DocsConceptsWhen Not to Rely on CRDTs

When Not to Use CRDTs

CRDTs shine for collaborative editing and offline-friendly applications. But they are not a universal replacement for coordination, transactions, or authorization. Use this guide to recognize when CRDTs are a poor fit and what to use instead.

Quick Reference

CRDTs merge; they do not reject. This clashes with requirements that demand global agreement at the time of the action:

  • Hard invariants: Must never be violated (e.g., balance ≥ 0)
  • Exclusive ownership: Only one winner per slot/resource

Why CRDTs fall short here

  • Merging is monotonic: remote edits cannot be “rolled back” on arrival
  • No locks or transactions across replicas
  • Check-then-set across peers needs coordination, not just convergence

Common problem scenarios

1) Exclusive resource management

Multiple users book the same room/time concurrently.

// With a CRDT-only model both intents exist after merge
// The app must pick a winner out-of-band
book("A101@2pm", { by: "A" })
book("A101@2pm", { by: "B" })
// After sync: both entries are present → invariant violated without coordination

Use instead:

  • Transactional authority (central server or database)
  • Distributed consensus (Raft/Paxos) for ownership
  • Hybrid: CRDT for UI drafts, authoritative booking via server

2) Financial transactions

Two withdrawals race on a $100 account.

// Each client applies its own withdrawal locally
withdraw(60) // A
withdraw(60) // B
// After merge, naive addition overdrafts without an authority

Use instead:

  • ACID transactions on an authoritative store
  • Command validation with a single writer per account
  • Event-sourced ledger with server-side checks

Other cases to avoid CRDT-as-the-only-source

  • Uniqueness constraints (usernames, slugs, one primary per group)
  • Authorization decisions that must be enforced at write time
  • Global invariants (referential integrity, cross-document totals)

When CRDTs work well

CRDTs excel when merged results are meaningful and temporary inconsistency is acceptable:

  • ✅ Collaborative text, lists, maps, rich presence
  • ✅ Comments, reactions, drafts, annotations
  • ✅ Offline edits where convergence is enough
How to Choose the Right CRDT TypesContainer ID