Caching

SLOzy implements a two-tier caching layer in internal/cache/ for Prometheus query results.

Architecture

Application → Memory Cache (sync.Map) → PostgreSQL (query_cache table)

The Service first checks the in-memory cache. On miss, it falls back to the database. Hits are promoted back into memory.

Cache Backend

The query_cache table (migrations/000018) stores:

Column	Type	Purpose
cache_key	VARCHAR(500) UNIQUE	query:{slo_id}:{time_window}:{sha256}
query_hash	VARCHAR(64)	SHA256 of query + parameters
result	JSONB	Cached Prometheus result
expires_at	TIMESTAMPTZ	TTL-based expiration
hit_count	INTEGER	Access frequency for hot/cold classification
is_fresh	BOOLEAN	Fresh vs. calculated data

Two repository implementations are available: SQLRepository (database/sql) and PgxRepository (pgx/v5 pool).

Configurable TTL

type CacheConfig struct {
    DefaultTTL            time.Duration // default 5 minutes
    MaxEntries            int           // max memory cache entries
    CleanupInterval       time.Duration // expired entry cleanup frequency
    EnableMemoryCache     bool
    EnableCompression     bool          // gzip compression for large results
    CompressionThreshold  int           // minimum bytes before compression
    EnableWarmup          bool
}

Invalidation Strategies

Strategy	Method	Scope
By key	Invalidate(ctx, cacheKey)	Single entry
By SLO ID	InvalidateBySLO(ctx, sloID)	All entries for an SLO
By pattern	InvalidateByPattern(ctx, pattern)	Key prefix match
By expiration	CleanupExpired(ctx)	All expired entries
Full flush	InvalidateAll(ctx)	Entire cache

LRU eviction runs automatically when MaxEntries is reached.

Statistics

type CacheStatistics struct {
    TotalEntries   int64
    TotalHits      int64
    TotalMisses    int64
    HitRate        float64
    MemoryUsage    int64   // MB
    HotEntriesCount  int64 // hit_count > 10
    ColdEntriesCount int64 // hit_count <= 10
}

API

Routes registered under /api/v1/cache/:

Endpoint	Description
GET /statistics	Cache performance metrics
GET /config	Current configuration (read-only)
PUT /config	Update configuration (TTL, maxEntries, compression)
GET /entries	Paginated list of cached entries
GET /entry/:key	Single entry by key
POST /entry	Manually create an entry
DELETE /entry/:key	Delete an entry
POST /invalidate	Invalidate by key/SLO/pattern/expired/all
POST /warmup	Preload cache with important queries

The Cache Management UI is available at /cache in the sidebar.