How to ensure data validity when using memcache php 7.2.x?

Question

PHP

If there is no data in memcache for a given key, data is retrieved from the database using PDO, and we write this data to memcache. Then we add a new record to the database. How to provide valid data for the user? Especially in the situation if the sql query is complex and has many interrelated elements. I heard about tag in memcache, but I didn't understand everything.