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Config entities

Client entity types and names are more dynamic than broker and topic entity types and names. For example, client quotas can be tied to a user principle that is associated with a session as well as a client-id which is a generic workload identifier. This is not as simple as a broker with a broker id, so dynamic client configs should also have similar expressibility and extensibility to that which was introduced in KIP-546 for client quotas. The ClientConfigs APIs will follow the design pattern of the ClientQuotas APIs with a few differences.

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Quota values are limited to double-precision 64-bit IEEE 754 format in the APIs introduced in KIP-546. However, client config values are strings in a .properties file until the config values are parsed into their respective types based on the client's config definition. Dynamic client configs should also be strings so that dynamic support can be added for any type of config in the future and remain consistent with static client configuration. This also allows dynamic client configs to be parsed and validated in the same way as static client configs.

Hierarchy for resolving dynamic client configs

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The user config is updated when the client-id component entity name in the AlterClientConfigsRequest is nullis missing from an AlterClientConfigsRequest. The <user, client-id> config is updated otherwise.Client configs will have the following order of precedence:

/config/users/<user>/clients/<client-id> 

/config/users/<user>

When the broker handles a DescribeClientConfigsRequest that a client is making for it's own dynamic configs (e.g. ResolveEntity field set to true), the user config and the <user, client-id> config will be returned as one entity whose configs are resolved with the above following order of precedence .from most precedent to least precedent:

/config/users/<user>/clients/<client-id> 

/config/users/<user>

For example, any config key value pairs found in /config/users/<user>/clients/<client-id> will override any config key value pairs found in /config/users/<user>. The final resolved map of configs will then be sent back to the client and will overwrite statically provided client configs.

Client quotas are stored in these znodes as well. However, all configs that are not Client quotas are stored in these znodes as well. However, all configs that are not quota configs are filtered out when constructing a DescribeClientQuotasResponse. Similar to this, all configs that are not dynamic client configs will be filtered out when constructing a DescribeClientConfigsResponse. The value for each key will also be validated against the allowed values for that key. For example, if the user tries to set acks=2, an InvalidRequest error code will be sent back. The client will also have to validate dynamic configs against user-provided configs, so the broker is only doing partial validation here. This is worth doing since partially validated configs may only be invalid for a subset of clients, whereas acks=2 would be invalid for all clients.

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The Java consumer will have a DynamicConsumerConfig that will periodically fetch dynamic configs. The interval on which dynamic configs are fetched will be the same amount of time as the interval for MetadataRequest, metadata.max.age.ms. It will use DescribeConfigsRequest as the RPC, validate the dynamic configs that are returned in DescribeConfigsResponse against the user provided configs and log any configurations that are accepted. The client will either reconfigure itself by changing the session timeout and heartbeat interval in the GroupRebalanceConfig, or discard the configs if the heartbeat interval is greater than or equal to the session timeout. The dynamic configs will take precedence over user provided client configs as long as the heartbeat interval is strictly less than the session timeout.

Command Line Changes

kafka-configs.sh will be extended to support the client configurations listed at the beginning of this KIP. The same entity types that are used for client quotas, users and clients, will be used for dynamic client configuration.

the GroupRebalanceConfig, or discard the configs if the heartbeat interval is greater than or equal to the session timeout. The dynamic configs will take precedence over user provided client configs as long as the heartbeat interval is strictly less than the session timeout.

Command Line Changes

kafka-configs.sh will be extended to support the client configurations listed at the beginning of this KIP. The same entity types that are used for client quotas, users and clients, will be used for dynamic client configuration.

For example, the user can add the new configs supported with this KIP along with the quota configs that are supported for the admin client in KIP-546 to their default dynamic config. In this example the user mixes some dynamic client configs that this KIP introduces with the quota config producer_byte_rate:

bin/kafka-configs.sh --bootstrap-server localhost:9092 \
  --alter \
  --entity-type users \
  --entity-name alice \
  --add-config acks=-1,session.timeout.ms=11000,producer_byte_rate=50000
  Completed updating config for user alice.

