...

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.

...

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

The hierarchy for resolving client quotas is rather complex. This is because quotas need to be set on every application in the system to achieve consistent results. If a quota is set for client A but not client B, client B can end up hoarding resources because it is not limited. This would potentially cause client A to do work at a lower rate than what the quota specifies. Having a robust hierarchy for quotas allows the user to set quotas on all applications rather easily and then fine tune as needed. However, the hierarchy for client configs does not need to have the same amount of depth. There will just be a dynamic default and a dynamic config so that dynamic client configuration is consistent with static client configuration (e.g. .properties file & client defaults). Since users should not be able to change each others dynamic configs, the entities in the hierarchy will be scoped by at least user principle and optionally by client-id.

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 may not necessarily support the same dynamic configs even if they are using the same <user, client-id> entity. A partial solution to this is to store a map of client software names and versions to lists of supported configs in each <user, client-id> entity config. The Java producer and consumer clients can register the configs they they support and the broker can tie the registered keys to the serialized ClientInformation from the RequestContext. This allows compatibility information to be displayed to the user. As long as the user knows what version of client each application is using, they will know which applications will be affected by each dynamic config.

A dynamic client config supported.configs can be added to hold the serialized value of this map.

This allows the compatibility information to be refreshable since `supported.configs` can be deleted from an entity. This could be useful as clients go offline and are upgraded to clean up stale compatibility data. Clients register configs every time they request configs, so the compatibility information will eventually be added again and continuously updated as clients of different versions register configs with the entity. 

The big flaw in storing the registration alongside configs is that the entity config could be updated concurrently if multiple clients with the same entity names register at the same time. A better alternative to adding `supported.configs` is to store config registrations in an internal topic and perform ad hoc aggregations on the data in the topic to gather compatibility information. 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.

Public Interfaces

Admin Client Changes

Admin client calls will be added to support {Describe, Alter}ClientConfigs.

Network Protocol

Public Interfaces

...

Network Protocol

{
  "apiKey": 50,
  "type": "request",
  "name": "DescribeClientConfigsRequest",
  "validVersions": "0",
  "flexibleVersions": "none",
  "fields": [
    { "name": "Components", "type": "[]ComponentData", "versions": "0+",
      "about": "Filter components to apply to config entities.", "fields": [
      { "name": "EntityType", "type": "string", "versions": "0+",
        "about": "The entity type that the filter component applies to." },
      { "name": "MatchType", "type": "int8", "versions": "0+",
        "about": "How to match the entity {0 = exact name, 1 = default name, 2 = any specified name}." },
  

{
  "apiKey": 50,
  "type": "request",
  "name": "DescribeClientConfigsRequest",
  "validVersions": "0",
  "flexibleVersions": "none",
  "fields": [
    { "name": "ComponentsMatch", "type": "[]ComponentDatastring", "versions": "0+", "nullableVersions": "0+",
        "about": "FilterThe componentsstring to applymatch toagainst, config entities.", "fields": [
  or null if unused for the match type." }
    ]},
    { "name": "EntityTypeSupportedConfigs", "type": "[]string", "versions": "0+",
 "nullableVersions": "0+",
      "about": "The entity type that the filter component applies to configuration keys to register, or null if not registering configuration keys." },
      { "name": "MatchTypeResolveEntity", "type": "int8bool", "versions": "0+",
        "about": "HowTrue toif matchan theapplication entityis {0requesting =dynamic exactconfigs name, 1 = default namefor itself, 2 = any specified name}false otherwise." },
      { "name": "MatchStrict", "type": "stringbool", "versions": "0+", "nullableVersions": "0+",
        "about": "TheWhether stringthe tomatch matchis againststrict, or null if unused for the match typei.e. should exclude entities with unspecified entity types." }
    ]
},
  


{
  { "nameapiKey": "SupportedConfigs"50,
  "type": "[]stringresponse",
  "versionsname": "0+DescribeClientConfigsResponse",
  "nullableVersionsvalidVersions": "0+",
      "aboutflexibleVersions": "The configuration keys to register, or null if not registering configuration keys." },"none",
  "fields": [
    { "name": "ResolveEntityThrottleTimeMs", "type": "boolint32", "versions": "0+",
      "about": "True if an application is requesting dynamic configs for itself, false otherwiseThe duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "StrictErrorCode", "type": "boolint16", "versions": "0+",
      "about": "WhetherThe the match is strict, i.e. should exclude entities with unspecified entity types." }
  ]
}


