--- title: How we Validate a Legacy Application After a Migration description: Discover how GO4IT Echo automates legacy application validation after migration, comparing behavior across versions to reduce production incidents. image: https://go4it.solutions/hubfs/ChatGPT%20Image%205%20oct%202026%2c%2016_21_53.png --- [![go4it-logo](https://go4it.solutions/hubfs/Logos/go4it-logo.svg "go4it-logo")](https://go4it.solutions/?hsLang=en) - [Our Technology](https://go4it.solutions/en/nuestra-tecnologia) - [Success Stories](https://go4it.solutions/en/casos-de-exito-go4it-solutions) - [Insights](https://go4it.solutions/en/insights) en - [English](https://go4it.solutions/en/insights/how-we-verify-legacy-application) - [Español](https://go4it.solutions/insights/como-homologamos-una-aplicacion-legacy-despues-de-una-migracion) [Contact](https://go4it.solutions/es/contacto?hsLang=en) - [Our Technology](https://go4it.solutions/en/nuestra-tecnologia) - [Success Stories](https://go4it.solutions/en/casos-de-exito-go4it-solutions) - [Insights](https://go4it.solutions/en/insights) [automated testing](https://go4it.solutions/en/insights/tag/automated-testing) [Legacy Modernization](https://go4it.solutions/en/insights/tag/legacy-modernization) [Artificial Intelligence](https://go4it.solutions/en/insights/tag/artificial-intelligence) # How we Validate a Legacy Application After a Migration [![Adrián Noguero | CEO](https://go4it.solutions/hs-fs/hubfs/Perfil.jpeg?width=48&height=48&name=Perfil.jpeg)](https://go4it.solutions/en/insights/author/adrian-noguero) [Adrián Noguero | CEO](https://go4it.solutions/en/insights/author/adrian-noguero) Oct 5, 2026, 10:25:14 AM ![](https://go4it.solutions/hubfs/ChatGPT%20Image%205%20oct%202026%2c%2016_21_53.png) One of the most critical phases of modernization is not transforming the code, but **demonstrating that the new application continues to do the same things as the original**. In our legacy application modernization projects, we have repeatedly encountered the same challenge: how can we validate an entire application when dealing with systems that have evolved over many years, contain thousands of lines of code, and include business logic that is not always documented? The answer led us to develop **GO4IT Echo**, a tool that turns real-world usage of the original application into automated tests and uses them to validate the modernized application, with the goal of reducing production incidents to zero. In this article, we explain how we approach this process at GO4IT Solutions and why we decided to change the traditional way of handling validation after a migration. ## The Challenge: Proving That the Application Still Works the Same Way The legacy applications we encounter in our projects are usually business-critical systems. They have been running for years, have evolved alongside the organization, and have accumulated a large number of rules and processes. The problem is that a significant part of this knowledge is not always documented. The application contains behaviors that have been incorporated over time: business rules, exceptions, validations, chained processes, or specific ways of responding to certain data. All of this is part of the system's actual functionality, even though it may not necessarily appear in a specification. Therefore, when we modernize an application, it is not enough to verify that the new code compiles or that the main functionalities are available. We need to be able to answer a much more demanding question: > «Does the modernized system behave like the original system?» ## The Problem with Manual Testing Once the migration has been completed, the most common way to verify that everything works correctly is to run tests on the new application and compare the results with those expected. For small applications, this approach may be sufficient. However, when working with large enterprise applications, the number of scenarios that need to be validated can quickly become overwhelming. Manual testing also presents another problem: it depends heavily on the knowledge of the people who design and execute the tests. This leads to several difficult questions: - Which parts of the application have been tested? - Which processes are covered? - What differences have been detected between the source and target systems? - At what point in the process did the problem occur? - What evidence do we have to analyze it? - How does coverage evolve as modernization progresses? Based on our experience, we saw that we needed a different approach to validation. Echo provides information about test execution and makes it possible to measure the coverage achieved, as well as record the results and differences detected. This provides something especially important in modernization projects: **verifiable evidence that the modernized system maintains the expected behavior**. ## Our Approach: Using Real Behavior as the Reference Instead of trying to manually reconstruct all the cases that should be tested, we took a different approach: > «Why not use the behavior of the legacy application itself as the source of our tests?» The original application is already running. Users are already using it. The real processes already exist. Therefore, we can capture those interactions and turn them into scenarios that can later be reproduced against the modernized application. The process starts with a simple idea: ***The original application becomes the functional reference against which we validate the new application.