Home → Magazine Archive → January 2016 (Vol. 59, No. 1) → Immutability Changes Everything → Abstract

Immutability Changes Everything

By Pat Helland

Communications of the ACM, Vol. 59 No. 1, Pages 64-70

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There is an inexorable trend toward storing and sending immutable data. We need immutability to coordinate at a distance, and we can afford immutability as storage gets cheaper. This article offers an amuse-bouche of repeated patterns of computing that leverage immutability. Climbing up and down the compute stack really does yield a sense of déjè vu all over again.

It was not that long ago that computation was expensive, disk storage was expensive, DRAM (dynamic random access memory) was expensive, but coordination with latches was cheap. Now all these have changed using cheap computation (with many-core), cheap commodity disks, and cheap DRAM and SSDs (solid-state drives), while coordination with latches has become harder because latch latency loses lots of instruction opportunities. Keeping immutable copies of lots of data is now affordable, and one payoff is reduced coordination challenges.


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