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Troubleshoot TiCDC

This document introduces the common issues and errors that you might encounter when using TiCDC, and the corresponding maintenance and troubleshooting methods.

Note:

In this document, the PD address specified in cdc cli commands is --pd=http://10.0.10.25:2379. When you use the command, replace the address with your actual PD address.

How do I choose start-ts when creating a task in TiCDC?

The start-ts of a replication task corresponds to a Timestamp Oracle (TSO) in the upstream TiDB cluster. TiCDC requests data from this TSO in a replication task. Therefore, the start-ts of the replication task must meet the following requirements:

  • The value of start-ts is larger than the tikv_gc_safe_point value of the current TiDB cluster. Otherwise, an error occurs when you create a task.
  • Before starting a task, ensure that the downstream has all data before start-ts. For scenarios such as replicating data to message queues, if the data consistency between upstream and downstream is not required, you can relax this requirement according to your application need.

If you do not specify start-ts, or specify start-ts as 0, when a replication task is started, TiCDC gets a current TSO and starts the task from this TSO.

Why can't some tables be replicated when I create a task in TiCDC?

When you execute cdc cli changefeed create to create a replication task, TiCDC checks whether the upstream tables meet the replication restrictions. If some tables do not meet the restrictions, some tables are not eligible to replicate is returned with a list of ineligible tables. You can choose Y or y to continue creating the task, and all updates on these tables are automatically ignored during the replication. If you choose an input other than Y or y, the replication task is not created.

How do I handle replication interruption?

A replication task might be interrupted in the following known scenarios:

  • The downstream continues to be abnormal, and TiCDC still fails after many retries.

    • In this scenario, TiCDC saves the task information. Because TiCDC has set the service GC safepoint in PD, the data after the task checkpoint is not cleaned by TiKV GC within the valid period of gc-ttl.
    • Handling method: You can resume the replication task via the HTTP interface after the downstream is back to normal.
  • Replication cannot continue because of incompatible SQL statement(s) in the downstream.

    • In this scenario, TiCDC saves the task information. Because TiCDC has set the service GC safepoint in PD, the data after the task checkpoint is not cleaned by TiKV GC within the valid period of gc-ttl.
    • Handling procedures:
      1. Query the status information of the replication task using the cdc cli changefeed query command and record the value of checkpoint-ts.
      2. Use the new task configuration file and add the ignore-txn-start-ts parameter to skip the transaction corresponding to the specified start-ts.
      3. Stop the old replication task via HTTP API. Execute cdc cli changefeed create to create a new task and specify the new task configuration file. Specify checkpoint-ts recorded in step 1 as the start-ts and start a new task to resume the replication.

How do I know whether a TiCDC replication task is interrupted?

  • Check the changefeed checkpoint monitoring metric of the replication task (choose the right changefeed id) in the Grafana dashboard. If the metric value stays unchanged, or the checkpoint lag metric keeps increasing, the replication task might be interrupted.
  • Check the exit error count monitoring metric. If the metric value is greater than 0, an error has occurred in the replication task.
  • Execute cdc cli changefeed list and cdc cli changefeed query to check the status of the replication task. stopped means the task has stopped and the error item provides the detailed error information. After the error occurs, you can search error on running processor in the TiCDC server log to see the error stack for troubleshooting.
  • In some extreme cases, the TiCDC service is restarted. You can search the FATAL level log in the TiCDC server log for troubleshooting.

What is gc-ttl in TiCDC?

Since v4.0.0-rc.1, PD supports external services in setting the service-level GC safepoint. Any service can register and update its GC safepoint. PD ensures that the key-value data smaller than this GC safepoint is not cleaned by GC. Enabling this feature in TiCDC ensures that the data to be consumed by TiCDC is retained in TiKV without being cleaned by GC when the replication task is unavailable or interrupted.

When starting the TiCDC server, you can specify the Time To Live (TTL) duration of GC safepoint through gc-ttl, which means the longest time that data is retained within the GC safepoint. This value is set by TiCDC in PD, which is 86,400 seconds by default.

How do I handle the OOM that occurs after TiCDC is restarted after a task interruption?

