ScyllaDB University LIVE, FREE Virtual Training Event | March 21
Register for Free
ScyllaDB Documentation Logo Documentation
  • Server
  • Cloud
  • Tools
    • ScyllaDB Manager
    • ScyllaDB Monitoring Stack
    • ScyllaDB Operator
  • Drivers
    • CQL Drivers
    • DynamoDB Drivers
  • Resources
    • ScyllaDB University
    • Community Forum
    • Tutorials
Download
ScyllaDB Docs ScyllaDB Open Source Features Change Data Capture (CDC) CDC Streams

Caution

You're viewing documentation for a previous version. Switch to the latest stable version.

CDC Streams¶

Streams are partitions in CDC log tables. They are identified by their keys: stream identifiers. When you perform a base table write, ScyllaDB chooses a stream ID for the corresponding CDC log entries based on two things:

  • the currently operating CDC generation (CDC Stream Generations),

  • the base write’s partition key.

Example:

CREATE TABLE ks.t (pk int, ck int, v int, PRIMARY KEY (pk, ck)) WITH cdc = {'enabled': true};
INSERT INTO ks.t (pk,ck,v) values (0,0,0);
INSERT INTO ks.t (pk,ck,v) values (0,1,0);
INSERT INTO ks.t (pk,ck,v) values (0,2,0);
INSERT INTO ks.t (pk,ck,v) values (2,0,0);
INSERT INTO ks.t (pk,ck,v) values (2,1,0);
INSERT INTO ks.t (pk,ck,v) values (2,2,0);
SELECT "cdc$stream_id", pk, ck FROM ks.t_scylla_cdc_log;

returns:

 cdc$stream_id                      | pk | ck
------------------------------------+----+----
 0x365fd1a9ae34373954529ac8169dfb93 |  2 |  0
 0x365fd1a9ae34373954529ac8169dfb93 |  2 |  1
 0x365fd1a9ae34373954529ac8169dfb93 |  2 |  2
 0x166eddaa68db9a95af83968998626f7c |  0 |  0
 0x166eddaa68db9a95af83968998626f7c |  0 |  1
 0x166eddaa68db9a95af83968998626f7c |  0 |  2

(6 rows)

Observe that in the example above, all base writes made to partition 0 were sent to the same stream. The same is true for all base writes made to partition 1.

Underneath, ScyllaDB uses the token of the base write’s partition key to decide the stream ID. It stores a mapping from the token ring (the set of all tokens, which are 64-bit integers) to the set of stream IDs associated with the currently operating CDC generation. Thus, choosing a stream proceeds in two steps:

base partition key |--- partitioner ---> token |--- stream ID mapping ---> stream ID

Therefore, at any given moment, the stream ID chosen for a single base partition key will be the same, but two different partition keys might get mapped to two different streams IDs. But the set of used stream IDs is much smaller than the set of all tokens, so we will often see two base partitions appearing in a single stream:

CREATE TABLE ks.t (pk int, ck int, v int, PRIMARY KEY (pk, ck)) WITH cdc = {'enabled': true};
INSERT INTO ks.t (pk,ck,v) values (2,0,0);
INSERT INTO ks.t (pk,ck,v) values (5,0,0);
SELECT "cdc$stream_id", pk, ck FROM ks.t_scylla_cdc_log;

returns:

 cdc$stream_id                      | pk | ck
------------------------------------+----+----
 0x365fd1a9ae34373954529ac8169dfb93 |  2 |  0
 0x365fd1a9ae34373954529ac8169dfb93 |  5 |  0

(2 rows)

Note

To make the above example we simply kept inserting rows with different partition keys until we found two that went to the same stream.

Note

For a given stream there is no straightforward way to find a partition key which will get mapped to this stream, because of the partitioner, which uses the murmur3 hash function underneath (the truth is you can efficiently find such a key, as murmur3 is not a cryptographic hash, but it’s not completely obvious).

