seikaikyo/golang-samber-hot

Persona: You are a Go engineer who treats caching as a system design decision. You choose eviction algorithms based on measured access patterns, size caches from working-set data, and always plan for expiration, loader failures, and monitoring.

Using samber/hot for In-Memory Caching in Go

Generic, type-safe in-memory caching library for Go 1.22+ with 9 eviction algorithms, TTL, loader chains with singleflight deduplication, sharding, stale-while-revalidate, and Prometheus metrics.

Official Resources:

• pkg.go.dev/github.com/samber/hot
• github.com/samber/hot

This skill is not exhaustive. Please refer to library documentation and code examples for more information. For Go package docs, symbols, versions, importers, and known vulnerabilities, → See samber/cc-skills-golang@golang-pkg-go-dev skill (godig) — prefer it over Context7 for Go package facts. To navigate this library's usage in your own code (definitions, call sites, diagnostics), → See samber/cc-skills-golang@golang-gopls skill (gopls). Context7 remains a fallback for docs not indexed on pkg.go.dev.

go get -u github.com/samber/hot

Algorithm Selection

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:

1. 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).
2. 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.
3. 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

1. Forgetting WithJanitor() — without it, expired entries stay in memory until the algorithm evicts them. Always chain .WithJanitor() in the builder and defer cache.StopJanitor().
2. Calling SetMissing() without missing cache config — panics at runtime. Enable WithMissingCache(algorithm, capacity) or WithMissingSharedCache() in the builder first.
3. WithoutLocking() + WithJanitor() — mutually exclusive, panics. WithoutLocking() is only safe for single-goroutine access without background cleanup.
4. 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.
5. Ignoring loader errors — Get() returns (zero, false, err) on loader failure. Always check err, not just found.

Best Practices

1. Always set TTL — unbounded caches serve stale data indefinitely because there is no signal to refresh
2. Use WithJitter(lambda, upperBound) to spread expirations — without jitter, items created together expire together, causing thundering herd on the loader
3. Monitor with WithPrometheusMetrics(cacheName) — hit rate below 80% usually means the cache is undersized or the algorithm is wrong for the workload
4. 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