Question 1

What does ACID mean in databases?

Accepted Answer

ACID names four guarantees a database provides around transactions.

Atomicity means all or nothing. A transaction that updates several rows either applies completely or leaves no trace. A crash midway cannot produce half the change.

Consistency means the database moves from one valid state to another, respecting the constraints you defined. A transaction that would violate a uniqueness rule or a foreign key is rejected rather than committed.

Isolation means concurrent transactions do not interfere in ways that produce incorrect results. This is the most nuanced of the four, because it is not binary. Databases offer isolation levels trading strictness for concurrency, and the default is often weaker than people assume, permitting anomalies such as reading a value that another transaction changes before you write based on it.

Durability means once a transaction commits, it survives a crash. The data is on stable storage, not merely in memory.

The practical significance is that these move burden off your application. Without them, safely transferring a value between two records requires implementing your own recovery.

Two caveats matter in real systems. Isolation is the one to actually think about, because the default level allows behaviours that break read-then-write logic under concurrency; if that pattern matters, use explicit locking or a stricter level rather than assuming the default protects you. And durability depends on configuration, since some databases and cloud setups acknowledge a commit before it is fully persisted, trading a small window of risk for latency.

Many non-relational databases now offer transactions too, so the old framing of ACID as exclusively relational is out of date.

Question 2

When should I use a database transaction?

Accepted Answer

The rule is simple to state: use a transaction whenever two or more writes must either all succeed or all fail. If a partial result would leave your data in a state that should never exist, those

Question 3

What is database normalisation, and when should you denormalise?

Accepted Answer

Normalisation organises data so that each fact is stored exactly once, eliminating redundancy and the update anomalies that come with it. Denormalisation deliberately reintroduces redundancy to make

Question 4

What is state management, and why is it hard?

Accepted Answer

State is any data that changes over time and affects behaviour. Managing it is hard because state creates dependencies across time — and time is the dimension our reasoning handles worst. Why it is

Question 5

What are the common caching strategies?

Accepted Answer

A small number of patterns that differ in who writes to the cache and when — and choosing the wrong one for your consistency requirements is where cache bugs come from. The read patterns:

Question 6

What is the difference between SQL and NoSQL databases?

Accepted Answer

The distinction is less about query language than about what the database guarantees and what shape of data it is optimised for. Relational databases store data in tables with a fixed schema and

Question 7

What are mutexes, semaphores and atomic operations?

Accepted Answer

Three mechanisms for coordinating concurrent access to shared state, at different levels of abstraction — and choosing the wrong one is how concurrency bugs are introduced. Atomic operations. A

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