When to close MongoDB database connection in Nodejs

It's best to use a pooled connection and then call db.close() in cleanup function at the end of your application's life:

process.on('SIGINT', cleanup);
process.on('SIGTERM', cleanup);

See http://mongodb.github.io/node-mongodb-native/driver-articles/mongoclient.html

A bit old thread, but anyway.

Here's a potential solution based on the counting approach (I haven't tested it and there's no error trapping, but it should convey the idea).

The basic strategy is: Acquire the count of how many records need to be updated, save each record asynchronously and a callback on success, which will decrement the count and close the DB if the count reaches 0 (when the last update finishes). By using {safe:true} we can ensure that each update is successful.

The mongo server will use one thread per connection, so it's good to either a) close unused connections, or b) pool/reuse them.

db.open(function (err, db) {
  db.collection('foo', function (err, collection) {
    var cursor = collection.find({});
    cursor.count(function(err,count)){
      var savesPending = count;

      if(count == 0){
        db.close();
        return;
      }

      var saveFinished = function(){
        savesPending--;
        if(savesPending == 0){
          db.close();
        }
      }

      cursor.each(function (err, doc) {
        if (doc != null) {
          doc.newkey = 'foo'; // Make some changes
          db.save(doc, {safe:true}, saveFinished);
        }
      });
    })
  });
});