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When Yarn support was added to Flink, the Client remained agnostic of the cluster setup. The command-line interface (CliFrontend) would manage the cluster lifecycle by setting up the Yarn cluster, extracting the cluster's jobmanager address, and setting up a Client to submit jobs. On the one hand it meant that Standalone, Yarn, and soon Mesos cluster setup and management had different code paths which had to be maintained in the CliFrontend. On the other hand it also inherently tied cluster management dependencies (Yarn) to the "flink-clients" module.
FLINK-3667 coupled the Client (now ClusterClient) with cluster related lifecycle methods. The original Client class became the base class of the cluster modes Flink supports (Standalone, Yarn, tbd Mesos). This enabled us to delegate all cluster-specific code to the implementation of the cluster. In the course, we also created an interface for custom command-line code (CustomCommandLine) which enabled us to clearly separated the general command-line client code from the specific command-lines for other cluster managers. The result is a clear seperation between the "flink-clients" and the "flink-yarn" modules.
In addition to unifying the lifecycle management, we saw another shortcoming in the process of resuming a cluster. It wasn't possible to programmatically resume running clusters. A resumed cluster would simply be treated as a standalone Flink cluster. In the case of Yarn, it was not possible to shutdown a resumed cluster or to resume the cluster with a Yarn-specific attribute, e.g. the Yarn application id.
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