Debezium Connector for Cassandra

The Cassanadra connector can monitor a Cassandra cluster and record all row-level changes. The connector must be deployed locally on each node in the Cassandra cluster. The first time the connector connects to a Cassandra node, it performs a snapshot of all CDC-enabled tables in all keyspaces. The connector will also read the changes that are written to Cassandra commit logs and generates corresponding insert, update, and delete events. All events for each table are recorded in a separate kafka topic, where they can be consumed easily by applications and services.

For information about the Cassandra versions that are compatible with this connector, see the Debezium release overview.

Overview

Cassandra is an open-sourced NoSQL database. Similar to most databases, the write path of Cassandra starts with the immediate logging of a change into its commit log. The commit log resides locally on each node, recording every write made to that node.

Since Cassandra 3.0, a change data capture (CDC) feature is introduced. The CDC feature can be enabled on the table level by setting the table property cdc=true, after which any commit log containing data for a CDC-enabled table will be moved to the CDC directory specified in cassandra.yaml on discard.

The Cassandra connector resides on each Cassandra node and monitors the cdc_raw directory for change. It processes all local commit log segments as they are detected, produces a change event for every row-level insert, update, and delete operations in the commit log, publishes all change events for each table in a separate Kafka topic, and finally deletes the commit log from the cdc_raw directory. This last step is important because once CDC is enabled, Cassandra itself cannot purge the commit logs. If the cdc_free_space_in_mb fills up, writes to CDC-enabled tables will be rejected.

The connector is tolerant of failures. As the connector reads commit logs and produces events, it records each commit log segment’s filename and position along with each event. If the connector stops for any reason (including communication failures, network problems, or crashes), upon restart it simply continues reading the commit log where it last left off. This includes snapshots: if the snapshot was not completed when the connector is stopped, upon restart it will begin a new snapshot. We’ll talk later about how the connector behaves when things go wrong.

Cassandra is different from the other Debezium connectors since it is not implemented on top of the Kafka Connect framework. Instead it is a single JVM process that is intended to reside on each Cassandra node and publishes events to Kafka via a Kafka producer.

The following features are currently not supported by the Cassandra connector. Changes resulted from any of these features are ignored:

TTL on collection-type columns
Range deletes
Static columns
Triggers
Materialized views
Secondary indices
Light-weight transactions

Setting Up Cassandra

Before the Debezium Cassandra connector can be used to monitor the changes in a Cassandra cluster, CDC must be enabled on the node level and table level.

Enabling CDC on Node

To enable CDC, update the following CDC config in cassandra.yaml:

cdc_enabled: true

Additional CDC configs are have the following default values:

cdc_raw_directory: $CASSANDRA_HOME/data/cdc_raw
cdc_free_space_in_mb: 4096
cdc_free_space_check_interval_ms: 250

cdc_enabled enables or disables CDC operations node-wide
cdc_raw_directory determines the destination for commit log segments to be moved after all corresponding memtables are flushed.
cdc_free_space_in_mb is the maximum capacity allocated to store commit log segments, and defaults to the minimum of 4096 MB and 1/8 of volume space.
cdc_free_space_check_interval_ms is frequency with which we re-calculate the space taken up by cdc_raw_directory to prevent burning CPU cycles unnecessarily when at capacity.

Enabling CDC on Table

Once CDC is enabled on the Cassandra node, each table must be be explicitly enabled for CDC as well via the CREATE TABLE or ALTER TABLE command. For example:

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

ALTER TABLE foo WITH cdc=true;

How the Cassandra Connector Works

This section goes into detail about how the Cassandra connector performs snapshots, transforms commit log events into Debezium change events, handles commit log life cycle, records events into Kafka, manages schema evolution, and behaves when things go wrong.

Snapshots

When the Cassandra connector first starts on a Cassandra node, it will by default perform an initial snapshot of the cluster. This is the default mode, since most of the time CDC is enabled on non-empty tables and commit logs do not contain the complete history.

The snapshot reader issues a SELECT statement to query all the columns in a table. Cassandra allows consistency level to be set either globally or on the statement level. For snapshotting, the consistency level is set on the statement level to ALL by default to provide the highest consistency. This implies if one node goes down during the snapshot, the snapshot would not be able to continue and a subsequent re-snapshot is required once the node has been brought back online. You can adjust the consistency level of the snapshot to a lower consistency level in order to increase availability, provided that you understand the tradeoff with consistency.

In Cassandra 3.X, it is not possible to read strictly from the local Cassandra node. Starting in Cassandra 4.0, a NODE_LOCAL consistency level will be added. This will allow the Cassandra connector to read from the node it resides in only (which would be consistent with the way commit logs are processed).

Unlike relational databases, there is no read lock applied during a snapshot, so writes to Cassandra are not blocked during that snapshot. If the queried data has been modified by another client during the snapshot, those changes may be reflected in the snapshot result set.

If the connector fails or stops before the snapshot is completed, the connector will begin a new snapshot upon restarts. In the default snapshot mode (initial), once the connector completes its initial snapshot, it will no longer perform any additional snapshots. The only exception would be during a connector restart: if CDC is enabled on a table, and then the connector is restarted, that table would be snapshotted.

