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crdt

May 25, 2025, 5 min read

tags : peer-to-peer,Alternative Internet, Local First Software, Distributed Systems

FAQ

Relationships

CRDT vs OT

CRDT vs Cloudflare Durable Objects

  • Core Relationship

    • DOs are foundational: They provide strongly consistent, stateful serverless units with storage. Think of them as a single source of truth or a coordination point.
    • CRDTs are a pattern for data: They are special data structures designed for multi-user collaboration, automatically merging concurrent edits to reach eventual consistency without central locking during the edits.
    • DOs can host CRDTs: You can use a DO to store the state of a CRDT, manage its updates, and coordinate clients.
    • DOs are not limited to CRDTs: For many tasks, using a DO as the single authoritative state manager is simpler and sufficient, avoiding CRDT complexity.

  • Pragmatic Points & Considerations

    • Use Case Dictates Choice:
      • Need simple coordination/state? Use a standard Durable Object.
      • Need real-time, multi-user simultaneous editing (like Google Docs)? Consider building with CRDTs, potentially using DOs for persistence and coordination.
    • CRDT Complexity:
      • Implementing CRDTs correctly is hard.
      • Not all data types easily map to CRDTs.
      • Specific CRDT libraries (like Yjs) might have APIs that clash with DO’s asynchronous nature or present challenges for complex multi-document scenarios.
    • DO Integration Challenges (when using CRDTs):
      • Storage Limits: DOs have storage size limits (e.g., 128KB per value). CRDT updates might need chunking or merging before saving.
      • Performance: Optimize how you write CRDT updates to DO storage (e.g., batching/debouncing) to manage costs and latency.
      • Feature Interaction: You may need to adapt CRDT features (like presence detection) to work well with DO features (like Hibernation).

CRDT and Synchronization

I recommend learning about CRDTs. Not because a sync system should be built on CRDTs (it probably shouldn’t unless you’re building a truly decentralized, peer-to-peer system), but because everyone’s talking about them and it’s useful to understand their limitations (and strengths). Also the literature contains many concepts that are helpful even if you don’t need costly decentralization because can rely on a centralized server. - pesterhazy

What’s relationship between CRDT and Eventual Consistency?

  • See WebRTC
  • You don’t really need webrtc to collaborative tech as you’ll anyway need a signaling server.
  • You might aswell use a centralized sync server with something like WebSockets.

When do you need to use CRDT?

TypeVerdict
General collaborative appscan “get away with” not using CRDTs in collaborative apps, especially with a server.
collaborative text editing and list manipulationCRDTs probably best option

What’s the crux of the problem?

Often the challenge was less technical, and centered around what a conflicted state means in the absence of a human discussion, i.e. how would you resolve a git merge conflict without human intervention, or if two people reschedule the meeting simultaneously one of them is likely to miss it unless they talk it out

What?

Assumptions and context

  • CRDT’s assume that you can model and automate conflict resolution for your data-model. Depending on your needs this may be easy or difficult. If you can’t do that modeling for some reason then you can’t really use CRDT.

Different definitions

  • CRDTs are a family of generic data structures that can withstand network partitions without conflicts
  • CRDTs are a buzzname for lattices storing state. Whats needed is to think latticewise (monotonically) about how you use state, not just store it.
  • Keep CALM and CRDT on: Paper explores a richer API than that of CRDTs, which guarantees both update and query are meaningful

CRDT and Text editing

Resources

Tools

Primitives

Frameworks/Libraries

  • Traditional CRDT libraries (Yjs, Automerge, Collabs) give you a data structure
  • The data structure also controls the state of the whole list/text (or possibly your whole app).
Library/FrameworkPrimary Language(s)Key Focus / Distinguishing FeaturesHistory / Status Note
YjsJavaScriptHigh performance (esp. text), modular, network agnostic, rich editor bindings, widely used.Mature, popular (web), early high-performance (~2015).
YrsRust (core); Bindings (WASM, Python, Ruby, Swift, etc.)Rust rewrite of Yjs core logic for performance, portability, and memory safety.Active dev, Yjs format compat, enables non-JS use.
AutomergeRust (core); Bindings (JS/WASM, Swift, C)JSON-like data model, immutable history (stores all changes), automerge-repo for sync/storage.Research-backed, immutable history (audit/time-travel), ~2017.
Diamond TypesRust (with WASM bindings)Extreme performance optimization, specifically for plain text and list operations.Claims superior text performance vs. Yjs/Automerge.
CollabsTypeScriptProvides a collection of common CRDT building blocks; designed for extensibility.Formerly crdt (npm); base for custom CRDTs.
json-joyTypeScript / JavaScriptImplements the JSON CRDT specification for standardized JSON collaboration.Focuses on JSON CRDT standard compliance.
LoroRust (with WASM bindings)Uses Replayable Event Graph approach; supports rich text, lists, maps, movable trees.Newer lib, alternative internal representation (REG).
sql_crdtDartIntegrates CRDT concepts directly with SQLite for synchronizing database state in Flutter apps.Specific to Dart/Flutter; local-first SQLite sync.
Akka Distributed DataScala / JavaProvides CRDTs as part of the Akka framework for building resilient, distributed JVM applications.Part of Akka toolkit; JVM enterprise use.

Toy Implementations

Graph View

crdtpeer-to-peerAlternative InternetLocal First Software (LoFi)Distributed SystemsSynchronizationEventual ConsistencyWebRTCWebSocketsSubjects I am curious aboutHTTPFormal MethodssqliteuuidCALMData ReplicationTwo Phase Locking (2PL) & Two Phase Commit (2PC)Clocks

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