If the replication task is interrupted for a long time and a large volume of new data has been written to TiDB, Out of Memory (OOM) might occur when TiCDC is restarted. In this situation, you can enable unified sorter, TiCDC's experimental sorting engine. This engine sorts data in the disk when the memory is insufficient. To enable this feature, pass --sort-engine=unified and --sort-dir=/path/to/sort_dir to the cdc cli command when creating a replication task. For example:

cdc cli changefeed update -c [changefeed-id] --sort-engine="unified" --sort-dir="/data/cdc/sort" --pd=http://10.0.10.25:2379

Note:

  • Since v4.0.9, TiCDC supports the unified sorter engine.
  • TiCDC (the 4.0 version) does not support dynamically modifying the sorting engine yet. Make sure that the changefeed has stopped before modifying the sorter settings.
  • Currently, the unified sorter is an experimental feature. When the number of tables is too large (>=100), the unified sorter might cause performance issues and affect replication throughput. Therefore, it is not recommended to use it in a production environment. Before you enable the unified sorter, make sure that the machine of each TiCDC node has enough disk capacity. If the total size of unprocessed data changes might exceed 1 TB, it is not recommend to use TiCDC for replication.

What is the complete behavior of TiCDC garbage collection (GC) safepoint?

If a replication task starts after the TiCDC service starts, the TiCDC owner updates the PD service GC safepoint with the smallest value of checkpoint-ts among all replication tasks. The service GC safepoint ensures that TiCDC does not delete data generated at that time and after that time. If the replication task is interrupted, the checkpoint-ts of this task does not change and PD's corresponding service GC safepoint is not updated either. The Time-To-Live (TTL) that TiCDC sets for a service GC safepoint is 24 hours, which means that the GC mechanism does not delete any data if the TiCDC service can be recovered within 24 hours after it is interrupted.

How do I handle the Error 1298: Unknown or incorrect time zone: 'UTC' error when creating the replication task or replicating data to MySQL?

This error is returned when the downstream MySQL does not load the time zone. You can load the time zone by running mysql_tzinfo_to_sql. After loading the time zone, you can create tasks and replicate data normally.

mysql_tzinfo_to_sql /usr/share/zoneinfo | mysql -u root mysql -p

If the output of the command above is similar to the following one, the import is successful:

Enter password:
Warning: Unable to load '/usr/share/zoneinfo/iso3166.tab' as time zone. Skipping it.
Warning: Unable to load '/usr/share/zoneinfo/leap-seconds.list' as time zone. Skipping it.
Warning: Unable to load '/usr/share/zoneinfo/zone.tab' as time zone. Skipping it.
Warning: Unable to load '/usr/share/zoneinfo/zone1970.tab' as time zone. Skipping it.

If the downstream is a special MySQL environment (a public cloud RDS or some MySQL derivative versions) and importing the time zone using the above method fails, you need to specify the MySQL time zone of the downstream using the time-zone parameter in sink-uri. You can first query the time zone used by MySQL:

  1. Query the time zone used by MySQL:

    show variables like '%time_zone%';
    +------------------+--------+
    | Variable_name    | Value  |
    +------------------+--------+
    | system_time_zone | CST    |
    | time_zone        | SYSTEM |
    +------------------+--------+
  2. Specify the time zone when you create the replication task and create the TiCDC service:

    cdc cli changefeed create --sink-uri="mysql://root@127.0.0.1:3306/?time-zone=CST" --pd=http://10.0.10.25:2379

    Note:

    CST might be an abbreviation for the following four different time zones:

    • Central Standard Time (USA) UT-6:00
    • Central Standard Time (Australia) UT+9:30
    • China Standard Time UT+8:00
    • Cuba Standard Time UT-4:00

    In China, CST usually stands for China Standard Time.

How to understand the relationship between the TiCDC time zone and the time zones of the upstream/downstream databases?

Upstream time zoneTiCDC time zoneDownstream time zone
Configuration methodSee Time Zone SupportConfigured using the --tz parameter when you start the TiCDC serverConfigured using the time-zone parameter in sink-uri
DescriptionThe time zone of the upstream TiDB, which affects DML operations of the timestamp type and DDL operations related to timestamp type columns.TiCDC assumes that the upstream TiDB's time zone is the same as the TiCDC time zone configuration, and performs related operations on the timestamp column.The downstream MySQL processes the timestamp in the DML and DDL operations according to the downstream time zone setting.

Note:

Be careful when you set the time zone of the TiCDC server, because this time zone is used for converting the time type. Keep the upstream time zone, TiCDC time zone, and the downstream time zone consistent. The TiCDC server chooses its time zone in the following priority:

  • TiCDC first uses the time zone specified using --tz.
  • When --tz is not available, TiCDC tries to read the time zone set using the TZ environment variable.
  • When the TZ environment variable is not available, TiCDC uses the default time zone of the machine.

