Delta CRDTs: Bandwidth‑Efficient Edge Collaboration

Deep Dive: Stage 1 — The Solution – Delta CRDTs

Delta CRDTs store only the change (delta) that occurred, not the whole state. Each operation produces a small delta object that is merged into the local state and propagated to peers. When a peer receives a delta, it merges it into its state and can optionally send back a compressed acknowledgment. Because deltas are usually a few bytes, bandwidth usage drops dramatically, and the time to apply changes remains low.

Key properties:

• State‑based semantics: The merge function remains the same as a full‑state CRDT, ensuring correctness.
• Delta generation: Each operation records the minimal information needed to reconstruct the change.
• Compaction: Periodic full snapshots can be sent to new peers or to recover from divergence.

Observable outcome: Compaction: Periodic full snapshots can be sent to new peers or to recover from divergence.

Deep Dive: Stage 2 — Code Snippet – GCounter Delta CRDT (TypeScript)

type Delta = { id: string; inc: number };
type GCounter = Map<string, number>;

function deltaInc(counter: GCounter, nodeId: string, n: number): Delta {
  const current = counter.get(nodeId) ?? 0;
  counter.set(nodeId, current + n);
  return { id: nodeId, inc: n }; // delta to send
}

function mergeDelta(counter: GCounter, delta: Delta): void {
  const current = counter.get(delta.id) ?? 0;
  counter.set(delta.id, current + delta.inc); // idempotent merge
}

When a device updates its counter, it calls deltaInc to get the delta and broadcasts it. Peers run mergeDelta on receipt. The full state can be reconstructed by summing all values in the map.

Observable outcome: The full state can be reconstructed by summing all values in the map.

Deep Dive: Stage 3 — Architecture Kit – Delta Propagation Flow

Observable outcome: You can explain how “Architecture Kit – Delta Propagation Flow” changes the architecture or implementation path.

• Delta CRDTs are a practical pattern for edge‑first collaborative apps: they keep network traffic minimal, preserve low‑latency updates, and maintain the strong guarantees of CRDTs.
• Use them whenever you need to stay responsive on bandwidth‑constrained, unreliable links.
• Delta CRDTs store only the change (delta) that occurred, not the whole state.
• When a device updates its counter, it calls deltaInc to get the delta and broadcasts it.
• Apply consistency-replication to Architecture Kit – Delta Propagation Flow.