Object-oriented programming & transactions

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A little intro:

Class contains fields and methods (let me skip properties this time). Fields represent a state of the class. Methods describe behavior of the class.

In a well-designed class, a method won't change the class's state if it throws an exception, right? (In other words, whatever happens, class's state shouldn't be corrupted)

Question:

Is there a framework, a design pattern, best practice or a programming language to call a sequence of methods in a transactional style, so that either class's state don't get changed (in case of exception), or everything succeeds?

E.g.:

// the class foo is now in the state S1
foo.MoveToState2();
// it is now (supposed to be) in the state S2
foo.MoveToFinalState();
// it is now (supposed to be) in the state, namely, S3

Surely, an exception might occur both in MoveToState2() and MoveToFinalState(). But from this block of code I want the class foo to be either in the state S1 or S3.

This is a simple scenario with a single class involved, no if's, no while's, no side effects, but I hope the idea is clear.

Take a look at the Memento pattern

The memento pattern is a software design pattern that provides the ability to restore an object to its previous state (undo via rollback).

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The simplest and most reliable "pattern" to use here is an immutable data structure.

Instead of writing:

foo.MoveToState2();
foo.MoveToFinalState();

You write:

MyFoo foo2 = foo.MoveToState2();
MyFoo finalFoo = foo2.MoveToFinalState();

And implement the methods accordingly - that is, MoveToState2 does not actually change anything about MyFoo, it creates a new MyFoo that is in state 2. Similarly with the final state.

This is how the string classes in most OO languages work. Many OO languages are also starting to implement (or have already implemented) immutable collections. Once you have the building blocks, it's fairly straightforward to create an entire immutable "entity".

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Not the most efficient method, but you could have an object that represents your transactional data. When you start a transaction, make a copy of the data and perform all operations on that. When the transaction ends successfully, move the copy to your real data - this can be done using pointers, so need not be too inefficient.

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Functional programming is a paradigm that seems to fit well to transactional computations. Since no side-effects are allowed without explicit declaration, you have full control of all data flow.

Therefore software transactional memory can be expressed easily in functional terms - See STM for F#

The key idea is the concept of monads. A monad can be used to model an arbitrary computation through two primitives: Return to return a value and Bind to sequence two computations. Using these two, you can model a transactional monad that controls and saves all state in form of continuations.

One could try to model these in an object-oriented way through a State+Memento pattern, but generally, transactions in imperative languages (like the common OO-ones) are much more difficult to implement since you can perform arbitrary side-effects. But of course you can think of an object defining a transaction scope, that saves, validates and restores data as needed, given they expose a suitable interface for this (the patterns I mentioned above).

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This would be pretty ugly to implement everywhere, but just saving the state locally, then restoring it in the case of an exception would work in simple scenarios. You'd have to catch and rethrow the exception, which may lose some context in some languages. It might be better to wrap it if possible to retain the context.

try {
   save state in local variables
   move to new state
} catch (innerException) {
   restore state from local variables
   throw new exception( innerException )
}

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Comments
  • As I say in my post, the memento is used more in a gui environment, for persistence and undo/redo. Although the basic concept is the same (create another class that represents the state) you shouldn't transit to the new state until it's completely safe to do so. That suggest to try to create the new state, and only if this step succeed, to change the state in the new one.