Go Cache

Go Cache

Generate two files for every cache: a port interface and a Redis-backed implementation.

When to Use

• Create a cache layer for any module
• Redis-backed TTL storage (OTP, sessions, OAuth state)
• Rate limiting storage
• Boolean flag caching (existence checks)
• JSON data caching (structured objects)

Which Variant?

Pick before writing anything:

For TTL:

• Fixed TTL — short-lived or individually written entries (OTPs, OAuth state, rate limits, sessions)
• Randomized TTL — long-lived entries written in bulk (activation flags, daily metrics) — prevents cache stampede

Two-File Pattern

Every cache requires exactly two files:

1. Port interface: internal/modules/<module>/ports/<cache_name>_cache.go 2. Cache implementation: internal/modules/<module>/cache/<cache_name>_cache.go

File Layout Order

1. Constants (key prefix, TTL) 2. Implementation struct (XxxCache) 3. Compile-time interface assertion 4. Constructor (NewXxxCache) 5. Methods (Set, Get, Delete) 6. Helper methods (buildKey, calculateTTL)

---

Boolean Flag Cache

Use when caching simple existence flags, presence checks, or rate limit states.

• Store "1" as the value
• Return false, nil when the key doesn't exist (not an error)

Port

package ports

import "context"

// XxxCache describes ...
type XxxCache interface {
	Set(ctx context.Context, id uint64) error
	Get(ctx context.Context, id uint64) (bool, error)
	Delete(ctx context.Context, id uint64) error
}

Implementation

package cache

import (
	"context"
	"errors"
	"fmt"
	"time"

	"github.com/cristiano-pacheco/bricks/pkg/redis"
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/ports"
	redislib "github.com/redis/go-redis/v9"
)

const (
	entityCacheKeyPrefix = "entity_name:"
	entityCacheTTL       = 10 * time.Minute
)

type EntityCache struct {
	redisClient redis.UniversalClient
}

var _ ports.EntityCache = (*EntityCache)(nil)

func NewEntityCache(redisClient redis.UniversalClient) *EntityCache {
	return &EntityCache{
		redisClient: redisClient,
	}
}

func (c *EntityCache) Set(ctx context.Context, id uint64) error {
	key := c.buildKey(id)
	return c.redisClient.Set(ctx, key, "1", entityCacheTTL).Err()
}

func (c *EntityCache) Get(ctx context.Context, id uint64) (bool, error) {
	key := c.buildKey(id)
	result := c.redisClient.Get(ctx, key)
	if err := result.Err(); err != nil {
		if errors.Is(err, redislib.Nil) {
			return false, nil
		}
		return false, err
	}
	return true, nil
}

func (c *EntityCache) Delete(ctx context.Context, id uint64) error {
	key := c.buildKey(id)
	return c.redisClient.Del(ctx, key).Err()
}

func (c *EntityCache) buildKey(id uint64) string {
	return fmt.Sprintf("%s%d", entityCacheKeyPrefix, id)
}

---

JSON Data Cache

Use when caching structured data. Data structs are defined in the dto package, never in ports.

• Serialize with json.Marshal before storing
• Deserialize with json.Unmarshal when retrieving
• Return nil, nil on missing key — unless the key is always expected to exist, in which case return a domain error (e.g., errs.ErrXxxNotFound)
• Use distinct variable names (getErr, unmarshalErr) to avoid shadowing

Port

package ports

import (
	"context"

	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/dto"
}

// XxxCache describes ...
type XxxCache interface {
	Set(ctx context.Context, key string, data dto.XxxData) error
	Get(ctx context.Context, key string) (dto.XxxData, error)
	Delete(ctx context.Context, key string) error
}

Implementation

package cache

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"time"

	"github.com/cristiano-pacheco/bricks/pkg/redis"
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/dto"
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/ports"
	redislib "github.com/redis/go-redis/v9"
)

const (
	entityCacheKeyPrefix = "entity_name:"
	entityCacheTTL       = 10 * time.Minute
)

type EntityCache struct {
	redisClient redis.UniversalClient
}

var _ ports.EntityCache = (*EntityCache)(nil)

func NewEntityCache(redisClient redis.UniversalClient) *EntityCache {
	return &EntityCache{
		redisClient: redisClient,
	}
}

func (c *EntityCache) Set(ctx context.Context, key string, data dto.EntityData) error {
	cacheKey := c.buildKey(key)
	jsonData, err := json.Marshal(data)

	if err != nil {
		return fmt.Errorf("marshal entity data: %w", err)
	}

	return c.redisClient.Set(ctx, cacheKey, jsonData, entityCacheTTL).Err()
}

func (c *EntityCache) Get(ctx context.Context, key string) (dto.EntityData, error) {
	cacheKey := c.buildKey(key)
	result := c.redisClient.Get(ctx, cacheKey)