The set of used stream IDs is independent from the table. It’s a global property of the ScyllaDB cluster:

CREATE TABLE ks.t1 (pk int, ck int, v int, primary key (pk, ck)) WITH cdc = {'enabled':'true'};
CREATE TABLE ks.t2 (pk int, ck int, v int, primary key (pk, ck)) WITH cdc = {'enabled':'true'};
INSERT INTO ks.t1 (pk,ck,v) values (0,0,0);
INSERT INTO ks.t2 (pk,ck,v) values (0,0,0);
SELECT "cdc$stream_id", pk, ck FROM ks.t1_scylla_cdc_log;

returns:

 cdc$stream_id                      | pk | ck
------------------------------------+----+----
 0x166eddaa68db9a95af83968998626f7c |  0 |  0

(1 rows)
SELECT "cdc$stream_id", pk, ck FROM ks.t2_scylla_cdc_log;

returns:

 cdc$stream_id                      | pk | ck
------------------------------------+----+----
 0x166eddaa68db9a95af83968998626f7c |  0 |  0

(1 rows)

As the example above illustrates, even writes made to two different tables will use the same stream ID for their corresponding CDC log entries if their partition keys are the same, assuming that the operating CDC generation doesn’t change in between those writes.

More generally, two base writes will use the same stream IDs if the tokens of their partition keys get mapped to the same stream ID by the CDC generation.

Ordering¶

All considerations related to partition and clustering keys apply to CDC log tables. In particular, when performing a partition scan of the CDC log table, all entries from one stream will appear before all entries from another:

CREATE TABLE ks.t (pk int, ck int, v int, PRIMARY KEY (pk, ck)) WITH cdc = {'enabled':'true'};
INSERT INTO ks.t (pk,ck,v) values (0,0,0);
INSERT INTO ks.t (pk,ck,v) values (2,0,0);
INSERT INTO ks.t (pk,ck,v) values (0,1,0);
INSERT INTO ks.t (pk,ck,v) values (2,1,0);
INSERT INTO ks.t (pk,ck,v) values (0,2,0);
INSERT INTO ks.t (pk,ck,v) values (2,2,0);
SELECT "cdc$stream_id", totimestamp("cdc$time"), pk, ck FROM ks.t_scylla_cdc_log;

returns:

 cdc$stream_id                      | system.totimestamp(cdc$time)    | pk | ck
------------------------------------+---------------------------------+----+----
 0x365fd1a9ae34373954529ac8169dfb93 | 2020-03-25 13:12:59.195000+0000 |  2 |  0
 0x365fd1a9ae34373954529ac8169dfb93 | 2020-03-25 13:12:59.196000+0000 |  2 |  1
 0x365fd1a9ae34373954529ac8169dfb93 | 2020-03-25 13:12:59.197000+0000 |  2 |  2
 0x166eddaa68db9a95af83968998626f7c | 2020-03-25 13:12:59.194000+0000 |  0 |  0
 0x166eddaa68db9a95af83968998626f7c | 2020-03-25 13:12:59.195000+0000 |  0 |  1
 0x166eddaa68db9a95af83968998626f7c | 2020-03-25 13:12:59.197000+0000 |  0 |  2

(6 rows)

Therefore there is no global time ordering between all writes in the CDC log; you only get time-based ordering within a stream, for each stream.

Was this page helpful?

PREVIOUS
Basic operations in CDC
NEXT
CDC Stream Generations
  • Create an issue
  • Edit this page

