Event Sourcing is a powerful architectural pattern where system state is derived from a sequence of immutable events. Interestingly, Git itself is an event-sourced system! Understanding Git’s internals can help us grasp event sourcing concepts intuitively.

In this blog, we’ll explore:

✅ How Git follows Event Sourcing principles

✅ Key Git concepts and their Event Sourcing counterparts

✅ Lessons we can apply when implementing Event Sourcing in software

1. What is Event Sourcing?

1.1 Traditional State-Based vs. Event-Sourced Systems

In a traditional state-based system, we store only the latest state:

Each update overwrites the previous state. This means we lose historical data!

In an event-sourced system, we store every change as an event:

Now, we can reconstruct the system's state at any point in time by replaying events.

2. How Git Uses Event Sourcing

2.1 Git Commits as Events

Git stores changes as commits, rather than overwriting files. Each commit represents an immutable event in a project’s history.

✅ Commit Hash (ID): Unique identifier of the event

✅ Tree (State Snapshot): Current state after applying all past commits

✅ Parent Commit: The previous state before this commit

Example Git Commit:

git commit -m "Added user authentication"

This creates an event (commit) recording the change.

💡 In Event Sourcing terms:

• Commit = Domain Event
• Repository = Event Store
• HEAD (Latest Commit) = Current Aggregate State

2.2 Git Log as an Event Stream

When you run:

git log

You see a list of past events (commits) in chronological order:

commit abc123 (HEAD -> main)
Author: Dev
Message: Added login feature

commit def456
Author: Dev
Message: Added user model

💡 In Event Sourcing terms:

• git log = Replay of events
• Every commit can be replayed to reconstruct state

2.3 Rolling Back to Previous States (Event Replay)

Git allows you to restore any previous state using:

git checkout <commit-hash>

This reconstructs the project’s state at that commit, just like how an event-sourced system replays events to restore past states.

💡 Key takeaway: Event Sourcing provides time-travel-like capabilities, just like Git!

2.4 Branching & Merging as Parallel Event Streams

When you create a Git branch:

git checkout -b feature-x

You start a parallel event stream, which you can later merge:

git merge feature-x

💡 In Event Sourcing terms:

• Branches = Event Streams for different aggregates
• Merging = Eventual Consistency in Distributed Systems

3. Lessons from Git for Event Sourcing

3.1 Immutability is Key

Just like Git never modifies past commits, event-sourced systems never update or delete past events.

Instead, new events derive new states, ensuring a perfect audit log and debugging capabilities.

3.2 Snapshots Improve Performance

Event Sourcing systems, like Git, sometimes need snapshots to avoid replaying all events.

Example:

• In Git, a tree object is a snapshot of the project’s files.
• In Event Sourcing, snapshots store aggregate states after every X events.

This speeds up state reconstruction.

3.3 Event-Based Collaboration

In Git, multiple developers work in parallel, committing events asynchronously.

In distributed systems, Event Sourcing enables event-driven architectures, where multiple microservices consume and react to events independently.

4. Final Thoughts

Git is a real-world example of Event Sourcing that developers use daily. By studying Git, we gain intuitive insights into:

✅ Storing immutable event histories

✅ Reconstructing state by replaying events

✅ Handling parallel event streams (branching/merging)

✅ Using snapshots to improve performance

If you’re building an event-driven system, take inspiration from Git!

Would you like a follow-up blog on CQRS with Event Sourcing? Let me know in the comments!