thanos-io/thanos · error

marshal content of cache backend configuration

Error message

marshal content of cache backend configuration

What it means

After extracting the generic cache backend blob, NewCacheConfig re-marshals cacheConfig.Config to YAML for backend-specific unmarshaling. Failure to re-marshal indicates the parsed raw config cannot be serialized, wrapped as 'marshal content of cache backend configuration'.

Solutions

1. Ensure cacheConfig.Config holds a map or yaml-supported value; check the input YAML 'config' section is a mapping, not a scalar or list.
2. Verify the Thanos version; older versions had limited Config types — upgrade if using a new backend.
3. If constructing config in Go, populate Config with a yaml.Marshal-encodable type (map[string]interface{}).

Example fix

# before (config must be a mapping)
type: REDIS
config: redis://localhost
# after
type: REDIS
config:
  endpoints: [localhost:6379]

var p struct {
    Type   string      `yaml:"type"`
    Config interface{} `yaml:"config"`
}
if err := yaml.UnmarshalStrict(data, &p); err != nil { return err }
if _, ok := p.Config.(map[interface{}]interface{}); !ok {
    return fmt.Errorf("cache config section must be a mapping")
}

cfg, err := queryfrontend.NewCacheConfig(logger, yamlBytes)
if err != nil {
    if strings.Contains(err.Error(), "marshal content of cache backend configuration") {
        return fmt.Errorf("cache backend 'config' section must be a YAML mapping: %w", err)
    }
    return err
}

When it happens

Trigger: yaml.Marshal(cacheConfig.Config) fails after a successful strict unmarshal of the provider config — typically a nil or yaml-incompatible value in the Config field.

Common situations: Custom or programmatically built cache provider configs holding types yaml.Marshal cannot encode; rarely hit via plain YAML files, more common when constructing config in code or tests.

Related errors

• parsing config YAML file
• initializing the query range cache config
• initializing the labels cache config
• response cache with type
• invalid ResultsCache config for labels tripperware

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/d4b1ca1299fcc11e. Report an issue: GitHub.

Appendix: source

Thrown at pkg/queryfrontend/config.go:97

}

// CacheProviderConfig is the initial CacheProviderConfig struct holder before parsing it into a specific cache provider.
// Based on the config type the config is then parsed into a specific cache provider.
type CacheProviderConfig struct {
	Type   ResponseCacheProvider `yaml:"type"`
	Config any                   `yaml:"config"`
}

// NewCacheConfig is a parser that converts a Thanos cache config yaml into a cortex cache config struct.
func NewCacheConfig(logger log.Logger, confContentYaml []byte) (*cortexcache.Config, error) {
	cacheConfig := &CacheProviderConfig{}
	if err := yaml.UnmarshalStrict(confContentYaml, cacheConfig); err != nil {
		return nil, errors.Wrap(err, "parsing config YAML file")
	}

	backendConfig, err := yaml.Marshal(cacheConfig.Config)
	if err != nil {
		return nil, errors.Wrap(err, "marshal content of cache backend configuration")
	}

	switch strings.ToUpper(string(cacheConfig.Type)) {
	case string(INMEMORY):
		var config InMemoryResponseCacheConfig
		if err := yaml.Unmarshal(backendConfig, &config); err != nil {
			return nil, err
		}

		return &cortexcache.Config{
			EnableFifoCache: true,
			Fifocache: cortexcache.FifoCacheConfig{
				MaxSizeBytes: config.MaxSize,
				MaxSizeItems: config.MaxSizeItems,
				Validity:     config.Validity,
			},
		}, nil
	case string(MEMCACHED):

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