Status

Current state: Accepted

Discussion thread: thread

JIRA: KAFKA-9366

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

Motivation

In May 2012, the log4j dev team released log4j 1.2.17 and stopped their support to 1.x releases. And from then on, Apache Kafka is still using it in its core and the other subprojects.

The problem caused by the obsolete log4j version is not limited to security problems like CVE-2019-17571. Most users are now familiar with log4j2 (2.x) syntax, not 1.x. For this reason, when they are trying to customize the logging of Apache Kafka or Kafka Connect, they have to work with outdated, dismissed old configuration format.

This KIP proposes to upgrade the log4j 1.x dependencies into log4j2 from the Server-side of Kafka. (For the exact definition of 'server-side', please refer to the 'Which modules will be influenced?' subsection.)

Public Interfaces

This KIP proposes the following:

1. Replace server-side dependency from log4j into log4j2, along with their slf4j bindings.
2. User-interfacing configurations (like broker logging config), provide additional log4j2-equivalent configuration with backward compatibility.
3. For non-user interfacing configurations (like test config), all of them will be migrated into log4j2.

Proposed Changes

Which modules will be influenced?

The following modules will be updated:

• clients: core, metadata, raft, server-common, and storage modules are directly dependent on clients module. So, We should include it.
• connect
• core
• metadata
• raft
• storage
• streams: this module directly depends on clients.

The following modules are not the scope of this proposal with some reasons:

• log4j-appender: This module should not be touched for the users, and its log4j2 equivalent should be provided independently. However, it is above the scope of this proposal.
• tools: VerifiableLog4jAppender depends on log4j-appender. So, we can't migrate them until log4j2-appender is ready.
• trogdor: As of this KIP was passed, trogdor was a part of tools. So, it was excluded.

1. Core

For backward compatibility, Kafka broker will use the log4j configuration file by default. But for informational purpose, the following message will be shown in default mode:

DEPRECATED: using log4j 1.x configuration. To use log4j 2.x configuration, run with: 'export KAFKA_LOG4J_OPTS="-Dlog4j.configurationFile=file:$base_dir/../config/log4j2.properties"'

As the message above states, the user can run Kafka broker with log4j2 config file by setting `export KAFKA_LOG4J_OPTS="-Dlog4j.configurationFile={log4j2-config-file-path}"`. Thanks to log4j12-api, a compatibility bridge between log4j and log4j2, Kafka broker can be run without any changes. Since a log4j2 equivalent for traditional built-in log4j config (log4j2.properties) will be provided, the user can make use of it if they want.

The test logging configuration (src/test/resources/log4j.properties) will be updated into log4j2. In this case, we don't care about the backward-compatibility.

2. Client

The test logging configuration (src/test/resources/log4j.properties) will be updated into log4j2.

3. Connect

Like core, connect will use the log4j configuration file by default. it will show the following message also if it is in default mode:

DEPRECATED: using log4j 1.x configuration. To use log4j 2.x configuration, run with: 'export KAFKA_LOG4J_OPTS=\"-Dlog4j.configurationFile=file:$base_dir/../config/connect-log4j2.properties\"'

A log4j2 equivalent for traditional log4j config (connect-log4j2.properties) will be also provided, like other subprojects.

The test logging configuration (src/test/resources/log4j.properties) will be updated into log4j2, also.

4. Streams

Archetype log4j configuration will be updated into log4j2 equivalent (log4j2.properties).

5. Log4j-appender

Log4j-appender is the only module that does not affected by this update, since it provides a log4j 1.x appender.

Its log4j2 counterpart should be provided in near future. However, it is above the scope of this KIP.

6. Tools

Like core and connect, it will run with log4j configuration by default, but the users can use it by setting `export KAFKA_LOG4J_OPTS="-Dlog4j.configurationFile={log4j2-config-file-path}"`. A log4j2 equivalent for traditional log4j config (tools-log4j2.properties) will also be provided.

7. ETC

All template properties for test scripts (see tests/ directory) will be updated into log4j2 equivalent.

Compatibility, Deprecation, and Migration Plan

1. Core

At some time or other,the default logging configuration format will be switched into log4j2. However, without removing log4j12-api it can also support log4j configuration format. In that point, The informational message will be also changed into:

Using log4j 2.x configuration. To use log4j 1.x configuration, run with: 'export KAFKA_LOG4J_OPTS="-Dlog4j.configuration=file:$base_dir/../config/log4j.properties"'

2. Client

None.

3. Connect

Same to core.

4. Strams

None.

5. Log4j-appender

This subproject will be remained for backward-compatibility, when the default logging format is changed into log4j2. Instead, its log4j2 counterpart, log4j2-appender, will be provided at that moment.

6. Tools

Same as core and connect.

7. ETC

None.

Rejected Alternatives

None.