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Global Logger vs. Dependency Injection

Notice that our requestLogger middleware accepts a *log.Logger as a parameter:

func requestLogger(logger *log.Logger) func(next http.Handler) http.Handler

But why not just keep using the global logger we already declared everywhere? Wouldn't that be simpler? On the surface, perhaps.

But globals are generally a bad idea because they make testing and debugging more difficult... and a global logger is no exception!

Using Dependency Injection

Dependency injection (DI) is a really fancy term for a really simple idea: pass a function or method's dependencies in as arguments. Generally it makes testing much easier, because you don't need to continuously mutate shared state.

By passing the logger object as an argument to our middleware function, we can avoid these problems. It also means we can pass a distinct logger object for each test, and even run tests in parallel without worrying about global state.

So, non-global loggers are great because:

It's easy to use different loggers in different parts of your app if you'd like
You can more easily pass context information to the logger as needed
There's no global state to worry about when you're writing unit tests that use the logger

Assignment

Replace the global logger with injected loggers.

Add a logger field to the server struct, update newServer to accept and set the logger, and update server logging to use it.
Add a logger field to the Store, update store.New to accept a logger, and use the injected logger in the store package.
Create two non-global loggers in run:
- An "access" logger that writes to a file named linko.access.log with an INFO: prefix.
- A "standard" logger that writes to STDERR with a DEBUG: prefix.
Use the access logger for server/request logs, and use the standard logger for your Store and shutdown messages.
Remove the old global logger.

Restart your server:

go run .

Run and submit the CLI tests from the root of the Linko repo.