Idempotent API (v2.6.0)

Understanding Idempotency

A distributed system consists of multiple components located on different networked computers, which communicate and coordinate their actions by passing messages to one another from any system. For example, I am sure that you have heard of the microservices architecture, which is a kind of distributed system.

Creating Web APIs for distributed systems is challenging because of distribution pitfalls such as process failures, communication failures, asynchrony, and concurrency. One common requirement and challenge when building fault-tolerant distributed applications is the need to be idempotent.

• In mathematics and computer science, an operation is idempotent when applied multiple times without changing the result beyond the initial application.
• Fault-tolerant applications can continue operating despite the system, hardware, and network faults of one or more components, ensuring high availability and business continuity for critical applications or systems.

Idempotence in Web APIs ensures that the API works correctly (as designed) even when consumers (clients) send the same request multiple times. For example, this case can happen when the API failed to generate the response (due to process failures, temporary downtime, etc.) or because the response was generated but could not be transferred (network issues).

Imagine a scenario in which the user clicks a “Pay” button to make a purchase. For unknown reasons, the user receives an error, but the payment was completed. If the user clicks the “Pay” button again or the request is re-sent by a retry library, we would result in two payments! Using idempotency, the user will get a successful message (e.g., on the second try), but only one charge would be performed.

Creating Idempotent Web APIs is the first step before using a resilient and transient-fault-handling library, such as Polly. The Polly .NET library allows developers to express policies such as Retry, Circuit Breaker, Timeout, Bulkhead Isolation, and Fallback in a fluent and thread-safe manner.

The IdempotentAPI library provides an easy way to develop idempotent Web APIs. In the following sections, we will see the idempotency in the different HTTP methods, how the IdempotentAPI library works, its code, and finally, how to use the IdempotentAPI NuGet packages.

Idempotency in HTTP (Web)

HTTP defines a set of request methods (HTTP verbs: GET, POST, PUT, PATCH, etc.) to indicate the desired action to be performed for a given resource. An idempotent HTTP method can be called many times without resulting in different outcomes. Safe methods are HTTP methods that do not modify the resources. In Table 1, we can see details about which HTTP methods are idempotent or/and safe.

Table 1. - Idempotent or/and Safe HTTP methods (verbs).

| HTTP Method | Idempotent | Safe | Description | | --------------- | -------------- | -------- | ------------------------------------------------------------ | | GET | Yes | Yes | Safe HTTP methods do not modify resources. Thus, multiple calls with this method will always return the same response. | | OPTIONS | Yes | Yes | Same as the previous HTTP method. | | HEAD | Yes | Yes | Same as the previous HTTP method. | | PUT | Yes | No | The PUT HTTP method is idempotent because calling this HTTP method multiple times (with the same request data) will update the same resource and not change the outcome. | | DELETE | Yes | No | The DELETE HTTP method is idempotent because calling this HTTP method multiple times will only delete the resource once. Thus, numerous calls of the DELETE HTTP method will not change the outcome. | | POST | No | No | Calling the POST method multiple times can have different results and will create multiple resources. For that reason, the POST method is not idempotent. | | PATCH | No | No | The PATCH method can be idempotent depending on the implementation, but it isn’t required to be. For that reason, the PATCH method is not idempotent. |

Idempotent Web Consumer

The creation of an idempotent consumer is an essential factor in HTTP idempotency. The API server would need a way to recognize subsequent retries of the same request. Commonly, the consumer generates a unique value, called idempotency-key, which the API server uses for that purpose. In addition, when building an idempotent consumer, it is recommended to:

• Use "V4 UUIDs" for the creation of the idempotency unique keys (e.g. “07cd2d27-e0dc-466f-8193-28453e9c3023”).
• Use techniques like the exponential backoff and random jitter, i.e., including an exponential and random delay between continuous requests.

The IdempotentAPI Library

The IdempotentAPI is an open-source NuGet library, which implements an ASP.NET Core attribute (filter) to handle the HTTP write operations (POST and PATCH) that can affect only once for the given request data and idempotency-key.

How IdempotentAPI Works

The API consumer (e.g., a Front-End website) sends a request including an Idempotency-Key header unique identifier (default name: IdempotencyKey). The API server checks if that unique identifier has been used previously for that request and either returns the cached response (without further execution) or save-cache the response along with the unique identifier. The cached response includes the HTTP status code, the response body, and headers.

Storing data is necessary for idempotency, but if the data are not expired after a certain period, it will include unneeded complexity in data storage, security, and scaling. Therefore, the data should have a retention period that makes sense for your problem domain.

The IdempotentAPI library performs additional validation of the request’s hash-key to ensure that the cached response is returned for the same combination of Idempotency-Key and Request to prevent accidental misuse.

The following figure shows a simplified example of the IdempotentAPI library flow for two exact POST requests. As shown, the IdempotentAPI library includes two additional steps, one before the controller’s execution and one after constructing the controller’s response.

Figure 1. - A simplified example of the IdempotentAPI flow for two exact POST requests.

✔ Features

• ⭐ Simple: Support idempotency in your APIs easily with simple steps 1️⃣2️⃣3️⃣.
• 🔍 Validations: Performs validation of the request’s hash-key to ensure that the cached response is returned for the same combination of Idempotency-Key and Request to prevent accidental misuse.
• 🌍 Use it anywhere!: IdempotentAPI targets .NET Standard 2.0. So, we can use it in any compatible .NET implementation (.NET Framework, .NET Core, etc.). Click here to see the minimum .NET implementation versions that support each .NET Standard version.
• ⚙ Configurable: Customize the idempotency in your needs.
  • Configuration Options (see below for more details)
  • Logging Level configuration
  • ✳ NEW ✳ - Configure the Newtonsoft SerializerSettings based on your needs.
• 🔧 Caching Implementation based on your needs.
  • 🏠 DistributedCache: A build-in caching that is based on the standard IDistributedCache interface.
  • 🦥 FusionCache: A high performance and robust cache with an optional distributed 2nd layer and some advanced features.
  • ... or you could use your own implementation 😉
• 🔀 Support idempotency in a Cluster Environment (i.e., a group of multiple server instances) using Distributed Locks.
  • samcook/RedLock.net: Supports the Redis Redlock algorithm.
  • madelson/DistributedLock: Supports multiple technologies such as Redis, SqlServer, Postgres and many more.
• 💪Powerful: Can be used in high-load scenarios.
• 🌟 Supports Minimal APIs.
• ✳ NEW ✳ - 🌟 Support FastEndpoints, a developer-friendly alternative to Minimal APIs and MVC.

📦 Main NuGet Packages

| Package Name | Description | Release | | ------------------------------------------------------------ | ------------------------------------------------------------ | ------------------------------------------------------------ | | [IdempotentAPI](h