What is the best way to test APIs for backward compatibility?

1) Avoid Breaking Changes: Ensuring new API modifications don't disrupt existing functions. 2) Prevent Client Failures: Reducing unexpected failures in customers' applications due to API updates. 3) Maintain Consistency: Ensuring seamless API integration, regardless of updates. 4) Client Trust and Experience: Assuring stability and reliability, fostering a smoother user experience and client trust. 5) Reduced Costs and Time: Lowering troubleshooting and repair costs by preempting service disruptions. 6) Ease of Use: Facilitating seamless upgrades, helping developers adapt without relearning the API. 7) Longevity of APIs: Enhancing API longevity, reducing service disruptions during a new release, leading to higher customer satisfaction.

First, implement unit tests that check your API's functionality on all versions. These tests should focus on confirming that the API endpoints consistently accept identical request schemas and return the same response schemas. Complement this with integration testing to assess the interaction between API components, end-to-end testing to evaluate the complete workflow, and contract testing to ensure adherence to established API contracts. Lastly, adopt a comprehensive versioning system and a clear deprecation policy.

Well... this may look easy, but it's not. As it happens, API/service owners don't know completely how API/service is being used. An interesting technique, besides the obvious need for regression tests, is to use live site 'traffic forking' where same request is sent to the old and new API versions and then results are compared and discrepancies logged for further analysis.

1- Introduce a strong versioning process 2- Introduce backward compatibility tests 3-Implement logging to analyze the behavior of the API before and after each update 4- Always involve clients in early stages to identify issues before widespread releases .Provide a mechanism for clients to report problems and offer support during the migration process

A robust automated testing strategy and execution with the right dataset is great way to test and deliver backward compatibility ensuring consistent and correct outputs for old and new consumers.

A single API doesn't operate in isolation; its modifications can disrupt interconnected elements. For instance, even a simple change in the payload key requires UI adaptation to prevent dysfunction. Syntactic compatibility is crucial; structural changes necessitate synchronized adjustments for uninterrupted functionality. Equally vital is semantic compatibility. Consider an example where two chained APIs are involved; now, if an upstream API's new version delays, downstream processes might timeout, affecting user interactions. Ensuring backward compatibility is vital in API development, directly impacting the seamless operation of interconnected components. Automated suites validating backward compatibility can prove helpful.

Use versioning for API set. Have compatibility associated with each APIs. Ex. Build APIs changed / Newly Added ,Compatible etc.

An example I've seen is the impact of one additional functionality to a homepage where filter on data was missing and it resulted in loading thousands of records to the cache which hugely increased the loading time. So other than structure or format changes, keeping track of your changes in terms of loading of webpage/response, checking performance indicators are also crucial.

Testing APIs for syntactic backward compatibility involves: 1) Change Identification: Recognize alterations in the API. 2) Previous State Documentation: Log the existing API syntax and structure. 3) Post-change Testing: Conduct tests after changes and compare results with the prior state. 4) Contract Testing: Use specific tools to check the component's intercommunication. 5) API Mocking: Mimic the old version and compare its responses with the new one. 6) Test Automation: Implement automated testing tools for efficiency and time-saving. The goal is to ensure modifications don't affect communication between the new API version and existing clients.

If we keep constant approach of testing backward compatibly than it would be best to check with the oldest version to be supported only. Assuming tests are properly written and cover enough area of the api, it should be sufficient testing the oldest to assure newer version will comply as well.

Unit tests that effectively mock methods in your code help protect the existing contract between API functions. Any new functionality that breaks the contract should cause an error somewhere in your unit test hierarchy. Breaking tests can help guide your remediation strategy with respect to managing changes to your API without sacrificing backward compatibility because they preserve the intent of the original functionality.

Testing APIs for syntactic compatibility is much easier compared with semantic testing. As long as the data exchange format is consistent with updates, we can say that API is backward compatible. Postman is a good tool for testing request/response templates of an API while being agnostic about its implementation. Robustness Principle teaches us to 'Be conservative in what you send, be liberal in what you accept.' This principle helps a lot in keeping the syntactic backward compatibility.

