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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

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participant "Deployment\nFactory\n(df)" as df

Example Code Block

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Architecture

New provider will be added (descendant ProviderDeploymentContributorBase class) with a set of filters. See Pic.#1 for common architecture.

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Pic. #1 – Providers architecture

Definition.

Alias – set of Hadoop name-nodes configured for High Availability mode.

Definition.

High Availability Strategy – plan of defining active name-node and switching between active and stand-by name-nodes. Strategy may contain such parameters as retryCount and timeoutInterval. See Pic.#2 for class diagram for HA mode.

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Pic.#2 Class diagram for HA mode.

See Table #1 for class description.

Table #1. – HA mode new classes description.

See Pic.#2 for  UML sequence diagram for UrlRewriteProcessor.

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Pic #3 – UML sequence diagram for UrlRewriteProcessor.

Provider configuration example

Please look at the WebHDFS HA section http://knox.apache.org/books/knox-0-5-0/knox-0-5-0.html#WebHDFS