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| <h1>Jobs and Scheduling</h1> |
| |
| |
| |
| <p>This document briefly describes how Flink schedules jobs and |
| how it represents and tracks job status on the JobManager.</p> |
| |
| <ul id="markdown-toc"> |
| <li><a href="#scheduling" id="markdown-toc-scheduling">Scheduling</a></li> |
| <li><a href="#jobmanager-data-structures" id="markdown-toc-jobmanager-data-structures">JobManager Data Structures</a></li> |
| </ul> |
| |
| <h2 id="scheduling">Scheduling</h2> |
| |
| <p>Execution resources in Flink are defined through <em>Task Slots</em>. Each TaskManager will have one or more task slots, |
| each of which can run one pipeline of parallel tasks. A pipeline consists of multiple successive tasks, such as the |
| <em>n-th</em> parallel instance of a MapFunction together with the <em>n-th</em> parallel instance of a ReduceFunction. |
| Note that Flink often executes successive tasks concurrently: For Streaming programs, that happens in any case, |
| but also for batch programs, it happens frequently.</p> |
| |
| <p>The figure below illustrates that. Consider a program with a data source, a <em>MapFunction</em>, and a <em>ReduceFunction</em>. |
| The source and MapFunction are executed with a parallelism of 4, while the ReduceFunction is executed with a |
| parallism of 3. A pipeline consists of the sequence Source - Map - Reduce. On a cluster with 2 TaskManagers with |
| 3 slots each, the program will be executed as described below.</p> |
| |
| <div style="text-align: center;"> |
| <img src="fig/slots.svg" alt="Assigning Pipelines of Tasks to Slots" height="250px" style="text-align: center;" /> |
| </div> |
| |
| <p>Internally, Flink defines through <a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/jobmanager/scheduler/SlotSharingGroup.java">SlotSharingGroup</a> |
| and <a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/jobmanager/scheduler/CoLocationGroup.java">CoLocationGroup</a> |
| which tasks may share a slot (permissive), respectively which tasks must be strictly placed into the same slot.</p> |
| |
| <h2 id="jobmanager-data-structures">JobManager Data Structures</h2> |
| |
| <p>During job execution, the JobManager keeps track of distributed tasks, decides when to schedule the next task (or set of tasks), |
| and reacts to finished tasks or execution failures.</p> |
| |
| <p>The JobManager receives the <a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/jobgraph/">JobGraph</a>, |
| which is a representation of the data flow consisting of operators (<a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/jobgraph/AbstractJobVertex.java">JobVertex</a>) |
| and intermediate results (<a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/jobgraph/IntermediateDataSet.java">IntermediateDataSet</a>). |
| Each operator has properies, like the parallelism and the code that it executes. |
| In addition, the JobGraph has a set of attached libraries, that are neccessary to execute the code of the operators.</p> |
| |
| <p>The JobManager transforms the JobGraph into an <a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/executiongraph/">ExecutionGraph</a>. |
| The ExecutionGraph is a parallel version of the JobGraph: For each JobVertex, it contains an <a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/executiongraph/ExecutionVertex.java">ExecutionVertex</a> per parallel subtask. An operator with a parallelism of 100 will have one JobVertex and 100 ExecutionVertices. |
| The ExecutionVertex tracks the state of execution of a particular subtask. All ExecutionVertices from one JobVertex are held in an |
| <a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/executiongraph/ExecutionJobVertex.java">ExecutionJobVertex</a>, |
| which tracks the status of the operator as a whole. |
| Besides the vertices, the ExecutionGraph also contains the <a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/executiongraph/IntermediateResult.java">IntermediateResult</a> and the <a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/executiongraph/IntermediateResultPartition.java">IntermediateResultPartition</a>. The former tracks the state of the <em>IntermediateDataSet</em>, the latter the state of each of its partitions.</p> |
| |
| <div style="text-align: center;"> |
| <img src="fig/job_and_execution_graph.svg" alt="JobGraph and ExecutionGraph" height="400px" style="text-align: center;" /> |
| </div> |
| |
| <p>During its execution, each parallel task goes through multiple stages, from <em>created</em> to <em>finished</em> or <em>failed</em>. The diagram below illustrates the |
| states and possible transitions between them. A task may be executed multiple times (for example in the course of failure recovery). |
| For that reason, the execution of an ExecutionVertex is tracked in an <a href="https://github.com/apache/flink/blob/master//flink-runtime/src/main/java/org/apache/flink/runtime/executiongraph/Execution.java">Execution</a>. Each ExecutionVertex has a current Execution, and prior Executions.</p> |
| |
| <div style="text-align: center;"> |
| <img src="fig/state_machine.svg" alt="States and Transitions of Task Executions" height="300px" style="text-align: center;" /> |
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