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Add Local and Sequential Executors to Doc (apache#8084)
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.. 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. | ||
.. _executor:LocalExecutor: | ||
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Local Executor | ||
============== | ||
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:class:`~airflow.executors.local_executor.LocalExecutor` runs tasks by spawning processes in a controlled fashion in different modes. | ||
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Given that BaseExecutor has the option to receive a ``parallelism`` parameter to limit the number of process spawned, | ||
when this parameter is ``0`` the number of processes that LocalExecutor can spawn is unlimited. | ||
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The following strategies are implemented: | ||
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- | **Unlimited Parallelism** (``self.parallelism == 0``): In this strategy, LocalExecutor will | ||
| spawn a process every time ``execute_async`` is called, that is, every task submitted to the | ||
| :class:`~airflow.executors.local_executor.LocalExecutor` will be executed in its own process. Once the task is executed and the | ||
| result stored in the ``result_queue``, the process terminates. There is no need for a | ||
| ``task_queue`` in this approach, since as soon as a task is received a new process will be | ||
| allocated to the task. Processes used in this strategy are of class :class:`~airflow.executors.local_executor.LocalWorker`. | ||
- | **Limited Parallelism** (``self.parallelism > 0``): In this strategy, the :class:`~airflow.executors.local_executor.LocalExecutor` spawns | ||
| the number of processes equal to the value of ``self.parallelism`` at ``start`` time, | ||
| using a ``task_queue`` to coordinate the ingestion of tasks and the work distribution among | ||
| the workers, which will take a task as soon as they are ready. During the lifecycle of | ||
| the LocalExecutor, the worker processes are running waiting for tasks, once the | ||
| LocalExecutor receives the call to shutdown the executor a poison token is sent to the | ||
| workers to terminate them. Processes used in this strategy are of class :class:`~airflow.executors.local_executor.QueuedLocalWorker`. | ||
Arguably, :class:`~airflow.executors.sequential_executor.SequentialExecutor` could be thought as a ``LocalExecutor`` with limited | ||
parallelism of just 1 worker, i.e. ``self.parallelism = 1``. | ||
This option could lead to the unification of the executor implementations, running | ||
locally, into just one :class:`~airflow.executors.local_executor.LocalExecutor` with multiple modes. |
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.. 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. | ||
.. _executor:SequentialExecutor: | ||
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Sequential Executor | ||
=================== | ||
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The :class:`~airflow.executors.sequential_executor.SequentialExecutor` is the default executor when you first install ``airflow``. | ||
It is the only executor that can be used with ``sqlite`` since ``sqlite`` doesn't support multiple connections. | ||
This executor will only run one task instance at a time. For production use case, please use other executors. |