Consistency is not a localized property

# Consistency is not a localized property Source: [https://n-the-loop.com/blog/consistency-is-not-a-localized-property/](https://n-the-loop.com/blog/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 \([Gustafson et al\. 2016](https://n-the-loop.com/blog/consistency-is-not-a-localized-property/#citeproc_bib_item_1)\)\.[1](https://n-the-loop.com/blog/consistency-is-not-a-localized-property/#fn: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\.\([Narkhede and Wang 2017](https://n-the-loop.com/blog/consistency-is-not-a-localized-property/#citeproc_bib_item_2)\) 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\.[https://cwiki\.apache\.org/confluence/display/KAFKA/KIP\-98\+\-\+Exactly\+Once\+Delivery\+and\+Transactional\+Messaging](https://cwiki.apache.org/confluence/display/KAFKA/KIP-98+-+Exactly+Once+Delivery+and+Transactional+Messaging)\. Narkhede, Neha, and Guozhang Wang\. 2017\. “Exactly\-Once Semantics Are Possible: Here’s How Kafka Does It\.” Confluent\. 2017\.[https://www\.confluent\.io/blog/exactly\-once\-semantics\-are\-possible\-heres\-how\-apache\-kafka\-does\-it/](https://www.confluent.io/blog/exactly-once-semantics-are-possible-heres-how-apache-kafka-does-it/)\. Olshan, Justine, and Calvin Liu\. 2022\. “KIP\-890: Transactions Server\-Side Defense\.” Apache Software Foundation\. 2022\.[https://cwiki\.apache\.org/confluence/display/KAFKA/KIP\-890:\+Transactions\+Server\-Side\+Defense](https://cwiki.apache.org/confluence/display/KAFKA/KIP-890:+Transactions+Server-Side+Defense)\. [← All posts](https://n-the-loop.com/blog/)