is last write wins redundant for immutable keys?

Question

In the book Designing Data-Intensive Applications > chapter-5 > Leaderless replication > Detecting Concurrent Writes, below is what Author says while talking about Last write wins (LWW)

The only safe way of using a database with LWW is to ensure that a key is only written once and thereafter treated as immutable, thus avoiding any concurrent updates to the same key. For example, a recommended way of using Cassandra is to use a UUID as the key, thus giving each write operation a unique key

If keys are immutable, why do I even need LWW? Am I missing something here?

<opinion>LWW sounds like a stupid way to design a database.</opinion>. Most modern RDBMs have "transactions" to deal with "simultaneous" writes. That usually involves "rolling back" one of the competing clients. I have used databases for over 30 years; I have never heard of LWW. — Rick James
– Rick James, Commented May 12 at 21:39

Shah Kushal · Accepted Answer · 2025-03-16 05:52:05Z

0

I believe you are understanding this in a wrong sense.

You do not

Need LWW if keys are immutable.

You should

Use keys that are immutable if DB is of type LWW.

The pre-condition here is not that "keys are immutable". Precondition is "DB is LWW"; and conclusion is not "Need LWW". Conclusion is "given pre-condition DB is LWW, you need to make keys immutable"

answered Mar 16 at 5:52

Shah Kushal

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4 Comments

wenn32 Mar 16 at 6:18

Please correct, why would I need LWW either way? if keys are immutable then technically there shouldn't be any conflicts. Hence no need for LWW conflict resolution? Thank you

Shah Kushal Mar 16 at 6:53

Keys are not immutable by default. Keys being immutable is a guideline you follow to make the best of an LWW db. You "might" as well have no immutable keys and an LWW db, if that suits ur purpose. Immutable keys is just a suggestion for strongest consistency.

wenn32 Mar 16 at 7:13

I think I get your point. Would you say if I use UUID as the key, does it makes any sense to use LWW (or any particular usecase)?

Shah Kushal Mar 16 at 7:34

Nope, it does not make sense for the particular case. However, the choice you have is about making UUID as the key and not about having LWW database. However, I can suggest an example where even mutable keys make sense with LWW database. Say, a counter whose only work is to add one to the db.

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