Don't do it now! Part 3. Hangfire details - jobs

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This is the third part of a series discussing job scheduling and Hangfire details:

This part will focus on the basic scheduling API of Hangfire. The easiest way to create a fire and forget job is by using the classHangfire.BackgroundJob and its minimalistic (and this is a complement) API of static functions:

Enqueue

As the name suggests it is the method for enqueuing jobs. It comes in different overrides:

static string Enqueue(Expression<Func<Task>> methodCall);
static string Enqueue(Expression<Action> methodCall);
static string Enqueue<T>(Expression<Func<T, Task>> methodCall);
static string Enqueue<T>(Expression<Action<T>> methodCall);

Two of them are non generic implementations of the generic version, so it comes down to whatever we want to:

  • schedule an async function Func<T, Task>
  • schedule a synchronous function Action<T>

What will be important further on is that all this methods return a string which is a unique identifier of the job. It will come in handy in the next posts.

Schedule

As the name suggests - it allows to enqueue a job, but delay its execution by some time period. The overrides are:

static string Schedule(Expression<Func<Task>> methodCall, DateTimeOffset enqueueAt);
static string Schedule(Expression<Func<Task>> methodCall, TimeSpan delay);
static string Schedule(Expression<Action> methodCall, DateTimeOffset enqueueAt);
static string Schedule(Expression<Action> methodCall, TimeSpan delay);
static string Schedule<T>(Expression<Func<T, Task>> methodCall, DateTimeOffset enqueueAt);
static string Schedule<T>(Expression<Func<T, Task>> methodCall, TimeSpan delay);
static string Schedule<T>(Expression<Action<T>> methodCall, TimeSpan delay);
static string Schedule<T>(Expression<Action<T>> methodCall, DateTimeOffset enqueueAt);

A few overrides, but it boils down to:

  • generic or nongeneric version
  • asynchronous vs. synchronous (Func<T, Task> vs. Action<T>)
  • delay it by a period from now, or execute it at specific time in future (TimeSpan delay vs. DateTimeOffset)

Similar to the Enqueue this functions also returns a string identifier of a job.

This two function are enough to start to worry about corner cases when using SqlServerStorage (probably the most common usage), so here we go:

Corner cases

Does Hangfire using SQLServer storage requires an opened transaction?

No Hangfire will create its own transaction scope and manage it. Code handling it an be found in Hangfire.SqlServer.SqlServerStorage.CreateTransaction.

If there is an opened transaction will Hangfire use it?

Yes Hangfire will enlist to an opened transaction, and use it. This means that the job will be enqueued only if rest of the process succeeded and we commit the transaction.

Does Hangfie using SQLServer storage support distributed transactions?

Yes. Again the code can be found in Hangfire.SqlServer.SqlServerStorage.CreateTransaction.

Can I rely on object state?

No. Only things being serialized are:

  • object type
  • called method
  • passed parameters

This means that we can’t rely on any state that the object had at the time of scheduling its function because it won’t be recreated. This is why I wrote that Hangfire enables to schedule functions. Because in C# functions aren’t first class citizens, but they are tied to a object instance you have to be more careful. To give an example of how a “Hello world” job looks in storage:

{  
   "Type":"System.Console, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089",
   "Method":"WriteLine",
   "ParameterTypes":"[\"System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089\"]",
   "Arguments":"[\"\\\"Hello, world!\\\"\"]"
}

Based on this info Hangfire will recreate the object and execute the function. This also brings us to a small performance tip to keep in mind: keep your parameters small, and be sure to know how they will be serialized

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