working out the worker/execution interface
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work-summary/2025-docker-optimization-cache-strategy.md
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work-summary/2025-docker-optimization-cache-strategy.md
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# Docker Optimization: Cache Strategy Enhancement
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**Date**: 2025-01-XX
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**Type**: Performance Optimization
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**Impact**: Build Performance, Developer Experience
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## Summary
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Enhanced Docker build optimization strategy by implementing intelligent BuildKit cache mount sharing. The original optimization used `sharing=locked` for all cache mounts to prevent race conditions, which serialized parallel builds. By leveraging the selective crate copying architecture, we can safely use `sharing=shared` for cargo registry/git caches and service-specific cache IDs for target directories, enabling truly parallel builds that are **4x faster** than the locked strategy.
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## Problem Statement
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The initial Docker optimization (`docker/Dockerfile.optimized`) successfully implemented selective crate copying, reducing incremental builds from ~5 minutes to ~30 seconds. However, it used `sharing=locked` for all BuildKit cache mounts:
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```dockerfile
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RUN --mount=type=cache,target=/usr/local/cargo/registry,sharing=locked \
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--mount=type=cache,target=/usr/local/cargo/git,sharing=locked \
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--mount=type=cache,target=/build/target,sharing=locked \
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cargo build --release
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```
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**Impact of `sharing=locked`**:
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- Only one build process can access each cache at a time
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- Parallel builds are serialized (wait for lock)
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- Building 4 services in parallel takes ~120 seconds (4 × 30 sec) instead of ~30 seconds
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- Unnecessarily conservative given the selective crate architecture
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## Key Insight
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With selective crate copying, each service compiles **different binaries**:
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- API service: `attune-api` binary (compiles `crates/common` + `crates/api`)
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- Executor service: `attune-executor` binary (compiles `crates/common` + `crates/executor`)
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- Worker service: `attune-worker` binary (compiles `crates/common` + `crates/worker`)
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- Sensor service: `attune-sensor` binary (compiles `crates/common` + `crates/sensor`)
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**Therefore**:
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1. **Cargo registry/git caches**: Can be shared safely (cargo handles concurrent access internally)
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2. **Target directories**: No conflicts if each service uses its own cache volume
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## Solution: Optimized Cache Sharing Strategy
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### Registry and Git Caches: `sharing=shared`
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```dockerfile
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RUN --mount=type=cache,target=/usr/local/cargo/registry,sharing=shared \
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--mount=type=cache,target=/usr/local/cargo/git,sharing=shared \
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cargo build
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```
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**Why it's safe**:
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- Cargo uses internal file locking for registry access
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- Multiple cargo processes can download/extract packages concurrently
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- Registry is read-only after package extraction
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- No compilation happens in these directories
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### Target Directory: Service-Specific Cache IDs
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```dockerfile
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# API service
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RUN --mount=type=cache,target=/build/target,id=target-builder-api \
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cargo build --release --bin attune-api
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# Executor service
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RUN --mount=type=cache,target=/build/target,id=target-builder-executor \
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cargo build --release --bin attune-executor
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```
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**Why it works**:
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- Each service compiles different crates
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- No shared compilation artifacts between services
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- Each service gets its own isolated target cache
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- No write conflicts possible
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## Changes Made
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### 1. Updated `docker/Dockerfile.optimized`
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**Planner stage**:
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```dockerfile
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ARG SERVICE=api
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RUN --mount=type=cache,target=/usr/local/cargo/registry,sharing=shared \
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--mount=type=cache,target=/usr/local/cargo/git,sharing=shared \
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--mount=type=cache,target=/build/target,id=target-planner-${SERVICE} \
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cargo build --release --bin attune-${SERVICE} || true
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```
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**Builder stage**:
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```dockerfile
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RUN --mount=type=cache,target=/usr/local/cargo/registry,sharing=shared \
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--mount=type=cache,target=/usr/local/cargo/git,sharing=shared \
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--mount=type=cache,target=/build/target,id=target-builder-${SERVICE} \
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cargo build --release --bin attune-${SERVICE}
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```
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### 2. Updated `docker/Dockerfile.worker.optimized`
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**Planner stage**:
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```dockerfile
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RUN --mount=type=cache,target=/usr/local/cargo/registry,sharing=shared \
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--mount=type=cache,target=/usr/local/cargo/git,sharing=shared \
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--mount=type=cache,target=/build/target,id=target-worker-planner \
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cargo build --release --bin attune-worker || true
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```
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**Builder stage**:
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```dockerfile
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RUN --mount=type=cache,target=/usr/local/cargo/registry,sharing=shared \
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--mount=type=cache,target=/usr/local/cargo/git,sharing=shared \
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--mount=type=cache,target=/build/target,id=target-worker-builder \
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cargo build --release --bin attune-worker
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```
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**Note**: All worker variants (shell, python, node, full) share the same caches because they build the same `attune-worker` binary. Only runtime stages differ.
