# Sunday, 02 June 2013

A "Mini" workflow engine. Part 2: on the serialization of Bookmarks

Last time, we have seen that Activities are composed in a way that allows them to be paused and resumed, using continuations in the form of bookmarks:
[Serializable]
internal class Bookmark
{
    public string Name { get; set; }
    public Action<ActivityExecutionContext, object> ContinueAt { get; set; }  
    public ActivityExecutionContext ActivityExecutionContext { get; set; }
}
A Bookmark has a relatively simple structure: a Name, a continuation (in the form of an Action) and a Context. Bookmarks are created by the Context, and the continuation is always a ContinueAt method implemented in an activity. Thus, serializing a Bookmark (and, transitively, its Context) should lead to the serialization of an object graph that holds the entire workflow code, its state and its "Program Counter".

However, serializing a delegate sounds kind of scary to me: are we sure that we can safely serialize something that is, essentially, a function pointer?
That's why I started looking for an answer to a particular question: what happens when I serialize a delegate, a Func<T> or an Action<T>?

I found some good questions and answers on StackOverflow; in particular, a question about serialization of lambdas and closures (related to prevalence, a technique to provide transactional implementing checkpoints and redo journals in terms of serialization) and a couple of questions about delegate (anonymous and not) serialization (1, 2 and 3).

It looks like:
  • delegate objects (and Func<>) are not XML serializable, but they are BinarySerializable.
  • even anonymous delegates, and delegates with closures (i.e. anonymous classes) are serializable, provided that you do some work to serialize the compiler-generated classes (see for example this MSDN article, and this blog post)
  • still, there is a general agreement that the whole idea of serializing a delegate looks very risky

BinarySerializable looks like the only directly applicable approach; in our case, we are on the safe side: every function  we "point to" from delegates and bookmarks is in classes that are serialized as well, and objects and data that accessed by the delegate are serialized as well (they access only data that is defined in activities themselves, which is saved completely in the context). BinarySerilization will save everything we need to pause and resume a workflow in a different process.

I implemented a very simple persistence of workflow instances using BinarySerializer for MiniWorkflow, and it does indeed work as expected.
You can grab the code on github and see it yourself.

However, I would like to provide an alternative serialization mechanism for MiniWorkflow, less "risky". I do not like the idea of not knowing how my Bookmarks are serialized, I would like something more "visible", to understand if what I need really goes from memory to disk, and to be more in control.
Also, I am really curious to see how WF really serializes bookmarks; workflows are serialized/deserialized as XAML, but a workflow instance is something else. In fact, this is a crucial point: in the end, I will probably end up using WF4 (MiniWorkflow is -probably- just an experiment to understand the mechanics better), but I will need to implement serialization anyway. I do not want to take a direct dependency on SQL, so I will need to write a custom instance store, alternative to the standard SQL instance store.

I will explore which alternatives we have to a BinarySerializer; but before digging into this problem, I want to work out a better way to handle the flow of execution. Therefore, next time calling continuations: recursion, work item queue or... call/cc?

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