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 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.

Please note that in rare chances, e.g. slow disk, it is possible for the consumer to read an empty value from the _cdc.idx file because update is achieved with first truncating the file then write to the file. In such case, the consumer should retry read the index file.

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_total_space: (default: min of 4096MiB 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 (default: 250ms)

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)