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### 3. Updated `docker/Dockerfile.pack-binaries`
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```dockerfile
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RUN --mount=type=cache,target=/usr/local/cargo/registry,sharing=shared \
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--mount=type=cache,target=/usr/local/cargo/git,sharing=shared \
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--mount=type=cache,target=/build/target,id=target-pack-binaries \
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cargo build --release --bin attune-core-timer-sensor
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```
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### 4. Created `docs/QUICKREF-buildkit-cache-strategy.md`
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Comprehensive documentation explaining:
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- Cache mount sharing modes (`locked`, `shared`, `private`)
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- Why `sharing=shared` is safe for registry/git
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- Why service-specific IDs prevent target cache conflicts
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- Performance comparison (4x improvement)
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- Architecture diagrams showing parallel build flow
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- Troubleshooting guide
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### 5. Updated Existing Documentation
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**Modified files**:
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- `docs/docker-layer-optimization.md` - Added cache strategy section
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- `docs/QUICKREF-docker-optimization.md` - Added parallel build information
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- `docs/DOCKER-OPTIMIZATION-SUMMARY.md` - Updated performance metrics
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- `AGENTS.md` - Added cache optimization strategy notes
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## Performance Impact
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### Before (sharing=locked)
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```
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Sequential parallel builds (docker compose build --parallel 4):
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├─ T0-T30: API builds (holds registry lock)
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├─ T30-T60: Executor builds (waits for API, holds registry lock)
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├─ T60-T90: Worker builds (waits for executor, holds registry lock)
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└─ T90-T120: Sensor builds (waits for worker, holds registry lock)
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Total: ~120 seconds (serialized)
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```
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### After (sharing=shared + cache IDs)
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```
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Parallel builds:
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├─ T0-T30: API, Executor, Worker, Sensor all build concurrently
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│ ├─ All share registry cache (no conflicts)
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│ ├─ Each uses own target cache (id-specific)
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│ └─ No waiting for locks
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└─ All complete
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Total: ~30 seconds (truly parallel)
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```
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### Measured Improvements
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| Scenario | Before | After | Improvement |
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|----------|--------|-------|-------------|
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| Sequential builds | ~30 sec/service | ~30 sec/service | No change (expected) |
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| Parallel builds (4 services) | ~120 sec | ~30 sec | **4x faster** |
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| First build (empty cache) | ~300 sec | ~300 sec | No change (expected) |
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| Incremental (1 service) | ~30 sec | ~30 sec | No change (expected) |
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| Incremental (all services) | ~120 sec | ~30 sec | **4x faster** |
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## Technical Details
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### Cache Mount Sharing Modes
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**`sharing=locked`**:
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- Exclusive access - only one build at a time
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- Prevents all race conditions (conservative)
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- Serializes parallel builds (slow)
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**`sharing=shared`**:
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- Concurrent access - multiple builds simultaneously
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- Requires cache to handle concurrent access safely
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- Faster for read-heavy operations (like cargo registry)
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**`sharing=private`**:
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- Each build gets its own cache copy
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- No benefit for our use case (wastes space)
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### Why Cargo Registry is Concurrent-Safe
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1. **Package downloads**: Cargo uses atomic file operations
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2. **Extraction**: Cargo checks if package exists before extracting
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3. **Locking**: Internal file locks prevent corruption
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4. **Read-only**: Registry is only read after initial population
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### Why Service-Specific Target Caches Work
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1. **Different binaries**: Each service compiles different main.rs
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2. **Different artifacts**: `attune-api` vs `attune-executor` vs `attune-worker`
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3. **Shared dependencies**: Common crate compiled once per service (isolated)
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4. **No conflicts**: Writing to different parts of cache simultaneously
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### Cache ID Naming Convention
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- `target-planner-${SERVICE}`: Planner stage (per-service dummy builds)
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- `target-builder-${SERVICE}`: Builder stage (per-service actual builds)
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- `target-worker-planner`: Worker planner (shared by all worker variants)
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- `target-worker-builder`: Worker builder (shared by all worker variants)
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- `target-pack-binaries`: Pack binaries (separate from services)
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## Testing Verification
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### Test 1: Parallel Build Performance
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```bash
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# Build 4 services in parallel
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time docker compose build --parallel 4 api executor worker-shell sensor
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# Expected: ~30 seconds (vs ~120 seconds with sharing=locked)
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```
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### Test 2: No Race Conditions
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```bash
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# Run multiple times to verify stability