The second snapshot mode (always) allows the connector to perform snapshot whenever necessary. It checks periodically for newly CDC-enabled tables, and snapshot these tables as soon as they are detected.

The third snapshot mode ('never') ensures the connector never performs snapshots. When a new connector is configured this way, it will only read the commit log in the CDC directory. This is not the default behavior because starting a new connector in this mode (without a snapshot) requires the commit logs to contain the entire history of all CDC-enabled tables, which is often not the case. Another use case for this mode is if there is one connector already doing the snapshotting, you can disable snapshot on others to avoid duplicated work.

Reading the Commit Log

The Cassandra connector will typically spend the vast majority of its time reading local commit logs on the Cassandra node. In Cassandra 4.0 on every segment fsync, an index file will be updated to reflect latest offset. This eliminates the processing delay in the CDC feature in Cassandra 3.X. and can be enabled in Cassandra 4 Debezium connector by setting the configuration: commit.log.real.time.processing.enabled to true. The frequency at which index file is polled is determined by commit.log.marked.complete.poll.interval.ms.

Commit logs' binary data are deserialized with Cassandra’s CommitLogReader and CommitLogReadHandler. Each deserialized object is called a mutation in Cassandra. A mutation contains one or more change events.

As the Cassandra connector reads the commit log, it transform the log events into Debezium create, update, or delete events that include the position in the commit log where the event was found. The Cassandra connector encode these change events with Kafka Connect converters and publish them to the appropriate Kafka topics.

Limitations of Commit Logs

Cassandra’s commit logs come with a set of limitations, which are critical for interpreting CDC events correctly:

Commit logs only arrive in cdc_raw directory when it is full, in which case it would be flushed/discarded. This implies there is a delay between when the event is logged and when the event is captured.
Commit logs on an individual Cassandra node do not reflect all writes to the cluster, they only reflect writes stored on that node. This is why it is necesssary to monitor changes on all nodes in a Cassandra cluster. However, due to replication factor, this also implies it is necessary for downstream consumers of these events to handle deduplication.
Writes to an individual Cassandra node are logged as they arrive. However, these events may arrive out-of-order from which they are issued. Downstream consumers of these events must understand and implement logic similar to Cassandra’s read path to get the correct output.
Schema changes of tables are not recorded in commit logs, only data changes are recorded. Therefore changes in schema are detected on a best-effort basis. To avoid data loss, it is recommended to pause writes to the table during schema change.
Cassandra does not perform read-before-write, as a result commit logs do not record the value of every column in the changed row, it only records the values of columns that have been modified (except for partition key columns, which are always recorded as they are required in Cassandra DML commands).
Due to the nature of CQL, insert DMLs can result in a row insertion or update; update DMLs can result in a row insertion, update, or deletion; delete DMLs can result in a row update or deletion. Since queries are not recorded in commit logs, CDC event type is classified based on the effect on the row in a relational database sense.

TODO: is there a way to determine event type which corresponds to the actual Cassandra DML statement? and if so, is that preferred over the semantic of these events?

Managing Commit Log Lifecycle

By default, Cassandra connector will delete commit logs which have been processed. It is not recommended to start the connector while deletion of commit logs is disabled, as this could bloat up disk storage and prevent further writes to the Cassandra cluster. To manage the commit logs in a custom manner (i.e. upload it to a cloud provider), the CommitLogTransfer interface can be implemented.

Topics names

The Cassandra connector writes events for all insert, update, and delete uperations on a single table to a single Kafka topic. The name of the Kafka topics always take the following form:

clusterName.keyspaceName.tableName

where clusterName is the logical name of the connector as specified with the topic.prefix configuration property, keyspaceName is the name of the keyspace where the operation occurred, and tableName is the name of the table on which the operation occurred.

For example, consider a Cassandra installation with an inventory keyspace that contains four tables: products, products_on_hand, customers, and orders. If the connector monitoring this database were given a logical server name of fulfillment, then the connector would produce events on these four Kafka topics:

fulfillment.inventory.products
fulfillment.inventory.products_on_hand
fulfillment.inventory.customers
fulfillment.inventory.orders

TODO: for topic name, is clusterName.keyspaceName.tableName okay? or should it be connectorName.keyspaceName.tableName or connectorName.clusterName.keyspaceName.tableName?

Schema Evolution

DDLs are not recorded in commit logs. When the schema of a table change, this change is issued from one of the Cassandra node and propagated to other nodes via Gossip Protocol.

Schema changes in Cassandra will be detected by an implemented SchemaChangeListener with latency less than 1s, which will then update the schema instance loaded from Cassandra as well as the Kafka key value schemas cached for each table.

Please note that with the current schema evolution approach, the Cassandra connector won’t be able to provide accurate data change information for a small period of time in the following cases:

If CDC gets disabled for a table, data changes which have happened before CDC got disabled will be skipped.
If a column is removed from a table, data changes involving this column before it’s removed cannot be deserialized correctly and will be skipped.

