Change Data Capture¶

Overview¶

Change data capture (CDC) provides a mechanism to flag specific tables for archival as well as rejecting writes to those tables once a configurable size-on-disk for the combined flushed and unflushed CDC-log is reached. An operator can enable CDC on a table by setting the table property cdc=true (either when creating the table or altering it), after which any CommitLogSegments containing data for a CDC-enabled table are moved to the directory specified in cassandra.yaml on segment discard. A threshold of total disk space allowed is specified in the yaml at which time newly allocated CommitLogSegments will not allow CDC data until a consumer parses and removes data from the destination archival directory.

Configuration¶

Enabling or disable CDC on a table¶

CDC is enable or disable through the cdc table property, for instance:

CREATE TABLE foo (a int, b text, PRIMARY KEY(a)) WITH cdc=true;

ALTER TABLE foo WITH cdc=true;

ALTER TABLE foo WITH cdc=false;

cassandra.yaml parameters¶

The following cassandra.yaml are available for CDC:

cdc_enabled (default: false): Enable or disable CDC operations node-wide.
cdc_raw_directory (default: $CASSANDRA_HOME/data/cdc_raw): Destination for CommitLogSegments to be moved after all corresponding memtables are flushed.
cdc_free_space_in_mb: (default: min of 4096 and 1/8th volume space): Calculated as sum of all active CommitLogSegments that permit CDC + all flushed CDC segments in cdc_raw_directory.
cdc_free_space_check_interval_ms (default: 250): When at capacity, we limit the frequency with which we re-calculate the space taken up by cdc_raw_directory to prevent burning CPU cycles unnecessarily. Default is to check 4 times per second.

Reading CommitLogSegments¶

This implementation included a refactor of CommitLogReplayer into CommitLogReader.java. Usage is fairly straightforward with a variety of signatures available for use. In order to handle mutations read from disk, implement CommitLogReadHandler.

Warnings¶

Do not enable CDC without some kind of consumption process in-place.

The initial implementation of Change Data Capture does not include a parser (see Reading CommitLogSegments above) so, if CDC is enabled on a node and then on a table, the cdc_free_space_in_mb will fill up and then writes to CDC-enabled tables will be rejected unless some consumption process is in place.