Its also extremely important to look into the use case of the setup.
It is quite possible that only small handful of data operations would need such level of database consistency. Where you can have large scale distributed DB for almost everything but one API endpoint.
Facebook for example is known to still extensively use MySQL throughout their system, during such situations (exactly where however is unknown).
Majority of data operations would then typically use "eventual consistency" where it makes alot more sense at their scale.
With some rather careful planning of API and data flows, along with DB sharding. And of course time to invest in the relevant dev work... it can be quite surprising how little data operations need true complete consistency.
Of course yes. Going that big, it's important to have a well-defined operations workflow.
Once you're using replicated MySQL with nontrivial lag among replicas (thing US to Europe or Asia), it's eventually consistent anyway unless you force your reads to go to the master.
Facebook's backend is still pretty much all MySQL, but backing another system called Tao.
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