diff --git a/README.md b/README.md
index 11ab314..b420599 100644
--- a/README.md
+++ b/README.md
@@ -3,10 +3,12 @@
 Provides `Lock` and `RWLock` (read write lock) synchronization primitives for
 protecting in-memory state across multiple tasks and/or microtasks.
 
+This is a wholesale fork of [`@rocicorp/lock`](https://github.com/rocicorp/lock) but adds `RWLockMap` functionality.
+
 # Installation
 
 ```
-npm install @rocicorp/lock
+npm install @ccorcos/lock
 ```
 
 # Usage
@@ -14,7 +16,7 @@ npm install @rocicorp/lock
 `Lock` is a mutex that can be used to synchronize access to a shared resource.
 
 ```ts
-import {Lock} from '@rocicorp/lock';
+import {Lock} from '@ccorcos/lock';
 
 const lock = new Lock();
 
@@ -35,7 +37,7 @@ void f(2);
 `RWLock` is a read write lock. There can be mutlipe readers at the same time but only one writer at the same time.
 
 ```js
-import {RWLock} from '@rocicorp/lock';
+import {RWLock} from '@ccorcos/lock';
 
 const rwLock = new RWLock();
 
@@ -73,3 +75,19 @@ const release = await lock.lock();
 // do something
 release();
 ```
+
+`RWLockMap` will dynamically create and clean up locks keyed by a string.
+
+```js
+import {RWLockMap} from '@ccorcos/lock';
+
+const lockMap = new RWLockMap();
+const release1 = await lockMap.read("node1");
+const release2 = await lockMap.write("node2");
+const v3 = await lockMap.withRead("node3", async () => {
+  return 3;
+});
+const v4 = await lockMap.withWrite("node4", async () => {
+  return 4;
+});
+```
\ No newline at end of file
diff --git a/package.json b/package.json
index 78d3340..08eb650 100644
--- a/package.json
+++ b/package.json
@@ -1,8 +1,8 @@
 {
-  "name": "@rocicorp/lock",
+  "name": "@ccorcos/lock",
   "description": "Implements Lock and RWLock synchronization primitives.",
   "version": "1.0.3",
-  "repository": "github:rocicorp/lock",
+  "repository": "github:ccorcos/lock",
   "license": "Apache-2.0",
   "engines": {
     "node": "^12.20.0 || ^14.13.1 || >=16.0.0"
diff --git a/src/lock.test.ts b/src/lock.test.ts
index a67fc17..bc824ea 100644
--- a/src/lock.test.ts
+++ b/src/lock.test.ts
@@ -1,6 +1,6 @@
 import {expect} from 'chai';
 import {SinonFakeTimers, useFakeTimers} from 'sinon';
-import {RWLock} from './lock.js';
+import {RWLock, RWLockMap} from './lock.js';
 
 /**
  * Creates a promise that resolves after [[ms]] milliseconds. Note that if you
@@ -50,7 +50,9 @@ test('Multiple reads', async () => {
     release();
   })();
 
+  expect(lock.unlocked).to.equal(false);
   const [v1, v2, v3] = await Promise.all([r1, r2, r3]);
+  expect(lock.unlocked).to.equal(true);
   expect(v1).to.equal(1);
   expect(v2).to.equal(2);
   expect(v3).to.equal(undefined);
@@ -83,9 +85,11 @@ test('Multiple reads with sleep', async () => {
     release();
   })();
 
+  expect(lock.unlocked).to.equal(false);
   await clock.runAllAsync();
 
   const [v1, v2, v3] = await Promise.all([r1, r2, r3]);
+  expect(lock.unlocked).to.equal(true);
   expect(v1).to.equal(1);
   expect(v2).to.equal(2);
   expect(v3).to.equal(undefined);
@@ -122,9 +126,11 @@ test('Multiple write', async () => {
     release();
   })();
 
+  expect(lock.unlocked).to.equal(false);
   await clock.runAllAsync();
 
   const [v1, v2, v3] = await Promise.all([w1, w2, w3]);
+  expect(lock.unlocked).to.equal(true);
   expect(v1).to.equal(1);
   expect(v2).to.equal(2);
   expect(v3).to.equal(undefined);
@@ -161,9 +167,11 @@ test('Write then read', async () => {
     release();
   })();
 
+  expect(lock.unlocked).to.equal(false);
   await clock.runAllAsync();
 
   const [v1, v2, v3] = await Promise.all([w1, r2, r3]);
+  expect(lock.unlocked).to.equal(true);
   expect(v1).to.equal(1);
   expect(v2).to.equal(2);
   expect(v3).to.equal(undefined);
@@ -208,9 +216,11 @@ test('Reads then writes', async () => {
     return 4;
   })();
 
+  expect(lock.unlocked).to.equal(false);
   await clock.runAllAsync();
 
   const [v1, v2, v3, v4] = await Promise.all([r1, r2, w3, w4]);
+  expect(lock.unlocked).to.equal(true);
   expect(v1).to.equal(1);
   expect(v2).to.equal(2);
   expect(v3).to.equal(undefined);
@@ -257,9 +267,11 @@ test('Reads then writes (withRead)', async () => {
     return 4;
   });
 
+  expect(lock.unlocked).to.equal(false);
   await clock.runAllAsync();
 
   const [v1, v2, v3, v4] = await Promise.all([r1, r2, w3, w4]);
+  expect(lock.unlocked).to.equal(true);
   expect(v1).to.equal(1);
   expect(v2).to.equal(2);
   expect(v3).to.equal(undefined);
@@ -276,3 +288,71 @@ test('Reads then writes (withRead)', async () => {
     'w4b',
   ]);
 });
+
+test('RWLockMap will cleanup locks.', async () => {
+  const locks = new RWLockMap();
+
+  function runTest(key: string) {
