

To configure WAL extensions, use the wal_ext configuration property. Inside the wal_ext block, you can enable storing old and new tuples as follows:

Set the old and new options to true to store old and new tuples in a write-ahead log for all spaces.
```
box.cfg {
    wal_ext = { old = true, new = true }
}
```

To adjust these options for specific spaces, use the spaces option.

box.cfg {
    wal_ext = {
        old = true, new = true,
        spaces = {
            space1 = { old = false },
            space2 = { new = false }
        }
    }
}

The configuration for specific spaces has priority over the global configuration, so only new tuples are added to the log for space1 and only old tuples for space2.

Note that records with additional fields are replicated as follows:

If a replica doesn’t support the extended format configured on a master, auxiliary fields are skipped.
If a replica and master have different configurations for WAL records, a master’s configuration is ignored.

Example

The table below demonstrates how write-ahead log records might look for the specific CRUD operations if storing old and new tuples is enabled for the bands space.

Operation	Example	WAL information
insert	`bands:insert{4, 'The Beatles', 1960}`	new_tuple: [4, ‘The Beatles’, 1960] tuple: [4, ‘The Beatles’, 1960]
delete	`bands:delete{4}`	key: [4] old_tuple: [4, ‘The Beatles’, 1960]
update	`bands:update({2}, {{'=', 2, 'Pink Floyd'}})`	new_tuple: [2, ‘Pink Floyd’, 1965] old_tuple: [2, ‘Scorpions’, 1965] key: [2] tuple: [[‘=’, 2, ‘Pink Floyd’]]
upsert	`bands:upsert({2, 'Pink Floyd', 1965}, {{'=', 2, 'The Doors'}})`	new_tuple: [2, ‘The Doors’, 1965] old_tuple: [2, ‘Pink Floyd’, 1965] operations: [[‘=’, 2, ‘The Doors’]] tuple: [2, ‘Pink Floyd’, 1965]
replace	`bands:replace{1, 'The Beatles', 1960}`	old_tuple: [1, ‘Roxette’, 1986] new_tuple: [1, ‘The Beatles’, 1960] tuple: [1, ‘The Beatles’, 1960]

Storing both old and new tuples is especially useful for the update operation because a write-ahead log record contains only a key value.

Note

You can use the tt cat command to see the contents of a write-ahead log.

Version:

WAL extensions

Configuration

Example