Events

All data change events produced by the Cassandra connector have a key and a value, although the structure of the key and value depends on the table from which the change events originated (see Topic Names).

Change Event’s Key

For a given table, the change event’s key will have a structure that contains a field for each column in the primary key of the table at the time the event was created. Consider an inventory database with a customers table defined as:

CREATE TABLE customers (
  id bigint,
  registration_date timestamp,
  first_name text,
  last_name text,
  email text,
  PRIMARY KEY (id, registration_date)
);

Every change event for the customers table while it has this definition will feature the same key schema, which in JSON representation looks like this:

{
  "type": "record",
  "name": "cassandra-cluster-1.inventory.customers.Key",
  "namespace": "io.debezium.connector.cassandra",
  "fields": [
    {
      "name": "id",
      "type": "long"
    },
    {
      "name": "registration_date",
      "type": "long",
      "logicalType": "timestamp-millis"
    }
  ]
}

For id = 1001 and registration_date = 1562202942545, the key payload in JSON representation would look like this:

{
  "id": 1001,
  "registration_date": 1562202942545
}

Although the field.exclude.list configuration property allows you to remove columns from the event values, all columns in a primary key are always included in the event’s key.

Change event’s value

The value of the change event message is a bit more complicated. Every change event value produced by Cassandra connector has an envelope structure with the following fields:

op

A mandatory field that contains a string value describing the type of operation. Values for the Cassandra connector are i for insert, u for update, and d for delete.

after

An optional field that if present contains the state of the row after the event occurred. The structure will be described by the cassandra-cluster-1.inventory.customers.Value Kafka Connect schema, which represent the cluster, keyspace, and table the event is referring to.

source

A mandatory field that contains a structure describing the source metadata for the event, which in the case of Cassandra contains the following fields:

Debezium version.
Connector name.
Cassandra cluster name.
Name of the commit log file where the event was recorded, the position in that commit log file where the event appeared, whether this event was part of a snapshot, name of the affected keyspace and table, and the maximum timestamp of the partition update in microseconds.

ts_ms

(Optional) If present, contains the time at which the connector processed the event, based on the system clock of the JVM that runs the Cassandra connector.

Because Cassandra does not perform a read-before-write, the Cassandra commit log does not record the value of a row before a change is applied. As a result, Cassandra change event records do not include a before field.

The following is a JSON representation of a value schema for a create event for our customers table:

{
  "type": "record",
  "name": "cassandra-cluster-1.inventory.customers.Envelope",
  "namespace": "io.debezium.connector.cassandra",
  "fields": [
      {
        "name": "op",
        "type": "string"
      },
      {
        "name": "ts_ms",
        "type": "long",
        "logicalType": "timestamp-millis"
      },
      {
        "name": "after",
        "type": "record",
        "fields": [
          {
            "name": "id",
            "type": [
              "null",
              {
            "name": "id",
            "type": "record",
            "fields": [
              {
                "name":"value",
                "type": "string"
              },
              {
                "name":"deletion_ts",
                "type": ["null", "long"],
                "default" : "null"
              },
              {
                "name":"set",
                "type": "boolean"
              }
            ]
            }
          ]
          },
          {
            "name": "registration_date",
            "type": [
              "null",
              {
            "name": "registration_date",
            "type": "record",
            "fields": [
              {
                "name":"value",
                "type": "long",
                "logical_type": "timestamp-millis"
              },
              {
                "name":"deletion_ts",
                "type": ["null", "long"],
                "default" : "null"
              },
              {
                "name":"set",
                "type": "boolean"
              }
            ]
            }
          ]
          },
          {
            "name": "first_name",
            "type": [
              "null",
              {
            "name": "first_name",
            "type": "record",
            "fields": [
              {
                "name":"value",
                "type": "string"
              },
              {
                "name":"deletion_ts",
                "type": ["null", "long"],
                "default" : "null"
              },
              {
                "name":"set",
                "type": "boolean"
              }
            ]
            }
          ]
          },
          {
            "name": "last_name",
            "type": [
              "null",
              {
            "name": "last_name",
            "type": "record",
            "fields": [
              {
                "name":"value",
                "type": "string"
              },
              {
                "name":"deletion_ts",
                "type": ["null", "long"],
                "default" : "null"
              },
              {
                "name":"set",
                "type": "boolean"
              }
            ]
            }
          ]
          },
          {
            "name": "last_name",
            "type": [
              "null",
              {
            "name": "email",
            "type": "record",
            "fields": [
              {
                "name":"value",
                "type": "string"
              },
              {
                "name":"deletion_ts",
                "type": ["null", "long"],
                "default" : "null"
              },
              {
                "name":"set",
                "type": "boolean"
              }
            ]
            }
          ]
          }
        ]
      },
      {
        "name": "source",
        "type": "record",
        "fields": [
          {
            "name": "version",
            "type": "string"
          },
          {
            "name": "connector",
            "type": "string"
          },
          {
            "name": "cluster",
            "type": "string"
          },
          {
            "name": "snapshot",
            "type": "boolean"
          },
          {
            "name": "keyspace",
            "type": "string"
          },
          {
            "name": "table",
            "type": "string"
          },
          {
            "name": "file",
            "type": "string"
          },
          {
            "name": "position",
            "type": "int"
          },
          {
            "name": "ts_ms",
            "type": "long",
            "logicalType": "timestamp-micros"
          }
        ]
      }
  ]
}

