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cloudkit-expert skill

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This skill analyzes CloudKit sync design, conflict resolution, and offline strategies to optimize multi-device data consistency and resilience.

npx playbooks add skill willsigmon/sigstack --skill cloudkit-expert

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---
name: CloudKit Expert
description: CloudKit sync - schema design, conflict resolution, offline support, multi-device sync validation
allowed-tools: Read, Edit, Grep
---

# CloudKit Expert

CloudKit architecture for Leavn's multi-device sync.

## Core Concepts
- **Containers**: App's CloudKit namespace
- **Databases**: Private (user), Public (shared), Shared (collaboration)
- **Record Types**: Schema definitions
- **Zones**: Custom zones for atomic operations

## Sync Architecture
1. Local SwiftData/CoreData as source of truth
2. CloudKit as sync transport
3. CKSyncEngine for automatic sync (iOS 17+)

## Conflict Resolution
- **Last-write-wins**: Simple, may lose data
- **Field-level merge**: Merge non-conflicting fields
- **Custom resolver**: App-specific logic

## Offline Support
- Queue changes locally when offline
- Sync when connectivity restored
- Handle partial sync failures

## Validation Checklist
- [ ] Schema matches local model
- [ ] Indexes on query fields
- [ ] Conflict handling tested
- [ ] Offline → online sync works
- [ ] Delete propagation correct

Use when: CloudKit bugs, sync issues, schema design, conflict resolution

Overview

This skill helps design, validate, and troubleshoot CloudKit sync for multi-device apps. I focus on schema alignment, conflict resolution strategies, offline handling, and practical validation steps to ensure reliable sync across private, public, and shared databases. The guidance targets TypeScript-backed tooling and iOS architectures using SwiftData/CoreData with CloudKit as transport.

How this skill works

I inspect your CloudKit schema and compare it with the local model to find mismatches and missing indexes. I evaluate your conflict resolution approach (last-write-wins, field-level merge, or custom resolvers) and recommend changes. I review offline queuing and retry logic, and validate end-to-end sync using a checklist that covers zones, record types, and delete propagation. I also provide test plans for multi-device sync validation and partial failure scenarios.

When to use it

You see inconsistent data across devices after sync
Schema migrations or model changes are being deployed
Conflicts occur frequently and need a deterministic strategy
Offline edits don’t propagate correctly when connectivity returns
You need to validate multi-device collaboration or shared database behavior

Best practices

Keep CloudKit schema in sync with the local model and version migrations explicitly
Index fields you query often to improve fetch performance and reduce network cost
Prefer field-level merges for composed objects; use custom resolvers for domain logic
Queue local changes reliably and persist pending operations for offline support
Test delete flows and tombstone propagation to avoid ghost records
Automate multi-device tests that simulate network loss, partial failures, and simultaneous edits

Example use cases

Audit and align CloudKit record types and indexes to a local SwiftData/CoreData model
Design a conflict resolution strategy: when to use last-write-wins vs. custom merge
Implement offline queuing with persisted pending changes and retry/backoff logic
Validate multi-device sync by simulating concurrent edits and verifying deterministic merges
Review CKSyncEngine integration (iOS 17+) and migration steps from older sync layers

FAQ

How do I choose between last-write-wins and custom merging?

Use last-write-wins for simple, infrequently edited records. Choose field-level or custom merging when different fields are updated independently or domain rules require preserving specific values.

What should I test for offline support?

Persist pending changes, simulate network loss and restoration, verify retry/backoff, and confirm partial syncs don’t corrupt state.

How do I verify schema compatibility?

Compare local model types, required fields, and indexes with CloudKit schema. Run migration tests and ensure record type versions are backward compatible.