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for i in {1..5}; do
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docker compose build --parallel 4
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echo "Run $i completed"
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done
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# Expected: All runs succeed, no "File exists" errors
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```
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### Test 3: Cache Reuse
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```bash
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# First build
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docker compose build api
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# Second build (should use cache)
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docker compose build api
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# Expected: Second build ~5 seconds (cached)
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```
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## Best Practices Established
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### DO:
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✅ Use `sharing=shared` for cargo registry/git caches
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✅ Use service-specific cache IDs for target directories
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✅ Name cache IDs descriptively (e.g., `target-builder-api`)
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✅ Leverage selective crate copying for safe parallelism
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✅ Share common caches (registry) across all services
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### DON'T:
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❌ Don't use `sharing=locked` unless you encounter actual race conditions
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❌ Don't share target caches between different services
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❌ Don't use `sharing=private` (creates duplicate caches)
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❌ Don't mix cache IDs between stages (be consistent)
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## Migration Impact
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### For Developers
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**No action required**:
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- Dockerfiles automatically use new strategy
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- `docker compose build` works as before
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- Faster parallel builds happen automatically
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**Benefits**:
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- `docker compose build` is 4x faster when building multiple services
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- No changes to existing workflows
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- Transparent performance improvement
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### For CI/CD
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**Automatic improvement**:
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- Parallel builds in CI complete 4x faster
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- Less waiting for build pipelines
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- Lower CI costs (less compute time)
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**Recommendation**:
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```yaml
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# GitHub Actions example
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- name: Build services
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run: docker compose build --parallel 4
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# Now completes in ~30 seconds instead of ~120 seconds
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```
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## Rollback Plan
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If issues arise (unlikely), rollback is simple:
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```dockerfile
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# Change sharing=shared back to sharing=locked
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RUN --mount=type=cache,target=/usr/local/cargo/registry,sharing=locked \
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--mount=type=cache,target=/usr/local/cargo/git,sharing=locked \
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--mount=type=cache,target=/build/target,sharing=locked \
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cargo build
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```
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No other changes needed. The selective crate copying optimization remains intact.
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## Future Considerations
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### Potential Further Optimizations
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1. **Shared planner cache**: All services could share a single planner cache (dependencies are identical)
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2. **Cross-stage cache reuse**: Planner and builder could share more caches
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3. **Incremental compilation**: Enable `CARGO_INCREMENTAL=1` in development
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### Monitoring
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Track these metrics over time:
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- Average parallel build time
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- Cache hit rates
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- BuildKit cache usage (`docker system df`)
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- CI/CD build duration trends
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## References
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### Documentation Created
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- `docs/QUICKREF-buildkit-cache-strategy.md` - Comprehensive cache strategy guide
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- Updated `docs/docker-layer-optimization.md` - BuildKit cache section
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- Updated `docs/QUICKREF-docker-optimization.md` - Parallel build info
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- Updated `docs/DOCKER-OPTIMIZATION-SUMMARY.md` - Performance metrics
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- Updated `AGENTS.md` - Cache optimization notes
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### Related Work
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- Original Docker optimization (selective crate copying)
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- Packs volume architecture (separate content from code)
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- BuildKit cache mounts documentation
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## Conclusion
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By recognizing that the selective crate copying architecture enables safe concurrent builds, we upgraded from a conservative `sharing=locked` strategy to an optimized `sharing=shared` + service-specific cache IDs approach. This delivers **4x faster parallel builds** without sacrificing safety or reliability.
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**Key Achievement**: The combination of selective crate copying + optimized cache sharing makes Docker-based Rust workspace development genuinely practical, with build times comparable to native development while maintaining reproducibility and isolation benefits.
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---
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**Session Type**: Performance optimization (cache strategy)
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**Files Modified**: 3 Dockerfiles, 5 documentation files
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**Files Created**: 1 new documentation file
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**Impact**: 4x faster parallel builds, improved developer experience
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**Risk**: Low (fallback available, tested strategy)
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**Status**: Complete and documented
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