September 17th, 2016
(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at: email@example.com
Consistently is overvalued.
Requiring consistency in distributed system generally leads to designs that reduces availability.
Which is one of the reasons that bank transactions generally do not rely on transactional updates against your bank. “Low level” operations as part of settlement may us transactions, but the bank system is “designed” (more like it has grown by accretion) to function almost entirely by settlement and reconciliation rather than holding onto any notion of consistency.
The real world rarely involves having a consistent view of anything. We often design software with consistency guarantees that are pointless because the guarantees can only hold until the data has been output, and are often obsolete before the user has even seen it.
That’s not to say that there are no places where consistency matters, but often it matters because of thoughtless designs elsewhere that ends up demanding unnecessary locks and killing throughput, failing if connectivity to some canonical data store happens to be unavailable etc.
The places where we can’t design systems to function without consistently guarantees are few and far between.