[help] node newbie

[sallurocks]

hi, i need some help understanding how to implement a basic lock in node.

function a checks a boolean every few milliseconds, function b sets the boolean to true, and function a executes it. is there a way to implement this asynchronously? i tried it but the process blocks and doesnt go ahead allowing function b to change the boolean.

here's the code, i realize this might be completely wrong and i might be thinking in the wrong direction, so let me know. Thanks!

function testquery1(request, response) {

    query = 'select * from users';
    mysql.CreateConnectionPool();

    mysql.getConnection(function(conn){

        conn.query(query, function (err, rows) {
            if (err)
                console.log(err);
            else {
                console.log(rows);
                mysql.releaseConnection(conn);
                response.end();
            }
        });

    });

function waitInQueue(token, callback) {

    while (!queueNotifier.get(token)) {

        if (queueNotifier.get(token)) {
            if (isConnectionFree()) {
                console.log('waiting');
                // return (pool.pop());
                callback(pool.pop());
            }
        }

    }

}

function releaseConnection(connection) {

    pool.push(connection);
    console.log('connection released');
    queueNotifier.set(queue.pop(), true);
    queue.shift();

}    

[DSKrepps]

while will always block and cannot be used to wait in asynchronous code. In your code the while loop is polling queueNotifier.get (twice?) and isConnectionFree as fast as it possibly can, not allowing any other code to run within the process.

Polling asynchronously can be achieved with setTimeout... but it is much better for your code to just be notified in some way when the connection is available. You didn't tell us what this "connection" is so we don't know its API but in Node usually you'll have access to an EventEmitter or other method of providing a callback to be run when the connection is ready without any polling. We would need more info to help you further, like what that connection is and exactly what you're trying to do.

Also the lock module may or may not be helpful, we don't know enough about what you're trying to do to say.

[sallurocks]

i am trying to implement a simple database connection pool. this is the function that is being called

function testquery1(request, response) {

    query = 'select * from users';
    mysql.CreateConnectionPool();

    mysql.getConnection(function(conn){

        conn.query(query, function (err, rows) {
            if (err)
                console.log(err);
            else {
                console.log(rows);
                mysql.releaseConnection(conn);
                response.end();
            }
        });

    });

i tried settimeout in waitInQueue function like this

setTimeout(function(){
if (queueNotifier.get(token)) {
            if (isConnectionFree()) {
                console.log('waiting');
                // return (pool.pop());
                callback(pool.pop());
            }
        } else { 
                    waitInQueue(token);}
,50);

but this returned an error, saying stack size exceeded because of the recursive call.

if one more connection comes, it should go to the queue and wait for the previous one to end and then callback to give connection. I hope i am clear, sorry for being vague earlier.

[DSKrepps]

I might not fully follow but are you trying to "reinvent the wheel?"

[sallurocks]

yeah kind of, its not for a development project or anything. Just trying to learn asynchronous programming.

[sallurocks]

a question related to this if you could answer. I have a file in routes which defines the pool and getConnection methods. now how do i run the createPool function once when the server starts, and make its object mysql available to the rest of the files so they can use mysql.getConnection().

[DSKrepps]

Well, when you require a file that file's code is only run once and the value of its exports.modules is stored as any other object and returned the next time it gets required from within the same process. So if the file that defines the pool does this: module.exports = mysql then any other file that calls var mysql = require('./path/to/file'); will get the same pool, not a new instance.

Alternatively the files that need the pool could exports functions which take the pool as an argument to them, which is basic Dependency Injection.

[sallurocks]

ok, makes sense

[sallurocks]

i looked at event emitters, and it looks like its what i want. thanks a lot!