What is Contract Testing?

Contract testing is software testing that provides an effective and efficient way to ensure that a system’s components are functioning as expected. It offers numerous benefits to businesses, such as improved reliability, increased speed of development, and reduced cost.

In this blog post, we’ll discuss what API contract testing is, how it works, and the advantages it can provide to your organization. We’ll also provide examples of how contract testing can be used to benefit your business and its API integrations.

How Is Contract Testing Different Than Schema Testing?

As Matt Fellows puts it, “Schemas are not a contract.” While schemas can provide some value in establishing shared understanding between partners, they don’t capture all the aspects of a contract. Contract testing is an approach to software development that focuses on the interactions between different parts of a system.

This type of approach aims to ensure that any communication between components is consistent and meets the standards required for the system to function as expected. It differs from schema testing in that it focuses on what happens when components interact rather than how they are structured.

In schema testing, the focus is on ensuring that the data structures used in the system are correct and valid. This type of testing is important for maintaining data integrity, but it does not cover the interaction between different parts of the system.

Contract testing takes this a step further by verifying that when one component sends information to another, the receiving component will respond correctly. This allows developers to ensure that all elements of their system are working together as expected and can catch errors before they become issues.

The Benefits of Contract Testing

Contract testing is also useful for identifying potential security vulnerabilities. By testing how components communicate with each other, developers can identify any weak points in their systems that could be exploited by malicious actors. With schema testing, it is difficult to detect these types of problems since it only tests the structure of the data and not how it is being used or manipulated. Contract testing, however, allows developers to find any issues with communication between components before they become a problem.

Overall, contract testing provides a more comprehensive approach to ensuring a system works properly than testing schemas alone. By validating the interactions between components, developers can ensure their systems are secure and reliable. Additionally, contract testing can help catch errors early on and save developers time and money in the long run.

Contract Tests Can Identify Issues That Schema Testing Can’t

When it comes to defining the agreement between two software applications, many people tend to think that a JSON schema is the perfect solution. But schemas do not capture all the key aspects of a contract. Schema testing ensures that the data being exchanged matches a given specification, such as in XML or JSON. However, this approach does not capture all the aspects of a contract. For example, it can’t guarantee that a certain field contains the right kind of data or that certain requests are supported.

Additionally, schemas can introduce ambiguity which can lead to misinterpretations. Consumers may generate code or types from a schema and consume all of the available API surface area (requests, fields, etc.), even if the application doesn’t actually need all of them. This makes evolution more difficult because you can’t track the specific needs of each consumer: you must assume they use the entire set of features. With contract tests, however, it’s not a breaking change if you remove an endpoint that no client uses or remove a field that no client expects.

Moreover, some data types may be more specific than what is represented in a schema. For example, in Protobufs, there are only a few primitive data types that may be encoded in the Protobuf definition. However, knowing that a certain field requires an actual UUID rather than any string is useful from an integration contract perspective. The guarantees you get here will often vary depending on how specific the schema definition is for your use case. Schemas and definitions can be outdated and not reflect the actual world conditions. We like to think API developers are building specification-first or documentation-first, but that’s often not the case.

Using Contract Testing to Establish and Test Agreements Between Partners

Contract testing is an integral part of the software development process, used to ensure that any agreements between two or more parties are functioning as expected. It ensures that all stakeholders clearly understand their obligations and the terms and conditions under which those obligations will be carried out.

Contract testing involves verifying that all services, inputs, outputs, and other interactions between different systems meet the expectations of each party involved in the agreement. This helps ensure that agreements are being honored and all expectations are met.

To ensure that contract testing is successful, developers must first create contracts for all interactions between different systems. These contracts should describe the exact input and output parameters, response times, expected behavior, and other important factors. Once these contracts are in place, developers can then set up automated tests to check that each service or interaction is meeting the expectations described in the agreement.

Contract testing can be used to test various types of agreements, including software-as-a-service (SaaS) agreements, platform-as-a-service (PaaS) agreements, and microservices agreements. Additionally, contract testing can also be used to test the performance of APIs and other integration points between different systems.

Some of the popular tools that enable contract testing for APIs are:

Final Thoughts

Ultimately, contract testing is a beneficial type of API testing. Using contract testing helps verify that all parties involved in an agreement understand the expectations, obligations, and commitments required of them. By ensuring that all contracts and agreements are functioning correctly and meeting expectations, contract testing helps software providers maintain strong relationships with their partners and customers.