Why is the in-sync replica (ISR) set important for durability?

The ISR is the set of replicas — leader plus any followers — that are fully caught up with the leader's log at a given moment. It matters because it's the actual pool of candidates eligible to become the new leader if the current one fails, and it's what acks=all and min.insync.replicas are checking against, not the full, nominal replica count.

A topic can have a replication factor of 3 on paper, but if two of those three replicas have fallen behind (say, due to a slow disk or network issue) and dropped out of the ISR, the effective durability at that moment is whatever a single in-sync replica provides — the nominal replication factor overstates the real protection until those followers catch back up and rejoin the ISR. This is exactly why min.insync.replicas exists: it lets a broker refuse writes outright rather than silently accepting them with weaker-than-intended durability during a period when the ISR has shrunk, making the durability gap visible as write failures instead of an invisible risk.

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.