diff --git a/lib/bcoin/ldb.js b/lib/bcoin/ldb.js
index 4dacb10a..6d2e2087 100644
--- a/lib/bcoin/ldb.js
+++ b/lib/bcoin/ldb.js
@@ -12,12 +12,15 @@ var db = {};
 
 module.exports = function ldb(name, options) {
   var file = bcoin.prefix + '/' + name + '-' + network.type + '.db';
-  var backend = process.env.BCOIN_DB;
+  var backend = typeof options.db === 'string'
+    ? options.db
+    : process.env.BCOIN_DB;
 
   if (!db[file]) {
-    if (options.db) {
+    if (options.db && typeof options.db !== 'string') {
+      bcoin.ensurePrefix();
       backend = options.db;
-    } else if (bcoin.isBrowser) {
+    } else if (bcoin.isBrowser && backend !== 'memdown') {
       backend = require('level-js');
     } else {
       if (!backend || backend === 'leveldb')
@@ -26,6 +29,11 @@ module.exports = function ldb(name, options) {
         backend = 'rocksdown';
       else if (backend === 'lmdb')
         backend = 'lmdb';
+      else if (backend === 'memory')
+        backend = 'memdown';
+
+      if (backend !== 'memdown')
+        bcoin.ensurePrefix();
 
       backend = require(backend);
     }
@@ -33,8 +41,6 @@ module.exports = function ldb(name, options) {
     if (!options)
       options = {};
 
-    bcoin.ensurePrefix();
-
     db[file] = new LowlevelUp(file, {
       keyEncoding: 'ascii',
       valueEncoding: 'binary',
diff --git a/lib/bcoin/mempool.js b/lib/bcoin/mempool.js
index 16f38f96..b6f7b8fd 100644
--- a/lib/bcoin/mempool.js
+++ b/lib/bcoin/mempool.js
@@ -29,6 +29,13 @@ function Mempool(node, options) {
   this.node = node;
   this.chain = node.chain;
   this.db = node.chain.db;
+
+  if (this.options.memory) {
+    this.db = bcoin.ldb('mempool', {
+      db: 'memdown'
+    });
+  }
+
   this.tx = new bcoin.txdb('m', this.db, {
     indexSpent: true,
     indexExtra: false,
@@ -100,7 +107,7 @@ Mempool.prototype._lock = function _lock(func, args, force) {
 
     self.busy = false;
 
-    if (func === Chain.prototype.add) {
+    if (func === Mempool.prototype.add) {
       if (self.pending.length === 0)
         self.emit('flush');
     }
diff --git a/lib/bcoin/txdb.js b/lib/bcoin/txdb.js
index 71d24bfb..22a54cad 100644
--- a/lib/bcoin/txdb.js
+++ b/lib/bcoin/txdb.js
@@ -182,7 +182,7 @@ TXPool.prototype.mapAddresses = function mapAddresses(address, callback) {
   })();
 };
 
-TXPool.prototype._addOrphan = function add(key, hash, index, callback) {
+TXPool.prototype._addOrphan = function _addOrphan(key, hash, index, callback) {
   var prefix = this.prefix + '/';
   var orphans;
 
@@ -243,12 +243,12 @@ TXPool.prototype._getOrphans = function _getOrphans(key, callback) {
   });
 };
 
-TXPool.prototype.add = function add(tx, callback) {
+TXPool.prototype.add = function add(tx, callback, force) {
   var self = this;
 
   if (Array.isArray(tx)) {
     return utils.forEachSerial(tx, function(tx, next) {
-      self.add(tx, next);
+      self.add(tx, next, force);
     }, callback);
   }
 
@@ -261,7 +261,7 @@ TXPool.prototype.add = function add(tx, callback) {
         return callback(null, false);
     }
 
-    return self._add(tx, map, callback);
+    return self._add(tx, map, callback, force);
   });
 };
 
@@ -281,20 +281,16 @@ TXPool.prototype._add = function add(tx, map, callback, force) {
   if (!unlock)
     return;
 
-  function done(err, result) {
-    unlock();
-    if (callback)
-      callback(err, result);
-  }
+  callback = utils.wrap(callback, unlock);
 
   // Attempt to confirm tx before adding it.
   this._confirm(tx, map, function(err, existing) {
     if (err)
-      return done(err);
+      return callback(err);
 
     // Ignore if we already have this tx.
     if (existing)
-      return done(null, true);
+      return callback(null, true);
 
     batch = self.db.batch();
 
@@ -353,7 +349,7 @@ TXPool.prototype._add = function add(tx, map, callback, force) {
 
           // Skip invalid transactions
           if (!tx.verify(i))
-            return done(null, false);
+            return callback(null, false);
 
           updated = true;
 
@@ -390,16 +386,16 @@ TXPool.prototype._add = function add(tx, map, callback, force) {
 
         self.isSpent(input.prevout.hash, input.prevout.index, function(err, result) {
           if (err)
-            return done(err);
+            return callback(err);
 
           // Are we double-spending?
           if (result)
-            return done(new Error('Transaction is double-spending.'));
+            return callback(new Error('Transaction is double-spending.'));
 
           // Add orphan, if no parent transaction is yet known
           self._addOrphan(key, hash, i, function(err, orphans) {
             if (err)
-              return done(err);
+              return callback(err);
 
             batch.put(prefix + 'o/' + key, orphans);
 
@@ -409,7 +405,7 @@ TXPool.prototype._add = function add(tx, map, callback, force) {
       });
     }, function(err) {
       if (err)
-        return done(err);
+        return callback(err);
 