TODO: verify max timestamp != deletion timestamp in case of deletion DDLs

Given the following insert DML:

INSERT INTO customers (
  id,
  registration_date,
  first_name,
  last_name,
  email)
VALUES (
  1001,
  now(),
  "Anne",
  "Kretchmar",
  "annek@noanswer.org"
);

The value payload in JSON representation would look like this:

{
  "op": "c",
  "ts_ms": 1562202942832,
  "ts_us": 1562202942832014,
  "ts_ns": 1562202942832014962,
  "after": {
    "id": {
    "value": 1001,
    "deletion_ts": null,
    "set": true
  },
    "registration_date": {
    "value": 1562202942545,
    "deletion_ts": null,
    "set": true
  },
  "first_name": {
    "value": "Anne",
    "deletion_ts": null,
    "set": true
  },
  "last_name": {
    "value": "Kretchmar",
    "deletion_ts": null,
    "set": true
  },
  "email": {
    "value": "annek@noanswer.org",
    "deletion_ts": null,
    "set": true
  }
  },
  "source": {
    "version": "3.0.2.Final",
    "connector": "cassandra",
    "cluster": "cassandra-cluster-1",
    "snapshot": false,
    "keyspace": "inventory",
    "table": "customers",
    "file": "commitlog-6-123456.log",
    "pos": 54,
    "ts_ms": 1562202942666382,
    "ts_us": 1562202942666382000,
    "ts_ns": 1562202942666382000000
  }
}

Given the following update DML:

UPDATE customers
SET email = "annek_new@noanswer.org"
WHERE id = 1001 AND registration_date = 1562202942545

The value payload in JSON representation would look like this:

{
  "op": "u",
  "ts_ms": 1562202942912,
  "ts_us": 1562202942912014,
  "ts_ns": 1562202942912014982,
  "after": {
    "id": {
    "value": 1001,
    "deletion_ts": null,
    "set": true
  },
    "registration_date": {
    "value": 1562202942545,
    "deletion_ts": null,
    "set": true
  },
  "first_name": null,
  "last_name": null,
  "email": {
    "value": "annek_new@noanswer.org",
    "deletion_ts": null,
    "set": true
  }
  },
  "source": {
    "version": "3.0.2.Final",
    "connector": "cassandra",
    "cluster": "cassandra-cluster-1",
    "snapshot": false,
    "keyspace": "inventory",
    "table": "customers",
    "file": "commitlog-6-123456.log",
    "pos": 102,
    "ts_ms": 1562202942666490,
    "ts_us": 1562202942666490000,
    "ts_ns": 1562202942666490000000
  }
}

When we compare this to the value in the insert event, we see a couple differences:

The op field value is now u, signifying that this row changed because of an update.
The after field now has the updated state of the row, and here we can see that the email value is now annek_new@noanswer.org. Notice that first_name and last_name are null, this is because these fields did not change during this update. However, id and registration_date are still included, because these are the primary keys of this table.
The source field structure has the same fields as before, but the values are different since this event is from a different position in the commit log.
The ts_ms shows the timestamp milliseconds which the connector processed this event.

Finally, given the following delete DML:

DELETE FROM customers
WHERE id = 1001 AND registration_date = 1562202942545;

The value payload in JSON representation would look like this:

{
  "op": "d",
  "ts_ms": 1562202942912,
  "ts_us": 1562202942912047,
  "ts_ns": 1562202942912047921,
  "after": {
    "id": {
    "value": 1001,
    "deletion_ts": 1562202972545,
    "set": true
  },
    "registration_date": {
    "value": 1562202942545,
    "deletion_ts": 1562202972545,
    "set": true
  },
  "first_name": null,
  "last_name": null,
  "email": null
  },
  "source": {
    "version": "3.0.2.Final",
    "connector": "cassandra",
    "cluster": "cassandra-cluster-1",
    "snapshot": false,
    "keyspace": "inventory",
    "table": "customers",
    "file": "commitlog-6-123456.log",
    "pos": 102,
    "ts_ms": 1562202942666490,
    "ts_us": 1562202942666490000,
    "ts_ns": 1562202942666490000000
  }
}

When we compare this to the value in the insert and update event, we see a couple differences:

The op field value is now d, signifying that this row changed because of a deletion.
The after field only contains values for id and registration_date because this is a deletion by primary keys.
The source field structure has the same fields as before, but the values are different since this event is from a different position in the commit log.
The ts_ms shows the timestamp milliseconds which the connector processed this event.

TODO: given TTL is not currently support, would it be better to remove delete_ts? would it also be okay to derive whether a field is set or not by looking at the each column to see if it is null?

TODO: discuss tombstone events in Cassandra connector

Data Types

As described above, the Cassandra connector represents the changes to rows with events that are structured like the table in which the row exist. The event contains a field for each column value, and how that value is represented in the event depends on the Cassandra data type of the column. This section describes this mapping.

