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.
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