       // Add unspent outputs or resolve orphans
       utils.forEachSerial(tx.outputs, function(output, next, i) {
@@ -429,7 +425,7 @@ TXPool.prototype._add = function add(tx, map, callback, force) {
           var some = false;
 
           if (err)
-            return done(err);
+            return callback(err);
 
           if (!orphans)
             return finish();
@@ -459,7 +455,7 @@ TXPool.prototype._add = function add(tx, map, callback, force) {
               if (err)
                 return next(err);
               return next();
-            });
+            }, true);
           }, function(err) {
             if (err)
               return next(err);
@@ -493,11 +489,11 @@ TXPool.prototype._add = function add(tx, map, callback, force) {
         });
       }, function(err) {
         if (err)
-          return done(err);
+          return callback(err);
 
         batch.write(function(err) {
           if (err)
-            return done(err);
+            return callback(err);
 
           self.emit('tx', tx, map);
 
@@ -508,11 +504,11 @@ TXPool.prototype._add = function add(tx, map, callback, force) {
             self.emit('updated', tx, map);
           }
 
-          return done(null, true);
+          return callback(null, true);
         });
       });
     });
-  });
+  }, true);
 };
 
 TXPool.prototype.isSpent = function isSpent(hash, index, callback, checkCoin) {
@@ -548,12 +544,18 @@ TXPool.prototype.isSpent = function isSpent(hash, index, callback, checkCoin) {
   });
 };
 
-TXPool.prototype._confirm = function _confirm(tx, map, callback) {
+TXPool.prototype._confirm = function _confirm(tx, map, callback, force) {
   var self = this;
   var prefix = this.prefix + '/';
   var hash = tx.hash('hex');
   var batch;
 
+  var unlock = this._lock(_confirm, [tx, map, callback], force);
+  if (!unlock)
+    return;
+
+  callback = utils.wrap(callback, unlock);
+
   this.getTX(hash, function(err, existing) {
     if (err)
       return callback(err);
@@ -634,16 +636,16 @@ TXPool.prototype._confirm = function _confirm(tx, map, callback) {
   });
 };
 
-TXPool.prototype.remove = function remove(hash, callback) {
+TXPool.prototype.remove = function remove(hash, callback, force) {
   var self = this;
   var first = Array.isArray(hash) ? hash[0] : hash;
 
   if (!this.options.indexExtra && (first instanceof bcoin.tx))
-    return this.lazyRemove(hash, callback);
+    return this.lazyRemove(hash, callback, force);
 
   if (Array.isArray(hash)) {
     return utils.forEachSerial(hash, function(hash, next) {
-      self.remove(hash, next);
+      self.remove(hash, next, force);
     }, callback);
   }
 
@@ -668,17 +670,17 @@ TXPool.prototype.remove = function remove(hash, callback) {
           return callback(null, false);
       }
 
-      return self._remove(tx, map, callback);
+      return self._remove(tx, map, callback, force);
     });
   });
 };
 
-TXPool.prototype.lazyRemove = function lazyRemove(tx, callback) {
+TXPool.prototype.lazyRemove = function lazyRemove(tx, callback, force) {
   var self = this;
 
   if (Array.isArray(tx)) {
     return utils.forEachSerial(tx, function(tx, next) {
-      self.lazyRemove(tx, next);
+      self.lazyRemove(tx, next, force);
     }, callback);
   }
 
@@ -691,15 +693,23 @@ TXPool.prototype.lazyRemove = function lazyRemove(tx, callback) {
         return callback(null, false);
     }
 
-    return self._remove(tx, map, callback);
+    return self._remove(tx, map, callback, force);
   });
 };
 
-TXPool.prototype._remove = function remove(tx, map, callback) {
+TXPool.prototype._remove = function remove(tx, map, callback, force) {
   var self = this;
   var prefix = this.prefix + '/';
   var hash = tx.hash('hex');
-  var batch = this.db.batch();
+  var batch;
+
+  var unlock = this._lock(remove, [tx, map, callback], force);
+  if (!unlock)
+    return;
+
+  callback = utils.wrap(callback, unlock);
+
+  batch = this.db.batch();
 
   batch.del(prefix + 't/t/' + hash);
 
@@ -795,12 +805,12 @@ TXPool.prototype._remove = function remove(tx, map, callback) {
   });
 };
 
-TXPool.prototype.unconfirm = function unconfirm(hash, callback) {
+TXPool.prototype.unconfirm = function unconfirm(hash, callback, force) {
   var self = this;
 
   if (Array.isArray(hash)) {
     return utils.forEachSerial(hash, function(hash, next) {
-      self.unconfirm(hash, next);
+      self.unconfirm(hash, next, force);
     }, callback);
   }
 
@@ -827,18 +837,26 @@ TXPool.prototype.unconfirm = function unconfirm(hash, callback) {
           return callback(null, false);
       }
 
-      return self._unconfirm(tx, map, callback);
+      return self._unconfirm(tx, map, callback, force);
     });
   });
 };
 
-TXPool.prototype._unconfirm = function unconfirm(tx, map, callback) {
+TXPool.prototype._unconfirm = function unconfirm(tx, map, callback, force) {
   var self = this;
   var prefix = this.prefix + '/';
-  var hash = tx.hash('hex');
-  var batch = this.db.batch();
-  var height = tx.height;
-  var ts = tx.ts;
+  var hash, batch, height, ts;
+
+  var unlock = this._lock(unconfirm, [tx, map, callback], force);
+  if (!unlock)
+    return;
+
+  callback = utils.wrap(callback, unlock);
+
+  hash = tx.hash('hex');
+  batch = this.db.batch();
+  height = tx.height;
+  ts = tx.ts;
 
   if (height !== -1)
     return callback(null, false);