Build fault tolerant applications with Cassandra API for Azure Cosmos DB

Build fault tolerant applications with Cassandra API for Azure Cosmos DB

Learn how to build fault tolerant applications using the Azure Cosmos DB Cassandra API with a custom retry policy for 429 errors.

Azure Cosmos DB is a resource governed system that allows you to execute a certain number of operations per second based on the provisioned throughput you have configured. If clients exceed that limit and consume more request units than what was provisioned, it leads to rate limiting of subsequent requests and exceptions being thrown – they are also referred to as 429 errors.

With the help of a practical example, I’ll demonstrate how to incorporate fault-tolerance in your Go applications by handling and retrying operations affected by these rate limiting errors. To help you follow along, the sample application code for this blog is available on GitHub and it uses the gocql driver for Apache Cassandra. In this post, we’ll go through: 

  • Initial setup and configuration before running the sample application
  • Execution of various load test scenarios and analyze the results
  • A quick overview of the Retry Policy implementation. 

One way of tackling rate limiting is by adjusting provisioned throughput to meet your application requirements. There are multiple ways to do this, including using Azure portal, Azure CLI, and CQL (Cassandra Query Language) commands. 

But, what if you wanted to handle these errors in the application itself? 

The good thing is that the Cassandra API for Azure Cosmos DB translates the rate limiting exceptions into overloaded errors on the Cassandra native protocol. Since the gocql driver allows you to plugin your own RetryPolicy, you can write a custom implementation to intercept these errors and retry them after a certain (cool down) time period. This policy can then be applied to each Query or at a global level using a ClusterConfig

The Azure Cosmos DB extension library makes it quite easy to use Retry Policies in your Java applications. An equivalent Go version is available on GitHub and has been used in the sample application for this blog post. 

Retry Policy in action

As promised, you will walk through the entire process using a simple yet practical example. The sample application used to demonstrate the concepts is a service that exposes a REST endpoint to POST orders data which is persisted to a Cassandra table in Azure Cosmos DB. 

You will run a few load tests on this API service to see how rate limiting manifests itself and how it’s handled. 


Start by installing hey, a load testing program. You can download OS specific binaries (64-bit) for LinuxMac and Windows (please refer to the GitHub repo for latest information in case you face issues downloading the utility)

You can use any other tool that allows you to generate load on an HTTP endpoint

Clone this GitHub repo and change into the right directory: 

git clone 
cd cosmos-go-rate-limiting

cassandra api apache cassandra appdev cassandra go paas

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