[perlgeek]

So, the first solution to consider is if this needs a technical solution at all.

If it happens infrequently enough, you can have simply have a proceed "change mortgage rate" that touches the 4 legacy databases. Done.

If it does need a technical solution, you have to establish the leading system / source of truth / whatever you call it. This can be either an existing system or a new one. And then all the others need to follow the leading system.

A more distributed approach (every of the 4 legacy systems can generate events that the other 3 work with) likely won't work with 4 legacy systems. It's often hard enough to get high-fidelity events/deltas out of a single system.

[PaulRobinson]

There might be an addendum to this good answer: OP seems to be thinking they all need to show the same rates at the same time, so in essence you're looking for a "distributed transaction", which, as we all know... well, exercise for the reader and all that.

However, one technique I've used which works with all your approaches and guarantees consistency if the data model allows it is to have "future value" entries.

If you just have a single value:

    mortgage_rate_percent: 3.0

And this is in 4 databases, there will be a period where they don't match. But if you put a future "valid_from" in there:

    mortgage_rate_percent: 3.0
    valid_from: 2020-04-01 00:00:00-0000

Well, you now have the ability to have the rates all input at different times in different databases, and by querying for only the most recent one (or use from/to dates and find the intersection), they all become active at the exact same moment in time (clock drift aside) as time passes. No magic needed.

This seems really obvious once you've seen it, but it's amazing how many people try and worry about source of truth, synchronisation and distributed transaction problems when in many cases (like this), the change can be planned so you can just say "this value will apply in the future" and you have ample time to make sure it gets synch'ed before that time and everything "just works"

[chrisacky]

> This seems really obvious once you've seen it

I think that's true for 90% of patterns/problems in dev. Did you see that Prince of Persia post the other day? How the original creator came up with Shadow Man? Kind of resonates with what you are getting at in being obvious.

I embarrassingly have a 8 year old top StackOverflow post asking what a fluent interface was. Keeping in mind I'd been devving for 5+ years already at that point. It was just a "ahh F* yeah, that's obvious" moment.

[todaysAI]

There could be a rate-change once a week on average. Of course if there is a central bank change then probably every mortgage product will need to be changed that day.

Rates are the important data-item but some of the legacy DBs will need to be notified of changes like fees, as well.