On this page

  • CDC Streams
    • Ordering
ScyllaDB Open Source
  • 6.2
    • master
    • 6.2
    • 6.1
    • 6.0
    • 5.4
    • 5.2
    • 5.1
  • Getting Started
    • Install ScyllaDB
      • Launch ScyllaDB on AWS
      • Launch ScyllaDB on GCP
      • Launch ScyllaDB on Azure
      • ScyllaDB Web Installer for Linux
      • Install ScyllaDB Linux Packages
      • Install scylla-jmx Package
      • Run ScyllaDB in Docker
      • Install ScyllaDB Without root Privileges
      • Air-gapped Server Installation
      • ScyllaDB Housekeeping and how to disable it
      • ScyllaDB Developer Mode
    • Configure ScyllaDB
    • ScyllaDB Configuration Reference
    • ScyllaDB Requirements
      • System Requirements
      • OS Support by Linux Distributions and Version
      • Cloud Instance Recommendations
      • ScyllaDB in a Shared Environment
    • Migrate to ScyllaDB
      • Migration Process from Cassandra to ScyllaDB
      • ScyllaDB and Apache Cassandra Compatibility
      • Migration Tools Overview
    • Integration Solutions
      • Integrate ScyllaDB with Spark
      • Integrate ScyllaDB with KairosDB
      • Integrate ScyllaDB with Presto
      • Integrate ScyllaDB with Elasticsearch
      • Integrate ScyllaDB with Kubernetes
      • Integrate ScyllaDB with the JanusGraph Graph Data System
      • Integrate ScyllaDB with DataDog
      • Integrate ScyllaDB with Kafka
      • Integrate ScyllaDB with IOTA Chronicle
      • Integrate ScyllaDB with Spring
      • Shard-Aware Kafka Connector for ScyllaDB
      • Install ScyllaDB with Ansible
      • Integrate ScyllaDB with Databricks
      • Integrate ScyllaDB with Jaeger Server
      • Integrate ScyllaDB with MindsDB
    • Tutorials
  • ScyllaDB for Administrators
    • Administration Guide
    • Procedures
      • Cluster Management
      • Backup & Restore
      • Change Configuration
      • Maintenance
      • Best Practices
      • Benchmarking ScyllaDB
      • Migrate from Cassandra to ScyllaDB
      • Disable Housekeeping
    • Security
      • ScyllaDB Security Checklist
      • Enable Authentication
      • Enable and Disable Authentication Without Downtime
      • Creating a Custom Superuser
      • Generate a cqlshrc File
      • Reset Authenticator Password
      • Enable Authorization
      • Grant Authorization CQL Reference
      • Certificate-based Authentication
      • Role Based Access Control (RBAC)
      • Encryption: Data in Transit Client to Node
      • Encryption: Data in Transit Node to Node
      • Generating a self-signed Certificate Chain Using openssl
      • Configure SaslauthdAuthenticator
    • Admin Tools
      • Nodetool Reference
      • CQLSh
      • Admin REST API
      • Tracing
      • ScyllaDB SStable
      • ScyllaDB Types
      • SSTableLoader
      • cassandra-stress
      • SSTabledump
      • SSTableMetadata
      • ScyllaDB Logs
      • Seastar Perftune
      • Virtual Tables
      • Reading mutation fragments
      • Maintenance socket
      • Maintenance mode
      • Task manager
    • ScyllaDB Monitoring Stack
    • ScyllaDB Operator
    • ScyllaDB Manager
    • Upgrade Procedures
      • ScyllaDB Versioning
      • ScyllaDB Open Source Upgrade
      • ScyllaDB Open Source to ScyllaDB Enterprise Upgrade
      • ScyllaDB Image
      • ScyllaDB Enterprise
    • System Configuration
      • System Configuration Guide
      • scylla.yaml
      • ScyllaDB Snitches
    • Benchmarking ScyllaDB
    • ScyllaDB Diagnostic Tools
  • ScyllaDB for Developers
    • Develop with ScyllaDB
    • Tutorials and Example Projects
    • Learn to Use ScyllaDB
    • ScyllaDB Alternator
    • ScyllaDB Drivers
      • ScyllaDB CQL Drivers
      • ScyllaDB DynamoDB Drivers
  • CQL Reference
    • CQLSh: the CQL shell
    • Appendices
    • Compaction
    • Consistency Levels
    • Consistency Level Calculator
    • Data Definition
    • Data Manipulation
      • SELECT
      • INSERT
      • UPDATE
      • DELETE
      • BATCH
    • Data Types
    • Definitions
    • Global Secondary Indexes
    • Expiring Data with Time to Live (TTL)
    • Functions
    • Wasm support for user-defined functions
    • JSON Support
    • Materialized Views
    • Non-Reserved CQL Keywords
    • Reserved CQL Keywords
    • Service Levels
    • ScyllaDB CQL Extensions
