--- title: "Consistency is not a localized property" slug: consistency-is-not-a-localized-property url: https://listedarticles.com/articles/consistency-is-not-a-localized-property canonical_url: https://n-the-loop.com/blog/consistency-is-not-a-localized-property/ content_type: essay language: en published_at: 2026-10-06T00:00:00.000Z updated_at: 2026-10-11T02:08:40.969Z authored_by: human publisher: "N The Loop" publisher_url: https://n-the-loop.com/ topics: ["Distributed Systems", "Software Engineering"] license: all-rights-reserved word_count: 305 reading_minutes: 1 citation: "N The Loop. \"Consistency is not a localized property.\" 6 Oct 2026. https://n-the-loop.com/blog/consistency-is-not-a-localized-property/ (all-rights-reserved)" # The full text follows. The web page shows an extract and sends readers # to the source above; quote the citation and link the canonical URL. --- # Consistency is not a localized property > A short N The Loop essay using Kafka's years-long exactly-once effort, which still needed redesign eight years later, to argue that consistency is an end-to-end property no single component can guarantee, so someone must understand and own the whole system rather than trusting machines to hold it. # Consistency is not a localized property Oct 6, 2026 It is a common pattern for developers to assume that consistency is something they can achieve inside a single component. Apache Kafka is the cautionary tale. Making writes exactly-once took a team of distributed systems engineers years and several new components and a complete rewrite of older components (Gustafson et al. 2016).1 And after all of that, the people who built it are blunt about what you get: > Exactly-once processing is an end-to-end guarantee and the application > has to be designed to not violate the property as > well.(Narkhede and Wang 2017) If the component cannot promise the property, the promise has to live somewhere else. Two practical consequences: * Someone has to **understand** the system end to end. Since no component can confirm it, something outside the components has to keep it all consistent. * Someone has to **own** it. Component owners will each correctly say their part works. A global property needs a person who is liable for the whole thing, with some guarantee that it makes sense. The lesson: do not hand global, valuable properties to machines and assume they are held. Such guarantees are grounded in understanding, not code. ## References Gustafson, Jason, Flavio Junqueira, Apurva Mehta, Sriram Subramanian, and Guozhang Wang. 2016. “KIP-98: Exactly Once Delivery and Transactional Messaging.” Apache Software Foundation. 2016. . Narkhede, Neha, and Guozhang Wang. 2017. “Exactly-Once Semantics Are Possible: Here’s How Kafka Does It.” Confluent. 2017. . Olshan, Justine, and Calvin Liu. 2022. “KIP-890: Transactions Server-Side Defense.” Apache Software Foundation. 2022. . --- 1. Eight years after it shipped, the protocol still had to be redesigned to close correctness holes that could `violate EOS` (Olshan and Liu 2022). ↩︎