Reacting to File Changes Using the Observer Design Pattern in Go
Science has shown that shy people are clever because they spend more time listening and observing and less time speaking and showing off. They absorb more information and spend countless hours reasoning them. They do it quietly and are rarely recognized by their intellects. What science has not shown is that the Observer Design Pattern is also a humble part of a crafted designed software but rarely recognized as well.
You know you are in front of a observer implementation when an event happens and one or multiple routines react to that. The source of the event is normally called publisher and the code that reacts to that is called subscriber. You can actually have a propagation of events where subscribers also act as publishers, triggering other subscribers in a chain reaction. These two concepts are also popular in messaging systems, which is a way to implement the observer pattern in a distributed and decoupled fashion.
To illustrate the observer pattern in Go, we are going to watch for changes in a local folder. Every time a folder or a file is created, modified, or removed, an event is published and propagated to subscribers. To watch the local file system we rely on fsnotify. When something happens, we get events from fsnotify and propage the event to our subscribers. The full implementation is available in my Github repo. Let’s review it, starting with two interfaces:
Publisher interface requires the implementer to
unregister() subscribers, and
notify() subscribers about events. The
observe() behaviour is specific for this case because the publisher is also a subscriber of fsnotify events. To be honest, the Publisher interface is not really necessary but, as we saw in the article about the adapter design pattern, it helps to encapsulate the fsnotify library.
Subscriber interface is simpler, pushing the implementation of a
receive() method that gets the message from the publisher. Let’s first look at the Publisher implementation: the
observe() method get a watcher from the
fsnotify library and, with the help of
filepath.Walk(), watches the target path and all its sub-folders. Then, a goroutine starts an infinite loop, waiting for events from the file system. When they happen, the
notify() method is called with information about the event.
We have two subscribers for this publisher: the
PathIndexer, which would keep a database of references to the files, and the
PathFileMD5, which would calculate the checksum of the files for consistence checks.
These subscribers are not fully implemented because the goal is to show the observer pattern, but we will eventually implement them to push files to an Azure Storage Account. For the moment, let’s see how the publisher and the subscribers are put together in the
The publisher is created with the attribute
rootPath set with the absolute path to the folder we want to watch. Then we create the subscribers and add them to the publisher. Finally, we call
pathWatcher.observer() to start observing the file system for changes.
As usual, you can find the full implementation in my Github repo. When you find some time, run the application with:
$ cd azure/storage $ go run .
and in another console, run these commands:
In a console, run some basic operations:
$ cd /home/[username]/liftbox $ mkdir pictures $ echo "Blog Post" > post.txt $ rm post.txt
Liftbox produces the following output:
Indexing: /home/htmfilho/liftbox/pictures, CREATE Checksuming: /home/htmfilho/liftbox/pictures, CREATE Indexing: /home/htmfilho/liftbox/post.txt, CREATE Checksuming: /home/htmfilho/liftbox/post.txt, CREATE Indexing: /home/htmfilho/liftbox/post.txt, WRITE Checksuming: /home/htmfilho/liftbox/post.txt, WRITE Indexing: /home/htmfilho/liftbox/post.txt, REMOVE Checksuming: /home/htmfilho/liftbox/post.txt, REMOVE
This experience of revisiting the design patterns in Go has been an amazing experience so far. The challenge is to come up with ideas to describe them through realistic use cases. I take this challenge with pleasure because it is really cool to see useful cases materialized in Go.