I am now running a few personal Go applications and am working on a Go web application that will eventually be ready for public consumption. Since Go gives us the ability to act as a FastCGI backend or a stand alone web application, it only seems reasonable that in a production system you’d want the ability to use it like any other web server. This includes the ability to reload the configuration on the fly and start and stop the application.
I thought I’d put together an example of how I’m doing it right now. From what I’ve been able to gather, RPC seems to be the best way to interact with your application. If you are wanting to run the application yourself, you can try it by doing:
git clone https://github.com/icub3d/go-ipc-with-rpc.git cd go-ipc-with-rpc/servicer go install cd ../greeter go install cd .. emacs greeter.service # Modify greeter.service to use the path to your GOBIN where the # above apps were installed. sudo cp greeter.services /etc/systemd/system cp config.json /tmp
You can then run it and test if like so:
$ sudo systemctl start greeter $ curl http://localhost:9000 Howdy Doodie! $ emacs /tmp/config.json $ sudo systemctl reload greeter $ curl http://localhost:9000 Hey there, partner! $ sudo systemctl stop greeter $ curl http://localhost:9000 curl: (7) couldn't connect to host at localhost:9000
At a high level there are three parts to the application. The first part is the web application itself which can be found in the greeter directory. The second is a helper application that makes RPC calls to the greeter. It can be found in the servicer directory. Finally, we have a default configuration for the greeter application and a systemd service configuration file.
Within the greeter application, I’ve divided the code into four parts. The file config.go manages reading the config file and translating it into a map that the rest of the application can use. Using a JSON config file makes reading and loading the configuration unbelievably simple. I also use a mutex to protect access to the configuration while it is being reloaded.
The file flag.go just initializes the command line flags for parsing. It isn’t really related to this post, but it was helpful during testing because I could point to various configuration files. Under normal circumstances, you’d likely have more complex flags or maybe none at all.
The file main.go contains the main function which starts up the application. It first parses the command line flags and then reads the configuration file. It then starts the listener for our web application. Next, we start the RPC services. We pass the web application’s listener to this function so that the RPC services know which listener to interact with. Finally, we setup our web server.
The last file in the greeter application is rpc.go, which is the meat of what I wanted to talk about. If this looks foreign to you, don’t fret. Go has some great documentation on using RPC. We create a ServiceHandler struct which will manage all of the RPC calls. RPC requires a very specific method definition, so even though we don’t use the incoming arguments, they need to be present. I used an integer for simplicity.
The Stop function checks to see if the listener is available and if so closes it. This has the net effect of stopping the web application because the http.Serve function exits when the listener is closed. The Reload function is equally simple because we need only call the ReadConfig function again.
The StartRPC function is what sets up the RPC service. You register objects with the rpc package and then it takes care of determining which functions meet the criteria of being exposed. In our case we register a ServiceHandler. We then set up RPC to handle HTTP requests and open up a port to listen for those request. Finally, we use the http’s Serve function to start accepting connections to the RPC service.
With that done, we now just need some way to talk to the RPC service. I wrote a little helper application that does this in the servicer directory. It is all contained in servicer.go. First, it gets our operation from the command line and translates that into the RPC method we want to call. Next, we create a client RPC connection with the DialHTTP function. We then use the client to Call the method we wanted.
Now all that remains is to hook it up into your service manager. I run Arch Linux and use systemd. If you run another service manager, you’ll have to set up your own configuration for it. I’ve provided a greeter.service which starts up the greeter application and uses the servicer to reload and stop the application.
Overall, I’m extremely happy with the outcome. I can see this being a common pattern and plan to eventually make a package out of it. If for some reason all my links above weren’t enough, here is where I got most of my information from and my example:
There was some discussion about the security of using RPC. RPC itself doesn’t care about security so that can leave you open to attack. Obviously a firewall would help solve this. Alternatively, you can use a named pipe instead of a socket and go’s rpc package will use it just the same.