Hangfire takes regular classes and regular methods to perform them in the background, because it is simple:
This snippet says that the
Console.WriteLine method will be called in background. But note that the name of the method is
Enqueue, and not the
Invoke and so on.
I’ve chosen the name of this method to highlight, that invocation of a given method is only queued in the current execution context and returns the control to a calling thread immediately after queueing. Hangfire will invoke the method in a different execution context.
What does this mean? Several things, that may break your usual expectations about method invocation process. You should be aware of them.
Differences between local and background method invocation
Method invocation is serialized
Before creating a background job, Hangfire serializes the information about the given method (its type, method name and parameter types) to a string. MethodInfo serialization process is invisible to a user, unlike the arguments serialization.
NOTE. Since Hangfire 1.1, the Json.NET package is used to serialize all the things, including custom classes. The information below is obsolete.
Arguments are also serialized to string, but arguments serialization process uses the
TypeConverter class. All standard classes like numbers, strings, dates and so on already have the corresponding
TypeConverter implementation, but if you want to pass an instance of a custom class as an argument, you should write the custom converter first.
Furthermore, serialized arguments can take more space, and it often is more efficient to pass database identifiers or file names instead of their contents.
Execution context is changed
In the simplest case, such as using
Task.Factory.StartNew methods, only thread is changed. But in Hangfire, you can use different process, or different server to process background jobs. So, the execution context term includes not only thread context, request context and so on, but also static data, including local locks, local filesystem, etc.
That is why if you are querying data inside a background job that corresponds to the execution context where the job was enqueued, it may fail. If you need to pass the current state to a job, use arguments or shared storage.
Background job method is not invoked immediately. It is placed on a queue and waits until any worker pick it up. This leads to undefined start time and end time.
Undefined start-up time
Your method can be invoked tomorrow, after two weeks or six months (always true for scheduled jobs, but works with “fail-deploy-retry” practice as well). If it is true even for regular method calls, that application data can be changed or arguments can become stale during the method invocation, especially in a highly concurrent web applications, the probability of these situations in background job processing is very high.
Always double-check the data that you pass as arguments and think about its changing nature. Here are some examples:
- You want to publish an article tomorrow, think, what do you need to do: publish its current state, or publish the article itself. In the most cases you’d choose to publish the article itself, changed or unchanged. That is why in this case you need to pass an article identifier as an argument.
- You want to check comments for spam and it is possible to change them. You are creating a new background job on each edit attempt. In this case, you need to check exactly the given text for each edit, so pass the whole text as an argument. And after the check is completed, you can compare this text with the current one. If it was changed, then just don’t do anything.
Undefined end time
We are thinking about the end time, when we want to tell our users about the job was completed. If you do something inside the request processing synchronously, you can rely on that fact, that this information will be available for a user immediately. That is why you can redirect her to a just created article page.
But when, for example, you are creating the same article in a background job, you can redirect user to a non-existing page yet, because you can not guarantee that the enqueued job will be processed in time.
Of course, you always can tune your system to perform background jobs as soon as possible, but you can not eliminate the delay between enqueue time and a real invocation completely.
So you need to think how to show yet unprocessed entities (with loaders, progress bars and so on), and how to report the job completion. For a latter task, you can always use polling, server push (for example, using SignalR), or force users to reload the page manually.
If you pass user input as an argument of your job, you should validate them first. Otherwise the job will always fail, retry, fail again, retry again and so on. It will never be completed without manual intervention.
You can disable automatic retry on failure, but there is another problem – since the immediate invocation is not guaranteed (see the section Delayed invocation), your user may leave the site and she will not be able to correct the data.
So, validate your arguments early, and instead of doing this:
Method can be called multiple times
Your method can be retried manually (through the Monitor interface) or automatically on failure (i.e on unexpected exception that is unhandler by the method itself).
So, be prepared for this situation. Try to do all your background job methods idempotent. If it is impossible, be prepared that in very rare cases it can fire multiple times.
You can always disable the automatic retry feature by applying the
[Retry(0)] filter to the exact method or globally. But to successfully fight with ASP.NET unexpected application domain unload in the middle of a job processing, Hangfire retries them automatically despite the given attribute. But don’t worry too much, these cases happen very rarely.
But as a general rule remember, that your job will be performed at least once. You can test your job for idempotence by calling it multiple times and compare the result:
Method may become unavailable
Your application can be redeployed with a different code base after a background job was enqueued. That is why you should change the signature of your background job methods carefully.
You can safely change parameter names, but the following things will lead to a broken job (however you are able to fix it and re-deploy the application):
- Addition and removal of method parameters.
- Parameter type changes.
- Parameters reordering.
- Method name changes.
- Method’s type name changes.
- Method removal.
- Method’s type removal.
- Type’s namespace changes.
- Type’s assembly changes.
Of course you can do all the above things, if there are no jobs in a storage. But instead of doing this, add a new method without touching the old one and call the new method from the old one until all old jobs become processed:
There are a lot of differences between local and background method invocation, but you likely know the most of them, because they relate to asynchronous and concurrent programming as well.
If you have any questions, ask them in the comments form below the post or start a new topic on https://discuss.hangfire.io if you want to share code snippets.