# Delta Sync vs Operation Log: Synthesized Comparison

**Synthesized From:** Gemini 2.5 Flash, GPT-5, Claude Opus 4.5
**Date:** December 2, 2025
**Branches Compared:** `feat/delta-sync` vs `feat/operation-logs`

---

## Implementation Status

**Both approaches have substantial implementations.**

### Current Implementation Status

| Branch                | Key Files                                                               | Lines  | Status                                                                            |
| --------------------- | ----------------------------------------------------------------------- | ------ | --------------------------------------------------------------------------------- |
| `feat/delta-sync`     | `super-sync.ts`, `diff-utils.ts`, `super-sync-api.ts`                   | ~1,600 | Full implementation: shadow state (IDB), diffing, watermarks, granular encryption |
| `feat/operation-logs` | `operation-log-store.service.ts`, `operation-log-sync.service.ts`, etc. | ~600+  | Core logic implemented: op store, sync service, effects, hydrator                 |

### Analysis Sources

| Model      | Delta Sync Source                           | Notes                                                       |
| ---------- | ------------------------------------------- | ----------------------------------------------------------- |
| **Gemini** | Earlier state of `feat/delta-sync`          | Analyzed before branch was updated with full implementation |
| **GPT-5**  | Design docs + code                          | Analyzed architectural design                               |
| **Opus**   | `feat/delta-sync` (940-line implementation) | Full code analysis of implemented delta sync                |

**Update:** Gemini's finding that delta sync was "vaporware" is now outdated. The `feat/delta-sync` branch has been updated and now contains the full 940-line `SuperSyncProvider` implementation with IDB shadow state, diff engine, and watermark tracking. Both approaches can now be compared on equal footing as implemented code.

---

## 1. Executive Summary

This document compares two synchronization **architectures** for Super Productivity, both of which are now substantially implemented:

- **Delta Sync:** State-based synchronization with shadow state, diffing, and watermarks
- **Operation Log:** Event-sourced synchronization with append-only operation log

### Approaches Compared

| Approach          | Branch                | Core Principle                                 | Implementation                          |
| ----------------- | --------------------- | ---------------------------------------------- | --------------------------------------- |
| **Delta Sync**    | `feat/delta-sync`     | Compare state snapshots, transmit differences  | **Implemented** (~1,600 lines)          |
| **Operation Log** | `feat/operation-logs` | Record actions, transmit and replay operations | **Partially implemented** (~600+ lines) |

### Verdict (Unanimous)

All three models recommend **Operation Log** based on architectural analysis:

| Model  | Rationale                                                                       |
| ------ | ------------------------------------------------------------------------------- |
| Gemini | Delta sync has fundamental shadow state consistency issues                      |
| GPT-5  | Delta sync design has inherent LWW data-loss risk                               |
| Opus   | Code analysis reveals non-atomic state updates and vector clock governance gaps |

---

## 2. Architecture Comparison

### 2.1 Delta Sync (As Implemented)

```
Client Storage (super-sync.ts):
├── App data (IndexedDB)
├── Shadow state (super-sync-shadow IDB)
│   ├── shadow_state: last synced state per model
│   └── watermarks: revision cursors per model
└── Memory cache (lastSyncedState Map)

Sync Flow:
1. Load shadow state from IDB (or memory cache)
2. Compute diff via createDiff() in diff-utils.ts
3. Upload changes to /api/sync/changes
4. Server shallow-merges (LWW)
5. Update shadow + watermark in IDB
```

**Code-Verified Concerns (from Opus analysis of super-sync.ts):**

- Shadow state exists in both memory and IDB (can drift)
- Watermark and shadow saved in separate IDB transactions (not atomic)
- Diff computation is O(N) using JSON.stringify comparison
- LWW merge loses concurrent independent changes
- Empty vector clocks treated as CONCURRENT (false conflicts)

### 2.2 Operation Log (As Implemented)

```
Client Storage (actual):
├── SUP_OPS IndexedDB
│   ├── ops: append-only operation log
│   └── state_cache: periodic snapshots
└── NgRx Store (derived from ops)

Sync Flow:
1. Local action → append to ops log
2. Upload pending ops to remote
3. Download remote ops
4. Detect conflicts via vector clock comparison
5. Apply non-conflicting ops; surface conflicts to user
```

