Asynchronous calls using postgres as an example in NodeJS

Question

When implementing this code (example taken directly from https://github.com/brianc/node-postgres):

var pg = require('pg'); 

var conString = "tcp://postgres:1234@localhost/postgres";

pg.connect(conString, function(err, client) {
  client.query("SELECT NOW() as when", function(err, result) {
      console.log("Row count: %d",result.rows.length);  // 1
      console.log("Current year: %d", result.rows[0].when.getFullYear());
      //Code halts here
  });
});

After the last console.log, node hangs. I think this is because the asynchronous nature, and I suspect at this point, one should call a callback function.

I have two questions:

Is my thinking correct?
If my thinking is correct, then how does the mechanics work. I know NodeJS is using an event loop, but what is making this event loop halt at this point?

Jonathan Lonowski · Accepted Answer · 2012-11-21 04:27:58Z

6

It appears to hang because the connection to Postgres is still open. Until it's closed, or "ended"...

client.end(); // Code halts here

Node will continue to wait in idle for another event to be added to the queue.

Not quite. This is a detail of node-postgres and its dependencies, not of Node or of its "asynchronous nature" in general.
The idling is due to and documented for the generic-pool module that node-postgres uses:

If you are shutting down a long-lived process, you may notice that node fails to exit for 30 seconds or so. This is a side effect of the idleTimeoutMillis behavior -- the pool has a setTimeout() call registered that is in the event loop queue, so node won't terminate until all resources have timed out, and the pool stops trying to manage them.

And, as it explains under Draining:
If you know would like to terminate all the resources in your pool before their timeouts have been reached, you can use destroyAllNow() in conjunction with drain():
```
pool.drain(function() {
    pool.destroyAllNow();
});
```
One side-effect of calling drain() is that subsequent calls to acquire() will throw an Error.
Which is what pg.end() does and can certainly be done if your intention is to exit at the end of a serial application, such as unit testing or your given snippet.

edited Nov 21, 2012 at 4:27

answered Nov 20, 2012 at 4:43

Jonathan Lonowski

124k35 gold badges203 silver badges202 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Barry Steyn Over a year ago

No, I don't (think) that is the answer. By calling pg.end(), you destroy all connections in a pool. In fact, the maker says that one should avoid using this. I quote: "Disconnects all clients within a pool if poolKey is provided, or disconnects all clients in all pools. Not very clean and can potentially interrupt query executions. Primarily used during testing to allow the node process to shutdown after all the tests are executed. I'm currently evaluating routes for cleaning up and shutting down client pools as gracefully as possible."

Jonathan Lonowski Over a year ago

@BarrySteyn Perhaps that should be client.end() rather than pg.end(). But, as long as the connection pool is open, the Node process will continue to run. Your quote covers this: "Primarily used during testing to allow the node process to shutdown after all the tests are executed." And, as the snippet you provided really has no reason to keep it open, being done after a single query, pg.end() is fine in this case.

Barry Steyn Over a year ago

client.end() does not work either (I tries it first). I understand what you are sying, but this still does not answer my two questions posed.

Collectives™ on Stack Overflow

Asynchronous calls using postgres as an example in NodeJS

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related