aljoscha commented on a change in pull request #14114: URL: https://github.com/apache/flink/pull/14114#discussion_r528831442
########## File path: docs/dev/datastream_execution_mode.md ########## @@ -0,0 +1,377 @@ +--- +title: "Execution Mode (Batch/Streaming)" +nav-id: datastream_execution_mode +nav-parent_id: streaming +nav-pos: 1 +--- +<!-- +Licensed to the Apache Software Foundation (ASF) under one +or more contributor license agreements. See the NOTICE file +distributed with this work for additional information +regarding copyright ownership. The ASF licenses this file +to you under the Apache License, Version 2.0 (the +"License"); you may not use this file except in compliance +with the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + +Unless required by applicable law or agreed to in writing, +software distributed under the License is distributed on an +"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +KIND, either express or implied. See the License for the +specific language governing permissions and limitations +under the License. +--> + +The DataStream API supports different runtime execution modes from which you +can choose depending on the requirements of your use case and the +characteristics of your job. + +There is the "classic" execution behavior of the DataStream API, which we call +`STREAMING` execution mode. This should be used for unbounded jobs that require +continuous incremental processing and are expected to stay online indefinitely. + +Additionally, there is a batch-style execution mode that we call `BATCH` +execution mode. This executes jobs in a way that is more reminiscent of batch +processing frameworks such as MapReduce. This should be used for bounded jobs +for which you have a known fixed input and which do not run continuously. + +Apache Flink's unified approach to stream and batch processing means that a +DataStream application executed over bounded input will produce the same +results regardless of the configured execution mode. By enabling `BATCH` +execution, we allow Flink to apply additional optimizations that we can only do +work when we know that our input is bounded. For example, different +join/aggregation strategies can be used, in addition to a different shuffle +implementation that allows more efficient failure recovery behavior. We will go +into some of the details of the execution behavior below. + +* This will be replaced by the TOC +{:toc} + +## When can/should I use BATCH execution mode? + +The `BATCH` execution mode can only be used for Jobs/Flink Programs that are +_bounded_. Boundedness is a property of a data source that tells us whether all +the input coming from that source is known before execution or whether new data +will show up, potentially indefinitely. A job, in turn, is bounded if all its +sources are bounded, and unbounded otherwise. + +`STREAMING` execution mode, on the other hand, can be used for both bounded and +unbounded jobs. + +As a rule of thumb, you should be using `BATCH` execution mode when your program +is bounded because this will be more efficient. You have to use `STREAMING` +execution mode when your program is unbounded because only this mode is general +enough to be able to deal with continuous data streams. + +One obvious outlier case is when you want to use a bounded job to bootstrap +some job state that you then want to use in an unbounded job. For example, by +running a bounded job using `STREAMING` mode, taking a savepoint, and then +restoring that savepoint on an unbounded job. This is a very specific use case +and one that might soon become obsolete when we allow producing a savepoint as +additional output of a `BATCH` execution job. + +Another case where you might run a bounded job using `STREAMING` mode is when +writing tests for code that will eventually run with unbounded sources. For +testing it can be more natural to use a bounded source in those cases. + +## Configuring BATCH execution mode + +The execution mode can be configured via the `execution.runtime-mode` setting. +There are three possible values: + + - `STREAMING`: The classic DataStream execution mode (default) + - `BATCH`: Batch-style execution on the DataStream API + - `AUTOMATIC`: Let the system decide based on the boundedness of the sources + +This can be configured via command line parameters of `bin/flink run ...