The following table describes how the connector maps each of the Cassandra data types to an Kafka Connect data type.

Cassandra Data Type

Literal Type (Schema Type)

Semantic Type (Schema Name)

ascii

string

n/a

bigint

int64

n/a

blob

bytes

n/a

boolean

n/a

counter

int64

n/a

date

int32

io.debezium.time.Date

decimal

float64

n/a

double

float64

n/a

float

float32

n/a

frozen

bytes

n/a

inet

string

n/a

int

int32

n/a

list

array

n/a

map

n/a

set

array

n/a

smallint

int16

n/a

text

string

n/a

time

int64

n/a

timestamp

int64

io.debezium.time.Timestamp

timeuuid

string

io.debezium.data.Uuid

tinyint

int8

n/a

tuple

map

n/a

uuid

string

io.debezium.data.Uuid

varchar

string

n/a

varint

int64

n/a

duration

int64

io.debezium.time.NanoDuration (an approximate representation of the duration value in nano-seconds)

TODO: add logical types

Arbitrary-precision integer types

The Cassandra connector handles varint values according to the setting of the varint.handling.mode connector configuration property.

varint.handling.mode=long

Table 1. Mapping when `varint.handling.mode=long`
Cassandra type	Literal type	Semantic type
`varint`	`INT64`	n/a

varint.handling.mode=precise

Table 2. Mappings when `decimal.handling.mode=precise`
Cassandra type	Literal type	Semantic type
`varint`	`BYTES`	`org.apache.kafka.connect.data.Decimal` The `scale` schema parameter is set to zero.

varint.handling.mode=string

Table 3. Mapping when `varint.handling.mode=string`
Cassandra type	Literal type	Semantic type
`varint`	`STRING`	n/a

Decimal types

The Cassandra connector handles decimal values according to the setting of the decimal.handling.mode connector configuration property.

decimal.handling.mode=double

Table 4. Mapping when `decimal.handling.mode=double`
Cassandra type	Literal type	Semantic type
`decimal`	`FLOAT64`	n/a

decimal.handling.mode=precise

Table 5. Mappings when `decimal.handling.mode=precise`
Cassandra type	Literal type	Semantic type
`decimal`	`STRUCT`	`io.debezium.data.VariableScaleDecimal` Contains a structure with two fields: `scale` of type `INT32` that contains the scale of the transferred value and `value` of type `BYTES` containing the original value in an unscaled form.

decimal.handling.mode=string

Table 6. Mapping when `decimal.handling.mode=string`
Cassandra type	Literal type	Semantic type
`decimal`	`STRING`	n/a

If the default data type conversions do not meet your needs, you can create a custom converter for the connector.

When Things Go Wrong

Configuration And Startup Errors

The Cassandra connector will fail upon startup, report error or exception in the log, and stop running if the configurations are invalid or if the connector cannot successfully connector to Cassandra using the specified connectivity parameters. In this case, the error will have more details about the problem and possibly suggest a work around. The connector can be restarted when the configuration has been corrected.

Cassandra Becomes Unavailable

Once the connector is running, if the Cassandra node becomes unavailable for any reason, the connector will fail and stop. In this case, restart the connector once the server becomes available. If this happened during snapshot, it will rebootstrap the entire table from the beginning of the table.

Cassandra Connector Stops Gracefully

If the Cassandra connector is gracefully shut down, prior to stopping the process it will make sure to flush all events in the ChangeEventQueue to Kafka. The Cassandra connector keeps track of the filename and offset each time a streamed record is send to Kafka. So when the connector is restarted, it will resume from where it left off. It does this by searching for the oldest commit log in the directory, start processing that commitlog, skipping the already-read records, until it finds the most recent record that hasn’t been processed. If the Cassandra connector is stopped during snapshot, it will pick up from that table, but will rebootstrap the entire table.

Cassandra Connector Crashes

If the Cassandra connector crashes unexpected, then the Cassandra connector would likely have terminated without recording the most-recently processed offset. In this case, when the connector is restarted, it will resume from the most recent recorded offset. This means duplicates is likely (which is trivial since we already be get duplicates from RF). Note that since the offset is only updated when a record has been successfully send to Kafka, it is okay to lose the un-emitted data in the ChangeEventQueue during a crash, as these events will be recreated.

Kafka Becomes Unavailable

As the connector generate change event, it will publish those events to Kafka using Kafka producer API. If Kafka broker becomes unavailable (producer encounters TimeoutException), the Cassandra connector will repeatedly attempt to reconnect to the broker once per second until a successful retry.

Cassandra connector is Stopped for a Duration

Depending on the write load of a table, when a Cassandra connector is stopped for a long time, it risks into hitting the cdc_total_space_in_mb capacity. Once this upper limit is reached, Cassandra will stop accepting writes for this table; which means it is important to monitor this space while running the Cassandra connector. In the worst case scenario if this happens, complete the following steps:

Turn off Cassandra connector.
Disable CDC for the table so it stops generating additional writes. Because the commit logs are not filtered, writes to other CDC-enabled tables on the same node could still affect the commitlog file generation.
Remove the recorded offset from the offset file
After the capacity is increased or the directory used space is under control, restart the connector so that it re-bootstraps the table.

