KNOX-88: Support HDFS HA

Design details and discussion for KNOX-88 

 Definition 

Knox HA is a set of routines for transparent work with Hadoop service that stands in HA mode.

 Purpose of Knox HA service

1. Automatic failover. (Example: switch request from not responding name-node to active name-node.)
2. Pluggable support of failover strategies.
3. Daemon-service for regular ping of Hadoop service state (Performance optimization to keeping actual state of service).

Provider configuration example

Enables or disables HA Provider and binds strategy and provider together. Alias contains list of Hadoop services (name-nodes in our case: active and standby) grouped into one entity.

<topology>
  <gateway>
    ...
    <provider>
    <role>ha</role>
    <name>HAProvider</name>
    <param>
        <name>webhdfs.ha</name>
        <value>failover_strategy=BaseStrategy;retryCount=3;timeoutInterval=5000;enabled=true</value>
    </param>
</provider>
    ...
  <gateway>
  ...
  <service>
    <role>WEBHDFS</role>
    <url>machine1.example.com:50070</url>
    <url>machine2.example.com:50070</url>
  </service>
  ...
<service>
    <role>NAMENODE</role>
    <url>machine1.example.com:50070</url>
    <url>machine2.example.com:50070</url>
</service>
...
</topology>

• failover_strategy – indicates how to define active service and contains some configuration parameters. Default value is BaseStrategy. BaseStrategy for failover has following parameters:
• retryCount – indicates how many times knox will ping name-node before  knox decides that namenode is down.
• timeoutInterval – interval for connection timeout. 
• enabled – indicates whether  HAProvider  is active or not for service.

Example UML

Example Code Block

class X {
}