Project 21: Cross-Device Sync Plugin

A plugin that syncs settings across multiple devices/machines, supports profile-aware behavior, and adapts per hardware capability.

Quick Reference

Attribute Value
Difficulty Level 5
Time Estimate 24-40h
Main Programming Language TypeScript
Alternative Programming Languages JavaScript, Go, Rust
Coolness Level Level 5 (Professional workflow reliability)
Business Potential Level 4 (Team productivity value)
Prerequisites Consistency and convergence models, Scoped configuration keys, Hardware capability negotiation
Key Topics Synchronization, profile-aware state, capability detection

1. Learning Objectives

By completing this project, you will:

  1. Build a production-quality implementation of Cross-Device Sync Plugin.
  2. Apply concept boundaries around Consistency and convergence models, Scoped configuration keys, and Hardware capability negotiation.
  3. Validate behavior with explicit outcomes and failure-mode tests.
  4. Produce evidence artifacts suitable for review, support, and iteration.

2. All Theory Needed (Per-Concept Breakdown)

2.1 Consistency and convergence models

  • Fundamentals: This concept defines the first architectural boundary for this project. You should know the invariant conditions that must remain true during normal operation and failure operation. In Stream Deck plugin work, the most useful mindset is to treat interaction paths as explicit contracts, not ad-hoc callbacks, so behavior remains deterministic under context churn and profile switching.
  • Deep Dive into the concept: For this project, Consistency and convergence models is where correctness begins. Model state transitions explicitly, define allowed events, and reject illegal transitions early. Tie every side effect to context identity and traceability fields so debugging can reconstruct the full sequence. Design your test plan around race-prone paths first. Add failure classes and recovery transitions before polishing UX. This creates robust behavior under load and avoids hidden coupling across action instances.
  • How this fit on projects: This concept is the primary driver of runtime correctness in this project.
  • Definitions & key terms: invariant, transition contract, failure class, recovery path.
  • Mental model diagram:
Intent -> Validate -> Reduce -> Persist -> Render
  ^                                       |
  +--------------- Recover/Retry <--------+
  • How it works: model inputs, validate boundaries, reduce deterministic state, emit minimal side effects, then observe and recover.
  • Minimal concrete example:
PSEUDOCODE
if !isValid(event, state):
  return rejectWithHint()
next = reduce(state, event)
apply(next)
  • Common misconceptions: fast prototypes do not remove the need for explicit invariants.
  • Check-your-understanding questions: Which invalid transition causes highest user impact? Why?
  • Check-your-understanding answers: Any transition that mutates irreversible state without confirmation.
  • Real-world applications: production plugins that must survive long sessions and rapid profile switches.
  • Where you will apply it: project runtime handlers and teardown logic.
  • References: Stream Deck SDK docs + main sprint Theory Primer concepts 1/2/6.
  • Key insights: deterministic state design scales better than callback patching.
  • Summary: make invalid states unrepresentable and observable.
  • Homework/Exercises to practice the concept: draw one transition table and one failure table.
  • Solutions to the homework/exercises: each transition/failure should map to explicit UI feedback and test case.

2.2 Scoped configuration keys

  • Fundamentals: Scoped configuration keys handles data integrity and long-lived behavior. Treat user configuration, entitlement, and environment state as a schema-governed domain.
  • Deep Dive into the concept: Build validation at every boundary: PI input, backend receive, persistence write, and migration load. Use explicit versioning and conflict policy so stale updates cannot silently win. If sensitive fields exist, isolate them through secret-safe adapters and redact all diagnostics. This prevents corruption, race bugs, and support incidents that usually appear only after release.
  • How this fit on projects: ensures reliable persistence and predictable restart/recovery behavior.
  • Definitions & key terms: schema, migration, revision, redaction.
  • Mental model diagram:
Input Delta -> Merge -> Validate -> Version -> Commit -> Observe
  • How it works: merge safely, validate strictly, commit atomically, expose clear error feedback.
  • Minimal concrete example:
PSEUDOCODE
merged = merge(prev, delta)
assert schemaValid(merged)
save(merged, revision+1)
  • Common misconceptions: compile-time types are not runtime safety.
  • Check-your-understanding questions: Why must backend revalidate PI values?
  • Check-your-understanding answers: PI can be stale/malformed; backend is source of truth.
  • Real-world applications: paid plugins, sync features, and multi-account integrations.
  • Where you will apply it: persistence, entitlement checks, and API credential handling.
  • References: Stream Deck settings/secrets docs + RFC security guidance where applicable.
  • Key insights: data integrity is a user-visible feature.
  • Summary: strict boundaries prevent expensive post-release bugs.
  • Homework/Exercises to practice the concept: define v1/v2 schema and migration tests.
  • Solutions to the homework/exercises: include defaults, backward compatibility, and rollback path.

