Add Redis database selection support for SingleFlight configuration

[jmooo]

Problem

Athens currently always connects to Redis database 0 when using Redis for SingleFlight coordination. There is no way to configure which database index to use.

Current configuration:

singleFlight:
  redis:
    endpoint: "redis.example.com:6379"
    password: "secret"
    # No database parameter available

This limitation makes it difficult to run Athens alongside other applications that share the same Redis instance, as all applications compete for keys in database 0.

Proposed Solution

Add a database parameter to the Redis SingleFlight configuration:

singleFlight:
  redis:
    endpoint: "redis.example.com:6379"
    password: "secret"
    database: 4  # Specify Redis database index (0-15)

Use Case

We're deploying Athens alongside Harbor in a shared Valkey (Redis-compatible) cluster. Our database allocation looks like:

Application	Database	Purpose
Harbor Core	DB 0	Registry metadata cache
Harbor JobService	DB 1	Job queue
Harbor Registry	DB 2	Layer cache
Harbor Trivy	DB 3	Vulnerability scan cache
Athens	DB 0	Currently forced to share with Harbor

Without database selection, Athens shares DB 0 with Harbor Core, which:

• Mixes different application keys in the same keyspace
• Makes troubleshooting difficult (can't separate Athens keys from Harbor keys)
• Prevents setting database-specific Redis policies (memory limits, eviction)
• Reduces operational clarity

Benefits

1. Multi-Tenancy: Support shared Redis instances across multiple applications
2. Isolation: Clear separation of concerns between applications
3. Observability: Database-level metrics via INFO keyspace command
4. Resource Management: Per-database memory limits and eviction policies
5. Best Practices: Follows standard Redis operational patterns

Implementation Scope

This feature would require changes in two repositories:

1. Athens Core (gomods/athens)

File: pkg/config/singleflight.go

Add Database field to Redis configuration:

type Redis struct {
    Endpoint   string            `envconfig:"ATHENS_REDIS_ENDPOINT"`
    Password   string            `envconfig:"ATHENS_REDIS_PASSWORD"`
    Database   int               `envconfig:"ATHENS_REDIS_DATABASE" default:"0"`  // NEW
    LockConfig *RedisLockConfig
}

Redis client initialization (wherever redis.NewClient is called):

client := redis.NewClient(&redis.Options{
    Addr:     config.Endpoint,
    Password: config.Password,
    DB:       config.Database,  // NEW: Use configured database
})

The go-redis v8 library (which Athens uses) already supports the DB option.

2. Athens Helm Chart (gomods/athens-charts)

File: charts/athens-proxy/values.yaml

singleFlight:
  redis:
    endpoint: ""
    password: ""
    database: 0  # NEW: Redis database index (0-15), defaults to 0
    lockConfig: {}

File: charts/athens-proxy/templates/deployment.yaml

Add environment variable:

{{- if .Values.singleFlight.redis.database }}
- name: ATHENS_REDIS_DATABASE
  value: {{ .Values.singleFlight.redis.database | quote }}
{{- end }}

Backwards Compatibility

✅ Fully backwards compatible - The parameter defaults to 0 (Redis default behavior), so existing deployments continue working without any changes.

Alternative Considered: Key Prefixes

We considered adding configurable key prefixes (e.g., athens:lock:{module}) instead of database selection. However, database selection is preferred because:

• Cleaner separation (Redis native feature)
• Better observability (INFO keyspace shows per-DB stats)
• Simpler implementation (single config parameter)
• Follows standard Redis operational practices
• Explicit isolation guarantees

Related Work

This is similar to issue #111 and PR #112, which improved SingleFlight Redis configuration by moving passwords to secrets. This would be a natural extension of that work.

Documentation Needs

• Update docs/content/configuration/singleflight.md with database parameter
• Add example to config.dev.toml
• Update Helm chart README with database configuration example

References

• Athens uses github.com/go-redis/redis/v8 which supports database selection via DB option: https://pkg.go.dev/github.com/go-redis/redis/v8#Options
• Athens Helm chart SingleFlight config: https://github.com/gomods/athens-charts/blob/main/charts/athens-proxy/values.yaml#L109-L113
• Athens config struct: https://github.com/gomods/athens/blob/main/pkg/config/singleflight.go

Willingness to Contribute

I'm willing to contribute a PR for this feature if the maintainers are open to it. The implementation is straightforward and I've already mapped out the required changes.

[DrPsychick]

Hi @jmooo , these suggest changes also make sense, feel free to provide a PR.