Automerge is a library of data structures for building collaborative applications. You can have a copy of the application state locally on several devices which may belong to the same user, or to different users. Each user can independently update the application state on their local device, even while offline, and save the state to local disk. This is similar to Git, which allows you to edit files and commit changes offline.
When a network connection is available, Automerge figures out which changes need to be synced from one device to another, and brings them into the same state.
(Similar to Git, which lets you push your own changes, and pull changes from other developers, when you are online.)
If the state was changed concurrently on different devices, Automerge automatically merges the changes together cleanly, so that everybody ends up in the same state, and no changes are lost.
(Git only supports merging of plain text; Automerge allows complex file formats to be merged automatically.)
Automerge keeps track of the changes you make to the state, so that you can view old versions, compare versions, create branches, and choose when to merge them.
(Similar to Git, which allows diffing, branching, merging, and pull request workflows.)
- Network-agnostic. Automerge is a pure data structure library that does not care about what kind of network you use. It works with any connection-oriented network protocol, which could be client/server (e.g. WebSocket), peer-to-peer (e.g. WebRTC), or entirely local (e.g. Bluetooth). Bindings to particular networking technologies are handled by separate libraries; see the page on real-time collaboration for examples. It also works with unidirectional messaging: you can send an Automerge file as email attachment, or on a USB drive in the mail, and the recipient will be able to merge it with their version.
- Immutable state. An Automerge object is an immutable snapshot of the application state at one point in time. Whenever you make a change, or merge in a change that came from the network, you get back a new state object reflecting that change. This fact makes Automerge compatible with the functional reactive programming style of React and Redux, for example.
- Automatic merging. Automerge is a Conflict-Free Replicated Data Type (CRDT), which allows concurrent changes on different devices to be merged automatically without requiring any central server. The conflict resolution approach is described in the documentation.
Automerge is designed for creating local-first software, i.e. software that treats a user's local copy of their data (on their own device) as primary, rather than centralising data in a cloud service. The local-first approach enables offline working while still allowing several users to collaborate in real-time and sync their data across multiple devices. By reducing the dependency on cloud services (which may disappear if someone stops paying for the servers), local-first software can have greater longevity, stronger privacy, and better performance, and it gives users more control over their data. The essay on local-first software goes into more detail on the philosophy behind Automerge, and the pros and cons of this approach.
However, if you want to use Automerge with a centralised server, that works fine too! You still get useful benefits, such as allowing several clients to concurrently update the data, easy sync between clients and server, being able to inspect the change history of your app's data, and support for branching and merging workflows.