2.3 Hardware capability negotiation

  • Fundamentals: Hardware capability negotiation translates implementation quality into user trust, adoption, and maintainability.
  • Deep Dive into the concept: Build release and support workflows in parallel with features. Define observability schema, packaging checks, and non-functional budgets (latency, memory, retry behavior). Add diagnostics UX so users can self-report actionable data. If this project targets commercial outcomes, connect operational quality to listing confidence and retention. For hardware-diverse use cases, ensure adaptive behavior is explicitly tested across capability subsets.
  • How this fit on projects: provides the delivery and sustainment layer beyond core functionality.
  • Definitions & key terms: SLA mindset, supportability, release gate, degraded mode.
  • Mental model diagram:
Feature Build -> Validation Gate -> Pack/Release -> Observe -> Support -> Improve
  • How it works: define quality gates, ship artifacts, monitor signals, feed incidents back into design.
  • Minimal concrete example:
PSEUDOCHECKLIST
validate pass
smoke install pass
diagnostics export pass
rollback artifact present
  • Common misconceptions: once it works locally, release risk is low.
  • Check-your-understanding questions: Which quality gate catches packaging regressions earliest?
  • Check-your-understanding answers: deterministic CLI validate/pack + smoke install checks.
  • Real-world applications: marketplace submission, enterprise team deployment, paid support.
  • Where you will apply it: release checklist, diagnostics, and post-launch iteration.
  • References: Stream Deck CLI docs, marketplace docs, and reliability references.
  • Key insights: sustainable plugins are operated products, not one-off scripts.
  • Summary: build supportability and release discipline into the first milestone.
  • Homework/Exercises to practice the concept: create one pre-release gate matrix and one incident response runbook.
  • Solutions to the homework/exercises: each gate/runbook step must include pass/fail evidence.

3. Project Specification

3.1 What You Will Build

A plugin that syncs settings across multiple devices/machines, supports profile-aware behavior, and adapts per hardware capability.

3.2 Functional Requirements

  1. Implement all user-facing behaviors listed in the source sprint project.
  2. Preserve deterministic state behavior under context churn and restart.
  3. Enforce boundary validation for configuration and external events.
  4. Expose clear feedback for success, pending, and failure modes.
  5. Provide release/support artifacts aligned with project scope.

3.3 Non-Functional Requirements

  • Performance: Remain responsive under expected event rates for this project.
  • Reliability: No orphaned timers/subscriptions after teardown paths.
  • Usability: Users can understand current state from key/PI feedback quickly.
  • Supportability: Logs and diagnostics must be actionable and redacted.

3.4 Example Usage / Output

“How do I keep user intent consistent across devices and profiles without creating hidden destructive conflicts?”

3.5 Real World Outcome

A user configures action settings on machine A and sees synchronized settings appear on machine B within expected convergence window. If both machines edit same setting offline, plugin surfaces conflict state with deterministic resolution rule and keeps both history and final applied value visible.

4. Solution Architecture

4.1 High-Level Design

Stream Deck Events -> Runtime Reducer -> Capability/Policy Layer -> Side Effects
        ^                                                          |
        +---------------------- Diagnostics/Observability <--------+

4.2 Key Components

  • Action Runtime Layer: Handles event routing, context scoping, and state reduction.
  • Policy Layer: Applies validation, feature gates, retries, throttles, and safety rules.
  • Feedback Layer: Produces deterministic key/dial/PI feedback from canonical state.
  • Persistence/Integration Layer: Manages settings, secrets, sync, and external API boundaries.

4.3 Design Questions (From Sprint)

  1. Sync topology
    • Which source is authoritative (cloud vs local)?
    • How do you recover from partial sync failures?
  2. Conflict strategy
    • Last-write-wins, merge, or manual resolution?
    • How is conflict presented in UI and logs?

5. Thinking Exercise (Before Building)

Conflict Simulation Matrix

Simulate three concurrent edit cases: same field same profile, same field different profile, different field same profile. Define expected final state for each.

6. Implementation Hints in Layers

Hint 1: Starting Point

  • Define one canonical sync envelope format.

Hint 2: Next Level

  • Include profileId, deviceId, and version in each setting mutation.

Hint 3: Technical Details

PSEUDOFLOW
local change -> versioned event -> sync queue -> remote apply -> ack -> reconcile

Hint 4: Tools/Debugging

  • Add a sync timeline view in debug mode to inspect event ordering.

7. Verification and Testing Plan

  1. Unit-level: transition validity, schema validation, and policy decisions.
  2. Integration-level: PI/backend flow, persistence/restart, and dependency adapters.
  3. Failure-level: network/auth/retry/teardown behavior under injected faults.
  4. Release-level: validate/pack/smoke workflow and artifact integrity checks.

8. Interview Questions

  1. “How do you prevent sync loops and duplicate updates?”
  2. “What metadata do you attach to each sync event?”
  3. “How do you handle devices that lack encoder/touch features?”
  4. “What is your stale data indicator strategy?”
  5. “How would you migrate sync schema without breaking old clients?”

9. Common Pitfalls and Debugging

Problem 1: “Settings oscillate between two values”

  • Why: Bidirectional sync loop with no deduplication token.
  • Fix: Add event IDs and source markers; ignore self-originating events.
  • Quick test: Run two-device edit race and confirm monotonic convergence.

10. Definition of Done

  • Settings sync across devices with deterministic convergence behavior.
  • Cloud sync can be enabled/disabled without breaking local operation.
  • Profile-aware behavior prevents unintended global overwrites.
  • Hardware capability detection drives adaptive behavior reliably.
  • Conflict states are observable and recoverable.

11. Additional Notes

  • Why this project matters: Multi-device consistency is a defining requirement for professional users.
  • Source sprint project file: P21-cross-device-sync-plugin.md
  • Traceability: Generated from ### Project 21 in the sprint guide.