Name: golang-samber-hot
Author: samber

SKILL.md

$27

Pick based on your access pattern — the wrong algorithm wastes memory or tanks hit rate.

Algorithm

Constant

Best for

Avoid when

W-TinyLFU

hot.WTinyLFU

General-purpose, mixed workloads (default)

You need simplicity for debugging

LRU

hot.LRU

Recency-dominated (sessions, recent queries)

Frequency matters (scan pollution evicts hot items)

LFU

hot.LFU

Frequency-dominated (popular products, DNS)

Access patterns shift (stale popular items never evict)

TinyLFU

hot.TinyLFU

Read-heavy with frequency bias

Write-heavy (admission filter overhead)

S3FIFO

hot.S3FIFO

High throughput, scan-resistant

Small caches (<1000 items)

ARC

hot.ARC

Self-tuning, unknown patterns

Memory-constrained (2x tracking overhead)

TwoQueue

hot.TwoQueue

Mixed with hot/cold split

Tuning complexity is unacceptable

SIEVE

hot.SIEVE

Simple scan-resistant LRU alternative

Highly skewed access patterns

FIFO

hot.FIFO

Simple, predictable eviction order

Hit rate matters (no frequency/recency awareness)

Decision shortcut: Start with hot.WTinyLFU. Switch only when profiling shows the miss rate is too high for your SLO.

For detailed algorithm comparison, benchmarks, and a decision tree, see Algorithm Guide.

Core Usage

Basic Cache with TTL

import "github.com/samber/hot"

cache := hot.NewHotCache[string, *User](hot.WTinyLFU, 10_000).

    WithTTL(5 * time.Minute).

    WithJanitor().

    Build()

defer cache.StopJanitor()

cache.Set("user:123", user)

cache.SetWithTTL("session:abc", session, 30*time.Minute)

value, found, err := cache.Get("user:123")

Loader Pattern (Read-Through)

Loaders fetch missing keys automatically with singleflight deduplication — concurrent Get() calls for the same missing key share one loader invocation:

cache := hot.NewHotCache[int, *User](hot.WTinyLFU, 10_000).

    WithTTL(5 * time.Minute).

    WithLoaders(func(ids []int) (map[int]*User, error) {

        return db.GetUsersByIDs(ctx, ids) // batch query

    }).

    WithJanitor().

    Build()

defer cache.StopJanitor()

user, found, err := cache.Get(123) // triggers loader on miss

Capacity Sizing

Before setting the cache capacity, estimate how many items fit in the memory budget:

Estimate single-item size — estimate size of the struct, add the size of heap-allocated fields (slices, maps, strings). Include the key size. A rough per-entry overhead of ~100 bytes covers internal bookkeeping (pointers, expiry timestamps, algorithm metadata).

Ask the developer how much memory is dedicated to this cache in production (e.g., 256 MB, 1 GB). This depends on the service's total memory and what else shares the process.

Compute capacity — capacity = memoryBudget / estimatedItemSize. Round down to leave headroom.

Example: *User struct ~500 bytes + string key ~50 bytes + overhead ~100 bytes = ~650 bytes/entry

         256 MB budget → 256_000_000 / 650 ≈ 393,000 items

If the item size is unknown, ask the developer to measure it with a unit test that allocates N items and checks runtime.ReadMemStats. Guessing capacity without measuring leads to OOM or wasted memory.

Common Mistakes

**Forgetting WithJanitor()** — without it, expired entries stay in memory until the algorithm evicts them. Always chain .WithJanitor() in the builder and defer cache.StopJanitor().

**Calling SetMissing() without missing cache config** — panics at runtime. Enable WithMissingCache(algorithm, capacity) or WithMissingSharedCache() in the builder first.

**WithoutLocking() + WithJanitor()** — mutually exclusive, panics. WithoutLocking() is only safe for single-goroutine access without background cleanup.

Oversized cache — a cache holding everything is a map with overhead. Size to your working set (typically 10-20% of total data). Monitor hit rate to validate.

Ignoring loader errors — Get() returns (zero, false, err) on loader failure. Always check err, not just found.

Best Practices

Always set TTL — unbounded caches serve stale data indefinitely because there is no signal to refresh

Use WithJitter(lambda, upperBound) to spread expirations — without jitter, items created together expire together, causing thundering herd on the loader

Monitor with WithPrometheusMetrics(cacheName) — hit rate below 80% usually means the cache is undersized or the algorithm is wrong for the workload

Use WithCopyOnRead(fn) / WithCopyOnWrite(fn) for mutable values — without copies, callers mutate cached objects and corrupt shared state

For advanced patterns (revalidation, sharding, missing cache, monitoring setup), see Production Patterns.

For the complete API surface, see API Reference.

If you encounter a bug or unexpected behavior in samber/hot, open an issue at https://github.com/samber/hot/issues.

Cross-References

→ See samber/cc-skills-golang@golang-performance skill for general caching strategy and when to use in-memory cache vs Redis vs CDN

→ See samber/cc-skills-golang@golang-observability skill for Prometheus metrics integration and monitoring

→ See samber/cc-skills-golang@golang-database skill for database query patterns that pair with cache loaders

→ See samber/cc-skills@promql-cli skill for querying Prometheus cache metrics via CLI

golang-samber-hot