  • Alternator: DynamoDB API in Scylla
    • Getting Started With ScyllaDB Alternator
    • ScyllaDB Alternator for DynamoDB users
  • Features
    • Lightweight Transactions
    • Global Secondary Indexes
    • Local Secondary Indexes
    • Materialized Views
    • Counters
    • Change Data Capture
      • CDC Overview
      • The CDC Log Table
      • Basic operations in CDC
      • CDC Streams
      • CDC Stream Generations
      • Querying CDC Streams
      • Advanced column types
      • Preimages and postimages
      • Data Consistency in CDC
    • Workload Attributes
  • ScyllaDB Architecture
    • Data Distribution with Tablets
    • ScyllaDB Ring Architecture
    • ScyllaDB Fault Tolerance
    • Consistency Level Console Demo
    • ScyllaDB Anti-Entropy
      • ScyllaDB Hinted Handoff
      • ScyllaDB Read Repair
      • ScyllaDB Repair
    • SSTable
      • ScyllaDB SSTable - 2.x
      • ScyllaDB SSTable - 3.x
    • Compaction Strategies
    • Raft Consensus Algorithm in ScyllaDB
    • Zero-token Nodes
  • Troubleshooting ScyllaDB
    • Errors and Support
      • Report a ScyllaDB problem
      • Error Messages
      • Change Log Level
    • ScyllaDB Startup
      • Ownership Problems
      • ScyllaDB will not Start
      • ScyllaDB Python Script broken
    • Upgrade
      • Inaccessible configuration files after ScyllaDB upgrade
    • Cluster and Node
      • Handling Node Failures
      • Failure to Add, Remove, or Replace a Node
      • Failed Decommission Problem
      • Cluster Timeouts
      • Node Joined With No Data
      • NullPointerException
      • Failed Schema Sync
    • Data Modeling
      • ScyllaDB Large Partitions Table
      • ScyllaDB Large Rows and Cells Table
      • Large Partitions Hunting
      • Failure to Update the Schema
    • Data Storage and SSTables
      • Space Utilization Increasing
      • Disk Space is not Reclaimed
      • SSTable Corruption Problem
      • Pointless Compactions
      • Limiting Compaction
    • CQL
      • Time Range Query Fails
      • COPY FROM Fails
      • CQL Connection Table
    • ScyllaDB Monitor and Manager
      • Manager and Monitoring integration
      • Manager lists healthy nodes as down
    • Installation and Removal
      • Removing ScyllaDB on Ubuntu breaks system packages
  • Knowledge Base
    • Upgrading from experimental CDC
    • Compaction
    • Consistency in ScyllaDB
    • Counting all rows in a table is slow
    • CQL Query Does Not Display Entire Result Set
    • When CQLSh query returns partial results with followed by “More”
    • Run ScyllaDB and supporting services as a custom user:group
    • Customizing CPUSET
    • Decoding Stack Traces
    • Snapshots and Disk Utilization
    • DPDK mode
    • Debug your database with Flame Graphs
    • How to Change gc_grace_seconds for a Table
    • Gossip in ScyllaDB
    • Increase Permission Cache to Avoid Non-paged Queries
    • How does ScyllaDB LWT Differ from Apache Cassandra ?
    • Map CPUs to ScyllaDB Shards
    • ScyllaDB Memory Usage
    • NTP Configuration for ScyllaDB
    • Updating the Mode in perftune.yaml After a ScyllaDB Upgrade
    • POSIX networking for ScyllaDB
    • ScyllaDB consistency quiz for administrators
    • Recreate RAID devices
    • How to Safely Increase the Replication Factor
    • ScyllaDB and Spark integration
    • Increase ScyllaDB resource limits over systemd
    • ScyllaDB Seed Nodes
    • How to Set up a Swap Space
    • ScyllaDB Snapshots
    • ScyllaDB payload sent duplicated static columns
    • Stopping a local repair
    • System Limits
    • How to flush old tombstones from a table
    • Time to Live (TTL) and Compaction
    • ScyllaDB Nodes are Unresponsive
    • Update a Primary Key
    • Using the perf utility with ScyllaDB
    • Configure ScyllaDB Networking with Multiple NIC/IP Combinations
  • Reference
    • AWS Images
    • Azure Images
    • GCP Images
    • Configuration Parameters
    • Glossary
    • Limits
    • API Reference (BETA)
    • Metrics (BETA)
  • ScyllaDB FAQ
  • Contribute to ScyllaDB
Docs Tutorials University Contact Us About Us
© 2025, ScyllaDB. All rights reserved. | Terms of Service | Privacy Policy | ScyllaDB, and ScyllaDB Cloud, are registered trademarks of ScyllaDB, Inc.
Last updated on 08 May 2025.
Powered by Sphinx 7.4.7 & ScyllaDB Theme 1.8.6