How can I test out a new feature on just a percentage of my user base?

When facebook rolls out a new version of th开发者_高级运维eir site, they show it to a percentage of users first.

How could I go about doing this cleanly?

Have your users sign up for your Beta.

Select a certain percentage of those who sign up for your Beta. As you make changes, keep incrementally adding some more testers. You don't want to let everyone in at once so you can get testing all the way up until the feature is complete and released. Look at stackoverflow as an example.

You would do this because most of the people who sign up will check out your beta version, then leave. They most likely will not come back / keep testing for you.

It is also better to opt-in than opt-out. Your users may not want to be your test subjects.

With a proxy that diverts some fraction of the sessions to one of two separate running instances. The proxy can be a software proxy on the hosting machine.

Well, depending on the change, if you have a farm of web servers you could apply the change to only some of the servers in the farm. That way only certain users who were "lucky" enough to hit one of the updated servers would see the change. Of course, this approach assumes that your web proxy will always route any given user to the same server (or group of updated servers) in the farm.