diff --git a/README.md b/README.md
index a66aee9..8b2217d 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,8 @@ An in-process, in-memory database for Go.
 
 ## TO DO
 
-* [ ] kvstore: tests
+* [ ] mdb: db exclusive lock
+* [ ] mdb: clean up tests
 
 ## Structure
 
diff --git a/database.go b/database.go
index 25e2ee7..a1c7243 100644
--- a/database.go
+++ b/database.go
@@ -3,35 +3,49 @@ package mdb
 import (
 	"net"
 	"os"
+	"path/filepath"
 	"sync"
 
 	"git.crumpington.com/private/mdb/kvstore"
+	"golang.org/x/sys/unix"
 )
 
 type Database struct {
 	root string
+	lock *os.File
 	kv   *kvstore.KV
 
 	collections map[string]dbCollection
 }
 
 func NewPrimary(root string) *Database {
-	must(os.MkdirAll(root, 0700))
-	db := &Database{
-		root:        root,
-		collections: map[string]dbCollection{},
-	}
-	db.kv = kvstore.NewPrimary(root)
-	return db
+	return newDB(root, true)
 }
 
 func NewSecondary(root string) *Database {
+	return newDB(root, false)
+}
+
+func newDB(root string, primary bool) *Database {
 	must(os.MkdirAll(root, 0700))
+
+	lockPath := filepath.Join(root, "lock")
+
+	// Acquire the lock.
+	lock, err := os.OpenFile(lockPath, os.O_RDWR|os.O_CREATE, 0600)
+	must(err)
+	must(unix.Flock(int(lock.Fd()), unix.LOCK_EX))
+
 	db := &Database{
 		root:        root,
 		collections: map[string]dbCollection{},
+		lock:        lock,
+	}
+	if primary {
+		db.kv = kvstore.NewPrimary(root)
+	} else {
+		db.kv = kvstore.NewSecondary(root, db.onStore, db.onDelete)
 	}
-	db.kv = kvstore.NewSecondary(root, db.onStore, db.onDelete)
 	return db
 }
 
@@ -55,9 +69,11 @@ func (db *Database) WALStatus() (ws WALStatus) {
 
 func (db *Database) Close() {
 	db.kv.Close()
+	db.lock.Close()
 }
 
-// ----------------------------------------------------------------------------
+// -----------------
+		db.kv = kvstore.NewSecondary(root)-----------------------------------------------------------
 
 func (db *Database) onStore(collection string, id uint64, data []byte) {
 	c, ok := db.collections[collection]