once_cell / LazyLock & dashmap

LazyLock (stdlib since Rust 1.80) initializes globals on first access. dashmap is a concurrent HashMap for caches and registries without a global mutex.

Recipe

use std::sync::LazyLock;
 
static CONFIG: LazyLock<String> = LazyLock::new(|| {
    std::env::var("APP_NAME").unwrap_or_else(|_| "app".into())
});

dashmap = "6"

use dashmap::DashMap;
use std::sync::Arc;
 
fn main() {
    let cache: Arc<DashMap<String, u64>> = Arc::new(DashMap::new());
    cache.insert("user:1".into(), 42);
    assert_eq!(*cache.get("user:1").unwrap(), 42);
}

When to reach for this:

• Process-wide HTTP clients, regex sets, config snapshots
• In-memory rate-limit or session tables (with eviction policy)
• Registry of named services in long-lived daemons

Working Example

use std::sync::LazyLock;
use dashmap::DashMap;
 
static METRICS: LazyLock<DashMap<&'static str, u64>> =
    LazyLock::new(DashMap::new);
 
fn inc(name: &'static str) {
    *METRICS.entry(name).or_insert(0) += 1;
}
 
fn main() {
    inc("requests");
    inc("requests");
    assert_eq!(*METRICS.get("requests").unwrap(), 2);
}

What this demonstrates:

• LazyLock replaces once_cell::sync::Lazy for new code
• DashMap allows concurrent insert/get
• Prefer Arc<DashMap> in Axum state for testability

Deep Dive

LazyLock vs once_cell

DashMap vs RwLock<HashMap>

DashMap shards locks internally; better for many readers/writers on independent keys. Single-key hot spots still contend.

Gotchas

• Mutable static without sync - compile error or UB. Fix: LazyLock, Mutex, or DashMap.
• DashMap as authoritative store - data lost on restart. Fix: persist to DB; cache only.
• Unbounded cache growth - OOM. Fix: TTL, LRU layer, or moka crate.
• LazyLock panics in init - fails at first access. Fix: return Result from inner setup or use OnceLock<Result<T>>.
• Iterating DashMap while mutating - deadlocks possible. Fix: snapshot keys first.

Alternatives

FAQs

Related

• Arc for shared state
• Mutex and RwLock
• Shared state in async
• Essential Crates Best Practices

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

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