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Status

Current state: "Under Discussion"

Discussion thread: here

JIRA: KAFKA-7800

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).


Note: This KIP is based on KIP-339: Create a new IncrementalAlterConfigs API

Motivation

Logging is a critical part of any system's infrastructure. It is the most direct way of observing what is happening with a system. In the case of issues, it helps us diagnose the problem quickly which in turn helps lower the MTTR.

Kafka supports application logging via the log4j library and outputs messages in various log levels (TRACE, DEBUG, INFO, WARN, ERROR). Log4j is a rich library that supports fine-grained logging configurations (e.g use INFO-level logging in kafka.server.ReplicaManager and use DEBUG-level in kafka.server.KafkaApis).
This is statically configurable through the log4j.properties file which gets read once at broker start-up.

A problem with this static configuration is that we cannot alter the log levels when a problem arises. It is severely impractical to edit a properties file and restart all brokers in order to gain visibility of a problem taking place in production.
It would be very useful if we support dynamically altering the log levels at runtime without needing to restart the Kafka process.

Log4j itself supports dynamically altering the log levels in a programmatic way and Kafka exposes a JMX API that lets you alter them. This allows users to change the log levels via a GUI (jconsole) or a CLI (jmxterm) that uses JMX.

There is one problem with changing log levels through JMX that we hope to address and that is Ease of Use:

  • Establishing a connection - Connecting to a remote process via JMX requires configuring and exposing multiple JMX ports to the outside world. This is a burden on users, as most production deployments may stand behind layers of firewalls and have policies against opening ports. This makes opening the ports and connections in the middle of an incident even more burdensome
  • Security - JMX and tools around it support authentication and authorization but it is an additional hassle to set up credentials for another system.
  • Manual process - Changing the whole cluster's log level requires manually connecting to each broker. In big deployments, this is severely impractical and forces users to build tooling around it.

Proposition

Ideally, Kafka would support dynamically changing log levels and address all of the aforementioned concerns out of the box.
We propose extending the IncrementalAlterConfig/DescribeConfig Admin API with functionality for dynamically altering the broker's log level.
This approach would also pave the way for even finer-grained logging logic (e.g log DEBUG level only for a certain topic) and would allow us to leverage the existing AlterConfigPolicy for custom user-defined validation of log-level changes.
These log-level changes will be temporary and reverted on broker restart - we will not persist them anywhere.

Public Interfaces

Protocol Changes

Users most likely need two operations for managing log levels - reading the currently-set log levels and altering them. Thus, we will add new functionality to the DescribeConfig and IncrementalAlterConfigs Admin APIs.

To differentiate between the normal Kafka config settings and the application's log level settings, we will introduce a new resource type - BROKER_LOGGERS

public final class ConfigResource {
 
    /**
     * Type of resource.
     */
    public enum Type {
        BROKER_LOGGER((byte) 8), BROKER((byte) 4), TOPIC((byte) 2), UNKNOWN((byte) 0);
    }
}

When resource_type=BROKER_LOGGER:

  • we will use the existing ACL for the Cluster resource (as used in IncrementalAlterConfigs/DescribeConfigs operations).
  • we will only support one of five values for the value of a config - the log levels (TRACE, DEBUG, INFO, WARN, ERROR)

Request/Response Overview

We will not be modifying the DescribeConfigs/IncrementalAlterConfigs request/response.
Let's go over the expected semantics when using them with the new resource type.

DescribeConfigs

DescribeConfigs Request (Version: 2) => [resource [config_name]] include_synonyms
    resource => resource_type resource_name
        resource_type => INT8 # BROKER_LOGGER (8)
        resource_name => STRING # ID of broker
    config_name => STRING
    include_synonyms => BOOLEAN # ignored
 
DescribeConfigs Response (Version: 2) => throttle_time_ms entities
    throttle_time_ms => INT32
    entities => error_code error_message resource configs
        error_code => INT16
        error_message => STRING
        resource => resource_type resource_name
            resource_type => INT8 # BROKER_LOGGER (8)
            resource_name => STRING # ID of broker
        configs => [config_entry synonym]
            config_entry =>
                config_name => STRING # logger name - e.g kafka.server.ReplicaManager
                config_value => NULLABLE_STRING # log level - e.g INFO. Null if not set explicitly yet
                read_only => BOOLEAN # false always
                config_source => INT8 # 2 (DYNAMIC_BROKER_CONFIG) or 4 (STATIC_BROKER_CONFIG), depending on what changed the log level
                is_sensitive => BOOLEAN # false always
            synonym => # empty always
                config_name => STRING
                config_value => NULLABLE_STRING
                config_source => INT8

