1. Problem Analysis
Dependency resolution is fundamentally an NP-complete problem (boolean satisfiability). However, real-world package ecosystems exhibit structure that makes practical resolution tractable. The primary bottleneck in Bundler is not algorithmic complexity—it is I/O latency.
Profiling reveals that Bundler spends ~85% of resolution time waiting on network requests to RubyGems.org. Each gem lookup incurs a full HTTP round-trip. For a Rails application with 300+ dependencies, this compounds to seconds of blocked I/O.
# Bundler's resolution pattern (simplified)
resolve(A) → fetch(A) → wait 50ms → resolve(B) → fetch(B) → wait 50ms → ...
# Total time: O(n) × network_latency
2. Speculative Prefetch
The key insight: when gem A is selected, its dependencies (B, C, D) will almost certainly be needed. Rather than waiting until the resolver explicitly requests B, we spawn prefetch tasks immediately.
t=0ms Resolve gem A
t=1ms Spawn prefetch for B, C, D (A's deps) — don't block
t=2ms Continue resolving A's constraints
t=48ms Prefetch completes in background
t=50ms Resolver needs B → cache hit ✓
In practice, speculative prefetch achieves 80-95% hit rates. The resolver rarely waits on network after the initial cold fetch. Combined with HTTP/2 connection multiplexing, we achieve near-optimal network utilization.
3. Conflict-Based Priority Inversion
Standard backtracking resolvers exhibit pathological behavior when two gems have incompatible version requirements. Consider gems A and B where A's newest versions conflict with B:
# Naive resolution (A first, B second)
Try A@3.0.0 + B@2.0.0 → conflict
Try A@2.9.0 + B@2.0.0 → conflict
Try A@2.8.0 + B@2.0.0 → conflict
... (3,847 more attempts) ...
Try A@1.0.0 + B@2.0.0 → success
Schwadler tracks conflict frequency per gem pair. After 5 consecutive conflicts between A and B, we invert their priority: B is resolved first. This technique, adapted from uv (Python's fast package manager), reduces worst-case resolution from O(n²) attempts to O(n).
4. Incremental Resolution
When updating a single gem in a 300-gem project, Bundler re-resolves the entire dependency tree. Schwadler computes the affected subgraph—only gems that transitively depend on the updated package.
# schwadl update sidekiq
1. Parse existing Gemfile.lock
2. Compute transitive dependents of sidekiq: {sidekiq-cron, activejob}
3. Affected gems: 3 of 300 (1%)
4. Lock remaining 297 gems, re-resolve only affected 3
5. Merge results → new Gemfile.lock
For typical single-gem updates, this yields 20-50x speedup over full re-resolution.
5. Zero-Copy Memory-Mapped Index
For offline resolution, Schwadler can download the entire RubyGems index (~200,000 gems) and serialize it using rkyv (zero-copy deserialization). The index is then memory-mapped at runtime.
# Traditional approach
Load file → Parse JSON/YAML → Allocate objects → ~500ms startup
# Zero-copy mmap
mmap(file) → Cast pointer → ~1ms startup
The rkyv format allows direct pointer access without parsing. Combined with mmap, the OS handles paging—only accessed portions of the index are loaded into RAM. This enables sub-millisecond index access regardless of total index size.
6. Parallelism Model
Schwadler employs a hybrid parallelism strategy:
- Rayon for CPU-bound work (version sorting, constraint matching)
- Tokio for I/O-bound work (HTTP requests, file operations)
- DashMap for lock-free concurrent caching
// CPU-bound: parallel sort across all cores
versions.par_sort_by(|a, b| compare_versions(a, b));
// I/O-bound: concurrent HTTP with connection pooling
let specs = client.fetch_deps_batch(&gem_names).await;
// Lock-free cache: no mutex contention
cache.insert(gem_name, versions); // Multiple threads, no blocking
7. Architecture Overview
┌─────────────────────────────────────────────────────────┐
│ CLI │
│ lock | update | install | index | cache │
└────────────────────────┬────────────────────────────────┘
│
┌────────────────────────┴────────────────────────────────┐
│ Resolver │
│ • BFS with constraint propagation │
│ • Conflict tracking + priority inversion │
│ • Speculative prefetch │
│ • Incremental mode for updates │
└────────────────────────┬────────────────────────────────┘
│
┌────────────────────────┴────────────────────────────────┐
│ RubyGems Client │
│ • Compact Index protocol (same as Bundler) │
│ • Parallel fetches (Tokio + reqwest) │
│ • Persistent disk cache with ETags │
│ • Conditional requests (If-Modified-Since) │
└────────────────────────┬────────────────────────────────┘
│
┌────────────────────────┴────────────────────────────────┐
│ Cache Layer │
│ • ~/.schwadler/cache/ — gem metadata │
│ • ~/.schwadler/index.rkyv — precomputed index │
│ • ~/.schwadler/git/ — cloned git sources │
└─────────────────────────────────────────────────────────┘
References
- uv #8157 — Conflict-based backjumping in Python package resolution
- uv #9843 — Priority inversion heuristics for version selection
- Dart PubGrub — Version solving algorithm used in Cargo and Pub
- rkyv — Zero-copy deserialization framework for Rust