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> * our enterprise db was bursting at the seams containing Literally Everything. Now, every part of the split up monolith has it's own self contained data store tailored to what is appropriate for that particular thing. (some use MariaDB, others Redis etc etc)

Why do you consider an enterprise DB "bursting at the seams" to be a bad thing? Isn't that what enterprise DBs are built for? Seems like you traded having everything in one large database to having everything scattered in different databases. You probably sacrificed some referential integrity in the process.

> * developing, building, testing and deploying took ages. Eg if I only needed to capture some new detail about a business partner user (eg their mfa preference app vs sms) I would still have to do deal with the unwieldy monolith. Now, I can do it in the dedicated business partner user service, which is much easier and faster.

You traded a clean codebase with a solid toolchain for probably a template repository that you hope your users use or everyone is reinventing some kind of linting/testing/deployment toolchain for every microservice

> * the whole monolith, including business partner facing operations, could go down because of issues to do with completely unrelated, non critical things like eg internal staff vacation hours.

This could apply to any software. Sure, a monolith can have a large blast radius, but I can guarantee one of your microservices is critical path and would cause the same outage if it goes offline.

> The few callers that do need to obtain both pieces of data just make concurrent calls to both and them zip them into a single result.

Almost like a database join?

Who says the monolith we had was a clean codebase with a solid toolchain? It was a giant piece of software that had existed since the 90s, with hundreds of developers working on it at any given moment, thousands of people coming through it over the years, full of bizarre things that would make anyone scream and tear their hair out. (each of which may have been the right thing to do at the time, but combined over the years wasn't optimal to say the least)

It's obvious nothing will convince you, but I maintain that eg internal vacation hours should not live in the same codebase as partner facing business critical things.

You can very well have a bad monolith and still think micro services is a bad idea, and his concerns especially around referential integrity are perfectly valid and valuable. Maybe all you needed is to fix the existing monolith incrementally?

Of course microservices should be sliced with referential integrity in mind. But a monolith by definition has everything, including utterly unrelated things that have absolutely nothing to do with each other, bundled up into a single, giant, well, monolith. What referential integrity could there possibly be between eg business partner user profiles and internal staff vacation hours?

There is nothing inherently bad about storing unrelated things in the same db, in general.

There might be security issues but good DBs offer security grants that are granular enough to deal with this.

Data sovereignty might be an issue forcing you to split, that depends on the domain and application requirements.

But in general two things being unrelated isn’t a reason to split the DB and splitting the DB isn’t a reason to split into services.

I’m not for or against microservices. I’m definitely for good reasoning though!

> Who says the monolith we had was a clean codebase with a solid toolchain?

You didn't. They're just trying to force the issue for some reason by straw manning your decision.

No offense, but someone coming in and splitting out a database into multiple bespoke data stores because the primary database is "bursting at the seams" is basically my nightmare. If your description of your reasoning is accurate, it's a completely broken mental model.

Well, that's not what happened. The whole org from devs to the board were highly aware of the problems and knew and agreed what had to be done. I'm just one of the many people who architected and implemented the change.

Could you be specific about the size of your company and the size of the data you're processing? And could you also be more specific about the business value it created? Are you an investor in the business? Is it your own money on the line?

Publicly listed, ~13 billion USD revenue, 22k employees, millions of b2b partners and users.

When the monolith goes down, it's hundreds of thousands, sometimes millions of dollars of revenue lost both during and as a result of an outage.

You really, really don't want a problem with eg internal staff vacation hours to do that.

Ok, interesting. That kind of scale is not exactly the norm though, so I don't agree with your implication that monoliths are always a bad idea.

I really don't know why anyone who wasn't so resource strapped as to have no alternative would want their $$$ services sharing a build-test-deploy-rollback lifecycle or database with the internal staff vacation hours tracker.

It seems like the real solution here is to outsource vacation hour tracking to one of the HR apps for the purpose (see Workday) instead of having the domain expert engineers split the company’s application apart into micro services so they can have a custom vacation tracker.

It was just one of the worst examples. A more obvious case would be the different lines of business. They have very little to do with each other and shouldn't share code or data. Another is business partners vs end users (regular people) - there's a lot if reasons, including legal ones, why they shouldn't share code or data.

That makes sense, but is nowhere near universally applicable. For folks working in most startups, talking about different lines of business is non-sensical - there's generally one product.

It sounds like you're describing a monolith that actually contained multiple completely independent applications - which I don't think anyone would disagree with being a good case for splitting.

In most cases I've seen, the decision to split into microservices usually involves a fairly high amount of dependency between different services to achieve a common business goal (hence the concerns about things like referential integrity)