Request semantics (as defined in KIP-133) are conserved where applicable:

  • an empty config_name in the request will return all existing loggers
  • can be sent to any broker
  • errors reported independently

IncrementalAlterConfigs

We will only support two out of the four operations for IncrementalAlterConfigs when the resource_type=BROKER_LOGGER.

SET: Set the log level to the desired value
REMOVE: Sets the log level to NULL. In log4j, this means using the next higher-up subpackage logger (or root logger). Most classes have a log level of NULL if inspected through JMX.

IncrementalAlterConfigsOp => INT8
0: SET
1: REMOVE # sets log level to NULL
2: APPEND # NOT SUPPORTED
3: SUBTRACT # NOT SUPPORTED
  
IncrementalAlterConfigsRequest (Version: 0) => [resources] validate_only
 validate_only => BOOLEAN
 resources => resource_type resource_name [configs]
    resource_type => INT8 # BROKER_LOGGER (8)
    resource_name => STRING # ID of broker
    configs => config_name config_op config_value
        config_name => STRING
        config_op => INT8 # support SET and APPEND only
        config_value => NULLABLE_STRING
 
IncrementalAlterConfigsResponse (Version: 0) => [responses] 
  responses => resource_type resource_name error_code error_message
  resource_type => INT8 # BROKER_LOGGER (8)
  resource_name => STRING # ID of broker
  error_code => INT16
  error_message => NULLABLE_STRING

Request semantics (as defined in KIP-133 and KIP-339) are conserved where applicable:

  • validate_only mode does not alter the log level
  • can be sent to any broker
  • non-transactional
  • if config key is duplicated, INVALID_REQUEST gets returned for all those duplicate keys
  • INVALID_REQUEST will always be returned for APPEND/SUBTRACT operations


Error Handling

In the case of an invalid config_value or an invalid/non-existent logger name, the broker will return an INVALID_CONFIG (40) error for that config.

Tools Changes

kafka-configs.sh will be extended to support the new resource type via --entity-type broker-logger.

Examples:

bin/kafka-configs.sh --bootstrap-server localhost:9092 --describe --entity-type broker-logger --entity-name 0  // show all the log levels for broker 0
 
bin/kafka-configs.sh --bootstrap-server localhost:9092 --alter --add-config "kafka.server.ReplicaManager=WARN,kafka.server.KafkaApis=DEBUG" --entity-type broker-logger --entity-name 0  // set some log levels for broker 0
 
bin/kafka-configs.sh --bootstrap-server localhost:9092 --alter --delete-config kafka.server.ReplicaManager --entity-type broker-logger --entity-name 0 // will set the log level to NULL


Compatibility, Deprecation, and Migration Plan

Compatibility

Since we are only adding new functionality under a new resource type, this KIP should not have compatibility issues with older versions.

Kafka will continue to expose the JMX API for configuring log levels.

Since we want to deprecate AlterConfigs, that API will not support altering log levels.

Testing

We should be able to create a JUnit integration test inside AK that can call the Admin API methods to modify the log-level and have access to Log4j in order to verify that the levels are changed.

Migration Plan

AlterConfigPolicy implementations will need to be updated to account for the new config type.

Rejected Alternatives

  1. Extend Kafka's JmxTool class to support changing log levels dynamically
    1. Does not properly address the concerns outlined in the Motivation section 
      1. Can abstract away a bit of the complexity but still has the fundamental flaws of no security and not being easy to use
  2. Create new Admin API commands for reading and altering log levels
    1. Will result in more boilerplate and some code duplication.
    2. Will result in seemingly needless Admin API command bloat
    3. Altering log levels can be seen as a form of altering configurations and seems intuitive use the same API
    4. Will require separate Policy class
  3. Add new config_type field in Alter/Describe request/responses or new map of logger=>log_level 
    1. Results in more code and field bloat in request/response without clear benefits of extendability


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