05Design patternsCollaborative editing
05 · Reusable pattern
Collaborative editing
Reason about operations, ordering, convergence, presence, snapshots, and offline edits.Lesson spine
What you need to understand.
Collaborative editing must make concurrent operations converge while keeping reconnect, presence, and history independently manageable.
The conflict problem
Concurrent inserts and deletes based on different document versions can produce divergent state when applied in different orders.
Operational transformation
A central sequencer transforms incoming operations against unseen committed operations before broadcast.
CRDTs
Give elements stable identities and merge operations with deterministic rules so replicas converge even after offline work.
Choose OT or CRDT
OT offers compact state with a central order; CRDTs favor decentralized or offline editing but carry metadata and semantic complexity.
Architecture
Route a document to a room owner, append operations, broadcast sequence numbers, snapshot periodically, and repair gaps from the log.
Presence and cursors
Publish ephemeral positions with TTLs and never let presence traffic block durable edit commits.
Before the boxes
Frame the decision.
What must work
Reason about operations, ordering, convergence, presence, snapshots, and offline edits.
What changes the design
Document size · editors · op rate · offline duration · snapshots · metadata
What owns the truth
Identify the component that commits authoritative state, then separate synchronous confirmation from derived work.
What stays simple
Do not add global coordination, multi-region writes, or a specialized store until a requirement earns the complexity.
Architecture map
Trace ownership, not just traffic.
Follow the decision from left to right. Every arrow should have a reason.
Creates operations
Authenticates
Orders ops
Persists history
Applies OT or CRDT rules
Compacts state
Carries cursors
Walk one representative request across every arrow. Say whether the handoff is synchronous or asynchronous, what identity makes a retry safe, and which step changes authoritative state.
- 01
Editor — Creates operations Define the output contract before moving to the next owner.
- 02
Sync gateway — Authenticates Define the output contract before moving to the next owner.
- 03
Room sequencer — Orders ops Define the output contract before moving to the next owner.
- 04
Operation log — Persists history Define the output contract before moving to the next owner.
- 05
Merge engine — Applies OT or CRDT rules Define the output contract before moving to the next owner.
- 06
Snapshotter — Compacts state Define the output contract before moving to the next owner.
- 07
Presence — Carries cursors Confirm the result and emit the evidence needed to reconcile it.
Decision table
Make the trade-offs explicit.
| Decision | Defensible position | Cost to acknowledge |
|---|---|---|
| Primary mechanism | OT centralizes order; CRDTs improve offline merge with more metadata and semantics. | The stronger guarantee usually adds coordination, latency, state, or operational work. |
| Sync vs. async | Keep only correctness-critical confirmation synchronous. Move derived views, notifications, analytics, and cleanup behind a durable boundary. | Async work needs idempotency, lag monitoring, replay, and a product definition for partial completion. |
| Simple vs. scaled | Begin with one logical owner and a clear API. Partition or replicate only the resource proven to be the first bottleneck. | Migration requires stable identities, versioned contracts, backfill, and a rollback path. |
Failure review
Design the recovery path.
Topic-specific risk
Offline edits, duplicate delivery, and tombstone growth can break convergence or size.
ResponsePersist enough identity and state to distinguish retry, resume, compensation, and operator repair.
Dependency timeout
A timeout is ambiguous: the remote side may have failed, succeeded, or still be running.
ResponseUse deadlines, bounded backoff with jitter, idempotency keys, and a status or reconciliation path.
Overload or skew
Average capacity can look healthy while a tenant, key, partition, region, or expensive request saturates one owner.
ResponseExpose queue depth and hot-key share, apply backpressure, isolate tenants, and degrade optional work before correctness.
Evidence + level bar
Prove the design can be operated.
Health of the promise
Measure user-visible latency or freshness, correctness drift, saturation, retry volume, and time to recover. Alert on the failed promise—not only CPU.
Complete and clear
Finish the happy path, identify the state owner, choose reasonable building blocks, and explain one scale mechanism.
Trade-offs and failure
Separate read and write paths, define consistency, explain partitioning, and make duplicate or partial failure safe.
Evolution and operations
Discuss multi-region boundaries, migration, tenant isolation, capacity, observability, and how the architecture changes over time.
Interview language
Open the deep dive with a claim.
“For Collaborative editing, the decision I want to make explicit is this: OT centralizes order; CRDTs improve offline merge with more metadata and semantics. I’ll trace the state-changing path first, show where the result becomes durable, then test the design against the highest-risk failure and our target scale.”
08 · Retrieval check
Can you defend it without the page?
- For Collaborative editing, where is the correctness boundary and which failure would you test first?
- Which component owns committed truth, and what event or response proves the commit?
- Where is the first scaling or coordination bottleneck under the stated envelope?
- What happens after an ambiguous timeout or duplicate operation?
- Which complexity would you remove at one hundredth of the scale?