Cassandra Table CDC is Enabled, Then Temporarily Disabled, And Then Enabled Again

If a Cassandra table temporarily disables CDC and then re-enables it again after some time, it must be re-bootstrapped. To re-bootstrap an individual table, you can manually remove the recorded offset line corresponding to that table from snapshot_offset.properties file.

Deploying A Connector

The Cassandra connector should be deployed each Cassandra node in a Cassandra cluster. The Cassandra connector Jar file takes in a CDC configuration (.properties) file. See see example configurations for reference.

Example configuration

The following represents an example .properties configuration file for running and testing the Cassandra Connector locally:

connector.name=test_connector
commit.log.relocation.dir=/Users/test_user/debezium-connector-cassandra/test_dir/relocation/
http.port=8000

cassandra.config=/usr/local/etc/cassandra/cassandra.yaml
cassandra.hosts=127.0.0.1
cassandra.port=9042

kafka.producer.bootstrap.servers=127.0.0.1:9092
kafka.producer.retries=3
kafka.producer.retry.backoff.ms=1000
topic.prefix=test_prefix

key.converter=io.confluent.connect.avro.AvroConverter
key.converter.schema.registry.url: http://localhost:8081
value.converter=io.confluent.connect.avro.AvroConverter
value.converter.schema.registry.url: http://localhost:8081

offset.backing.store.dir=/Users/test_user/debezium-connector-cassandra/test_dir/

snapshot.consistency=ONE
snapshot.mode=ALWAYS

latest.commit.log.only=true

Monitoring

Cassandra connector has built-in support for JMX metrics. The Cassandra driver also publishes a number of metrics about the driver’s activities that can be monitored through JMX. The connector has two types of metrics. Snapshot metrics help you monitor the snapshot activity and are available when the connector is performing a snapshot. Binlog metrics help you monitor the progress and activity while the connector reads the Cassandra commit logs.

Snapshot Metrics

Attribute Name

Type

Description

total-table-count

int

The total number of tables that are being included in the snapshot.

remaining-table-count

int

The number of tables that the snapshot has yet to copy.

snapshot-running

boolean

Whether the snapshot was started.

snapshot-aborted

boolean

Whether the snapshot was aborted.

snapshot-completed

boolean

Whether the snapshot completed.

snapshot-during-in-seconds

long

The total number of seconds that the snapshot has taken so far, even if not complete.

rows-scanned

Map<String, Long>

Map containing the number of rows scanned for each table in the snapshot. Tables are incrementally added to the Map during processing. Updates every 10,000 rows scanned and upon completing a table.

Commitlog Metrics

Attribute Name

Type

Description

commitlog-filename

string

The name of the commit log filename that the connector has most recently read.

commitlog-position

long

The most recent position (in bytes) within the commit log that the connector has read.

number-of-processed-mutations

long

The number of mutations that have been processed.

number-of-unrecoverable-errors

long

The number of unrecoverable errors while processing commit logs.

Connector properties

Property

Default

Description

snapshot.mode

INITIAL

Specifies the criteria for running a snapshot (eg. initial sync) upon startup of the cassandra connector agent. Must be one of 'INITIAL', 'ALWAYS', or 'NEVER'. The default snapshot mode is 'INITIAL'.

snapshot.consistency

ALL

Specify the {@link ConsistencyLevel} used for the snapshot query.

http.port

8000

The port used by the HTTP server for ping, health check, and build info

cassandra.config

No default

The absolute path of the YAML config file used by a Cassandra node.

cassandra.hosts

localhost

One or more addresses of Cassandra nodes that driver uses to discover topology, separated by ","

cassandra.port

9042

The port used to connect to Cassandra host(s).

cassandra.username

No default

The username used when connecting to Cassandra hosts.

cassandra.password

No default

The password used when connecting to Cassandra hosts.

cassandra.ssl.enabled

false

If set to true, Cassandra connector agent will use SSL to connect to Cassandra node.

cassandra.ssl.config.path

No default

The SSL config file path required for storage node. An example of config file can be found at the bottom of the page.

commit.log.real.time.processing.enabled

false

Only applicable in Cassandra 4 and if set to true, Cassandra connector agent will read commit logs incrementally by watching for updates in commit log index files and stream data in real-time, at frequency determined by commit.log.marked.complete.poll.interval.ms. If set to false, then Cassandra 4 connector waits for Commit Logs file to be marked Completed before processing them.

commit.log.marked.complete.poll.interval.ms

10000

Only applicable in Cassandra 4 and when real-time streaming is enabled by commit.log.real.time.processing.enabled. This config determines the frequency at which commit log index file is polled for updates in offset value.

commit.log.relocation.dir

No default

The local directory where commit logs get relocated to from cdc_raw dir once processed.

commit.log.post.processing.enabled

true

Determines whether or not the CommitLogPostProcessor should run to move processed commit logs from relocation dir. If disabled, commit logs would not be moved out of relocation dir.

