HelixGatewayParticipant only send target state implementation

[zpinto]

Issues

• Implementation to allow gateway participant to only send transitions to the targetState to the client application.

Description

The intended behavior for HelixGateway is to only send the targetState to the client application. A major refactor of Helix internals will be required to achieve this, as the controller will send a state transition message for each state in the state machine instead of just the final target state.

Until that refactor, the gateway participant will consult the IdealState of the resource for the pending state transition and forward a shard assignment message that has a the to state from the IdealState instead. All following ST messages from the controller will be auto-completed and NOT forwarded to the participant, as the client is already in the target state.

Tests

• TestHelixGatewayParticipant.testOnlySendTargetState

Changes that Break Backward Compatibility (Optional)

NA

Documentation (Optional)

NA

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. 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"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)