Introducing @upstash/lock: Distributed Locking using Upstash Redis

Today we're announcing our brand new SDK for distributed locking: @upstash/lock!

TL;DR

You can now use Upstash to create distributed locks for your applications. This is useful for applications that need to ensure only one client should access a resource at a time.

Note that because Upstash Redis uses async replication between replicas, a network failure may cause multiple clients to acquire the lock, so this library should be used for performance benefits and situations that require mostly consistent locking.

import { Lock } from "@upstash/lock";
import { Redis } from "@upstash/redis";
 
function handleOperation() {
  const lock = new Lock({
    id: "unique-lock-id",
    lease: 5000, // Hold the lock for 5 seconds
    redis: Redis.fromEnv(),
  });
 
  if (await lock.acquire()) {
    await handleCriticalSection();
    await lock.release();
  } else {
    handleLockNotAcquired();
  }
}

You can see a demo of this in action here.

If you're interested in learning more about locks, read on!

What is a lock?

First, let's explore what a lock is and what kind of applications might need them. A lock allows for mutual exclusion, meaning only one client can hold the lock at a time.

Take this example handles of code that handles requests for an ATM network. When the bank server receives a request (ie: deposit or withdraw), it should appropriately handle it:

// Code adapted from UC Berkeley's CS162
function bankServer() {
  while (true) {
    receiveRequest(operation, acctId, amount);
    await processRequest(operation, acctId, amount);
  }
}
 
async function processRequest(op, acctId, amount) {
  if (op == "deposit") {
    acct = await getAccount(acctId); // acct 1
    acct.balance = acct.balance + amount;
    await storeAccount(acct);
  }
  ...
}

Let’s say Alice and Bob both want to deposit $50 into one account. The expected behavior would be account 1 having $100. With the current implementation, the behavior is undefined. To demonstrate, here are 2 cases that produce different outputs:

Case 1: The requests are processed serially. This means that Alice’s request will fully execute, and then Bob’s request will fully execute.

Final account balance: $100

This is great! We get our expected result since both tasks ran serially. However, consider the following ordering:

Case 2: The requests are interleaved

Final account balance: $50 Uh oh, now we don’t have the expected behavior! We’ve run into a flaw with our synchronization, aka a race condition 👻. The primary problem here is that there is a section of code that we don’t want to be interleaved. That is, we want some portion of our code to be executed by only one thread at a time. This area of code is called the critical section. In our example, the critical section is

acct = getAccount(acctId);
acct.balance = acct.balance + amount;
storeAccount(acct);

We saw that running the 2 requests serially results in the correct behavior, and a lock helps us do exactly that. Specifically, a lock is a synchronization mechanism that allows only one thread to execute a specific section of code at a time (mutual exclusion). It ensures that multiple threads do not interfere with each other and helps maintain data consistency. Acquiring a lock around our critical section will give us the intended behavior.

How do locks work?

Locks in an operating system are implemented using atomic instructions. These instructions are guaranteed to be executed as a single unit, and cannot be interrupted. This means that if a thread is executing an atomic instruction, no other thread can execute any other instruction until the atomic instruction is finished. This is exactly what we want for our critical section! We want to ensure that once a thread enters the critical section, no other thread can enter until the first thread is done.

How do we implement locks with Redis?

It just so happens that Redis has its own atomic instructions that we can use to implement locks. Using Upstash Redis, we can use the SETNX attribute to set a key if it doesn’t already exist, making the locking process atomic. We can then use the EXPIRE command to set a timeout on the key, so that if the thread that acquired the lock crashes, the lock will be released after a certain amount of time.

For releasing, we can use the DEL command to delete the key, which is also atomic. However, simply using DEL to release the lock is not enough. Consider the following scenario:

1. Thread A acquires the lock
2. Thread A crashes
3. Thread A lease expires and the lock is released
4. Thread B acquires the lock
5. Thread A recovers and releases the lock This is a problem because Thread A was able to release the lock even though it was already acquired by Thread B.

To fix this, a unique UUID is assigned to each lock. When a thread acquires the lock, it stores the UUID and we are able to verify that the thread releasing the lock is the same thread that acquired it. A similar mechanism is used for extending the lock lease.

Future plans

A full distributed lock implementation would mean that we can use multiple Redis instances to improve performance and reliability. You can read more about distributed locks here. If you're interested in this, please let us know!