	if getErr := result.Err(); getErr != nil {
		if errors.Is(getErr, redislib.Nil) {
			return dto.EntityData{}, nil
		}
		return dto.EntityData{}, getErr
	}

	jsonData, err := result.Bytes()

	if err != nil {
		return dto.EntityData{}, fmt.Errorf("get bytes: %w", err)
	}

	var entityData dto.EntityData
	if unmarshalErr := json.Unmarshal(jsonData, &entityData); unmarshalErr != nil {
		return dto.EntityData{}, fmt.Errorf("unmarshal entity data: %w", unmarshalErr)
	}
	
	return entityData, nil
}

func (c *EntityCache) Delete(ctx context.Context, key string) error {
	cacheKey := c.buildKey(key)
	return c.redisClient.Del(ctx, cacheKey).Err()
}

func (c *EntityCache) buildKey(key string) string {
	return entityCacheKeyPrefix + key
}

---

Key Building

String ID (simple concatenation):

func (c *EntityCache) buildKey(id string) string {
	return entityCacheKeyPrefix + id
}

Uint64 ID:

func (c *EntityCache) buildKey(id uint64) string {
	return fmt.Sprintf("%s%d", entityCacheKeyPrefix, id)
}

Composite key:

func (c *EntityCache) buildKey(userID uint64, resourceID string) string {
	return fmt.Sprintf("%s%d:%s", entityCacheKeyPrefix, userID, resourceID)
}

TTL Configuration

Fixed TTL — for short-lived data where stampede is not a concern:

const (
	entityCacheKeyPrefix = "entity_name:"
	entityCacheTTL       = 10 * time.Minute
)

Randomized TTL — for long-lived data created in bulk (prevents cache stampede):

import "math/rand"

const (
	entityCacheKeyPrefix = "entity_name:"
	entityCacheTTLMin    = 23 * time.Hour
	entityCacheTTLMax    = 25 * time.Hour
)

func (c *EntityCache) calculateTTL() time.Duration {
	min := entityCacheTTLMin.Milliseconds()
	max := entityCacheTTLMax.Milliseconds()
	randomMs := min + rand.Int63n(max-min+1)
	return time.Duration(randomMs) * time.Millisecond
}

Common TTL ranges:

• 5-15 minutes — OTP codes, OAuth state, rate limits
• 50-70 minutes — User sessions
• 12-25 hours — Activation flags, daily metrics
• 6.5-7.5 days — Weekly aggregations

Naming

• Port interface: XxxCache (ports package, no suffix)
• Implementation struct: XxxCache (cache package — same name, disambiguated by package)
• Constructor: NewXxxCache, returns *XxxCache
• Constants: lowercase, package-level (e.g. entityCacheKeyPrefix, entityCacheTTL)

Fx Wiring

Add to internal/modules/<module>/module.go:

fx.Provide(
	fx.Annotate(
		cache.NewXxxCache,
		fx.As(new(ports.XxxCache)),
	),
),

Dependencies

• redis.UniversalClient from "github.com/cristiano-pacheco/bricks/pkg/redis"
• redislib "github.com/redis/go-redis/v9" for nil detection

Critical Rules

1. No standalone functions: When a file contains a struct with methods, do not add standalone functions. Use private methods on the struct instead. 2. Two files: Port in ports/, implementation in cache/ 2. Interface assertion: var _ ports.XxxCache = (*XxxCache)(nil) immediately below the struct 3. Constructor: Returns *XxxCache (pointer) 4. Context: Always accept ctx context.Context as first parameter — never call context.Background() internally 5. Redis nil: Import redislib "github.com/redis/go-redis/v9" and check with errors.Is(err, redislib.Nil) 6. TTL scope: TTL is an implementation detail — never expose it as a method parameter 7. buildKey: Always use a buildKey() helper; + for string IDs, fmt.Sprintf for numeric IDs 8. Missing keys: Boolean cache returns false, nil; JSON cache returns nil, nil (or a domain error if the key must exist) 9. DTOs in dto package: Data structs belong in dto/, never defined inline in ports/ 10. No method comments: Only port interfaces get doc comments; implementation methods do not 11. Error messages: "action noun: %w" format (e.g., "marshal oauth state: %w", "get bytes: %w")

Workflow

1. Decide variant: Boolean flag or JSON data? 2. Create port interface in ports/<name>_cache.go 3. Create cache implementation in cache/<name>_cache.go 4. Add Fx wiring to module.go 5. Run make lint 6. Run make nilaway