Foundations
What a database is, what a DBMS adds over a pile of files, the three-schema architecture, and the family of database systems.
A database is structured persistent state with concurrent access; a DBMS is the system that makes that practical to build apps against. The Foundations topic answers the questions an interviewer leads with: 'why do we have databases?', 'what does ACID actually mean?', 'how is a relational database different from a key-value store?'. Get these clear and the rest of the workbook becomes detail.
The move from file-based systems to databases is the original case study — once two programs share state, you need a system to manage redundancy, integrity, concurrency, and recovery. Every modern DBMS exists because file-based systems can't do those four jobs together.
Key concepts
- Database = structured, queryable, shared, persistent state
- DBMS = the system that gives those four properties guarantees
- Three-schema architecture decouples conceptual / logical / physical layers
- Data independence is the property that schema changes at one level don't cascade
- Different families (relational, document, key-value, graph, columnar) optimise for different workloads
Reference template
// Picking a database family
1. What's the access pattern? (point lookups? range scans? joins? graph traversal?)
2. What's the consistency need? (strong? eventual? per-key?)
3. What's the scale axis? (write throughput? read latency? data volume?)
4. What's the schema flexibility? (rigid? evolving? heterogeneous?)
5. What's the operational cost? (managed service? self-host? team skills?)Adapt to your problem; the structure is the load-bearing part.
Common pitfalls
- Reaching for NoSQL because 'it's web-scale' — the relational model is almost always the right starting point
- Conflating database (the data) with DBMS (the system) — interviewers will pick this up
- Treating ACID as binary — every property has gradations in production
- Picking a system before understanding the access pattern — the wrong choice is expensive to undo
Related topics
Items (4)
- What Is a Database?
A shared, structured, queryable, persistent collection of data — and the DBMS that gives those four properties teeth.
Concept Foundational - Why Not Files? The Case for a DBMS
The limits of file-based systems that drove the move to databases — redundancy, integrity, concurrency, recovery.
Concept Foundational - DBMS Architecture — The Three-Schema Model
Conceptual, logical, and physical schemas; data independence; how a schema change at one level shouldn't break the others.
Concept Foundational - Classification of Database Systems
Relational vs document vs key-value vs graph vs columnar vs time-series. What each is good at and where the boundaries blur.
Concept Foundational