Changefeedccl: ALTER CHANGEFEED Initial_scan = 'only' Behaves Strangely

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Introduction

In the world of distributed databases, change feeds play a crucial role in replicating data across different nodes. However, when it comes to altering change feeds, certain behaviors can be counterintuitive. In this article, we will delve into the specifics of the ALTER CHANGEFEED command, particularly when it comes to the initial_scan = 'only' option. We will explore how this option behaves when altering an existing change feed and discuss the implications of this behavior.

Understanding Change Feeds

Before we dive into the specifics of the ALTER CHANGEFEED command, let's take a step back and understand what change feeds are. A change feed is a mechanism that allows you to replicate data from one node to another. It provides a stream of changes made to the data, enabling you to keep your nodes in sync. Change feeds are particularly useful in distributed databases, where data is spread across multiple nodes.

The ALTER CHANGEFEED Command

The ALTER CHANGEFEED command is used to modify an existing change feed. This command allows you to add or remove tables from the change feed, as well as modify the configuration of the change feed. One of the options available when using the ALTER CHANGEFEED command is the initial_scan = 'only' option.

The initial_scan = 'only' Option

The initial_scan = 'only' option is used to specify whether an initial scan should be performed when adding a table to the change feed. An initial scan is a process where the change feed is scanned from the beginning to retrieve all the changes made to the data. This option is particularly useful when you want to ensure that all changes are replicated, even if the change feed is not up-to-date.

The Problem with ALTER CHANGEFEED initial_scan = 'only'

When you use the ALTER CHANGEFEED command with the initial_scan = 'only' option, unless the original change feed had initial_scan = 'only' configured, the behavior is not what you would expect. Not only will the initial scan not be performed, but the table will also be added to the change feed's watched targets. This means that future updates will be sent, even if the initial scan was not performed.

Implications of the Behavior

The behavior of the ALTER CHANGEFEED command with the initial_scan = 'only' option has significant implications. If you are relying on the initial scan to ensure that all changes are replicated, you may be surprised to find that the initial scan is not performed. This can lead to data inconsistencies and replication issues.

Jira Issue: CRDB-48472

The behavior of the ALTER CHANGEFEED command with the initial_scan = 'only' option has been reported as a bug in the Jira issue CRDB-48472. This issue highlights the need for a more intuitive and consistent behavior when using the ALTER CHANGEFEED command.

Conclusion

In conclusion, the behavior of the ALTER CHANGEFEED command with the initial_scan = 'only' option is not what you would expect. Unless the original change feed had initial_scan = 'only' configured, the initial scan will not be performed, and the table will be added to the change feed's watched targets. This behavior has significant implications and highlights the need for a more intuitive and consistent behavior when using the ALTER CHANGEFEED command.

Recommendations

Based on the behavior of the ALTER CHANGEFEED command with the initial_scan = 'only' option, we recommend the following:

  • Always check the configuration of the original change feed before using the ALTER CHANGEFEED command with the initial_scan = 'only' option.
  • Use the ALTER CHANGEFEED command with caution, as the behavior may not be what you expect.
  • Consider using a more intuitive and consistent behavior for the ALTER CHANGEFEED command.

Future Development

The behavior of the ALTER CHANGEFEED command with the initial_scan = 'only' option is a bug that needs to be addressed. We recommend that the development team considers the following:

  • Modify the behavior of the ALTER CHANGEFEED command to perform the initial scan when the initial_scan = 'only' option is used.
  • Provide a more intuitive and consistent behavior for the ALTER CHANGEFEED command.
  • Document the behavior of the ALTER CHANGEFEED command with the initial_scan = 'only' option to prevent similar issues in the future.

Conclusion

Introduction

In our previous article, we discussed the behavior of the ALTER CHANGEFEED command with the initial_scan = 'only' option. We highlighted the issue where the initial scan is not performed, and the table is added to the change feed's watched targets. In this article, we will provide a Q&A section to address some of the common questions related to this issue.

Q: What is the purpose of the initial_scan = 'only' option?

A: The initial_scan = 'only' option is used to specify whether an initial scan should be performed when adding a table to the change feed. An initial scan is a process where the change feed is scanned from the beginning to retrieve all the changes made to the data.

Q: Why is the initial scan not performed when using the ALTER CHANGEFEED command with the initial_scan = 'only' option?

A: The initial scan is not performed when using the ALTER CHANGEFEED command with the initial_scan = 'only' option because the original change feed did not have initial_scan = 'only' configured. This means that the change feed was not designed to perform an initial scan, and therefore, it does not perform one when the ALTER CHANGEFEED command is used.

Q: What are the implications of the initial scan not being performed?

A: The implications of the initial scan not being performed are significant. If you are relying on the initial scan to ensure that all changes are replicated, you may be surprised to find that the initial scan is not performed. This can lead to data inconsistencies and replication issues.

Q: How can I ensure that the initial scan is performed when using the ALTER CHANGEFEED command with the initial_scan = 'only' option?

A: To ensure that the initial scan is performed when using the ALTER CHANGEFEED command with the initial_scan = 'only' option, you need to configure the original change feed to have initial_scan = 'only'. This will ensure that the initial scan is performed when the ALTER CHANGEFEED command is used.

Q: What is the Jira issue CRDB-48472, and how does it relate to this issue?

A: The Jira issue CRDB-48472 is a bug that has been reported in the system. It relates to the behavior of the ALTER CHANGEFEED command with the initial_scan = 'only' option. The issue is that the initial scan is not performed, and the table is added to the change feed's watched targets. This is a known issue, and it is being addressed by the development team.

Q: How can I prevent this issue from occurring in the future?

A: To prevent this issue from occurring in the future, you need to ensure that the original change feed is configured to have initial_scan = 'only'. This will ensure that the initial scan is performed when the ALTER CHANGEFEED command is used. Additionally, you can use the ALTER CHANGEFEED command with caution and always check the configuration of the original change feed before using it.

Q: What is the recommended course of action when using the ALTER CHANGEFEED command with the initial_scan = 'only' option?

A: The recommended course of action when using the ALTER CHANGEFEED command with the initial_scan = 'only' option is to always check the configuration of the original change feed before using it. Additionally, you should use the ALTER CHANGEFEED command with caution and be aware of the implications of the initial scan not being performed.

Conclusion

In conclusion, the behavior of the ALTER CHANGEFEED command with the initial_scan = 'only' option is not what you would expect. This behavior has significant implications and highlights the need for a more intuitive and consistent behavior when using the ALTER CHANGEFEED command. We recommend that the development team addresses this issue and provides a more intuitive and consistent behavior for the ALTER CHANGEFEED command.