[LocalExecutor](https://airflow.apache.org/docs/apache-airflow/stable/_api/airflow/executors/local_executor/index.html#airflow.executors.local_executor.LocalExecutor) runs tasks by spawning processes in a controlled fashion in different modes.

    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.

    The following strategies are implemented:

    • 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 [LocalExecutor](https://airflow.apache.org/docs/apache-airflow/stable/_api/airflow/executors/local_executor/index.html#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 [LocalWorker](https://airflow.apache.org/docs/apache-airflow/stable/_api/airflow/executors/local_executor/index.html#airflow.executors.local_executor.LocalWorker).

    • Limited Parallelism (self.parallelism > 0):

    In this strategy, the [LocalExecutor](https://airflow.apache.org/docs/apache-airflow/stable/_api/airflow/executors/local_executor/index.html#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(摄取) 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 [QueuedLocalWorker](https://airflow.apache.org/docs/apache-airflow/stable/_api/airflow/executors/local_executor/index.html#airflow.executors.local_executor.QueuedLocalWorker).

    Arguably(可以说), [SequentialExecutor](https://airflow.apache.org/docs/apache-airflow/stable/_api/airflow/executors/sequential_executor/index.html#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 [LocalExecutor](https://airflow.apache.org/docs/apache-airflow/stable/_api/airflow/executors/local_executor/index.html#airflow.executors.local_executor.LocalExecutor) with multiple modes.