Is doing Transaction Management in the Controller bad practice?

Question

I\'m working on a PHP/MySQL app using the Yii framework.

I\'ve come across the following situation:

In my VideoController, I have a actionCrea

Answer 1

Best Practice: Put the the transactions in the model, do not put the transactions in the controller.

The primary advantage of the MVC design pattern is this: MVC makes model classes reusable without modification. Make maintenance and implementing new features easy.

For example, presumably you are primarily developing for a browser where a user enters one collection of data at a time, and you move data manipulation into the controller. Later you realize you need to support allowing the user to upload a large number of collections of data to be imported on the server from the command line.

If all the data manipulation was in the model, you could simply slurp in the data and pass it to the model to handle. If there is needful (transactional) functionality in the controller, you would have to replicate that in your CLI script.

On the other hand, perhaps you end up with another controller that needs to perform the same functionality, from a different point. You will need to replicate code in that other controller as well now.

To that end, you merely need to solve the transaction challenges in the model.

Assuming you have a Video class (model) with the setPrivacy() method that already has transaction build in; and you want to call it from another method persist() which needs to also wrap its functionality in a larger transaction, you could merely modify setPrivacy() to perform a conditional transaction.

Perhaps something like this.

class Video{
    private $privacy;
    private $transaction;

    public function __construct($privacy){

        $this->privacy = $privacy;
    }

    public function persist(){
        $this->beginTransaction();
        // ...action code...
        $this->setPrivacy($this->privacy, false);
        // ...action code...
        $this->commit();
    }

    public function setPrivacy($privacy, $transactional = true){
        if ($transactional) $this->beginTransaction();

        $this->privacy = $privacy;
        // ...action code..

        if ($transactional) $this->commit();
    }


    private function beginTransaction(){
        $this->transaction = Yii::app()->getDb()->beginTransaction();
    }

    private function commit(){
        $this->transaction->commit();
    }
}

In the end, your instincts are correct (re: That leads to duplicated code in a lot of places where I need transactions for the action.). Architect your models to support the myriad of transactional needs you have, and let the controller merely determine which entry point (method) it will use in it's own context.

Answer 2

The reason that I say transactions don't belong in the model layer is basically this:

Models can call methods in other models.

If a model tries to start a transaction, but it has no knowledge of whether its caller started a transaction already, then the model has to conditionally start a transaction, as shown in the code example in @Bubba's answer. The methods of the model have to accept a flag so that the caller can tell it whether it is permitted to start its own transaction or not. Or else the model has to have the ability to query its caller's "in a transaction" state.

public function setPrivacy($privacy, $caller){
    if (! $caller->isInTransaction() ) $this->beginTransaction();

    $this->privacy = $privacy;
    // ...action code..

    if (! $caller->isInTransaction() ) $this->commit();
}

What if the caller isn't an object? In PHP, it could be a static method or simply non-object-oriented code. This gets very messy, and leads to a lot of repeated code in models.

It's also an example of Control Coupling, which is considered bad because the caller has to know something about the internal workings of the called object. For example, some of the methods of your Model may have a $transactional parameter, but other methods may not have that parameter. How is the caller supposed to know when the parameter matters?

// I need to override method's attempt to commit
$video->setPrivacy($privacy, false);  

// But I have no idea if this method might attempt to commit
$video->setFormat($format); 

The other solution I have seen suggested (or even implemented in some frameworks like Propel) is to make beginTransaction() and commit() no-ops when the DBAL knows it's already in a transaction. But this can lead to anomalies if your model tries to commit and finds that its doesn't really commit. Or tries to rollback and has that request ignored. I've written about these anomalies before.

The compromise I have suggested is that Models don't know about transactions. The model doesn't know if its request to setPrivacy() is something it should commit immediately or is it part of a larger picture, a more complex series of changes that involve multiple Models and should only be committed if all these changes succeed. That's the point of transactions.

So if Models don't know whether they can or should begin and commit their own transaction, then who does? GRASP includes a Controller pattern which is a non-UI class for a use case, and it is assigned the responsibility to create and control all the pieces to accomplish that use case. Controllers know about transactions because that's the place all the information is accessible about whether the complete use case is complex, and requires multiple changes to be done in Models, within one transaction (or perhaps within several transactions).

The example I have written about before, that is to start a transaction in the beforeAction() method of an MVC Controller and commit it in the afterAction() method, is a simplification. The Controller should be free to start and commit as many transactions as it logically requires to complete the current action. Or sometimes the Controller could refrain from explicit transaction control, and allow the Models to autocommit each change.

But the point is that the information about what tranasction(s) are necessary is something that the Models don't know -- they have to be told (in the form of a $transactional parameter) or else query it from their caller, which would have to delegate the question all the way up to the Controller's action anyway.

You may also create a Service Layer of classes that each know how to execute such complex use cases, and whether to enclose all the changes in a single transaction. That way you avoid a lot of repeated code. But it's not common for PHP apps to include a distinct Service Layer; the Controller's action is usually coincident with a Service Layer.

Answer 3

Well, one disadvantage of these broad transactions (over the whole request) is that you limit concurrency capabilities of your database engine and you also increase deadlocks probability. From this point of view, it might pay off to put transactions only where you need them and let them cover only code that needs to be covered.

If possible, I would definitely go for placing transaction in models. The problem with overlapping transactions can be solved by introducing BaseModel (ancestors of all models) and variable transactionLock in that model. Then you simply wrap your begin/commit transaction directives into BaseModel methods that respect this variable.

Answer 4

No you are right. The transaction is delegated by the "create" method which is what a controller is supposed to do. Your suggestion of using a 'wrapper' like beforeAction() is the way to go. Just make the controller extend or implement this class. It looks like you are looking for an Observer type pattern or a factory-like implementation.