Testing APIs for semantic backward compatibility requires ensuring that the specific changes don't impact the intended functionality. 1) Check Changes: Understand the alterations made in the API 2) Maintain Previous State Records: Preserve records of the existing API behaviour 3) Explore API Behaviour: Run tests to understand if the API functionality is still the same. Check response data, status codes, and error messages 4) Regression Testing: Helps ensure new changes haven't affected old functionalities 5) Test Automation: Use automated testing tools to make the process more efficient and effective 6) Scenario Testing: Evaluate how the API performs under different conditions The objective is to ensure that behaviour is not changed

A common pitfall in regression testing is creating tests that are too tightly coupled to specific data or internal structures, making them fragile in the face of legitimate changes. To avoid this, focus on testing against stable, public-facing aspects of the API. Utilize abstracted, high-level test cases that concentrate on the API's intended functionality and output rather than its internal workings. Additionally, ensuring tests are clearly documented and regularly updated in line with API evolution helps in maintaining their relevance and effectiveness, thereby reducing the likelihood of false positives.

Compare snapshots. Take "snapshots" of request/response payloads and compare new snapshots against old ones to detect changes. Libraries like ApprovalTests.Json help automate this.

As your product evolves, it supports more and more functionality which require API enhancements. Your API support can get out of control easily if you don't keep track of your API changes via version control process. Version control and change management tools enable you to version your APIs and services. Developing regression test suites would also help you make sure that backward compatibility is not broken after major API changes. You should test your services by using CI/CD tools and convert these test cases into regression test suites to verify backward compatibility after future API changes. In summary, your clients depend on your service, you can ensure highest quality of service to your customers via automating your API testing.

Use automated regression testing. Have a regression test suite that runs with each build and detects breaking changes. Tools like Postman, Rest Assured, and Karate help automate API testing.

Sample TEST data can be created for your API testing. So, that you can automate them using unit test to check the response of your data in an automated environment. After each change in your code, make sure you run this automated test suite, which can be triggered using Jenkins during the build itself, or triggered via any other CI/CD deployment workflows. Also you can view the result using multiple tools or can send a test summary to the mails to monitor the impact.

If the regression set is fast it can run directly in the pull request process. As the set grows consider a nightly run on your develop branch or feature branches.or consider approaches to parallelize the work. This is where microservices shine. Breaking the problem down into smaller blocks. Continue to look at coverage: are there news gaps or overlap. Lastly compare against what you're seeing in your production telemetry. What is slow, critical or not used.

Backward or non-backward API compatibility should be tested with automation. CICD pipelines are perfect way to trigger and take necessary actions. These tests can be triggered on Pull Request creation to know if changes are breaking backward compatibility or not. If not at PR level, then before deployment pipeline can stop the deployment and notify that this deployment will result in breaking changes.

One more thing to consider when discussing testing API backward compatibility, "versioning" stands out as key. It's crucial when updates or new features might impact current users. Versioning allows for seamless introduction of changes, ensuring older versions remain functional. This approach maintains backward compatibility and eases user transition to updated versions.

Team enrollment: get everyone onboard with the value of your test suite. As new code/features come in how does this impact the regression suite. This will save the team lots of time later not having to put out urgent fires.

API backward compatibility is like Technical Debt. You can take it so much only, and have to pay it back to keep the system healthy. We can't keep the v1 of the API active until v100. We have to retire the old implementation and break the compatibility at some point. Considering this, every API should have a well documented life-cycle. This lifecycle should have depreciation date, versioning, expiry date, etc. Clients of APIs should handle the case of API expiry and handle it gracefully. Robustness Principle states: Be conservative in what you send, be liberal in what you accept. We should use it to fullest at both producer and consumer end to keep the backward compatibility at maximum.

To keep things extra simple, if you have good tests for your previous versions, and they run well on the newest, you’re fine.

Document changes clearly. In change log notes, call out any deprecated structures, endpoints, params etc. and when removal is scheduled so clients can prepare.