dgraph-io/badger · error

Cannot ban namespace in read-only mode.

Error message

Cannot ban namespace in read-only mode.

What it means

BanNamespace panics when the DB is opened in read-only mode. Banning a namespace writes a marker entry (under bannedNsKey) to the database, which requires write access; read-only handles cannot perform that write, so the library panics to prevent an invalid state.

Source

Thrown at db.go:1956

	if db.opt.NamespaceOffset < 0 {
		return nil
	}
	if len(key) <= db.opt.NamespaceOffset+8 {
		return nil
	}
	if db.bannedNamespaces.has(y.BytesToU64(key[db.opt.NamespaceOffset:])) {
		return ErrBannedKey
	}
	return nil
}

// BanNamespace bans a namespace. Read/write to keys belonging to any of such namespace is denied.
func (db *DB) BanNamespace(ns uint64) error {
	if db.opt.NamespaceOffset < 0 {
		return ErrNamespaceMode
	}
	if db.opt.ReadOnly {
		panic("Cannot ban namespace in read-only mode.")
	}
	db.opt.Infof("Banning namespace: %d", ns)
	// First set the banned namespaces in DB and then update the in-memory structure.
	key := y.KeyWithTs(append(bannedNsKey, y.U64ToBytes(ns)...), 1)
	entry := []*Entry{{
		Key:   key,
		Value: nil,
	}}
	req, err := db.sendToWriteCh(entry)
	if err != nil {
		return err
	}
	if err := req.Wait(); err != nil {
		return err
	}
	db.bannedNamespaces.add(ns)
	return nil
}

View on GitHub (pinned to 2a001d466f)

Solutions

1. Open a writable DB instance to call BanNamespace
2. Guard the call: skip or return an error when the DB options are read-only
3. Persist banned namespaces through your own config store and apply them only on writable instances

Example fix

// before
if err := roDB.BanNamespace(ns); err != nil { ... }

// after
if !roDB.IsClosed() && !opts.ReadOnly {
    if err := db.BanNamespace(ns); err != nil { ... }
}

if opts.ReadOnly { return errors.New("BanNamespace requires a writable DB") }

When it happens

Trigger: Calling db.BanNamespace(ns) on a DB opened with badger.Options.ReadOnly = true. Note it also panics-free returns ErrNamespaceMode if NamespaceOffset < 0 — the read-only panic is hit only when namespace mode is configured.

Common situations: Multi-tenant apps where a reader replica instance is asked to ban a tenant; a shared DB wrapper opened read-only for queries but also wired to admin handlers; forgetting that namespace bans are persisted writes, not in-memory-only flags.

Related errors

• Cannot use GetSequence in read-only mode.
• Cannot flatten in read-only mode.
• Attempting to drop data in read-only mode.
• ErrReadOnlyTxn
• ErrNamespaceMode

AI-assisted analysis of dgraph-io/badger@2a001d466f (2026-09-05). Data as JSON: /api/errors/a625a8b9f43c731b. Report an issue: GitHub.