ipfs/kubo · error

'prefix' field was missing or not a string

Error message

'prefix' field was missing or not a string

What it means

MeasureDatastoreConfig was asked to build a 'measure' datastore spec whose 'child' parsed fine but the spec has no (or a non-string) 'prefix' field; the measure wrapper needs the prefix to namespace its metrics.

Source

Thrown at repo/fsrepo/datastores.go:231

type measureDatastoreConfig struct {
	child  DatastoreConfig
	prefix string
}

// MeasureDatastoreConfig returns a measure DatastoreConfig from a spec.
func MeasureDatastoreConfig(params map[string]any) (DatastoreConfig, error) {
	childField, ok := params["child"].(map[string]any)
	if !ok {
		return nil, fmt.Errorf("'child' field is missing or not a map")
	}
	child, err := AnyDatastoreConfig(childField)
	if err != nil {
		return nil, err
	}
	prefix, ok := params["prefix"].(string)
	if !ok {
		return nil, fmt.Errorf("'prefix' field was missing or not a string")
	}
	return &measureDatastoreConfig{child, prefix}, nil
}

func (c *measureDatastoreConfig) DiskSpec() DiskSpec {
	return c.child.DiskSpec()
}

func (c measureDatastoreConfig) Create(path string) (repo.Datastore, error) {
	child, err := c.child.Create(path)
	if err != nil {
		return nil, err
	}
	return measure.New(c.prefix, child), nil
}

View on GitHub (pinned to 329838acdf)

Solutions

1. Add a string 'prefix' field to the measure datastore entry in Datastore.Spec
2. Example: {"type":"measure","prefix":"flatfs.datastore","child":{...}}

Example fix

// before
spec := map[string]any{"type": "measure", "child": childSpec}
// after
spec := map[string]any{"type": "measure", "prefix": "disk", "child": childSpec}

p, ok := params["prefix"].(string)
if !ok || p == "" { /* add prefix before calling */ }

func measurePrefix(params map[string]any) (string, bool) {
	p, ok := params["prefix"].(string)
	return p, ok && p != ""
}

ds, err := fsrepo.MeasureDatastoreConfig(spec)
if err != nil {
	if strings.Contains(err.Error(), "'prefix' field") {
		return fmt.Errorf("measure spec requires a prefix: %w", err)
	}
	return err
}

When it happens

Trigger: Calling MeasureDatastoreConfig with a valid "child" but where params["prefix"] is absent, nil, or a non-string (e.g. int).

Common situations: Spec of type "measure" where "prefix" was forgotten after adding "child"; prefix supplied as a non-string in hand-built maps; renamed key in generated specs.

Related errors

• 'mounts' field is missing or not an array
• expected map for mountpoint
• no 'mountpoint' on mount
• 'name' field was missing or not a string
• Provide.DHT.KeystoreBatchSize must be positive, got %d

AI-assisted analysis of ipfs/kubo@329838acdf (2026-09-03). Data as JSON: /api/errors/a8589eeb347d4aec. Report an issue: GitHub.