[Resolved] CROSSSLOT Keys error in Redis cluster-mode enabled

Valerio Barbera

Hi, I’m Valerio, founder and CTO at Inspector.

We recently decided to move our cache system from a single Redis server to an AWS managed Redis cluster.

Amazon ElastiCache is a fully managed caching service that can be configured in cluster mode with a simple click so it can be easily scaled-in and out.

Running a Redis cluster isn’t an easy task. Orchestrating multiple Redis nodes with the same efficiency of a managed service requires a lot of vertical skills. A managed service help us to stay focused on application development instead of dealing with infrastructure management issues.

But moving from a single server instance to a cluster enabled configuration caused a new error to appear:

CROSSSLOT Keys in request don't hash to the same slot

Where does this new error come from?

When Redis operates in cluster mode, it handles data storage differently than when it operates as a single instance.

This because it should be ready to distribute data across multiple nodes enabling horizontal scalability.

A Redis Cluster, if you are comparing it to the standalone Redis which is the non-distributed setup, use the “HASH SLOT” concept for key management, which implies some restrictions on key operations, necessary to ensure data consistency in a distributed environment.

If you take a look at the error description, it says “Keys in request don’t hash to the same slot“, it means that we are running some commands not comaptible with “HASH SLOT” restrictions mentioned above.

What is a Hash Slot in Redis?

Redis Cluster determine what instance the particular key shall be assigned to using a specfic algorithm.

To make it simple, when you create a new key, Redis will assign an integer to it , called hash-slot. Keys with the same hash-slot will reside on the same Redis node inside the cluster.

Keys distribution based on their hash slot

You can verify it by yourself with a quick example.

Connect to your Redis Cluster:

redis-cli -h [CLUSTER_ADDRESS] -p 6379

Create two new keys and checkout their assigned hash-slot:

redis> SET mykey1 "Hello"
"OK"
redis> SET mykey2 "Hello 2"
"OK"
redis> CLUSTER KEYSLOT mykey1
(integer) 650
redis> CLUSTER KEYSLOT mykey2
(integer) 8734

As you can see the cluster has assigned two different hash-slots to the keys, so they are probably stored on different nodes within the cluster.

This is exactly what happens when your application create or changes keys in Redis.

Why does the CROSSSLOT error occur?

This error occurs because the application is trying to run a command on multiple keys, but the keys involved in the operation aren’t in the same hash-slot.

This result in a “CROSS-SLOT” operation which is not allowed in a cluster.

Hash slots control how data is distributed within the cluster, so this constraint is necessary to avoid information corruption in a distributed environment.

In the example above we have verified that the two keys (mykey1, mykey2) have obtained two different hashes slots.

Trying to run a command that involve both keys we will get the error:

redis> SUNION mykey1 mykey2
(error) CROSSSLOT Keys in request don't hash to the same slot

How to solve CROSSSLOT error at the application level

When creating keys that could be used by multi-key operations, use hashtags ({…}) to force the keys into the same hash slot.

When the key contains a “{…}” pattern, only the substring between the braces, “{” and “}”, is considered to calculate the hash slot.

For example, the keys {user1}:mykey1 and {user1}:mykey2 are hashed to the same hash-slot, because only the string inside the braces “{” and “}”, that is, “user1”, is used to compute the hash-slot.

redis> CLUSTER KEYSLOT {user1}:mykey1
(integer) 8006
redis> CLUSTER KEYSLOT {user1}:mykey2
(integer) 8006

redis> SUNION {user1}:mykey1 {user1}:mykey2
1) "Hello"
2) "Hello 2"

Now multi-key operations can be performed since both keys are placed in the same hash-slot.

Almost all Redis client support the “tag” feature, so you should explore the documentation of your framework/library to understand how to use it.

Application monitoring

If you found this post interesting and want to drastically change your developers’ life for the better, you can give Inspector a try.

Inspector is an easy to use Code Execution Monitoring tool that helps developers to identify bugs and bottlenecks in their application automatically. Before customers do.

screenshot inspector code monitoring timeline

It is completely code-driven. You won’t have to install anything at the server level or make complex configurations in your cloud infrastructure.

It works with a lightweight software library that you can install in your application like any other dependency. Check out the supported technologies in the GitHub organization.

Create an account, or visit the website for more information: https://inspector.dev

Related Posts

cloud costs savings with code execution monitoring

How to save thousands of dollars in cloud costs with Code Execution Monitoring

Hi, I’m Valerio, software engineer, co-founder and CTO at Inspector. In this article I want to show you a monitoring strategy to help you save thousands of dollars a year on cloud costs. The typical scenario where you can get the most payback from this strategy is when your application runs on multiple servers. If

The 5 Best Application Monitoring Tools for 2022

When it comes to application monitoring software, there are some great tools available to make our lives easier. We’ve compiled a list of the very best. No offense intended but the truth is customers are no longer interested in what you have to do to make your product work when they need it. They only

What Are Source Maps and How to Properly Use Them

You are debugging a web app for a client but the minified version of the Javascript and CSS code makes it impossible to understand what statements the browser is actually executing. You could break down the original code line by line in your editor putting some “console.log()” statements here and there, or try debugging it

Join the "Scalable Applications" community

Learn from other developers' experience.
Join the international community of developers to build scalable applications.

Inspector customer feedback
2020 © Inspector S.R.L. - VAT: 09552901218 - Progetto agevolato con la misura Resto al SUD RSUD0000000 - CUP C61B21012300008

How to build scalable applications

Get the e-book about the Inspector scalability journey.