Azure Functions IsPastDue - when is it set to true?

I am trying to understand the IsPastDue flag.

I have written the following function in an attempt to simulate it.

[FunctionName("Function1")]
public static void Run([TimerTrigger("*/30 * * * * *")]TimerInfo myTimer, ILogger log)
{
    if (myTimer.IsPastDue)
    {
        log.LogInformation($"$Custom$C# Timer trigger was delayed at: {DateTime.Now}");
    }
    else
    {
        log.LogInformation($"$Custom$C# Timer trigger function started at: {DateTime.Now}");
        Thread.Sleep(45000);
        log.LogInformation($"$Custom$C# Timer trigger function ended at: {DateTime.Now}");
    }
}

The resulting logs are:

2020-11-05T11:17:00.984 [Information] Executing 'Function1' (Reason='Timer fired at 2020-11-05T11:17:00.9844456+00:00', Id=0080629c-8b23-4c64-ab2b-233e389115c0)
2020-11-05T11:17:00.985 [Information] $Custom$C# Timer trigger function started at: 11/5/2020 11:17:00 AM
2020-11-05T11:17:45.991 [Information] $Custom$C# Timer trigger function ended at: 11/5/2020 11:17:45 AM
2020-11-05T11:17:45.991 [Information] Executed 'Function1' (Succeeded, Id=0080629c-8b23-4c64-ab2b-233e389115c0, Duration=45007ms)
2020-11-05T11:17:46.006 [Information] Executing 'Function1' (Reason='Timer fired at 2020-11-05T11:17:46.0066396+00:00', Id=94521af2-c5ef-49d9-bb05-14e54581d3f8)
2020-11-05T11:17:46.007 [Information] $Custom$C# Timer trigger function started at: 11/5/2020 11:17:46 AM
2020-11-05T11:18:31.015 [Information] $Custom$C# Timer trigger function ended at: 11/5/2020 11:18:31 AM
2020-11-05T11:18:31.016 [Information] Executed 'Function1' (Succeeded, Id=94521af2-c5ef-49d9-bb05-14e54581d3f8, Duration=45009ms)
2020-11-05T11:18:31.031 [Information] Executing 'Function1' (Reason='Timer fired at 2020-11-05T11:18:31.0312837+00:00', Id=aaae053a-5d91-4d8e-bd1b-99cec8813eec)
2020-11-05T11:18:31.032 [Information] $Custom$C# Timer trigger function started at: 11/5/2020 11:18:31 AM
2020-11-05T11:19:16.045 [Information] $Custom$C# Timer trigger function ended at: 11/5/2020 11:19:16 AM
2020-11-05T11:19:16.046 [Information] Executed 'Function1' (Succeeded, Id=aaae053a-5d91-4d8e-bd1b-99cec8813eec, Duration=45015ms)
2020-11-05T11:19:16.062 [Information] Executing 'Function1' (Reason='Timer fired at 2020-11-05T11:19:16.0624948+00:00', Id=a00c778f-a64c-4f87-a4c3-d523100204cb)
2020-11-05T11:19:16.063 [Information] $Custom$C# Timer trigger function started at: 11/5/2020 11:19:16 AM
2020-11-05T11:20:01.073 [Information] $Custom$C# Timer trigger function ended at: 11/5/2020 11:20:01 AM
2020-11-05T11:20:01.074 [Information] Executed 'Function1' (Succeeded, Id=a00c778f-a64c-4f87-a4c3-d523100204cb, Duration=45012ms)
2020-11-05T11:20:01.089 [Information] Executing 'Function1' (Reason='Timer fired at 2020-11-05T11:20:01.0890795+00:00', Id=37ad85ca-a0c0-4b7d-a39e-f9a42a5bca77)
2020-11-05T11:20:01.090 [Information] $Custom$C# Timer trigger function started at: 11/5/2020 11:20:01 AM
2020-11-05T11:20:46.094 [Information] $Custom$C# Timer trigger function ended at: 11/5/2020 11:20:46 AM
2020-11-05T11:20:46.095 [Information] Executed 'Function1' (Succeeded, Id=37ad85ca-a0c0-4b7d-a39e-f9a42a5bca77, Duration=45005ms)
2020-11-05T11:20:46.110 [Information] Executing 'Function1' (Reason='Timer fired at 2020-11-05T11:20:46.1101441+00:00', Id=bd3e21a2-f574-4c8b-834b-bdcf19e67d20)
2020-11-05T11:20:46.110 [Information] $Custom$C# Timer trigger function started at: 11/5/2020 11:20:46 AM

The flag IsPastDue is never set to true. When is it set to true?

Solution

The isPastDue property is true when the current function invocation is later than scheduled. For example, a function app restart might cause an invocation to be missed. Here is the official document about it.

You can try to repro it by run the function -> stop the function -> re-run the function in visual studio locally. But this is not 100% repro. I tried it for more than 10 times, it only occurs 1 time. Here is the screenshot when isPastDue is true in my test: