WAL extensions
Enterprise Edition
WAL extensions are available in the Enterprise Edition only.
WAL extensions 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
:ext: new: true old: 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
andwal.ext.old
options. It means that only new tuples are added to the log forspace1
and only old tuples forspace2
.
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.
Note
You can use the Printing the contents of .snap and .xlog files command to see the contents of a write-ahead log.