Let us assume there is a Berserker Barbarian who is currently concentrating on a smite spell such as wrathful smite. Because they are concentrating on a spell the following applies:
Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration...
Notably this does not require their action, bonus action, or reaction at all. However, the Barbarian also has the Retaliation feature which states:
When you take damage from a creature that is within 5 feet of you, you can use your reaction to make a melee weapon attack against that creature.
This does require your reaction but it has the same trigger as a concentration check (taking damage).
Xanathar's Guide to Everything has an optional rule (page 77) on "Simultaneous Effects" which states:
In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table - whether player or DM - who controls that creature decides the order in which those things happen.
I am unsure whether these still count as happening at the same time when one of them requires a reaction and the other does not.
Do these effects occur in a specific order or does the active player/GM decide their order?