Since we're discussing misunderstandings in the community, it should be pointed out that a Database Management System (DBMS) is not merely a database, to say nothing of a data store. Oracle, Postgres, et al are genuine "DBMS". ATM you are very likely putting together a data store in your app code.
I interpreted the OP as DBMS=database, which absolutely includes application code that stores and retrieves data in proprietary formats.
Even a linked list mmap'd to disk can be a database, just maybe not a very good one.
To be fair this is a somewhat fuzzy categorization (DBMS, DB, Data Store) and it can cause confusion. An DBMS is a system, just like it says right on the acronym tin. A DBMS is a system that provides capabilities such DDL, DML, auxiliary processes, etc. to manage a database. There are various data models, e.g. a triple store, for databases, but the data model X is a orthogonal to XDBMS.