**Code-Verified Characteristics:**

- Single source of truth (operation log)
- Conflict detection is per-entity
- Compaction service bounds log growth
- Replay hydration from snapshot + tail ops

---

## 3. Criteria Comparison

### 3.1 Conflict Handling

This is the most significant differentiator and is **architecture-dependent, not implementation-dependent**.

| Aspect         | Delta Sync (Design)              | Operation Log (Implemented)           |
| -------------- | -------------------------------- | ------------------------------------- |
| Detection      | Whole-file vector clock          | Per-entity vector clock               |
| Merge          | Shallow LWW (`{...old, ...new}`) | Semantic (independent ops both apply) |
| User choice    | All-or-nothing                   | Per-entity granular                   |
| Data loss risk | **High** (silent overwrites)     | **Low** (conflicts surfaced)          |

**Example:**

```
Device A: Renames task to "Meeting"
Device B: Marks task complete

Delta Sync (LWW): One change lost
Operation Log: Both changes applied (independent operations)
```

**Winner: Operation Log** (unanimous, strong confidence)

### 3.2 Maintainability

| Aspect          | Delta Sync (Design)                 | Operation Log (Implemented) |
| --------------- | ----------------------------------- | --------------------------- |
| Data models     | 2 (app state + shadow)              | 1 (operation log)           |
| Adding features | Update shadow handling + diff rules | Add action to whitelist     |
| Debugging       | Compare shadow vs actual vs server  | Inspect operation sequence  |
| Source of truth | Ambiguous (shadow can drift)        | Clear (operation log)       |

**Winner: Operation Log** (unanimous)

### 3.3 Performance

| Metric         | Delta Sync                      | Operation Log           | Basis                                                   |
| -------------- | ------------------------------- | ----------------------- | ------------------------------------------------------- |
| Diff cost      | O(N) per sync                   | O(1) append             | **Code-verified** (diff-utils.ts iterates all entities) |
| Startup        | Fast (load state)               | Snapshot + replay tail  | **Code-verified**                                       |
| Bandwidth      | Smaller patches                 | Larger payloads         | **Code-verified** (delta sends only changed fields)     |
| Large datasets | UI freeze risk at 10k+ entities | Scales with change rate | **Needs measurement**                                   |

**Winner: Tie** — Delta sync wins on bandwidth; operation log wins on CPU. Needs benchmarking with real datasets.

### 3.4 Disk Usage

| Metric       | Delta Sync (Implemented)                  | Operation Log (Implemented)                   |
| ------------ | ----------------------------------------- | --------------------------------------------- |
| Steady-state | ~2X (shadow state in IDB)                 | ~1.2X (with compaction)                       |
| Basis        | **Code-verified** (super-sync-shadow IDB) | **Code-verified** (compaction service exists) |

**Winner: Operation Log** (if compaction works correctly)

---

## 4. Implementation Effort

### Delta Sync: Stabilize Existing Implementation

`feat/delta-sync` now has full implementation (~1,600 lines). Remaining work to stabilize:

| Component                       | Effort    | Notes                                          |
| ------------------------------- | --------- | ---------------------------------------------- |
| Atomic shadow/watermark updates | 1-2 weeks | Currently separate IDB transactions            |
| Web worker for diff             | 1 week    | Prevent UI freeze on large datasets            |
| Server delta API hardening      | 1-2 weeks | Pagination, error handling                     |
| Conflict UI improvements        | 1 week    | Auto-merge feedback                            |
| Testing/hardening               | 2-4 weeks | Edge cases, concurrent clients, crash recovery |