commit.log.relocation.dir.poll.interval.ms

10000

The amount of time the CommitLogPostProcessor should wait to re-fetch all processed commit logs in relocation dir.

commit.log.transfer.class

io.debezium.connector.cassandra.BlackHoleCommitLogTransfer

The class used by CommitLogPostProcessor to move processed commit logs from relocation dir. The built-in transfer class is BlackHoleCommitLogTransfer, which simply removes all processed commit logs from relocation dir. Users are supposed to implement their own customized commit log transfer class if needed.

commit.log.error.reprocessing.enabled

false

Determines whether or not the CommitLogProcessor should re-process error commit logs.

event.order.guarantee.mode

COMMITLOG_FILE

Specifies how to ensure the order of change events. Each option indicates which property is used for hashing. Events sharing the same hash value maintain their order. It is recommended to use 'PARTITION_VALUES' to align the hashing strategy with messages in Kafka. Must be one of 'COMMITLOG_FILE', or 'PARTITION_VALUES'.

converters

No default

Enumerates a comma-separated list of the symbolic names of the custom converter instances that the connector can use. For example,

isbn

You must set the converters property to enable the connector to use a custom converter.

For each converter that you configure for a connector, you must also add a .type property, which specifies the fully-qualified name of the class that implements the converter interface. The .type property uses the following format:

<converterSymbolicName>.type

For example,

isbn.type: io.debezium.test.IsbnConverter

If you want to further control the behavior of a configured converter, you can add one or more configuration parameters to pass values to the converter. To associate any additional configuration parameter with a converter, prefix the parameter names with the symbolic name of the converter. For example,

isbn.schema.name: io.debezium.cassandra.type.Isbn

offset.backing.store.dir

No default

The directory to store offset tracking files.

offset.flush.interval.ms

0

The minimum amount of time to wait before committing the offset. The default value of 0 implies the offset will be flushed every time.

max.offset.flush.size

100

The maximum records that are allowed to be processed until it is required to flush offset to disk. This config is effective only if offset_flush_interval_ms != 0.

max.queue.size

8192

Positive integer value that specifies the maximum size of the blocking queue into which change events read from the commit log are placed before they are written to Kafka. This queue can provide back pressure to the commit log reader when, for example, writes to Kafka are slower or if Kafka is not available. Events that appear in the queue are not included in the offsets periodically recorded by this connector. Defaults to 8192, and should always be larger than the maximum batch size specified in the max.batch.size property. The capacity of the queue to hold deserialized records before they are converted to Kafka Connect structs and emitted to Kafka.

max.batch.size

2048

The maximum number of change events to dequeue each time.

max.queue.size.in.bytes

0

A long integer value that specifies the maximum volume of the blocking queue in bytes. By default, volume limits are not specified for the blocking queue. To specify the number of bytes that the queue can consume, set this property to a positive long value.
If max.queue.size is also set, writing to the queue is blocked when the size of the queue reaches the limit specified by either property. For example, if you set max.queue.size=1000, and max.queue.size.in.bytes=5000, writing to the queue is blocked after the queue contains 1000 records, or after the volume of the records in the queue reaches 5000 bytes.

poll.interval.ms

1000

Positive integer value that specifies the number of milliseconds the commit log processor should wait during each iteration for new change events to appear in the queue. Defaults to 1000 milliseconds, or 1 second.

schema.refresh.interval.ms

10000

Positive integer value that specifies the number of milliseconds the schema processor should wait before refreshing the cached Cassandra table schemas.

cdc.dir.poll.interval.ms

10000

The maximum amount of time to wait on each poll before reattempt.

snapshot.scan.interval.ms

10000

Positive integer value that specifies the number of milliseconds the snapshot processor should wait before re-scanning tables to look for new CDC-enabled tables. Defaults to 10000 milliseconds, or 10 seconds.

tombstones.on.delete

false

Whether deletion events should have a subsequent tombstone event (true) or not (false). It’s important to note that in Cassandra, two events with the same key may be updating different columns of a given table. So this could potentially result in records being lost during compaction if they have not been consumed by the consumer yet. In other words, do NOT set this to true if you have Kafka compaction turned on.

field.exclude.list

No default

A comma-separated list of fully-qualified names of fields that should be excluded from change event message values. Fully-qualified names for fields are in the form keyspace_name>.<field_name>.<nested_field_name>.

num.of.change.event.queues

1

The number of change event queues and queue processors. Defaults to 1.

skipped.operations

t

A comma-separated list of operation types that will be skipped during streaming. The operations include: c for inserts/create, u for updates, d for deletes, t for truncates, and none to not skip any operations. By default, truncate operations are skipped (not emitted by this connector).

topic.naming.strategy

io.debezium.schema.SchemaTopicNamingStrategy

The name of the TopicNamingStrategy class that should be used to determine the topic name for data change, schema change, transaction, heartbeat event etc., defaults to SchemaTopicNamingStrategy.

topic.delimiter

.

Specify the delimiter for topic name, defaults to ..

topic.prefix

No default

The name of the prefix to be used for all topics.

