xcom简介

XComs(cross-communication)使得任务之间可以交换信息，允许更细粒度的控制和状态共享。XComs包含key, value, timestamp, 同时也包含创建xcom的任务实例的task_id, dag_id, execution_date等。

Task可以在运行时通过xcom_push(key, value)发送任意可序列化成JSON的对象。（其实支持pickle，但是已经被废弃）另外，task.execute()的返回值会默认发送到xcom，key为return_value。

Task中也可以通过xcom_pull(task_id(s), key)获取到一个或多个task的xcom值。不局限于下游。

详细介绍参考：xcom

详细分析

调用方式

在任务执行前，会先生成运行时的上下文context，然后调用task.execute(context)。因此在Operator的execute(context)方法中可以通过context[‘ti’]得到当前的TaskInstance，然后调用xcom_push和xcom_pull

这里的context包含很多字段，比如ti, task, dag对象，还有Jinja渲染需要的字段。

Context的字段定义

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{
            'dag': task.dag,
            'ds': ds,
            'next_ds': next_ds,
            'next_ds_nodash': next_ds_nodash,
            'prev_ds': prev_ds,
            'prev_ds_nodash': prev_ds_nodash,
            'ds_nodash': ds_nodash,
            'ts': ts,
            'ts_nodash': ts_nodash,
            'ts_nodash_with_tz': ts_nodash_with_tz,
            'yesterday_ds': yesterday_ds,
            'yesterday_ds_nodash': yesterday_ds_nodash,
            'tomorrow_ds': tomorrow_ds,
            'tomorrow_ds_nodash': tomorrow_ds_nodash,
            'END_DATE': ds,
            'end_date': ds,
            'dag_run': dag_run,
            'run_id': run_id,
            'execution_date': self.execution_date,
            'prev_execution_date': prev_execution_date,
            'prev_execution_date_success': lazy_object_proxy.Proxy(
                lambda: self.previous_execution_date_success),
            'prev_start_date_success': lazy_object_proxy.Proxy(lambda: self.previous_start_date_success),
            'next_execution_date': next_execution_date,
            'latest_date': ds,
            'macros': macros,
            'params': params,
            'tables': tables,
            'task': task,
            'task_instance': self,
            'ti': self,
            'task_instance_key_str': ti_key_str,
            'conf': configuration,
            'test_mode': self.test_mode,
            'var': {
                'value': VariableAccessor(), # NOTICE
                'json': VariableJsonAccessor()
            },
            'inlets': task.inlets,
            'outlets': task.outlets,
        }

xcom.py

Xcom是Model定义类，在里面实现了set(), get_one(), get_many(), delete()方法，对应数据库的增删查。

taskinstance.py

上面说的xcom_pull()和xcom_push在BaseOperator和TaskInstance中均有定义。BaseOperator中只是简单调用了TaskInstance中的方法。

TaskInstance部分代码：

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def xcom_push(
        self,
        key,
        value,
        execution_date=None):
    # 这里日期设置为未来一个时间，到时才可以被其他task发现
    """
    :param execution_date: if provided, the XCom will not be visible until
        this date. This can be used, for example, to send a message to a
        task on a future date without it being immediately visible.
    :type execution_date: datetime
    """

    if execution_date and execution_date < self.execution_date:
        raise ValueError(
            'execution_date can not be in the past (current '
            'execution_date is {}; received {})'.format(
                self.execution_date, execution_date))

    XCom.set(
        key=key,
        value=value,
        task_id=self.task_id,
        dag_id=self.dag_id,
        execution_date=execution_date or self.execution_date)

def xcom_pull(
        self,
        task_ids=None,
        dag_id=None,
        key=XCOM_RETURN_KEY,
        include_prior_dates=False):
    # 这里是为了得到那些设置为未来可见的xcom对象
    """
    :param include_prior_dates: If False, only XComs from the current
        execution_date are returned. If True, XComs from previous dates
        are returned as well.
    :type include_prior_dates: bool
    """

    if dag_id is None:
        dag_id = self.dag_id

    pull_fn = functools.partial(
        XCom.get_one,
        execution_date=self.execution_date,
        key=key,
        dag_id=dag_id,
        include_prior_dates=include_prior_dates)

    if is_container(task_ids):
        return tuple(pull_fn(task_id=t) for t in task_ids)
    else:
        return pull_fn(task_id=task_ids)