airflow 运行周期问题
最近开始正式使用airflow，关于 schedule_interval 和页面上显示的 last run一直有些不太清楚的地方，而在设置一个每周运行的任务时终于遇到了问题，任务并没有能够如期运行。
一系列google之后发现 airflow的 schedule_interval虽然可以使用cron表达式，但是还是和crontab有一些区别的。
关于 backfill
backfill命令是用来回填数据的，也就是说以之前的日期运行任务。
当任务是每天运行时只需要加上开始日期就可以了，例如
airflow backfill CKD_ALL_REPORT -s 2018-09-04但是当任务时多天运行一次时这样就不起作用了，会提示
No run dates were found for the given dates and dag interval.这是因为 airflow有一个窗口的概念
Airflow sets execution_date based on the left bound of the schedule period it is covering, not based on when it fires (which would be the right bound of the period)
stackoverflow上搜到比较合理的解释，意思就是说，airflow会在start_date开始后，符合schedule_interval定义的第一个时间点记为execution_date，但是会在下个时间点到达是才开始运行，也就是说由于这个窗口的原因，last run会滞后一个周期。
所以如何通过jinja来查看execution_date就会发现问题
Jinja模板
| JINJA表达式 | 含义 |
|---|---|
| {{ ds }} | the execution date as YYYY-MM-DD |
| {{ ds_nodash }} | the execution date as YYYYMMDD |
| {{ yesterday_ds }} | yesterday’s date as YYYY-MM-DD |
| {{ yesterday_ds_nodash }} | yesterday’s date as YYYYMMDD |
| {{ tomorrow_ds }} | tomorrow’s date as YYYY-MM-DD |
| {{ tomorrow_ds_nodash }} | tomorrow’s date as YYYYMMDD |
| {{ ts }} | same as execution_date.isoformat() |
| {{ ts_nodash }} | same as ts without - and : |
| {{ execution_date }} | the execution_date, (datetime.datetime) |
| {{ prev_execution_date }} | the previous execution date (if available)(datetime.datetime) |
| {{ next_execution_date }} | the next execution date (datetime.datetime) |
| {{ dag }} | the DAG object |
| {{ task }} | the Task object |
| {{ macros }} | a reference to the macros package, described below |
| {{ task_instance }} | the task_instance object |
| {{ end_date }} | same as {{ ds }} |
| {{ latest_date }} | same as {{ ds }} |
| {{ ti }} | same as {{ task_instance }} |
| {{ params }} | a reference to the user-defined params dictionary |
| {{ var.value.my_var }} | global defined variables represented as a dictionary |
| {{ var.json.my_var.path }} | global defined variables represented as a dictionary with deserialized JSON object, append the path to the key within the JSON object |
| {{ task_instance_key_str }} | a unique, human-readable key to the task instance formatted {dag_id}{task_id}{ds} |
| conf | the full configuration object located at airflow.configuration.conf which represents |
| run_id | the run_id of the current DAG run |
| dag_run | a reference to the DagRun object |
| test_mode | whether the task instance was called using the CLI’s test subcommand |