`, or +programmatically when creating/configuring the `StreamExecutionEnvironment`. + +Here's how you can configure the execution mode via the command line: + +```bash +$ bin/flink run -Dexecution.runtime-mode=BATCH examples/streaming/WordCount.jar +``` + +This example shows how you can configure the execution mode in code: + + ```java +Configuration config = new Configuration(); +config.set(ExecutionOptions.RUNTIME_MODE, RuntimeExecutionMode.BATCH); +StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment(config); + ``` + +## Execution Behavior + +This section provides an overview of the execution behavior of `BATCH` +execution mode and contrasts it with `STREAMING` execution mode. For more +details, please refer to the FLIPs that introduced this feature: +[FLIP-134](https://cwiki.apache.org/confluence/x/4i94CQ) and +[FLIP-140](https://cwiki.apache.org/confluence/x/kDh4CQ). As well as the +documentation about [task scheduling (TODO)](<TODO>). + +### Task Scheduling And Network Shuffle + +Flink jobs consist of different operations that are connected together in a +dataflow graph. The system decides how to schedule the execution of these +operations on different processes/machines (TaskManager) and how data is +shuffled (sent) between them. + +Multiple operations/operators can be chained together using a feature called +[chaining]({% link dev/stream/operators/index.md +%}#task-chaining-and-resource-groups). A group of one or multiple (chained) +operators that Flink considers as a unit of scheduling is called a _task_. +Often the term _subtask_ is used to refer to the individual instances of tasks +that are running in parallel on multiple TaskManagers but we will only use the +term _task_ here. + +Task scheduling and the network shuffle work differently for `BATCH` execution +mode and `STREAMING` execution mode. Mostly due to the fact that we know our +input data is bounded in `BATCH` execution mode, which allows Flink to use more +efficient data structures and algorithms. + +We will use this example to explain the differences in task scheduling and +network transfer: + +```java +StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment(); + +DataStreamSource<String> source = env.fromElements(...); + +source.name("source") + .map(...).name("map1") + .map(...).name("map2") + .rebalance() + .map(...).name("map3") + .map(...).name("map4") + .keyBy((value) -> value) + .map(...).name("map5") + .map(...).name("map6") + .sinkTo(...).name("sink"); +``` + +Operations that imply a 1-to-1 connection pattern between operations, such as +`map()`, `flatMap()`, or `filter()` can just forward data straight to the next +operation, which allows these operations to be chained together. This means +that Flink would not normally insert a network shuffle between them. + +Operation such as `keyBy()` or `rebalance()` on the other hand require data to +be shuffled between different parallel instances of tasks. This induces a +network shuffle. + +For the above example Flink would group operations together as tasks like this: + +- Task1: `source`, `map1`, and `map2` +- Task2: `map3`, `map4` +- Task3: `map5`, `map6`, and `sink` + +And we have a network shuffle between Tasks 1 and 2, and also Tasks 2 and 3. + +#### STREAMING Execution Mode + +In `STREAMING` execution mode, all tasks need to be online/running all the +time. This allows Flink to immediately process new records through the whole +pipeline, which we need for continuous and low-latency stream processing. This +also means that the TaskManagers that are allotted to a job need to have enough +resources to run all the tasks at the same time. + +Network shuffles are _pipelined_, meaning that records are immediately sent to +downstream tasks, with some buffering on the network layer. Again, this is +required because when processing a continuous stream of data there are no +natural points (in time) where data could be materialized between tasks (or +pipelines of tasks). This contrasts with `BATCH` execution mode where +intermediate results can be materialized, as explained below. + +#### BATCH Execution Mode + +In `BATCH` execution mode, the tasks of a job can be separated into stages that +can be executed one after another. We can do this because the input is bounded +and Flink can therefore fully process one stage of the pipeline before moving +on to the next. In the above example the job would have three stages that +correspond to the three tasks that are separated by the shuffle barriers. + +Instead of sending records immediately to downstream tasks, as explained above +for `STREAMING` mode, processing in stages requires Flink to materialize +intermediate results of tasks to some non-ephemeral storage which allows +downstream tasks to read them after upstream tasks have already gone off line. +This will increase the latency of processing but comes with other interesting +properties. For one, this allows Flink to backtrack to the latest available +results when a failure happens instead of restarting the whole job. Another +side effect is that `BATCH` jobs can execute on fewer resources (in terms of +available slots at TaskManagers) because the system can execute tasks +sequentially one after the other. + +TaskManagers will keep intermediate results at least as long as downstream +tasks have not consumed them. (Technically, they will be kept until the +consuming [pipelined regions TODO](<TODO>) have produced their output in turn.) Review comment: fixing ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: us...@infra.apache.org
