Have you ever thought about adding an undo feature to your application? This blog article is an attempt to approach that task by utilizing some internals of the Git version control system together with the Memento design pattern. But please, don’t take this too seriously :-).
As described in Pro Git - Git Internals, Git is defined as a content-addressable file system. According to the book, this simply means that at the core of the tool is a key-value store where you can put in arbitrary data and get back a unique key that you can use to access that content later on. You can use plumbing commands to access those internal features (in contrast to the more high-level porcelain commands like
As an experiment, let’s build on top of that file system and use it to design a simple undo feature, together with the Memento desgin pattern.
The Memento design pattern
The basic idea of the Memento pattern is to be able to save and restore the state of an object (called Originator) from the outside – without breaking its encapsulation.
Let’s take a look at an (yet incomplete) example Originator Python class:
def __init__(self): self._state = '' def set_state(self, state): self._state = state def get_state(self): return self._state def create_memento(self): # [gather internal state and put into memento object] # [return memento object] def restore(self, memento): # [restore internal state based on memento object]
Note that a client of the Originator class (Caretaker, as it’s called in the pattern) can create snapshots of the internal state without depending on how that state is actually maintained. One application of the pattern is an undo concept: By storing a memento object after every change to the Originator, we can step back in time quite easily.
Implement Memento using the Git file system
Now let’s go back to Git’s core concept – the content-addressable file system. How can we utilize it for our purpose of saving and restoring the state of an object?
Don’t you think the unique key that we get back for each stored data block looks very much like the content of a memento object? What if the Originator stored its internal state into Git’s key-value store and returned the key to the client? For restoring, it would then receive a Memento object containing a key and ask Git for the corresponding state data.
Let’s start working on that idea by filling in the missing parts of the Originator, assuming we already have some key_value_storage that behaves like Git internally:
class Memento: def __init__(self, key): self.key = key class Originator: # [...] def create_memento(self): key = self.key_value_storage.store( type(self).__name__ + '|' + self._state) return Memento(key) def restore(self, memento): self._state = self.key_value_storage.load( memento.key).split('|')
The data block that represents the internal state will then be the _state attribute, a string in the simplest case, preceded by the name of the class, separated by a
| character. The Memento’s only job here is to store the key returned from the storage.
The next step is to find an implementation for the key_value_storage. Its store and load operations can be fulfilled by Git’s hash-object and cat-file plumbing commands under the hood, so let’s just execute them as a process:
class GitStorage: def store(self, value): process = Popen( ['git', 'hash-object', '-w', '--stdin'], stdout=PIPE, stdin=PIPE) result = process.communicate(value.encode('utf-8')) return result.strip().decode('utf-8') def load(self, key): process = Popen(['git', 'cat-file', '-p', key], stdout=PIPE) return process.communicate().decode('utf-8')
That way we store every state as a new Git blob object. You can verify the object creation by taking a look at the
`find .git/objects -type f | xargs ls -lh --sort=time`
Here you may observe that a new hash is created only if the Originator’s state has actually changed.
Conclusion & References
Although this approach looks very promising, we need to take into account how Git is actually supposed to be used: as a version control system. In that sense, creating blob objects is only a part of the overall procedure of storing a snapshot of a repository. One thing it will certainly interfere with at some point is Git’s garbage collector (
git help gc). The reason for that is that the blob objects created are not referenced by any commit in the database.
You can find a working example of the code mentioned in the article at github/ronalterde/git_memento.
Feel free to send me your ideas on how to make this experimental concept a bit more ready for production.