Change Data Capture

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 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). Upon CommitLogSegment creation, a hard-link to the segment is created in the directory specified in cassandra.yaml. On segment fsync to disk, if CDC data is present anywhere in the segment a <segment_name>_cdc.idx file is also created with the integer offset of how much data in the original segment is persisted to disk. Upon final segment flush, a second line with the human-readable word “COMPLETED” will be added to the _cdc.idx file indicating that Cassandra has completed all processing on the file.

We we use an index file rather than just encouraging clients to parse the log realtime off a memory mapped handle as data can be reflected in a kernel buffer that is not yet persisted to disk. Parsing only up to the listed offset in the _cdc.idx file will ensure that you only parse CDC data for data that is durable.

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 files from the specified cdc_raw directory.

Configuration

Enabling or disabling 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 options 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

Use a 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.

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.

Change Data Capture (CDC)