Real Time Systems

What is a Circular Buffer (Ring Buffer)?

A circular buffer (or ring buffer) is a fixed-size array that wraps around on itself, using head and tail indices with modulo arithmetic so that writes and rea…

Online vs Offline Algorithms: What is the Difference?

An online algorithm processes input piece by piece as it arrives, making irrevocable decisions without knowledge of future input, while an offline algorithm ha…

What is Priority Scheduling?

Priority scheduling dispatches the ready process with the highest priority value next, rather than by arrival order or burst time, and can be implemented as ei…

What is Earliest Deadline First (EDF) Scheduling?

Earliest Deadline First (EDF) is a dynamic-priority, preemptive real-time scheduling algorithm that always runs whichever ready task has the closest absolute d…

What is Priority Inversion?

Priority inversion is a scheduling hazard where a high-priority task is indirectly blocked by a low-priority task holding a lock it needs, while an unrelated m…

Cooperative vs Preemptive Multitasking

Cooperative multitasking relies on each running task voluntarily yielding the CPU back to the scheduler, while preemptive multitasking lets the OS forcibly int…

What is Rate Monotonic Scheduling?

Rate Monotonic Scheduling (RMS) is a static-priority, preemptive real-time scheduling algorithm that assigns each periodic task a fixed priority inversely prop…

What is Dispatcher Latency?

Dispatcher latency is the time the OS dispatcher takes to stop one process and start another once the scheduler has decided which task should run next — it cov…

How Does Interrupt Handling Work in an OS?

Interrupt handling is the sequence by which the CPU detects an interrupt request, saves the currently running task’s state, looks up and runs a dedicated handl…

What Are Nested Interrupts and How Are They Handled?

A nested interrupt occurs when a higher-priority interrupt is allowed to preempt a currently executing lower-priority interrupt handler, so the CPU suspends th…

Symmetric vs Asymmetric Multiprocessing (SMP vs AMP)

Symmetric multiprocessing (SMP) has every CPU core run the same copy of the OS kernel and share equal access to memory and I/O, while asymmetric multiprocessin…

What is Energy-Aware Scheduling in an OS?

Energy-aware scheduling is an OS scheduling policy that chooses which task runs on which core, and at what performance level, based on energy cost rather than…

What Makes Embedded OS Design Different from a General-Purpose OS?

Embedded OS design prioritizes deterministic, bounded-latency execution and a tiny, predictable memory and code footprint over the throughput and general flexi…

How Would You Design a Ride-Matching System?

A ride-matching system is designed around a geospatial index (such as geohashing or an S2/H3 grid) that partitions drivers and riders into cells so nearby matc…

How Would You Design a Food Delivery System?

A food delivery system is a three-sided marketplace connecting customers, restaurants, and delivery drivers, built around order management, real-time location…

How Would You Design a Real-Time Leaderboard?

A real-time leaderboard ranks a large, constantly updating set of scores and answers “what is this player’s rank” and “who are the top N” in near-constant time…

How Would You Design an Online Code Editor (like Repl.it)?

Design an online code editor as three layers: a collaborative document layer using operational transformation or CRDTs for real-time multi-cursor editing, a sa…

How Do You Design a Live Streaming Service?

A live streaming service ingests a broadcaster’s video via a low-latency protocol (RTMP or WebRTC), transcodes it into multiple bitrates in near real time, pac…

How Do You Design a Comment System?

A comment system stores comments as nested or flat records keyed by the parent content ID, uses a database that supports efficient tree or adjacency-list queri…

How to Design a Parking Garage System?

A parking garage system models floors, rows and spots as a hierarchy with real-time availability counters, uses atomic decrement/increment operations at entry…

How to Design an Elevator System?

An elevator system is designed around a central dispatcher that models each car’s state (position, direction, queued stops) and assigns hall calls to the car t…

How to Design a Fraud Detection System

A fraud detection system scores every transaction in real time by combining a low-latency rules engine for known bad patterns with a machine-learning model for…

How to Design a Chat Presence System

A presence system tracks whether each user is online, offline, or away by having clients maintain a persistent connection (WebSocket) with periodic heartbeats…

How to Design a Group Chat System

A group chat system stores each group's membership and messages durably, fans out new messages to every online member's connection via a messaging service, and…