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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.

See also: Configure the write-ahead log.

WAL extensions are disabled by default. To configure them, use the wal.ext.* configuration options. Inside the wal.ext block, you can enable storing old and new tuples as follows:

  • To store old and new tuples in a write-ahead log for all spaces, set the wal.ext.old and wal.ext.new options to true:

  • To adjust these options for specific spaces, specify the wal.ext.spaces option:

    wal:
      ext:
        old: true
        new: true
        spaces:
          space1:
            old: false
          space2:
            new: false
    

    The configuration for specific spaces has priority over the configuration in the wal.ext.new and wal.ext.old options. It means that 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, the master’s configuration is ignored.

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.

Примечание

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

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