What is the default behavior of TiCDC if I create a replication task without specifying the configuration file in --config?

If you use the cdc cli changefeed create command without specifying the -config parameter, TiCDC creates the replication task in the following default behaviors:

  • Replicates all tables except system tables
  • Enables the Old Value feature
  • Skips replicating tables that do not contain valid indexes

How do I handle the incompatibility issue of configuration files caused by TiCDC upgrade?

Refer to Notes for compatibility.

Does TiCDC support outputting data changes in the Canal format?

Yes. To enable Canal output, specify the protocol as canal in the --sink-uri parameter. For example:

cdc cli changefeed create --pd=http://10.0.10.25:2379 --sink-uri="kafka://127.0.0.1:9092/cdc-test?kafka-version=2.4.0&protocol=canal" --config changefeed.toml

Note:

  • This feature is introduced in TiCDC 4.0.2.
  • TiCDC currently supports outputting data changes in the Canal format only to Kafka.

For more information, refer to Create a replication task.

How do I view the status of TiCDC replication tasks?

To view the status of TiCDC replication tasks, use cdc cli. For example:

cdc cli changefeed list --pd=http://10.0.10.25:2379

The expected output is as follows:

[{
    "id": "4e24dde6-53c1-40b6-badf-63620e4940dc",
    "summary": {
      "state": "normal",
      "tso": 417886179132964865,
      "checkpoint": "2020-07-07 16:07:44.881",
      "error": null
    }
}]
  • checkpoint: TiCDC has replicated all data before this timestamp to downstream.

  • state: The state of the replication task:

    • normal: The task runs normally.
    • stopped: The task is stopped manually or encounters an error.
    • removed: The task is removed.

Note:

This feature is introduced in TiCDC 4.0.3.

How do I know whether the replication task is stopped manually?

You can know whether the replication task is stopped manually by using cdc cli. For example:

cdc cli changefeed query --pd=http://10.0.10.25:2379 --changefeed-id 28c43ffc-2316-4f4f-a70b-d1a7c59ba79f

In the output of this command, admin-job-type shows the state of the replication task:

  • 0: In progress, which means that the task is not stopped manually.
  • 1: Paused. When the task is paused, all replicated processors exit. The configuration and the replication status of the task are retained, so you can resume the task from checkpiont-ts.
  • 2: Resumed. The replication task resumes from checkpoint-ts.
  • 3: Removed. When the task is removed, all replicated processors are ended, and the configuration information of the replication task is cleared up. Only the replication status is retained for later queries.

Why does the latency from TiCDC to Kafka become higher and higher?

  • Check how do I view the status of TiCDC replication tasks.

  • Adjust the following parameters of Kafka:

    • Increase the message.max.bytes value in server.properties to 1073741824 (1 GB).
    • Increase the replica.fetch.max.bytes value in server.properties to 1073741824 (1 GB).
    • Increase the fetch.message.max.bytes value in consumer.properties to make it larger than the message.max.bytes value.

When TiCDC replicates data to Kafka, does it write all the changes in a transaction into one message? If not, on what basis does it divide the changes?

No. According to the different distribution strategies configured, TiCDC divides the changes on different bases, including default, row id, table, and ts.

For more information, refer to Replication task configuration file.

When TiCDC replicates data to Kafka, can I control the maximum size of a single message in TiDB?

No. Currently TiCDC sets the maximum size of batch messages to 512 MB, and that of a single message to 4 MB.

When TiCDC replicates data to Kafka, does a message contain multiple types of data changes?

Yes. A single message might contain multiple updates or deletes, and update and delete might co-exist.

When TiCDC replicates data to Kafka, how do I view the timestamp, table name, and schema name in the output of TiCDC Open Protocol?

The information is included in the key of Kafka messages. For example:

{
    "ts":<TS>,
    "scm":<Schema Name>,
    "tbl":<Table Name>,
    "t":1
}

For more information, refer to TiCDC Open Protocol event format.

When TiCDC replicates data to Kafka, how do I know the timestamp of the data changes in a message?

You can get the unix timestamp by moving ts in the key of the Kafka message by 18 bits to the right.

How does TiCDC Open Protocol represent null?

In TiCDC Open Protocol, the type code 6 represents null.

TypeCodeOutput ExampleNote
Null6{"t":6,"v":null}

For more information, refer to TiCDC Open Protocol column type code.

