---
title: "How to test event-driven projections"
url: https://daily.dev/posts/how-to-test-event-driven-projections-xbtaxzu5p
source_url: https://event-driven.io/en/testing_event_driven_projections
type: article
source: "Event-Driven by Oskar Dudycz"
published: 2026-06-17T11:26:00.629Z
updated: 2026-06-17T11:31:42.534Z
tags: ["testing", "architecture", "typescript", "mongodb"]
reading_time: 11
upvotes: 0
comments: 0
language: en
---

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# How to test event-driven projections

**[Event-Driven by Oskar Dudycz](https://daily.dev/sources/event-driven-io)** · 11 min read · 0 upvotes · 0 comments

## Summary

A practical guide to integration testing event-driven projections using a shopping cart example with Node.js, EventStoreDB, and MongoDB. The author demonstrates a Given/When/Then specification pattern for verifying that projections correctly build read models from event sequences. Key challenges addressed include eventual consistency, idempotency, and at-least-once delivery guarantees. The post includes a custom 130-line test specification builder that handles event metadata, stream revisions, and retrying reads for eventually consistent MongoDB results.

## Full article

daily.dev links to this article rather than hosting it. Read it at the original source: <https://event-driven.io/en/testing_event_driven_projections>

---

Tags: [#testing](https://daily.dev/tags/testing), [#architecture](https://daily.dev/tags/architecture), [#typescript](https://daily.dev/tags/typescript), [#mongodb](https://daily.dev/tags/mongodb)

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