Data Consistency

What is Referential Integrity?

Referential integrity is a database rule ensuring that a foreign key value in one table always matches an existing primary key value in the referenced table, p…

Hot Backup vs Cold Backup: What is the Difference?

A hot backup is taken while the database stays online and serving reads and writes, using transaction-consistent snapshots or log coordination, whereas a cold…

What are the Main Cache Invalidation Strategies?

Cache invalidation removes or refreshes stale cached data using three common strategies: TTL-based expiration (entries auto-expire after a fixed time), explici…

What are the Trade-offs of Caching Database Query Results?

Caching query results trades a risk of serving stale data and added system complexity for dramatically lower read latency and reduced load on the database, and…

What is an Invariant in OOP?

A class invariant is a condition about an object’s state that must hold true at every point the object is observable from outside — after construction complete…

What is the Unit of Work Pattern?

The Unit of Work pattern tracks a set of business object changes made during a single logical operation and coordinates writing them out as one atomic transact…

What is Journaling in a File System and Why Does It Matter?

Journaling is a technique where a file system writes a record of an intended metadata (and optionally data) change to a dedicated log before applying that chan…

Write-Through vs Write-Back Cache: What is the Difference?

Write-through caching writes to the cache and the underlying database synchronously on every write, trading extra write latency for strong consistency, while w…

What is Database Replication in System Design?

Database replication is the process of copying data from a primary database to one or more replica databases so the system gains redundancy for fault tolerance…

How Would You Design a Payment System?

A payment system is designed around an idempotent, append-only ledger where every state transition (authorize, capture, settle, refund) is recorded as an immut…

How Would You Design an E-Commerce Checkout Flow?

An e-commerce checkout is designed as a short-lived, stateful order flow that reserves inventory optimistically, computes pricing and tax atomically at the mom…

How Would You Design a Hotel Booking System?

A hotel booking system is designed around a date-ranged inventory model where each room type’s availability is tracked per night, reservations acquire a short-…

How Would You Design a Distributed Lock?

A distributed lock is a coordination primitive that lets multiple independent processes across different machines agree that only one of them may hold a named…

How to Design an Inventory Management System?

An inventory management system is designed around a source-of-truth stock ledger per SKU-per-warehouse with atomic reserve/commit/release operations, an event…

How to Design a Collaborative Document Editor

A collaborative document editor (like Google Docs) lets multiple users edit the same document simultaneously by converting each keystroke into an operation, br…

How to Design a Bidding System?

A bidding system (such as a real-time ad auction or an online auction house) is designed around a strongly consistent bid-acceptance path that atomically valid…

What Are Cache Invalidation Strategies?

Cache invalidation strategies are the mechanisms a system uses to remove or refresh stale cached data once the underlying source of truth changes, and the two…

What is a Write-Ahead Log (WAL) and Why Do Systems Use It?

A write-ahead log is an append-only file that records every change durably before it is applied to the actual data structures on disk, so a crash can never los…

What are Distributed Transactions and How Do They Work?

A distributed transaction is a set of operations spanning multiple independent services or databases that must all succeed or all fail together, typically coor…

What is the Outbox Pattern?

The outbox pattern solves the dual-write problem by writing both a business state change and the event describing it into the same local database transaction,…

What is Change Data Capture (CDC)?

Change data capture is a technique for detecting and streaming every row-level insert, update, and delete made to a database as an ordered sequence of events,…

Exactly-Once vs At-Least-Once Delivery: What Is the Difference?

At-least-once delivery guarantees a message is delivered one or more times (never lost, but possibly duplicated), while exactly-once delivery guarantees a mess…

What Are Message Ordering Guarantees?

Message ordering guarantees define whether and how consumers are assured to see messages in the same sequence producers sent them, ranging from no ordering at…

How Do You Design Idempotent Writes in a Distributed System?

Designing idempotent writes in a distributed system means ensuring that applying the same write operation multiple times — due to retries, at-least-once messag…