Add zookeeper-api module to Apache Helix

[narendly]

Issues

• My PR addresses the following Helix issues and references them in the PR description:

Fixes #631 Fixes #556

Description

• Here are some details about my PR, including screenshots of any UI changes:

https://github.com/apache/helix/wiki/New-Modules-for-Apache-Helix

Tests

• The following tests are written for this issue:

All existing tests are kept.

• The following is the result of the "mvn test" command on the appropriate module:

helix-agent

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 9.706 s
[INFO] Finished at: 2020-02-10T09:45:49-08:00
[INFO] ------------------------------------------------------------------------

helix-rest

Results :

Tests run: 99, Failures: 0, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 29.275 s
[INFO] Finished at: 2020-02-10T09:47:05-08:00
[INFO] ------------------------------------------------------------------------

helix-admin-webapp

Results :

Failed tests:
TestJobQueuesResource.test:190 expected: but was:

Tests run: 20, Failures: 1, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 26.412 s
[INFO] Finished at: 2020-02-10T09:47:57-08:00
[INFO] ------------------------------------------------------------------------

-> This is a known failing test.

helix-core

Failed tests:
TestRebalancePipeline.testMsgTriggeredRebalance:222 expected: but was:
TestEnableCompression.testEnableCompressionResource:108 expected: but was:
TestZKWatch.testSubscribeDataChange:81 expected:<0> but was:<1>

These tests pass when run individually.

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation in the following wiki page:

https://github.com/apache/helix/wiki/New-modules-for-Apache-Helix

Code Quality

• My diff has been formatted using helix-style.xml

[junkaixue]

Overall, the structure looks good to me. Just one thing: for the module folder. org/apache/helix/zookeeper/api/api. Shall we leave the module folder as org/apache/helix/zookeeper/api/...

[junkaixue]

LGTM. Make sure you addressed other comments.

[jiajunwang]

LGTM! Thanks for working on this huge task.

One concern is the ivy file helix-core/helix-core-0.9.2-SNAPSHOT.ivy. 'commons-math3" is used by the WAGED rebalancer, I think we need to keep it in the ivy file.

[narendly]

LGTM! Thanks for working on this huge task.

One concern is the ivy file helix-core/helix-core-0.9.2-SNAPSHOT.ivy. 'commons-math3" is used by the WAGED rebalancer, I think we need to keep it in the ivy file.

Updated the .ivy file.

[narendly]

This PR is ready to be merged, approved by @dasahcc @jiajunwang

Add metrics-common, zookeeper-api, helix-common modules

We want to create a new module called zookeeper-api in order to decouple Helix's ZooKeeper APIs from helix-core. The goal is to enable non-Helix applications to use Helix's ZooKeeper APIs to use ZooKeeper.