Serverless 简明教程

Serverless - Scheduled Lambdas

通常，您需要让您的函数在固定间隔内被调用。它可以是一天一次、一周两次、工作日每分钟一次，依此类推。Serverless 提供两种类型的事件以固定频率调用函数。它们是 Cron 事件和速率事件。

Often, you require your function to be invoked at fixed intervals. It can be once a day, twice a week, once a minute during weekdays, and so on.Serverless provides two types of events to invoke functions at fixed frequencies. They are cron events and rate events.

Cron Event

Cron 事件比速率事件有更大的灵活性。唯一的缺点是它不像速率事件那样易于理解。Cron 表达式的语法定义在 AWS documentation 中 −

The cron event has much more flexibility than the rate event. The only downside is that it is not as straightforward to understand as the rate event. The syntax of the cron expression is defined in the AWS documentation −

cron(minutes hours day-of-month month day-of-week year)

可以看到，Cron 表达式由 6 个字段组成。每个字段都可以接受一些已接受的值，还有一些（如 AWS 所称）的通配符。

As can be seen, the cron expression consists of 6 fields. Each field can take in some accepted values, and some, as AWS calls them, wildcards.

让我们首先了解接受的值 −

Let’s have a look at the accepted values first −

  1. minutes − 0-59

  2. hours − 0-23

  3. day-of-month − 1-31

  4. month − 1-12 or JAN-DEC

  5. day-of-week − 1-7 or SUN-SAT

  6. year − 1970-2199

既然接受的值很明显了，让我们了解通配符。在 cron 表达式中有 8 个可能的通配符（一些允许用于所有 6 个字段，而另一些只允许用于特定字段）。在此列出它们 −

Now that the accepted values are clear, let us have a look at the wildcards. There are a total of 8 wildcards possible in the cron expression (some allowed for all 6 fields and some only for specific fields). Listing them down here −

  1. * (asterisk, allowed for all 6 fields) − This is the most popular wildcard. It simply says that all values of the field are included. A * in the hours field would mean that the cron would run every hour. A * in the day-of-month field would mean that the cron will run every day.

  2. , (comma, allowed for all 6 fields) − This is used to specify more than one value. For example. If you want your cron to run at the 5th, 7th, and 9th minute of every hour, your minute field would look like 5,7,9.Similarly, MON,TUE,WED,THU,FRI in the day-of-week field could mean that the cron should run only on weekdays.

  3. - (dash, allowed for all 6 fields) − This wildcard specifies ranges. In the previous wildcard example, in order to specify weekdays, instead of specifying 5 comma-separated-values, we could have simply written MON-FRI

  4. ? (question mark, only allowed for day-of-month and day-of-week) − This is like a don’t-care wildcard. If you have specified MON in the day-of-week field, you don’t care what date Monday falls on. Therefore, you will enter ? in place of day-of-month. Similarly, if you want the cron to run on the 5th of every month, you will enter 5 in the day-of-month field and ? in the day-of-week field, because you don’t care what the day is on the 5th of every month. Note that the AWS documentation clearly states that you cannot use * for both day-of-week and day-of-month fields. If you use * for one, you have to use ? for the other

  5. / (forward slash, allowed for the 5 fields except for day-of-month) − This field specifies increments. If you enter 0/2 in the hours field, this cron will run every even hour (0, 0+2, 0+2+2, and so on). If you specify 1/2 in the hours field, this cron will run every odd hour (1, 1+2, 1+2+2, and so on). As you would have guessed, the value preceding the / is the starting value and the value succeeding it defines the increment.

  6. L (only allowed for day-of-month and day-of-week) − Specifies the last day of the month or the last day of the week

  7. W (only allowed for day-of-month) − This specifies a weekday (Monday to Friday) nearest to that specific day of the month. So if you specify 8W in the day-of-month field, and it corresponds to a weekday, say Tuesday, then the cron will fire on 8th itself. But if 8 corresponds to a weekend day, say Saturday, then the cron will be triggered on 7th (Friday). If 8th corresponds to Sunday, then the cron will fire on 9th (Monday). This is one of the least used wildcards.

  8. # (only allowed for day-of-week) − This is a very special wildcard, best understood by an example. Say you want a cron to run on Mother’s day. Now, Mother’s day falls on 2nd Sunday of May every year.Therefore, your month field would contain MAY or 5. But how do you specify 2nd Sunday? Come in the hashtag. The expression is 0#2. The value preceding the wildcard is the day of the week (0 for Sunday,1 for Monday, and so on). The value succeeding the wildcard specifies the occurrence. Therefore, 2 here refers to the 2nd occurrence of Sunday or the second Sunday.

现在，为您的 lambda 函数定义 cron 触发器，您需要做的就是指定 serverless.yml 中函数 events 键内的 cron 表达式。

Now, to define a cron trigger for your lambda function, all you need to do is specify the cron expression within the events key in the function in serverless.yml.

functions:
   cron_triggered_lambda:
      handler: handler.hello
      events:
         - schedule: cron(10 03 * * ? *) #run at 03:10 (UTC) every day.

Some Examples

以下是 cron 表达式的一些示例 −

Given below are some examples of cron expressions −

  1. cron(30 15 ? * MON-FRI *) − Triggered at 15:30 (UTC) on every weekday

  2. cron(0 9 ? 6 0#3 *) − Triggered at 09:00 (UTC) on the third Sunday of June (Father’s Day)

  3. cron(0/15 * ? * MON *) − Triggered every 15 minutes on Mondays

  4. cron(0/30 9-18 ? * MON-FRI *) − Triggered every 30 minutes from 9 AM to 5:30 PM on weekdays (corresponding to office hours at several places)

Rate Event

与 cron 表达式相比，这样做更容易。语法很简单 rate(value unit) 。例如，rate(5 minutes)。

This is much more straightforward compared to cron expressions. The syntax is simply rate(value unit). For example, rate(5 minutes).

该值可以是任何正整数，允许的单位是分钟、小时、天。

The value can be any positive integer and the allowed units are minute(s), hour(s), day(s).

为 Lambda 函数定义速率触发器类似于定义 cron 触发器。

Defining a rate trigger for your lambda function is similar to defining a cron trigger.

functions:
   rate_triggered_lambda:
      handler: handler.hello
      events:
         - schedule: rate(10 minutes) #run every 10 minutes

Some Examples

  1. rate(2 hours) − Triggered every 2 hours

  2. rate(1 day) − Triggered every day (at 00:00 UTC)

  3. rate(90 minutes) − Triggered every 90 minutes

正如你所意识的那样，速率表达式的简洁性是以降低灵活性为代价的。你可以使用速率来运行每个 N 分钟/小时/天中的 lambda。要执行更复杂的操作，比如仅在工作日触发 lambda，你必须使用 cron 表达式。

As you would have realized, the simplicity of the rate expression comes at the price of reduced flexibility. You can use rate for lambdas running every N minutes/hours/days. To do anything more complex, like triggering your lambda only on the weekdays, you have to use a cron expression.

请注意，如果你的 cron 表达式以某种方式导致触发时间少于一分钟，则不支持该触发器。

Please note that if your cron expression somehow results in trigger times of less than a minute, it won’t be supported.

References

  1. AWS Scheduled Events

  2. Scheduled Expressions using a rate or cron