The user can also add configs specific to a client-id that will override the user's default dynamic configsFor example, the user can add the new configs supported with this KIP along with the quota configs that are supported for the admin client in KIP-546 to their default dynamic config. In this example the user mixes some dynamic client configs that this KIP introduces with the quota config producer_byte_rate:

bin/kafka-configs.sh --bootstrap-server localhost:9092 \
  --alter \
  --entity-type users \
  --entity-name alice \
  --entity-type clients \
  --entity-name clientid-override \
  --add-config acks=-10,sessionheartbeat.timeoutinterval.ms=110002000,producer_byte_rate=5000060000
  Completed updating config for user alice.

The user can also add configs specific to a client-id that will override the user's default dynamic configs:

user alice.

The user can describe these configs the same way that client quotas are described with the users and clients entity types. To make this possible, kafka-configs.sh will be sending a DescribeClientConfigsRequest as well as the DescribeClientQuotasRequest. Dynamic client configs must at least be scoped by a user:

bin/kafka-configs.sh --bootstrap-server localhost:9092 \
  --alter \
  --entity-type users-server localhost:9092 \
  --entity-namedescribe alice \
  --entity-type clientsusers \
  --entity-name clientid-override \
  --add-config acks=0,heartbeat.interval.ms=2000,alice
Quota configs for user-principal 'alice' are producer_byte_rate=6000050000.0
Dynamic configs Completed updating config for user-principal alice.

The user can describe these configs the same way that client quotas are described with the users and clients entity types. To make this possible, kafka-configs.sh will be sending a DescribeClientConfigsRequest as well as the DescribeClientQuotasRequest. Dynamic client configs must at least be scoped by a user:

'alice' are session.timeout.ms=11000, acks=-1

They may optionally be scoped by a client-id:

bin/kafka-configs.sh --bootstrap-server localhost:90929092 \
  --describe \
  --entity-type users \
  --describeentity-name alice \ 
  --entity-type usersclients \
  --entity-name aliceclientid-override
Quota configs for user-principal 'alice', client-id 'clientid-override' are producer_byte_rate=5000060000.0
Dynamic configs for user-principal 'alice', client-id 'clientid-override' are sessionheartbeat.timeoutinterval.ms=110002000, acks=-10

If They may optionally be scoped by a client-id is not specified when describing, all of the <user, client-id> entity configs will be returned:

bin/kafka-configs.sh --bootstrap-server localhost:9092 \
  --describe \
  --entity-type users \
  --entity-name alice \ 
  --entity-type clients \
  --entity-name clientid-override
Quota configs for user-principal 'alice', client-id 'clientid-override' are producer_byte_rate=60000.0
Dynamic configs for user-principal 'alice', client-id 'clientid-override' are heartbeat.interval.ms=2000, acks=0, 
SupportedDynamic dynamic configs for user-principal 'alice', client-id 'clientid-override""' are "{'ClientInformation(softwareName=apache-kafka-java, softwareVersion=x.y.z-SNAPSHOT)': 'acks'}"

The above output is under the assumption that a client with the ClientInformation above registered the supported dynamic configs acks under the entity <alice, clientid-override>. 

If an entity name is not specified when describing, all child entities of the entity type will be returned.

acks=-1

The default dynamic config will be used in the case that the client-id dynamic config does not contain a key that the default does contain, but only if the client is requesting configs with the ResolveEntity flag set to true.

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Compatibility, Deprecation, and Migration Plan

1. If a new client with this feature attempts to send a DescribeClientConfigsRequest to an old broker, the broker will send back an InvalidRequest error code and the client will disable this feature.

2. In the case that an old client is talking to a new broker, nothing will change since the old client will never send a DescribeClientConfigsRequest.

3. In the case that both the broker and client are up to date with this change, the client will take advantage of the feature.

4. The Java producers and consumers will register a list of configs that they support. This will be stored as the value of the dynamic config ‘supported.configs’ and can be returned to the user. If a new client registers with an entity the old value of this config will be overwritten.

Rejected Alternatives

Plan

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1. If a new client with this feature attempts to send a DescribeClientConfigsRequest to an old broker, the broker will send back an InvalidRequest error code and the client will disable this feature.