{
  "apiKey": 50,
  error code, or `0` if the config description succeeded." },
    { "name": "ErrorMessage", "type": "responsestring",
  "nameversions": "DescribeClientConfigsResponse0+",
  "validVersionsnullableVersions": "0+",
      "flexibleVersionsabout": "none",
  "fields": [The error message, or `null` if the config description succeeded." },
    { "name": "ThrottleTimeMsEntries", "type": "int32[]EntryData", "versions": "0+",
      "about "nullableVersions": "0+"The,
 duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
"about": "A result entry.", "fields": [
      { "name": "ErrorCodeEntity", "type": "int16[]EntityData", "versions": "0+",
        "about": "The error code, or `0` if the config description succeeded." },
config entity description.", "fields": [
        { "name": "ErrorMessageEntityType", "type": "string", "versions": "0+", "nullableVersions": "0+",

          "about": "The error message, or `null` if the config description succeededentity type." },
        { "name": "EntriesEntityName", "type": "[]EntryDatastring", "versions": "0+", "nullableVersions": "0+",
          "about": "AThe result entry.", "fields": [entity name, or null if the default." 
        ]},
      	{ "name": "EntityValues", "type": "[]EntityDataValueData", "versions": "0+",
      		  "about": "The config values for the entity description.", "fields": [
          { "name": "EntityTypeKey", "type": "string", "versions": "0+",
            "about": "The entityconfiguration typekey." },
          { "name": "EntityNameValue", "type": "string", "versions": "0+",
 "nullableVersions": "0+",
          "about": "The entity name, or null if the default." },
  configuration value." }
        ]}
      ]}

    ]}
  ]
}

{
  "nameapiKey": "EntityCompatibility"51,
  "type": "[]VersionDatarequest",
  "versionsname": "0+AlterClientConfigsRequest",
			"about  "validVersions": "Every0",
 software name and version of clients that registered configs with the broker along with the configs supported by that software name and version.", "flexibleVersions": "none",
  "fields": [
            { "name": "ClientInformationEntries", "type": "string[]EntryData", "versions": "0+", 
              "about": "The softwareconfiguration nameentries and version of one or more clients that registered configs under this entity." },
     to alter.", "fields": [
       { "name": "SupportedConfigsEntity", "type": "[]stringEntityData", "versions": "0+",
              "about": "The dynamicconfig configsentity available for the clients with this software name and version." }to alter.", "fields": [
        {  ]}
        ]},
      	{ "name": "Values"name": "EntityType", "type": "[]ValueDatastring", "versions": "0+",
		          "about": "The config values for the entityentity type.", "fields": [
  },
         { "name": "KeyEntityName", "type": "string", "versions": "0+",
 "nullableVersions": "0+",
          "about": "The configuration keyname of the entity, or null if the default." },
      ]},
      { "name": "ValueOps", "type": "string[]OpData", "versions": "0+",
            "about": "TheAn individual configuration entry value." }to alter.", "fields": [
        ]}
      ]}

{ "name": "Key", "type": "string", "versions": "0+",
      ]}
  ]
}

{
  "apiKeyabout": 51,
"The configuration key."type": "request" },
        { "name": "AlterClientConfigsRequestValue",
  "validVersionstype": "0string",
  "flexibleVersionsversions": "none0+",
  "fields": [
       { "nameabout": "Entries", "type": "[]EntryData", "versions": "0+",
      "about": "The configuration entries to alter.", "fields": [
The value to set, otherwise ignored if the value is to be removed." },
        { "name": "EntityRemove", "type": "[]EntityDatabool", "versions": "0+",
          "about": "The config entity to alter.", "fields": [Whether the configuration value should be removed, otherwise set." }
      ]}
    ]},
    { "name": "EntityTypeValidateOnly", "type": "stringbool", "versions": "0+",
          "about": "TheWhether entitythe type." },
        { "name": "EntityName", alteration should be validated, but not performed." }
  ]
}

{
  "apiKey": 51,
  "type": "stringresponse",
  "versionsname": "0+AlterClientConfigsResponse",
  "nullableVersionsvalidVersions": "0+",
          "aboutflexibleVersions": "The name of the entity, or null if the default." }
      ]},none",
  "fields": [
      { "name": "OpsThrottleTimeMs", "type": "[]OpDataint32", "versions": "0+",
        "about": "AnThe individualduration configurationin entrymilliseconds to alter.", "fields": [
    for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "KeyEntries", "type": "string[]EntryData", "versions": "0+",
          "about": "The configuration entries to keyalter." },
 "fields": [
      { "name": "ValueErrorCode", "type": "stringint16", "versions": "0+",
          "about": "The valueerror to setcode, otherwiseor ignored`0` if the valueconfig is to be removedalteration succeeded." },
        { "name": "RemoveErrorMessage", "type": "boolstring", "versions": "0+",
 "nullableVersions": "0+",
        "about": "Whether the configuration value should be removed, otherwise setThe error message, or `null` if the config alteration succeeded." }
      ]},
    ]},
    { "name": "ValidateOnlyEntity", "type": "bool[]EntityData", "versions": "0+",
        "about": "WhetherThe theconfig alterationentity should be validated, but not performed." }
  ]
}

