--- title: Replay testing is not magic, but it feels like it. It is... description: Replay testing is a technique for event-sourced systems that replays the entire production event history through a new code version to validate backward... canonical: https://daily.dev/posts/replay-testing-is-not-magic-but-it-feels-like-it-it-is-the-only-type-of-test-that-works-with-100--t2gce5srg twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Replay testing is not magic, but it feels like it. It is the only type of test that works with 100% realistic data without taking the risks of testing in production. | daily.dev og:description: Replay testing is a technique for event-sourced systems that replays the entire production event history through a new code version to validate backward... og:url: https://daily.dev/posts/replay-testing-is-not-magic-but-it-feels-like-it-it-is-the-only-type-of-test-that-works-with-100--t2gce5srg og:image: https://api.daily.dev/og/posts/t2GCe5srg.png og:image:alt: Replay testing is not magic, but it feels like it. It is the only type of test that works with 100% realistic data without taking the risks of testing in production. og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Replay testing is not magic, but it feels like it. It is the only type of test that works with 100% realistic data without taking the risks of testing in production. **[PHP Dev](https://daily.dev/sources/phpdev)** · [@sebastianbergmann](https://daily.dev/sebastianbergmann) · 1 upvotes · 0 comments ## Summary Replay testing is a technique for event-sourced systems that replays the entire production event history through a new code version to validate backward compatibility and correctness. Two oracle types are covered: implicit oracles (crash-free execution) and explicit oracles (comparing final state against known production snapshots). The approach enables A/B testing of new business logic against historical data without production risk. Key practical challenges include handling non-determinism (clocks, external APIs), event schema evolution via upcasting, silencing side-effect adapters to prevent real emails or payments, and GDPR compliance through crypto shredding or pseudonymisation. Replay testing complements—not replaces—unit and end-to-end tests, serving as the ultimate regression safety net for complex domain refactorings. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [Debugging as a design goal](https://daily.dev/posts/debugging-as-a-design-goal-p6g6p3nlp) · PHPUnit Expert · 13 upvotes · 1 comments - [To test, or not to Test? Part 3 – Make it easier to recover from a defect](https://daily.dev/posts/to-test-or-not-to-test-part-3-make-it-easier-to-recover-from-a-defect-u4fa1chcv) · We Are .NET · 0 upvotes · 0 comments - [A better way to end-to-end test your webapp](https://daily.dev/posts/a-better-way-to-end-to-end-test-your-webapp-gxhq7phhr) · swizec.com · 3 upvotes · 0 comments --- Tags: [#testing](https://daily.dev/tags/testing), [#architecture](https://daily.dev/tags/architecture), [#phpunit](https://daily.dev/tags/phpunit) [View this post on daily.dev](https://daily.dev/posts/replay-testing-is-not-magic-but-it-feels-like-it-it-is-the-only-type-of-test-that-works-with-100--t2gce5srg) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"DiscussionForumPosting","mainEntityOfPage":"https://daily.dev/posts/replay-testing-is-not-magic-but-it-feels-like-it-it-is-the-only-type-of-test-that-works-with-100--t2gce5srg","headline":"Replay testing is not magic, but it feels like it. 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