By Chee Leong
Introduction
Golang came bundled with a pretty comprehensive HTTP Standard Library a.k.a net/http.
I’m going to show you how to do a simple round robin HTTP server with Golang’s Duck Typing a.k.a Handler interface.
And because Golang Playground doesn’t work well with HTTP server, I’m not going to provide the link, so you have to download the script and run it yourself.
Basic
So, let’s start with the boilerplate we’re going to use for the rest of the guide.
We will be creating a program that starts a HTTP server and a client to request the HTTP server, then exit.
First, it’s very important to know Server.Handler accepts a Handler interface.
So right now we set up a server that respond Hello, World! that runs on 127.0.0.1:9090
For the following part is just the boilerplate of a HTTP request,
- Create a Request object.
- Call Client.Do.
- Read the response.
- Print the response.
Notice I call fatal whenever an error occurs, this is very important.
Rule, fail as soon as possible, fail quick and loudly is better than late and silently..
Custom Handler
Now, we’re going to create our own Handler so we can perform our tricks later.
As you can see, we’ve added RobinHood, an empty struct.
Since RobinHood implemented ServeHTTP(http.ResponseWriter, *http.Request), it fulfills and qualifies as Handler.
Basic Load Balancer
Let’s implement our basic round robin Handler
As you can see, I’ve added several attributes to the RobinHood struct, let’s go through
- counter - keep track of current state to know which handler to call.
- mu - mutual exclusive lock to maintain integrity of counter
- one, two, three - http handler funcs
ServeHTTP for RobinHood is also updated to serve the handler funcs in a round robin manner.
I’ve also put the bootstrapping of RobinHood as the function newRobinHood, just to avoid typing that over and over again.
newHandlerFunc is added also to save me from repeating the handler func boilerplate.
To demonstrate the working of our basic round robin engine, I’m running the the HTTP request in a loop of 5.
Run this and you should expect the same output as the included response.txt.
Chaining HTTP Handlers
Now, instead of hacking and limit ourselves to only 3 handler funcs, we can actually implement a round robin of a list of HTTP Handlers.
The changes involved are very small.
I’ve removed one, two, three, added handler a slice of http.Handler. Updated the newRobinHood bootstrapper.
newHandlerFunc is also removed and now newHandler is added since we are accepting Handler instead.
Now, we are not limited to the predefined handler func capacity that RobinHood used to have.
See line 45-46, with just a few lines of change, we made RobinHood more scalable. And that’s about it.
Bonus
However, there’s one more trick I want to show.
We can actually modify the pool of handlers.
I’ve added AddHandler, a way to slot in handler, of course with the similar code, you can remove or modify the sequence of Handlers.
Note The mutex lock is very important in this.
At line 86, I put in a new handler and request the server 5 times again. Check the results.