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Lesson Datastore

How to run

From the root directory of this repository;

go run *.go

This command is a little different to run, as we need Go to read all the files within this directory, we've split this into multiple files for simplicity.

curl -X POST --data '{"text": "Alex is awesome!", "author": "AJT"}' localhost:8080/quotes
curl localhost:8080/random

This would post a quote of Alex is awesome! by AJT into our system, we'd then retrieve a random item from our datastore (which is likely to be our text we just put in).

How it works

The main file

There are a few major components here as we begin to explore the power of Go's interfaces. The first thing we should explore is the main.go file as this is where our application kicks off. This is a simple file that just calls NewServer, with the arguments from a NewMemoryStore. After that we create an address, log out where our server is running. Finally we call http.ListenAndServe(addr, s), so long as s has to implement http.Handler and that allows us to bind that address to that server.

The server file

The interesting part of that is NewServer, defined in server.go. In that function we create a server struct with the database passed in (remember our main function passed in NewMemoryStore()), as well as a new router. With the router we bind 2 handler functions for our /quotes and /random functions and return the server. Below the psuedo constructor, our server implements ServeHTTP, we will discuss how that works further on, the short answer being interfaces.

At the top of this file there is a declaration of a Quote struct, containing the text of the quote, the author, and an ID of the quote. These fields also have a json tag, these allow us to easily serialise and deserialise this data.

The Datastore declaration is an interface, for more reading on interfaces try the tour of go or go by example. Our interface declares three methods, Insert, List, and Random, this enables us compile time guarantees that anywhere we use Datastore we have access to those three methods. Looking back at our server struct, it holds a Datastore field, function on the server struct can use any of these functions.

Last thing in this file is the handler functions themselves, quote and randomQuote. First thing is these are both functions on s, this allows us to use our Datastore interface from earlier. The quotes function accepts both POST and GET methods, so we need to switch based on the request info. In the POST we json decode the users input, call our datastores Insert function - which we know must exist as the application has compiled. After that we marshal the data, and write that to our ResponseWriter. In the get, we call our datastores List function, again marshal it and write out the bytes to our ResponseWriter. The randomQuote function behaves similarly; calling datastores Random and marshalling it.

The memory file

The entry point here is the NewMemoryStore() function, it takes no arguments and returns a memory struct. The memory struct contains a map of strings to Quotes, allowing us to store a lookup from IDs to Quotes. When initialising the quotes field in memory, we use Go's built in make, if we did not do this, we would end up assigning a key/value pair to a nil map resulting in a panic.

To comply with the Datastore interface described in the server file our memory struct must implement the Insert, Random, and List functions. The Insert function uses the random.go file to create a random 8 character key ID and assigns that ID to the quote, this function makes use of the rand package. The Random function iterates the map, returning the first element, or an error if there are no elements in the map. The List function iterates through the map returning it as a list.

Exercises

  • Try adding a Get function to the Datastore interface that allows a user to retrieve a quote by a specific ID
  • Try implementing a datastore using local file storage using the ioutil, and os
  • Try implementing a datastore using a database using the sql packages (drivers need to be imported separately).

Further Reading