node-mysql - when to release connection back into pool

Question

I'm using the node-mysql driver with connection pooling.

Releasing the connection back into the pool when there's only one query, is easy:

pool.getConnection(function(err, connection) {
  if (err) {
    throw err;
  }

  query = "SELECT * FROM user WHERE id = ?";
  connection.query(query, [id], function(err, users) {
    connection.release();

    if (err) {
      throw err;
    }

    // ...
  });
});

What if I need to use the connection a second time? I'd have to move the release() down a few lines. But what happens if the error is thrown? Is the connection never returned to the pool?

Do I have to use some control flow lib to have a "finally" moment in which I could release it?
Any better ideas?

Answer 1

One way this could be handled is promises. Since you're building a pool, you can construct your requests with something like q (or native promises, coming soon):

// assuming you have your local getConnection above or imported
exports.getConnection = function(queryParams) {
    var d = q.defer();
    local.getConnection(function(err, conn) {
        if(err) d.reject(err);
        d.resolve(conn);
    });
});

So, wrap few of your other calls into promises like that and then just compose your query:

db.getConnection()
.then(function(conn){
    makeRequest()
    .then(...)
    ...
.catch(function(err){
    // at the end you release the conn
});

Does it seem like something you're asking for?

Answer 2

When you perform a MySQL query, for that time it locks the database until query completes. After the successfull completion of query, it releases that database lock.

Same case here: connection.release(); simply releases the DB connection, nothing else.

Answer 3

You should use separate connections for that situation. That's what the connection pool is for. That way one doesn't have to wait for the other one to finish before it can start. I only use the same connection if one query cannot start until the other query has finished.