What I’ve learned moving from Azure WebJobs to Azure Function Apps

Updated: some previous of the content in this post is no longer applicable when you upgrade Microsoft.NET.Sdk.Functions to version 1.0.26, so I have added some notes and sections on that.

If you have been using Azure for some times, you might know about Azure WebJob, a great way to run your code in background on triggers and integrate with other services via bindings. Recently, Azure introduced a similar offering: Azure Function App. Function App is based on a very similar idea, where you can run your code in any supporting languages on certain triggers such as a Queue message or a HTTP request or a timer.

We have been using Azure WebJob for a couple of years to process our email/push notifications and to clean up of our database. WebJob served that purpose perfectly until our app growth reveals a big limitation: WebJob consumes all the resource of the web app itself. In theory, WebJob should be a separate background process and should not interfere with the web app. However, since it is inside the App Service, it utilizes the same CPU/Memory consumption constraint of the App Service Plan. We started getting downtime alert from our APIs because the WebJob is running too heavily, defeating the purpose of splitting up some processing to a separate process in the first place.

We look around for solutions. First of all, we tried scaling out the app service plan based on CPU and memory consumption. However, it was hard to set a proper rule to scale up and down, and the cost multiplied quite quickly. We started looking at moving to Azure Function App with consumption plan. On paper it looks great because we can completely separate our background processing and our API. We also does not have to waste computing resources for the WebJob dashboard as telemetry is now sent to Application Insights instead of Azure Storage (I heard the WebJob dashboard consumed App Service Plan’s CPU to perform certain indexing).

Unfortunately, the migration was not as smooth as we thought. We soon met many problems with our C# implementation:

• If you are using Timer trigger in Function with consumption plan, remember that you have to use the CRON schedule format (0 0 */8 * * *), not the Timer format (08:00:00).
• (This is no longer applied for Microsoft.NET.Sdk.Functions 1.0.26) Using QueueTrigger, you’ll get an amazingly cryptic error message on Azure Dashboard: The binding type(s) ‘queueTrigger’ are not registered. Please ensure the type is correct and the binding extension is installed. Visual Studio generated a functions.json with binding queueTrigger and can be seen on the Azure portal. We also added queues section to host.json file. We have also installed Microsoft.Azure.WebJobs.Extensions.Storage to enable QueueTrigger in code. So what did we miss here? Turn out you have to add
  Microsoft.Azure.WebJobs.Script.ExtensionsMetadataGenerator nuget package as well, so that Visual Studio will generate extensions.json. This is not mentioned anywhere in the docs but only a small page on GitHub, which is very irritating.
• If you are using Microsoft.NET.Sdk.Functions 1.0.26 and are using the above workaround, extensions.json might not be generated in the correct place, and you would suddenly get the same issue with unregistered binding types again when testing locally. You can try to remove Microsoft.Azure.WebJobs.Script.ExtensionsMetadataGenerator and let the SDK handle the json generation.
• ILogger is supported by the new WebJob SDK since the migration to .NET Core and also by Function SDK v2, which is great since our Functions can finally share code with ASP.NET Core. Unfortunately, you might get error “Cannot bind parameter ‘log’ to type ILogger. Make sure the parameter Type is supported by the binding.” Turn out you need to use the exact version of ILogger in the SDK (e.g. SDK 1.0.19 uses 2.1.0) (stackoverflow), which would be a big problem if one of the dependencies updates to higher versions.

There are also several limitations that is probably due to Function App being a quite young platform in Azure:

• It seems that Function is actually based on WebJob behind the scene, so the features set of Function SDK in C# is only a subset of that of WebJob:
  • (This is no longer applied for Microsoft.NET.Sdk.Functions 1.0.26) WebJob recently moved to .NET Standard and get the extremely convenient dependency injection. Meanwhile, there is no DI story in Function. This might be ok with small function, but ours get a lot of logic and we would like to have DI to get proper tests.
  • If you are using Microsoft.NET.Sdk.Functions 1.0.26, you can check out this blog post on the new DI support https://platform.deloitte.com.au/articles/performing-constructor-injections-on-azure-functions-v2.
  • A small extra note: if previously you have been using SetBasePath(context.FunctionAppDirectory) to read configuration in local.settings.json. You can now use SetBasePath(Directory.GetCurrentDirectory()) in your startup class.
• The current dashboard experience in Azure portal lacks the ability to replay the execution. This is not really critical for applications that have already been in production for a while as you should already have some retry mechanisms. However, it is quite convenient for development and small testing. Hopefully this will come soon in the future.

If you have any better solutions for those problems, please feel free to let me know in the comments.