{
  "apiKey": 51,
  "typeto alter.", "fields": [
        { "name": "responseEntityType",
  "nametype": "AlterClientConfigsResponsestring",
  "validVersionsversions": "0+",
          "flexibleVersionsabout": "none"The entity type." },
  "fields": [
      { "name": "ThrottleTimeMsEntityName", "type": "int32string", "versions": "0+",
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "Entries", "type": "[]EntryData", "versions": "0+",
      "about": "The configuration entries to alter.", "fields": [
      { "name": "ErrorCode", "type": "int16", "versions": "0+",
        "about": "The error code, or `0` if the config alteration succeeded." },
      { "name": "ErrorMessage", "type": "string", "versions": "0+", "nullableVersions": "0+",
        "about": "The error message, or `null` if the config alteration succeeded." },
      { "name": "Entity", "type": "[]EntityData", "versions": "0+",
        "about": "The config entity to alter.", "fields": [
        { "name": "EntityType", "type": "string", "versions": "0+",
          "about": "The entity type." },
        { "name": "EntityName", "type": "string", "versions": "0+", "nullableVersions": "0+",
          "about": "The name of the entity, or null if the default." }
      ]}
    ]}
  ]
}

Proposed Changes

Broker Changes

If a {Describe, Alter}ClientConfigsRequest is made with a null user component, an InvalidRequest error code will be returned to the client. Apart from this, when the EntityRequest field is not set to true, the mechanics of the <user, client-id> or user config entity descriptions are very similar to the mechanics outlined in KIP-546. This is because the bulk of the code in the brokers that handles fetching client quota entity configs from zookeeper can be reused for dynamic client configs.

Default dynamic client configs will be stored in the children of the znode /config/users, while client-id specific dynamic client configs will be stored in the children of /config/users/<user>/clients. 

The user config is updated when the client-id component entity name in the AlterClientConfigsRequest is null. 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 order of precedence.

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.

The same authorization that is necessary for {Describe,Alter}ClientQuotas, CLUSTER authorization, will be used when handling {Describe,Alter}ClientConfigsRequest.

Producer Changes

 "nullableVersions": "0+",
          "about": "The name of the entity, or null if the default." }
      ]}
    ]}
  ]
}

Proposed Changes

...

Admin Client Changes

Admin client calls will be added to support {Describe, Alter}ClientConfigs.

Broker Changes

If a {Describe, Alter}ClientConfigsRequest is made without a user component, an InvalidRequest error code will be returned to the client. Apart from this, when the EntityRequest field is not set to true, the mechanics of the <user, client-id> or user config entity descriptions are very similar to the mechanics outlined in KIP-546. This is because the bulk of the code in the brokers that handles fetching client quota entity configs from zookeeper can be reused for dynamic client configs.

Default dynamic client configs will be stored in the children of the znode /config/users, while client-id specific dynamic client configs will be stored in the children of /config/users/<user>/clients. 

The user config is updated when the client-id component is missing from an AlterClientConfigsRequest. The <user, client-id> config is updated otherwise.

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 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 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.

The same authorization that is necessary for {Describe,Alter}ClientQuotas, CLUSTER authorization, will be used when handling {Describe,Alter}ClientConfigsRequest.

Producer Changes

The Java producer will have a DynamicProducerConfig that will periodically fetch dynamic configs from the producer’s IO thread asynchronously. 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 DescribeClientConfigsRequest as the RPC, validate the dynamic configs returned in DescribeConfigsResponse against the user provided configs, and log any configurations that are accepted. The client will reconfigure its acks value by using a method in DynamicProducerConfig that gets the current value of acks. The dynamic acks config will take precedence over user provided acks config unless the user provided configs require acks to be a certain value, such as enable.idempotence=true. In this case the dynamic update will be ignored.