*** This means we do not have to try to define from scratch how the system should behave. We can use its actual behavior as the reference. ## This Is How We Developed GO4IT Echo This need led to the creation of **GO4IT Echo**, our technology for automating application validation after a modernization process. Echo allows us to record interactions performed on the original application and convert them into test scenarios, even before the migration takes place. Once the modernized application is available, those same scenarios can be reproduced against it. The system then compares the behavior of both versions and identifies any differences. The process we follow can be summarized in four steps: **1. Capture:** We record the interactions performed on the original application. **2. Generate:** We convert those interactions into reusable test scenarios. **3. Replay:** We execute the same scenarios against the modernized application. **4. Compare:** We automatically analyze the results to identify differences between both applications. The goal is to move validation away from a process based exclusively on manual testing and turn it into a process that is **automated, repeatable, and measurable**. ## Continuous Validation Throughout Modernization Another lesson we have learned from our projects is that validation should not be postponed until the end of the migration. If we wait until the entire process is complete before running the tests, any differences detected may be more difficult to analyze and fix. That is why our goal with Echo is to integrate validation directly into the modernization process. As the transformation progresses, we can rerun the scenarios and check how the behavior of the modernized application evolves. This allows us to establish a continuous cycle: Modernize→Test→Compare→Fix→↻ Test again In this way, validation stops being a final phase and becomes an integrated part of the process. ## What We Achieve with This Approach The goal behind developing Echo was not simply to automate a set of tests. We wanted to reduce one of the main sources of uncertainty in a modernization project: **how to demonstrate that functionality has not been lost during the transformation**. By automating validation, we can: - Reduce reliance on manual testing. - Reduce the amount of time required from functional experts. - Reuse scenarios based on real application usage. - Execute the same tests repeatedly. - Automatically compare the behavior of the original and modernized applications. - Detect differences more quickly. - Measure the level of coverage achieved. - Obtain traceability for the tests performed. - Integrate validation into the different phases of the project. ## From Modernization to Validation: Alight + Echo GO4IT Echo is part of a broader technology strategy within GO4IT Solutions. To automate code modernization, we have **GO4IT Alight**, our technology for transforming legacy applications into modern technologies. Alight uses an architecture based on grammars and deterministic rules to analyze the different elements of an application and generate their functional equivalents in the target technology. This allows us to approach transformation in a controlled, reproducible, and traceable way. However, transformation is only one part of the process. We need to verify that the result maintains the expected behavior, and that is where Echo comes in. ***Alight transforms and Echo validates.*** The combination of both technologies allows us to address two of the main challenges of modernization: automating the transformation and demonstrating that the result preserves the functionality of the original application. ## Frequently Asked Questions about modernized application validation What does it mean to validate an application after a migration? Validating an application after a migration means verifying that the modernized application maintains the behavior, processes, business logic, and response times of the original application. **Why is it difficult to validate a legacy application?** Legacy applications may have evolved over many years and contain a large number of rules, exceptions, and behaviors that are not always documented. In addition, the number of scenarios that need to be validated can make manual testing difficult to scale. **How does GO4IT Echo use the real behavior of an application?** Echo allows interactions performed on the original application to be recorded and converted into test scenarios that can later be replayed against the modernized application. The results from both versions can then be compared to identify differences. What is the difference between Alight and Echo? GO4IT Alight is used to automate the transformation of legacy applications into modern technologies. GO4IT Echo automates the validation of the modernized application. **What are the benefits of automating validation?** Automation reduces manual intervention, makes it possible to systematically repeat tests, enables differences to be detected more quickly, and provides metrics on testing and coverage achieved. When should validation be performed? Validation can be integrated throughout the different phases of modernization rather than being concentrated exclusively at the end of the project. This makes it possible to detect differences earlier and progressively validate the outcome of the transformation. GO4IT · No commitment Is your system a candidate for deterministic migration? Request a free POC ![Adrián Noguero | CEO](https://go4it.solutions/hs-fs/hubfs/Perfil.jpeg?width=96&height=96&name=Perfil.jpeg) ##### Adrián Noguero | CEO Adrián Noguero is CEO and software modernization specialist at GO4IT Solutions, where he leads projects focused on legacy application modernization, automated code transformation and functional validation. 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