Discussion thread: here
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The AlterConfigs RPC gives users a way to alter the configuration of a topic, broker, or other resource. However, the new configuration replaces any existing configuration. This makes AlterConfigs unusable in cases where the client does not know the full existing configuration before making the modification. This makes AlterConfigs less efficient, since it means that the client needs to call DescribeConfigs to retrieve the existing configuration before making any modification. It also introduces the possibility of "lost updates" when multiple clients enter a read-modify-write cycle around the same time.
Even worse, in some cases, the client may be unable to discover the existing configuration. "Sensitive" fields are not returned by DescribeConfigs. However, they will be overwritten by AlterConfigs.
In order to fix these issues, we should introduce a new RPC named incrementalAlterConfigs. The new RPC should operate incrementally, modifying only the configuration values that are specified. We should deprecate AlterConfigs.
The new IncrementalAlterConfigs API in AdminClient will take a collection of operations describing the configuration modifications. There are four types of operations.
- Set: set a configuration to a value. The value must not be null.
- Delete: delete a configuration key
- Append: if a configuration key is a list of values, add to the list.
- Subtract: if a configuration key is a list of values, subtract from the list.
We will use existing AlterConfigsOptions, AlterConfigsResult API classes to pass the API config options and to return the result of configuration resource modifications.
Similar to existing alterConfigs API, we will to keep the "transactionality" of updating several configs for the same ConfigResource at once. We guarantee that we never do a partial update of a collection of configs
for a ConfigResource from a single request. On validation/update error, we will return the error for the ConfigResource.
There will be a new Incremental AlterConfigsRequest.
The Incremental AlterConfigsResponse is the same as the AlterConfigsResponse.
There are two new error conditions.
- If a configuration key was specified more than once in the request for given ConfigResource, we fail ConfigResource update with the INVALID_REQUEST.
- If APPEND or SUBTRACT was specified for a configuration key that does not take a list of elements, we fail ConfigResource update with the INVALID_REQUEST.
Compatibility, Deprecation, and Migration Plan
This change is backwards compatible because the existing alterConfigs RPC is retained. Clients will migrate to the new incrementalAlterConfigs RPC as needed.
We could have changed alterConfigs so that it had an incremental mode. This would have avoided creating a new RPC. However, in order to avoid breaking compatibility, the incremental mode could not have been made the default for AdminClient. We would also not have been able to deprecate the non-incremental mode. This would create a confusing and dangerous stumbling block for new users. Because the problems with non-incremental mode are not immediately obvious, it is likely that many users would have made the wrong decision about what API to use.