Consumer Changes

The GroupCoordinator in the broker receives a group member’s session timeout upon the JoinGroupRequest and stores this with the rest of the group member's metadata. This means that to dynamically configure a consumer’s session timeout, the consumer must send a JoinGroupRequest. Currently, this could trigger an expensive rebalance operation when members are stable. JoinGroup behavior will be changed so that the session timeout can be updated using JoinGroup without triggering a rebalance in stable group members. The Java consumer's initial DescribeClientConfigsRequest will still be done synchronously before the first JoinGroupRequest to avoid sending an unnecessary JoinGroupRequest.

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 The Java producer will have a DynamicProducerConfig that will periodically fetch dynamic configs from the producer’s IO thread asynchronously. 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 DescribeClientConfigsRequest as the RPC, validate the dynamic configs returned in DescribeConfigsResponse against the user provided configs, and log any configurations that are accepted. The client will either reconfigure its acks value by using a method in DynamicProducerConfig that gets the current value of acks. The dynamic acks config 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 acks config unless the user provided configs require acks to be a certain value, such as enable.idempotence=true. In this case the dynamic update will be ignored.

Consumer Changes

The GroupCoordinator in the broker receives a group member’s session timeout upon the JoinGroupRequest and stores this with the rest of the group member's metadata. This means that to dynamically configure a consumer’s session timeout, the consumer must send a JoinGroupRequest. Currently, this could trigger an expensive rebalance operation when members are stable. To get around this, JoinGroup behavior will be changed in the broker so that the session timeout can be updated using JoinGroup without triggering a rebalance in stable group members. The Java consumer's initial DescribeClientConfigsRequest will still be done synchronously before the first JoinGroupRequest to avoid sending an unnecessary JoinGroupRequest.

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.

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:

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 configs:

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=0,heartbeat.interval.ms=2000,producer_byte_rate=60000
  Completed updating config for 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 \
  --alterdescribe \
  --entity-type users \
  --entity-name alice
Quota \
configs for --add-config acks=-1,session.timeout.ms=11000,user-principal 'alice' are producer_byte_rate=50000.0
  Completed updating configDynamic configs for user-principal alice.'alice' are session.timeout.ms=11000, acks=-1

They may optionally be scoped by The user can also add configs specific to a client-id that will override the user's default dynamic configs:

bin/kafka-configs.sh --bootstrap-server localhost:9092 \
  --alterdescribe \
  --entity-type users \
  --entity-name alice \ 
  --entity-type clients \
  --entity-name clientid-override
Quota \
configs for user--add-config acks=0,heartbeat.interval.ms=2000,principal 'alice', client-id 'clientid-override' are producer_byte_rate=60000.0
Dynamic configs for Completed updating config for user alice.user-principal 'alice', client-id 'clientid-override' are heartbeat.interval.ms=2000, acks=0

If a client-id is not specified when describing, all of the <user, client-id> entity configs will be returnedThe 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 \
  --describe \
  --entity-type users \
  --entity-name alice \
  --entity-type clients
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=-1

They may optionally be scoped by a client-id:

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, 
Supported 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.

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.

Any number of the configs that this KIP provides dynamic support for can be added or deleted with --add-config and --delete-config. They may optionally be mixed with quotas in the same command.

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. Clients will register their dynamic configs upon starting up through the initial DescribeClientConfigsRequest. This registration will be tied to <user, client-id> by adding the config supported.config to the entity config. The value of this config will be a JSON that maps software names and versions to the list of supported configs for that software name and version. The user will be able to refresh this compatibility information by deleting the config.

Rejected Alternatives

0
Dynamic configs for user-principal 'alice', client-id '""' are 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.

Any number of the configs that this KIP provides dynamic support for can be added or deleted with --add-config and --delete-config. They may optionally be mixed with quotas in the same command.

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

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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 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

• 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. Quota APIs are different and quotas are inherently hierarchical, so it seems reasonable to use a hierarchy of shallow depth for dynamic client configs
  • .

• Adding a config enable.dynamic.config to producers and consumers to enable the feature. Adding the dynamic config supported.configs is a better alternative since the user doesn't have to remember what types of clients support the capability but instead can just see which entities have clients that are registered along with the configs that are supported for each software name and versionThis defaulted to true anyway so it was removed.

• Making certain client configurations topic level configurations on the broker.

  • The semantic for the ProduceRequest API would be undefined since the producer would not receive a response with an offset for the ProduceRequests with acks=0.

  • If this were implemented for acks there would also be quite a bit of overhead associated with extra round trips since the RecordAccumulator sends batches that may contain records from multiple topics. If these topics have different acks configurations the records would need to be sent in different batches based on the acks value.

  • For example, if a producer is consistently producing to 2 different topics and one is configured as acks=0 while the other is acks=-1. This would require twice the amount of round trips to produce the same number of messages.