DashMap (Rust)
High-performance concurrent HashMap for Rust using a sharded lock design. DashMap provides thread-safe key-value storage that replaces Arc<Mutex<HashMap<K, V>>> with dramatically better concurrent performance. Uses fine-grained sharding (default 16 shards) so concurrent reads and writes to different shards don't contend. Drop-in ergonomic replacement for std::collections::HashMap in concurrent contexts.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
Memory safety guaranteed by Rust's borrow checker. Deadlock risks from cross-await reference holding — must be addressed at design time. No known security vulnerabilities.
⚡ Reliability
Best When
You need a shared HashMap accessible from multiple threads/async tasks in Rust and want better performance than Arc<Mutex<HashMap>>.
Avoid When
Single-threaded code — std HashMap is faster; or use tokio::sync::RwLock<HashMap> if you prefer explicit lock semantics.
Use Cases
- • Replace Arc<Mutex<HashMap>> with DashMap for concurrent caches, session stores, or shared state in async Rust services
- • Build high-throughput concurrent registries (plugin registries, connection pools) where multiple threads read and write concurrently
- • Implement distributed counters or metrics accumulators that are updated from many async tasks simultaneously
- • Use as a concurrent cache in Axum/Actix-web services as shared application state (State<DashMap>)
- • Store per-request or per-user state in concurrent request handlers without blocking on a global mutex
Not For
- • Single-threaded applications — std HashMap is simpler and faster without concurrent overhead
- • Distributed caching across multiple processes — use Redis or distributed cache; DashMap is in-process only
- • Databases or persistent storage — DashMap is an in-memory data structure
Interface
Authentication
Library with no auth requirement.
Pricing
Free and open source.
Agent Metadata
Known Gotchas
- ⚠ DashMap uses reference-counted shards — holding a Ref<K, V> or RefMut<K, V> reference across an .await point causes deadlocks; never hold DashMap references across async boundaries
- ⚠ DashMap's entry() API returns an OccupiedEntry or VacantEntry — the reference holds a shard lock; operations must complete before the guard is dropped
- ⚠ iter() on DashMap holds per-shard locks during iteration — long iterations block concurrent writers to the same shard; collect first if iteration is slow
- ⚠ DashMap is not fairness-guaranteed for writers — heavy read workloads can starve writers; consider tokio::sync::RwLock for write-priority scenarios
- ⚠ Cloning a DashMap clones the entire contents — for large maps, prefer Arc<DashMap> to share ownership without copying
- ⚠ DashMap versions have changed the API between major versions — ensure dashmap dependency version matches your usage; version 5.x and 6.x have incompatible changes
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for DashMap (Rust).
Scores are editorial opinions as of 2026-03-06.