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Build a CRDT library
Conflict-free merging as a math problem. Build the canonical CRDTs — counters, sets, registers, sequences — and feel why a commutative-associative-idempotent merge function buys you offline-first sync without a coordinator. The price is paid in metadata growth, and that's the load-bearing trade.SavedSaved on this device — Saved on this device
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About Build a CRDT library
Conflict-free merging as a math problem. Build the canonical CRDTs — counters, sets, registers, sequences — and feel why a commutative-associative-idempotent merge function buys you offline-first sync without a coordinator. The price is paid in metadata growth, and that's the load-bearing trade.
- Difficulty
- advanced
- Time
- about 80 minutes
- Stages
- 9
- Topic
- Consensus, Coordination & Durable Execution
How this problem is worked
Nine stages, from what the thing is for to how it compares with the real implementations. Each asks one question, and the simulator runs the architecture you draw against the requirements you wrote.
- 01Purpose & invariantsWhat is this for, and what must always be true of it?
- 02Workload characterizationWho writes, who reads, and in what shapes?
- 03Data model & on-disk formatWhat does the data look like at rest?
- 04Core algorithmsHow do the write path and the read path actually work?
- 05Distribution & replicationHow does this scale out and survive losing a machine?
- 06Consistency & correctnessUnder concurrency and failure, what is guaranteed?
- 07Failure modes & recoveryWhat actually happens when each part fails?
- 08Operational characteristicsCan a human run this at three in the morning?
- 09Trade-offs & comparisonWhere does this sit against the alternatives?
Primary sources for this problem
- Shapiro, Preguiça, Baquero, Zawirski — A Comprehensive Study of Convergent and Commutative Replicated Data Types (INRIA 2011)
- Shapiro et al. — Conflict-Free Replicated Data Types (SSS 2011)
- Almeida, Shoker, Baquero — Delta state replicated data types (JPDC 2018)
- Kleppmann — A Brief History of CRDTs (his talks and blog)
- Yjs internals — YATA + the Yjs document model
- Automerge documentation — RGA implementation in JavaScript/Rust
- Riak DT — production CRDT implementations
More in Consensus, Coordination & Durable Execution
Getting N machines to agree, and getting one job to happen exactly once: Raft, coordination services, CRDTs, locks, leader election, schedulers and durable workflows.
- Build Build Raft — consensus you can defendReplicate a deterministic state machine across N servers with safety as a theorem and liveness under partial synchrony. Build the protocol from term to commit to safety proof to reads, and feel why etcd, Cockroach, and TiKV ship slightly different Rafts.
- Build Build a workflow engine (Temporal / Airflow / Cadence style)A function that survives crashes, restarts, and re-deploys — and still finishes. Build a durable execution engine where workflow code is replayed deterministically from an event history, activities retry with exponential backoff, sagas compensate on failure, and the same workflow definition runs identically a year later. Internalize why 'just retry the cron job' breaks at the second step.
- Distributed LockRedlock controversy, fencing tokens, lease vs lock.
- Build Build a production AI agent (from one API call up)A model API is a function from text to text — it remembers nothing, does nothing and fails without saying so. Everything an agent actually does lives in the program around it: the loop, the tools, the window, the budgets, the gates, the sandbox, the run log. Build that program, one mechanism at a time, until it can resolve a support ticket and issue a refund exactly once.
- AI Agent PlatformLong-running multi-step LLM agents — durable workflow + sandboxed execution + LLM gateway. Brain / hands / state are independently replaceable. The agent run is a workflow, not a request.
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