Working with APIs: Why you should care about idempotency

Sander van Hooft's profile picture

Sander van Hooft


Mollie recently introduced idempotency support. At Sandorian we've been rooting for idempotency support a long time and adopted in all of our Mollie integrations as soon as it was available. Here's why.

As a software developer, you're familiar with the concept of APIs and how they work.

APIs, or application programming interfaces, are a set of rules and protocols that allow different software applications to communicate with each other. They are an essential part of modern software development and enable developers to build complex systems that can interact with a wide range of other applications and services.

However, one important concept that developers often overlook when working with APIs is idempotency.

Idempotency is the property of an operation in which it can be performed multiple times without causing any additional side effects beyond the initial operation.

In other words, if you perform the same operation multiple times, the result should be the same as if you had performed it only once.

So, why should you care about idempotency when designing and consuming an API?

Here are a few key reasons:

1. Avoiding Duplicate Operations

One of the most important reasons to care about idempotency is to avoid duplicate operations. If an API operation is not idempotent, performing it multiple times can result in unintended consequences.

For example, imagine that you have an API endpoint that triggers a creditcard charge. If the endpoint is not idempotent and you call it twice with the same parameters, you may end up charging your dearest customer twice instead of once.

By designing your API endpoints to be idempotent, you can ensure that duplicate operations have no effect beyond the initial operation. This can help you avoid costly mistakes and ensure that your application behaves predictably and reliably.

2. Handling Network Errors

Another important reason to care about idempotency is to handle network errors. When you make an API call, there is always a chance that the call will fail due to a network error or some other issue. If your API endpoint is not idempotent, a failed API call may result in inconsistent or unexpected behavior. For example, a dedicated API client may be configured to retry sending a request a few times when hitting network issues.

However, if your API endpoint is idempotent, you can simply retry the operation without worrying about the consequences of performing the operation multiple times. This can help you ensure that your application remains stable and responsive even in the face of network errors or other issues.

3. Scaling Your Application

Finally, designing your API endpoints to be idempotent can help you scale your application more effectively. If your API operations are not idempotent, you may need to build in additional logic to prevent duplicate operations or handle failed API calls. This can add complexity to your code and make it harder to scale your application effectively.

On the other hand, if your API endpoints are idempotent, you can simplify your code and make it easier to scale your application. You can rely on the idempotent nature of your API operations to ensure that your application behaves predictably and consistently even as you scale up your infrastructure.


Idempotency is a powerful concept to understand when working with APIs. By designing your API endpoints to be idempotent, you can avoid duplicate operations, handle network errors more effectively, and scale your application more easily. So, the next time you're working with an API, be sure to consider the idempotent nature of your operations and design them accordingly.

Sandorian Consultancy B.V.

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