如何使用 Promise Disposer 模式防止資源外洩？

Understanding the Promise Disposer Pattern

In asynchronous programming, managing resources like database connections securely without leaking them can be challenging. The promise disposer pattern emerges as a solution to this issue by linking resource ownership with the execution scope.

The Problem with Your Code

The provided code demonstrates a scenario where database connections may be leaked if not properly released after each call to getDb. This can eventually freeze the application as resources become exhausted.

The Disposer Pattern

The disposer pattern enforces a rigid relationship between a resource and the code that utilizes it. When a resource is bound to a specific execution context, it can be confidently released once the code finishes executing. This eliminates the risk of resource leakage. Its syntax resembles:

withResource(function(resource){
     return fnThatDoesWorkWithResource(resource); // returns a promise
 }).then(function(result){
    // resource disposed here
 });

Applying the Disposer Pattern

Applying the disposer pattern to your code would result in the following:

function withDb(work){
    var _db;
    return myDbDriver.getConnection().then(function(db){
        _db = db; // keep reference 
        return work(db); // perform work on db
    }).finally(function(){
        if (_db)
            _db.release();
    });
}

Using this function, the problematic code can be rewritten to:

 withDb(function(conn){
     return conn.query("SELECT name FROM users");
 }).then(function(users){
     // connection released here
 });

Benefits and Use Cases

The disposer pattern ensures that resources are released appropriately, eliminating leaks. Its implementation is prevalent in libraries like Sequelize and Knex, demonstrating its versatility. It can also be extended to control other tasks, such as hiding loaders after all AJAX requests complete.

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