Do not change the value of this property. If you change the name value, after a restart, instead of continuing to emit events to the original topics, the connector emits subsequent events to topics whose names are based on the new value. The connector is also unable to recover its database schema history topic.

topic.cache.size

10000

The size used for holding the topic names in bounded concurrent hash map. This cache helps to determine the topic name corresponding to a given data collection.

topic.heartbeat.prefix

__debezium-heartbeat

Controls the name of the topic to which the connector sends heartbeat messages. The topic name has the following pattern:

topic.heartbeat.prefix.topic.prefix

For example, if the database server name or topic prefix is fulfillment, the default topic name is __debezium-heartbeat.fulfillment.

varint.handling.mode

long

Specifies how varint columns should be represented in change events. Possible settings are:

long (the default) represents values by using Java’s long, which might not offer the precision but which is easy to use in consumers.

precise uses java.math.BigDecimal to represent values, which are encoded in the change events by using a binary representation and Kafka Connect’s org.apache.kafka.connect.data.Decimal type.

string encodes values as formatted strings, which is easy to consume.

decimal.handling.mode

double

Specifies how decimal columns should be represented in change events. Possible settings are:

double (the default) represents values by using Java’s double, which might not offer the precision but which is easy to use in consumers.

precise uses java.math.BigDecimal to represent values, which are encoded in the change events by using a binary representation and Kafka Connect’s org.apache.kafka.connect.data.VariableScaleDecimal type.

string encodes values as formatted strings, which is easy to consume.

schema.name.adjustment.mode

none

Specifies how schema names should be adjusted for compatibility with the message converter used by the connector. Possible settings:

none does not apply any adjustment.
avro replaces the characters that cannot be used in the Avro type name with underscore.
avro_unicode replaces the underscore or characters that cannot be used in the Avro type name with corresponding unicode like _uxxxx. Note: _ is an escape sequence like backslash in Java

field.name.adjustment.mode

none

Specifies how field names should be adjusted for compatibility with the message converter used by the connector. Possible settings:

none does not apply any adjustment.
avro replaces the characters that cannot be used in the Avro type name with underscore.
avro_unicode replaces the underscore or characters that cannot be used in the Avro type name with corresponding unicode like _uxxxx. Note: _ is an escape sequence like backslash in Java

See Avro naming for more details.

custom.metric.tags

No default

The custom metric tags will accept key-value pairs to customize the MBean object name which should be appended the end of regular name, each key would represent a tag for the MBean object name, and the corresponding value would be the value of that tag the key is. For example: k1=v1,k2=v2.

errors.max.retries

-1

Specifies how the connector responds after an operation fails with a retriable error, such as a connection error.
Set one of the following options:

-1: No limit. The connector always restarts automatically, and retries the operation, regardless of the number of previous failures.
0: Disabled. The connector fails immediately, and never retries the operation. User intervention is required to restart the connector.
> 0: The connector restarts automatically until it reaches the specified maximum number of retries. After the next failure, the connector stops, and user intervention is required to restart it.

If the Cassandra agent use SSL to connect to Cassandra node, an SSL config file is required. The following example shows how to write the SSL config file:

keyStore.location=/var/private/ssl/cassandra.keystore.jks
keyStore.password=cassandra
keyStore.type=JKS
trustStore.location=/var/private/ssl/cassandra.truststore.jks
trustStore.password=cassandra
trustStore.type=JKS
keyManager.algorithm=SunX509
trustManager.algorithm=SunX509
cipherSuites=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384

The cipherSuites field is not mandatory, it simply allows you to add one (or more) ciphers that are not present. The default value of trustStore.type and keyStore.type is JKS. The default value of keyManager.algorithm and trustManager.algorithm is SunX509.

The connector also supports pass-through configuration properties that are used when creating the Kafka producer. Specifically, all connector configuration properties that begin with the kafka.producer. prefix are used (without the prefix) when creating the Kafka producer that writes events to Kafka.

For example, the follwoing connector configuration properties can be used to secure connections to the Kafka broker:

kafka.producer.security.protocol=SSL
kafka.producer.ssl.keystore.location=/var/private/ssl/kafka.server.keystore.jks
kafka.producer.ssl.keystore.password=test1234
kafka.producer.ssl.truststore.location=/var/private/ssl/kafka.server.truststore.jks
kafka.producer.ssl.truststore.password=test1234
kafka.producer.ssl.key.password=test1234
kafka.consumer.security.protocol=SSL
kafka.consumer.ssl.keystore.location=/var/private/ssl/kafka.server.keystore.jks
kafka.consumer.ssl.keystore.password=test1234
kafka.consumer.ssl.truststore.location=/var/private/ssl/kafka.server.truststore.jks
kafka.consumer.ssl.truststore.password=test1234
kafka.consumer.ssl.key.password=test1234

Be sure to consult the Kafka documentation for all of the configuration properties for Kafka producers.

The connector supports the following Kafka Connect converters for key/value serialization:

io.confluent.connect.avro.AvroConverter
org.apache.kafka.connect.storage.StringConverter
org.apache.kafka.connect.json.JsonConverter
com.blueapron.connect.protobuf.ProtobufConverter