--- title: Using event metadata in event-driven projections | daily.dev description: When building event-driven projections, you sometimes need data (like a client ID) that isn't repeated across all events in a stream. Rather than adding... canonical: https://daily.dev/posts/using-event-metadata-in-event-driven-projections-quoiswh1k twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Using event metadata in event-driven projections | daily.dev og:description: When building event-driven projections, you sometimes need data (like a client ID) that isn't repeated across all events in a stream. Rather than adding... og:url: https://daily.dev/posts/using-event-metadata-in-event-driven-projections-quoiswh1k og:image: https://api.daily.dev/og/posts/QuOiSWH1k.png og:image:alt: Using event metadata in event-driven projections 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. # Using event metadata in event-driven projections **[Event-Driven by Oskar Dudycz](https://daily.dev/sources/event-driven-io)** · 9 min read · 0 upvotes · 0 comments ## Summary When building event-driven projections, you sometimes need data (like a client ID) that isn't repeated across all events in a stream. Rather than adding redundant fields to every event or coupling read models together, you can store contextual data in event metadata. Using a shopping cart example in TypeScript with Emmett and Pongo, the post demonstrates how to attach a clientId to event metadata at the command level, then use it in multi-stream projections to correlate events with the correct read model document. The approach is presented as a valid tradeoff, not a universal solution, with guidance on when metadata use is appropriate versus when it becomes a 'bag for random data'. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: --- Tags: [#architecture](https://daily.dev/tags/architecture), [#typescript](https://daily.dev/tags/typescript) [View this post on daily.dev](https://daily.dev/posts/using-event-metadata-in-event-driven-projections-quoiswh1k) ```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":"TechArticle","headline":"Using event metadata in event-driven projections","url":"https://daily.dev/posts/using-event-metadata-in-event-driven-projections-quoiswh1k","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/using-event-metadata-in-event-driven-projections-quoiswh1k"},"datePublished":"2026-06-17T11:26:16.337Z","dateModified":"2026-06-17T11:32:42.607Z","description":"When building event-driven projections, you sometimes need data (like a client ID) that isn't repeated across all events in a stream. 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