Azure Functions Timer Trigger

The Timer trigger allows you to run an Azure Function on a schedule. This is useful for scenarios like running a scheduled cleanup task, sending out daily reports, or synchronizing data at regular intervals.

How it Works

The Timer trigger uses a CRON expression or a TimeSpan to define the schedule. When the scheduled time arrives, the trigger activates your function. You can configure the trigger to run at specific times, recurring intervals, or even at a fixed rate.

Key Concepts

• CRON Expression: A string that defines the schedule. It's a powerful way to specify complex recurring schedules.
• TimeSpan: A duration that defines the interval between executions.
• IsPastDue: A boolean property that indicates if the trigger missed its intended execution time due to an outage or delay.
• ScheduleStatus: An object that provides information about the last execution time and the next execution time.

CRON Expressions Explained

A CRON expression consists of six fields (in order):

1. Minute (0-59)
2. Hour (0-23)
3. Day of the month (1-31)
4. Month (1-12)
5. Day of the week (0-6, where 0 is Sunday)
6. Year (optional, can be a specific year or * for any year)

Special characters include:

• *: Matches any value.
• ,: Specifies a list of values.
• -: Specifies a range of values.
• /: Specifies interval values.

Example CRON Expressions:

CRON Expression	Description
0 * * * * *	Every hour at minute 0.
0 0 * * * *	Every day at midnight.
0 0 12 * * *	Every day at 12:00 PM.
0 30 * * * *	Every hour at 30 minutes past the hour.
0 0 8 1 * *	On the first day of every month at 8:00 AM.
0 0 * * 1 *	Every Monday at midnight.

Using TimeSpan

You can also use a TimeSpan for simpler, fixed-interval schedules. For example, to run a function every 5 minutes:

"00:05:00"

Configuration

The timer trigger is typically configured in the function.json file (for JavaScript, Python, and other languages) or via attributes (for C#).

function.json Example (JavaScript/Python)

{
  "scriptFile": "../run.js",
  "bindings": [
    {
      "name": "myTimer",
      "type": "timerTrigger",
      "direction": "in",
      "schedule": "0 */5 * * * *" // Every 5 minutes
    }
  ]
}
        

C# Example (using Attributes)

using Microsoft.Azure.WebJobs;

public static class TimerTriggerCSharp
{
    [FunctionName("TimerTriggerCSharp")]
    public static void Run([TimerTrigger("0 */5 * * * *")]TimerInfo myTimer)
    {
        // Your function logic here
    }
}
        

Handling Missed Executions (IsPastDue)

The IsPastDue property is crucial for understanding if your function execution was delayed. If IsPastDue is true, it means the timer should have fired earlier but couldn't due to system issues or the function taking longer than expected to complete.

It's important to design your functions to be idempotent if they are susceptible to running multiple times due to missed executions. This means that running the function multiple times should produce the same result as running it once.

Common Use Cases

• Scheduled Data Processing: Process logs, analyze telemetry, or generate reports at regular intervals.
• Background Tasks: Perform maintenance operations, data synchronization, or cache invalidation.
• Notifications: Send out daily or weekly digest emails or alerts.
• Cron Jobs: Replicate traditional cron job functionality in a serverless environment.

Best Practices

• Keep Functions Small and Focused: Timer-triggered functions should ideally perform a single, well-defined task.
• Handle IsPastDue Appropriately: Design for idempotency to prevent unintended side effects from missed executions.
• Monitor Execution Times: Use Application Insights to track how long your timer functions are taking to run.
• Choose the Right Schedule: Select a schedule that balances your needs with resource consumption. Running too frequently can be costly.
• Consider Time Zones: CRON expressions are evaluated in UTC by default. Be mindful of this if your logic depends on specific local times.