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monolithic large relational databases are hard to scaleDB2 on z/OS was able handle billions of queries per day.
In 1999.
Some greybeards took great delight in telling me this sometime around 2010 when I was visiting a development lab.
> When you have one large database with tons of interdependencies, it makes migrating data, and making schema changes much harder.
Another way to say this is that when you have a tool ferociously and consistently protecting the integrity of all your data against a very wide range of mistakes, you have to sometimes do boring things like fix your mistakes before proceeding.
> In theory better application design would have separate upstream data services fetch the resources they are responsible for.
A join in the application is still a join. Except it is slower, harder to write, more likely to be wrong and mathematically guaranteed to run into transaction anomalies.
I think non-relational datastores have their place. Really. There are certain kinds of traffic patterns in which it makes sense to accept the tradeoffs.
But they are few. We ought to demand substantial, demonstrable business value, far outweighing the risks, before being prepared to surrender the kinds of guarantees that a RDBMS is able to provide.