use of multiple cache storage systems is not supported

Error message

use of multiple cache storage systems is not supported

What it means

New() in the cortex chunk cache package rejects configurations that configure more than one remote cache backend. It supports at most one of memcached or Redis; configuring both memcached and a Redis endpoint makes the tiered cache ambiguous, so construction fails fast with this error.

Solutions

Remove the memcached section (memcache_client.host / memcache_client.addresses) from the config, keeping only redis.endpoint.
Alternatively remove the redis section if memcached is the intended backend.
Validate the effective config before startup so only one cache backend is populated.

Example fix

// before
memcache_client:
  host: memcached:11211
redis:
  endpoint: redis:6379
// after
redis:
  endpoint: redis:6379

Defensive patterns

Strategy: validation

Validate before calling

if (cfg.MemcacheClient.Host != "" || cfg.MemcacheClient.Addresses != "") && cfg.Redis.Endpoint != "" {
    return errors.New("only one cache backend (memcached or redis) may be configured")
}

Prevention

Keep exactly one cache backend section populated in config templates.
Run Validate() on the cache config in CI before deploys.
When migrating backends, remove the old section in the same change.

When it happens

Trigger: Calling New() (via NewResultsCacheMiddleware) with cfg.MemcacheClient.Host or cfg.MemcacheClient.Addresses set AND cfg.Redis.Endpoint set at the same time.

Common situations: Migrating from memcached to Redis while leaving the old memcached host/addresses in the YAML config; merging two teams' cache config blocks; copying a sample config that includes both sections.

Understand the failure class

Background: Conflicting config options: "cannot be used together" — configuration validation errors across open-source libraries — this error's family across 162 libraries.

Related errors

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/1ed198b631a1fc61. Report an issue: GitHub.

Appendix: source

Thrown at internal/cortex/chunk/cache/cache.go:86

func New(cfg Config, reg prometheus.Registerer, logger log.Logger) (Cache, error) {
	if cfg.Cache != nil {
		return cfg.Cache, nil
	}

	caches := []Cache{}

	if cfg.EnableFifoCache {
		if cfg.Fifocache.Validity == 0 && cfg.DefaultValidity != 0 {
			cfg.Fifocache.Validity = cfg.DefaultValidity
		}

		if cache := NewFifoCache(cfg.Prefix+"fifocache", cfg.Fifocache, reg, logger); cache != nil {
			caches = append(caches, Instrument(cfg.Prefix+"fifocache", cache, reg))
		}
	}

	if (cfg.MemcacheClient.Host != "" || cfg.MemcacheClient.Addresses != "") && cfg.Redis.Endpoint != "" {
		return nil, errors.New("use of multiple cache storage systems is not supported")
	}

	if cfg.MemcacheClient.Host != "" || cfg.MemcacheClient.Addresses != "" {
		if cfg.Memcache.Expiration == 0 && cfg.DefaultValidity != 0 {
			cfg.Memcache.Expiration = cfg.DefaultValidity
		}

		client := NewMemcachedClient(cfg.MemcacheClient, cfg.Prefix, reg, logger)
		cache := NewMemcached(cfg.Memcache, client, cfg.Prefix, reg, logger)

		cacheName := cfg.Prefix + "memcache"
		caches = append(caches, NewBackground(cacheName, cfg.Background, Instrument(cacheName, cache, reg), reg))
	}

	if cfg.Redis.Endpoint != "" {
		if cfg.Redis.Expiration == 0 && cfg.DefaultValidity != 0 {
			cfg.Redis.Expiration = cfg.DefaultValidity
		}

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