+    // Same a test("Reads then writes (withRead)")
+    const log: string[] = [];
+    const r1: Promise<number> = locks.withRead(key, async () => {
+      log.push('r1a');
+      await sleep(8);
+      log.push('r1b');
+      return 1;
+    });
+    const r2: Promise<number> = locks.withRead(key, async () => {
+      log.push('r2a');
+      await sleep(6);
+      log.push('r2b');
+      return 2;
+    });
+    const w3: Promise<number> = locks.withWrite(key, async () => {
+      log.push('w3a');
+      await sleep(4);
+      log.push('w3b');
+      return -1;
+    });
+    const w4: Promise<number> = locks.withWrite(key, async () => {
+      log.push('w4a');
+      await sleep(2);
+      log.push('w4b');
+      return 4;
+    });
+
+    const result = Promise.all([r1, r2, w3, w4]);
+    return {log, result};
+  }
+
+  async function assertTest(args: {log: string[]; result: Promise<number[]>}) {
+    const [v1, v2, v3, v4] = await args.result;
+    expect(v1).to.equal(1);
+    expect(v2).to.equal(2);
+    expect(v3).to.equal(-1);
+    expect(v4).to.equal(4);
+
+    expect(args.log).to.deep.equal([
+      'r1a',
+      'r2a',
+      'r2b',
+      'r1b',
+      'w3a',
+      'w3b',
+      'w4a',
+      'w4b',
+    ]);
+  }
+
+  const test1 = runTest('key1');
+  const test2 = runTest('key2');
+
+  expect(locks['_locks'].get('key1')?.unlocked).to.equal(false);
+  expect(locks['_locks'].get('key2')?.unlocked).to.equal(false);
+  await clock.runAllAsync();
+
+  await assertTest(test1);
+  await assertTest(test2);
+
+  expect(locks['_locks'].get('key1')).to.equal(undefined);
+  expect(locks['_locks'].get('key2')).to.equal(undefined);
+});
diff --git a/src/lock.ts b/src/lock.ts
index 6781667..b5eb98b 100644
--- a/src/lock.ts
+++ b/src/lock.ts
@@ -8,25 +8,35 @@ export class Lock {
     const {promise, resolve} = resolver();
     this._lockP = promise;
     await previous;
-    return resolve;
+    return () => {
+      if (this._lockP === promise) this._lockP = null;
+      resolve();
+    };
   }
 
   withLock<R>(f: () => R | Promise<R>): Promise<R> {
     return run(this.lock(), f);
   }
+
+  get unlocked() {
+    return this._lockP === null;
+  }
 }
 
 export class RWLock {
   private _lock = new Lock();
   private _writeP: Promise<void> | null = null;
-  private _readP: Promise<void>[] = [];
+  private _readP: Set<Promise<void>> = new Set();
 
   read(): Promise<() => void> {
     return this._lock.withLock(async () => {
       await this._writeP;
       const {promise, resolve} = resolver();
-      this._readP.push(promise);
-      return resolve;
+      this._readP.add(promise);
+      return () => {
+        this._readP.delete(promise);
+        resolve();
+      };
     });
   }
 
@@ -40,14 +50,23 @@ export class RWLock {
       await Promise.all(this._readP);
       const {promise, resolve} = resolver();
       this._writeP = promise;
-      this._readP = [];
-      return resolve;
+      this._readP.clear();
+      return () => {
+        if (this._writeP === promise) this._writeP = null;
+        resolve();
+      };
     });
   }
 
   withWrite<R>(f: () => R | Promise<R>): Promise<R> {
     return run(this.write(), f);
   }
+
+  get unlocked() {
+    return (
+      this._lock.unlocked && this._writeP === null && this._readP.size === 0
+    );
+  }
 }
 
 async function run<R>(
@@ -61,3 +80,51 @@ async function run<R>(
     release();
   }
 }
+
+export class RWLockMap {
+  private _locks = new Map<string, RWLock>();
+
+  private getLock(key: string) {
+    const lock = this._locks.get(key);
+    if (lock) return lock;
+    const newLock = new RWLock();
+    this._locks.set(key, newLock);
+    return newLock;
+  }
+
+  private deleteLock(key: string) {
+    this._locks.delete(key);
+  }
+
+  async read(key: string): Promise<() => void> {
+    const lock = this.getLock(key);
+    const release = await lock.read();
+    return () => {
+      release();
+      if (lock.unlocked) this.deleteLock(key);
+    };
+  }
+
+  async withRead<R>(key: string, f: () => R | Promise<R>): Promise<R> {
+    const lock = this.getLock(key);
+    const result = await lock.withRead(f);
+    if (lock.unlocked) this.deleteLock(key);
+    return result;
+  }
+
+  async write(key: string): Promise<() => void> {
+    const lock = this.getLock(key);
+    const release = await lock.write();
+    return () => {
+      release();
+      if (lock.unlocked) this.deleteLock(key);
+    };
+  }
+
+  async withWrite<R>(key: string, f: () => R | Promise<R>): Promise<R> {
+    const lock = this.getLock(key);
+    const result = await lock.withWrite(f);
+    if (lock.unlocked) this.deleteLock(key);
+    return result;
+  }
+}