2. In the case that an old client is talking to a new broker, nothing will change since the old client will never send a DescribeClientConfigsRequest.

3. In the case that both the broker and client are up to date with this change, the client will take advantage of the feature.

4. The Java producers and consumers will register a list of configs that they support. This will be stored as the value of the dynamic config ‘supported.configs’ and can be returned to the user. If a new client registers with an entity the old value of this config will be overwritten.

Rejected Alternatives

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• Introducing new entity types for kafka-configs.sh that producers and consumers can associate themselves with. This would make the tool more cumbersome to use and it is most intuitive that client configurations be dynamically altered with the clients and users entity types.

• Use the {Describe, IncrementalAlter}Configs APIs. Client config entities are more dynamic than entities with a singular resource name and type which makes it hard to fit them into generic APIs that expect a distinct entity name and type.
• Use the <user/client-id> hierarchy implemented for client quotas in KIP-55 and extended for the admin client in KIP-546. Quotas are inherently hierarchical but client configs are not, so it seems reasonable to use a hierarchy of shallow depth for dynamic client configs.

• Making client config compatibility information available to the user

  The user should be able to see what dynamic configs are supported for each application. However, clients that are using the same <user, client-id> entity may not necessarily support the same dynamic configs, storing a list of supported configs alongs side quotas and configs is a flawed solution.

  A better solution is to store config registrations in an internal topic. The Java producer and consumer clients can register the configs that they support with a DescribeClientConfigsRequest. The broker can write a key-value pair to an internal topic upon receiving the request where the key is the <user, client-id> entity and the value is ClientVersion along with the list of supported configs.

  All versions of clients that registered with a <user, client-id> entity along with the supported configs for each version of client could be aggregated when a DescribeClientConfigsRequest from an admin client is received. This information would then be returned to the admin client in the DescribeClientConfigsResponse. For example, supported dynamic configs for user-principal 'alice', client-id 'clientid-override' are "{ 'ClientInformation(softwareName=apache-kafka-java, softwareVersion=x.y.a-SNAPSHOT)': 'acks', 'ClientInformation(softwareName=apache-kafka-java, softwareVersion=x.y.b-SNAPSHOT)': 'acks, enable.idempotence' }".

• Interesting hierarchies for config overrides

• Introducing new entity types for kafka-configs.sh that producers and consumers can associate themselves with. This would make the tool more cumbersome to use and it is most intuitive that client configurations be dynamically altered with the clients and users entity types.

• Use the {Describe, IncrementalAlter}Configs APIs. Client config entities are more dynamic than entities with a singular resource name and type which makes it hard to fit them into generic APIs that expect a distinct entity name and type.
• Use the <user/client-id> hierarchy implemented for client quotas in KIP-55 and extended for the admin client in KIP-546. Quotas are inherently hierarchical but client configs are not, so it seems reasonable to use a hierarchy of shallow depth for dynamic client configs.

• Making client config compatibility information available to the user

  The user should be able to see what dynamic configs are supported for each application. However, clients that are using the same <user, client-id> entity may not necessarily support the same dynamic configs. A flawed solution to this is to store a map of ClientInformation to lists of supported configs in 

  A better solution is to store config registrations in an internal topic. The Java producer and consumer clients can register the configs that they support with a DescribeClientConfigsRequest. The broker can write a key-value pair to an internal topic upon receiving the request where the key is the <user, client-id> entity and the value is ClientVersion along with the list of supported configs.

  Ad hoc aggregations of registration data for a particular entity could be performed to give descriptive information about client config compatibility to the user. For example, all versions of clients that registered with an entity along with the supported configs for each version of client could be aggregated and returned to the user in the DescribeClientConfigsResponse. 

• Interesting config hierarchies could be constructed if the Java producer and consumer resolved the dynamic configs instead of the broker. For example,  from most precedent to least precedent:
  • /config/users/<user>/clients/<client-id>
  • .properties file configs
  • /config/users/<user>
  • Static default configs defined in ProducerConfig and ConsumerConfig.

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