The start-ts timestamp of the TiCDC task is quite different from the current time. During the execution of this task, replication is interrupted and an error [CDC:ErrBufferReachLimit] occurs

Since v4.0.9, you can try to enable the unified sorter feature in your replication task, or use the BR tool for an incremental backup and restore, and then start the TiCDC replication task from a new time.

How can I tell if a Row Changed Event of TiCDC Open Protocol is an INSERT event or an UPDATE event?

If the Old Value feature is not enabled, you cannot tell whether a Row Changed Event of TiCDC Open Protocol is an INSERT event or an UPDATE event. If the feature is enabled, you can determine the event type by the fields it contains:

  • UPDATE event contains both "p" and "u" fields
  • INSERT event only contains the "u" field
  • DELETE event only contains the "d" field

For more information, refer to Open protocol Row Changed Event format.

Does TiCDC support replicating large transactions? Is there any risk?

TiCDC provides partial support for large transactions (more than 5 GB in size). Depending on different scenarios, the following risks might exist:

  • When TiCDC's internal processing capacity is insufficient, the replication task error ErrBufferReachLimit might occur.
  • When TiCDC's internal processing capacity is insufficient or the throughput capacity of TiCDC's downstream is insufficient, out of memory (OOM) might occur.

If you encounter an error above, it is recommended to use BR to restore the incremental data of large transactions. The detailed operations are as follows:

  1. Record the checkpoint-ts of the changefeed that is terminated due to large transactions, use this TSO as the --lastbackupts of the BR incremental backup, and execute incremental data backup.
  2. After backing up the incremental data, you can find a log record similar to ["Full backup Failed summary : total backup ranges: 0, total success: 0, total failed: 0"] [BackupTS=421758868510212097] in the BR log output. Record the BackupTS in this log.
  3. Restore the incremental data.
  4. Create a new changefeed and start the replication task from BackupTS.
  5. Delete the old changefeed.

When the downstream of a changefeed is a database similar to MySQL and TiCDC executes a time-consuming DDL statement, all other changefeeds are blocked. How should I handle the issue?

  1. Pause the execution of the changefeed that contains the time-consuming DDL statement. Then you can see that other changefeeds are no longer blocked.
  2. Search for the apply job field in the TiCDC log and confirm the StartTs of the time-consuming DDL statement.
  3. Manually execute the DDL statement in the downstream. After the execution finishes, go on performing the following operations.
  4. Modify the changefeed configuration and add the above StartTs to the ignore-txn-start-ts configuration item.
  5. Resume the paused changefeed.

After I upgrade the TiCDC cluster to v4.0.8, the [CDC:ErrKafkaInvalidConfig]Canal requires old value to be enabled error is reported when I execute a changefeed

Since v4.0.8, if the canal or maxwell protocol is used for output in a changefeed, TiCDC enables the old value feature automatically. However, if you have upgraded TiCDC from an earlier version to v4.0.8 or later, when the changefeed uses the canal or maxwell protocol and the old value feature is disabled, this error is reported. To fix the error, take the following steps:

  1. Set the value of enable-old-value in the changefeed configuration file to true.

  2. Execute cdc cli changefeed pause to pause the replication task.

    cdc cli changefeed pause -c test-cf --pd=http://10.0.10.25:2379
  3. Execute cdc cli changefeed update to update the original changefeed configuration.

    cdc cli changefeed update -c test-cf --pd=http://10.0.10.25:2379 --sink-uri="mysql://127.0.0.1:3306/?max-txn-row=20&worker-number=8" --config=changefeed.toml
  4. Execute cdc cli changfeed resume to resume the replication task.

    cdc cli changefeed resume -c test-cf --pd=http://10.0.10.25:2379

How much PD storage does TiCDC use?

TiCDC uses etcd in PD to store and regularly update the metadata. Because the time interval between the MVCC of etcd and PD's default compaction is one hour, the amount of PD storage that TiCDC uses is proportional to the amount of metadata versions generated within this hour. However, in v4.0.5, v4.0.6, and v4.0.7, TiCDC has a problem of frequent writing, so if there are 1000 tables created or scheduled in an hour, it then takes up all the etcd storage and returns the etcdserver: mvcc: database space exceeded error. You need to clean up the etcd storage after getting this error. See etcd maintaince space-quota for details. It is recommended to upgrade your cluster to v4.0.9 or later versions.

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