db backend.

lib/bcoin/ldb.js

@@ -11,10 +11,26 @@ var db = {};
 module.exports = function ldb(name, options) {
   var levelup = require('levelup');
   var file = bcoin.prefix + '/' + name + '-' + network.type + '.db';
+  var backend = process.env.BCOIN_DB;
 
   bcoin.ensurePrefix();
 
   if (!db[file]) {
+    if (bcoin.isBrowser) {
+      backend = 'level-js';
+    } else {
+      if (!backend || backend === 'rocksdb')
+        backend = 'rocksdown';
+      else if (backend === 'leveldb')
+        backend = 'leveldown';
+      else if (backend === 'lmdb')
+        backend = 'lmdb';
+      else if (backend === 'tree')
+        backend = 'tree';
+      else
+        assert(false, 'Bad DB.');
+    }
+
     db[file] = new levelup(file, {
       keyEncoding: 'ascii',
       valueEncoding: 'binary',
@@ -30,9 +46,7 @@ module.exports = function ldb(name, options) {
       mapSize: options.mapSize || 150 * (1024 << 20),
       writeMap: options.writeMap || false,
 
-      db: bcoin.isBrowser
-        ? require('level-js')
-        : require('level' + 'down')
+      db: require(backend)
     });
   }