In the this final part of this series we wrap up by briefly discussing some ways to check for blobs that have not been processed correctly.

When using Azure Functions, a timer trigger can be used to automatically periodically execute a function based on a CRON expression. The following code is and example of a timer-triggered function:

public static class CheckBlobs
{
    [FunctionName("CheckBlobs")]
    public static void Run(
        [TimerTrigger("0 */5 * * * *")]TimerInfo myTimer, 
        ILogger log)
    {
        log.LogInformation($"C# Timer trigger function executed at: {DateTime.Now}");

        // Blob checking logic here
    }
}

There are a number of ways to check for unprocessed blobs depending on the solution you are building, some examples:

• Check that an output blob exists for every input blob
• Use a database to keep track of blobs that were uploaded and compare this to actual output blobs
• If blobs are deleted after they have been processed, check there are no blobs in the container
• Etc.

Some things to bear in mind if implementing this kind of checking include:

• How often/when to run the function?
• How long after a blob is written should you give it to be processed normally?
• Will running this function interfere with any other processing in the system?
• What if a blob is due to be processed (e.g. message sitting in a queue but not yet processed)? Could this create false positives or cause duplication of processing?
• How long does the checking function take to execute? Will it take too long as the number of blobs increases and will the function be terminated by the runtime?
• How/who do you notify of missed blobs (email, SMS, create ticket in CRM/bug system, etc.)?
• Do you try to perform auto-retry of processing? Again, could this cause duplication, errors, etc.?

You could also use logging/Application Insights to provide you with information, or write every incoming blob name to a database and update that record when a blob has been processed, this way unprocessed blobs can be found with a simple “IF NOT PROCESSED” query.

If you want to fill in the gaps in your C# knowledge be sure to check out my C# Tips and Traps training course from Pluralsight – get started with a free trial.

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