**Total: 6-10 weeks** (high risk due to multi-state consistency issues identified in code analysis)

### Operation Log: Complete Implementation

`feat/operation-logs` has core logic (~600+ lines); remaining work:

| Component                    | Effort     | Notes                                   |
| ---------------------------- | ---------- | --------------------------------------- |
| Conflict resolution UI       | 1-2 weeks  | Currently stub                          |
| Dependency resolver retry    | 0.5-1 week | Handle missing parents                  |
| Smart resolution suggestions | 1 week     | LWW fallback for simple conflicts       |
| Genesis migration            | 1 week     | Convert existing data to first op       |
| Effect guards                | 0.5-1 week | Prevent side effects during replay      |
| Testing/hardening            | 1-2 weeks  | Compaction, large logs, concurrent tabs |

**Total: 4-6 weeks** (medium risk, primarily compaction correctness)

---

## 5. How to Validate Claims

Before committing to either approach, measure:

### For Delta Sync (if implemented)

1. **Diff cost:** Time `createDiff()` with 1k, 10k, 50k entities
2. **Shadow storage:** Measure IDB size with shadow vs without
3. **Watermark consistency:** Test crash recovery mid-sync
4. **LWW data loss:** Simulate concurrent edits, count lost changes

### For Operation Log

1. **Log growth:** Measure ops/day for typical usage; verify compaction bounds it
2. **Replay time:** Time startup with 1k, 10k, 100k operations
3. **Compaction correctness:** Verify no data loss after compaction
4. **Conflict detection accuracy:** Test concurrent edits, verify correct conflicts surfaced

---

## 6. Risk Assessment

### Delta Sync Risks (Code-Verified)

| Risk                    | Likelihood | Impact | Notes                                        |
| ----------------------- | :--------: | :----: | -------------------------------------------- |
| Shadow/watermark desync |    High    |  High  | Non-atomic IDB transactions in super-sync.ts |
| LWW data loss           |    High    |  High  | Fundamental to architecture                  |
| O(N) UI freeze          |   Medium   | Medium | Can be mitigated with web workers            |
| Vector clock governance |   Medium   | Medium | Empty clocks = false conflicts               |

### Operation Log Risks (Observed)

| Risk                   | Likelihood | Impact | Notes                                |
| ---------------------- | :--------: | :----: | ------------------------------------ |
| Compaction data loss   |    Low     |  High  | Mitigate with conservative retention |
| Replay non-determinism |   Medium   | Medium | Requires reducer purity              |
| Log growth unbounded   |   Medium   |  Low   | Compaction service exists            |

**Assessment:** Operation log has fewer high-likelihood, high-impact risks.

---

## 7. Recommendation

### Consensus

**Complete the Operation Log implementation** (`feat/operation-logs`).

### Rationale

1. **Both implementations exist**, but operation log has fewer code-verified architectural risks
2. **Conflict handling is architecturally superior** in operation log (per-entity vs whole-file)
3. **Risk profile is better** for operation log (no multi-state consistency issues)
4. **Effort is comparable**: Delta sync 6-10 weeks vs Operation log 4-6 weeks

### If Delta Sync Is Chosen

1. Address atomic shadow/watermark updates first (critical bug)
2. Move diff calculation to web worker (performance)
3. Fix vector clock governance (empty clocks = false conflicts)
4. Accept and document LWW limitations
5. Plan for extensive testing of crash recovery scenarios

---

## 8. Conclusion

| Aspect                | Delta Sync                     | Operation Log                       |
| --------------------- | ------------------------------ | ----------------------------------- |
| Implementation status | **Implemented** (~1,600 lines) | Partially implemented (~600+ lines) |
| Conflict handling     | LWW (lossy)                    | Per-entity (preserves intent)       |
| Effort to production  | 6-10 weeks                     | 4-6 weeks                           |
| Risk level            | High (multi-state consistency) | Medium (compaction correctness)     |
| Recommendation        | Viable but risky               | **Recommended**                     |

Both approaches are now substantially implemented. The recommendation for operation log is based on **code-verified architectural concerns** in the delta sync implementation:

- Non-atomic shadow/watermark updates create crash consistency risks
- LWW merge semantics cause data loss for concurrent independent changes
- Empty vector clocks trigger false conflicts

**Bottom line:** Operation log provides a more robust architecture for Super Productivity's multi-device use case. If delta sync is chosen, significant stabilization work is needed to address the identified issues.
