Which of the following is NOT a core benefit of load balancing?

Load balancing does not guarantee request ordering across servers. It distributes requests for performance and availability, but order depends on implementation details and network timing.

In the pizza delivery analogy for load balancing, what does the dispatcher represent?

The dispatcher intelligently assigns orders to drivers to prevent overload. This mirrors how a load balancer distributes requests across servers.

What happens to a system without load balancing when one popular server becomes a bottleneck?

Without load balancing, requests pile up in a queue on the bottlenecked server, degrading performance for all users as the system lacks mechanism to distribute load.

Which statement about fault tolerance in load balanced systems is correct?

Load balancing enables fault tolerance by detecting failed servers and rerouting traffic. A failed server doesn't bring down the whole system because traffic goes elsewhere.

In Round-Robin load balancing with 3 servers, how many requests arrive before Server 1 receives another request?

Round-Robin cycles through servers in order: S1, S2, S3, S1, S2, S3. Server 1 receives requests 1, 4, 7, etc. Three requests must arrive for Server 1's next turn.

When would Round-Robin load balancing perform poorly?

Round-Robin ignores actual server load. If one request takes 10 seconds while others take 10ms, that server sits idle while the slow-request server gets overloaded.

What is the main advantage of Weighted Round-Robin over standard Round-Robin?

Weighted Round-Robin lets you assign higher weights to more powerful servers so they receive proportionally more traffic, matching their capacity.

In Least-Connections algorithm, how does the load balancer decide which server gets the next request?

Least-Connections counts active connections on each server and routes to the one with the lowest count, mimicking choosing the shortest checkout line.

Which scenario favors Least-Connections over Round-Robin?

Least-Connections excels when request durations vary widely. It avoids overloading servers busy with slow requests, unlike Round-Robin which assigns blindly.

What is a key benefit of IP Hash load balancing?

IP Hash uses client IP to compute a hash, routing same IP to same server deterministically without requiring cookies or session state.

What happens in IP Hash when a server is removed from the load balancer?

IP Hash uses a static mapping function. Removing a server breaks most mappings, forcing clients to different servers. This is why consistent hashing was invented.

At which OSI layer does Layer 4 load balancing make routing decisions?

Layer 4 operates at the Transport layer, examining only IP addresses, ports, and TCP/UDP headers without reading application content.

Which of the following is a routing decision Layer 4 load balancer CANNOT make?

Layer 4 only sees IP and port. It cannot examine HTTP URLs or paths, so it cannot distinguish /api from /images requests.

What is the primary advantage of Layer 4 load balancing over Layer 7?

Layer 4 is very fast because it only inspects IP/port headers, avoiding the overhead of Layer 7's content parsing.

What does SSL termination in a Layer 7 load balancer accomplish?

SSL termination decrypts HTTPS traffic at the LB, forwarding plain HTTP to backends. This reduces backend CPU load and enables content inspection.

Why might content-based routing at Layer 7 be impossible at Layer 4?

Layer 4 only reads IP and port numbers from TCP/UDP headers. HTTP URLs and headers exist at Layer 7, invisible to Layer 4 routers.

Which protocols can a Layer 4 load balancer handle that a Layer 7 HTTP-based LB cannot?

Layer 4 works for any protocol using IP/TCP/UDP including SSH, FTP, DNS, databases, etc. Layer 7 LBs are typically HTTP-specific.

What is a key trade-off when using Layer 7 load balancing?

Layer 7 LBs must read the full HTTP request to make content-based decisions, requiring buffering. Layer 4 can stream packets without full inspection.

What does a TCP health check verify about a server?

TCP health check only attempts to establish a connection to the port. It verifies the port is open but says nothing about application state.

In the health check configuration, what does 'unhealthy: 3' mean?

The 'unhealthy: 3' parameter means the server is marked DOWN only after 3 consecutive failed health checks, preventing false positives.

What is the advantage of application-level health checks over HTTP health checks?

Application-level checks test real dependencies and functionality, detecting failures HTTP checks miss like database unavailability despite HTTP 200 responses.

When a server fails health checks and is marked DOWN, what happens next?

Load balancers continue health checking DOWN servers. When they pass checks again, the servers are marked UP and traffic resumes.

What is connection draining in the context of health checks?

Graceful draining allows existing connections to complete before LB stops sending new traffic to a failing server, avoiding abrupt disconnections.

What failure mode occurs when a health check endpoint itself crashes?

If /health endpoint crashes but the app serves real traffic, LB sees timeout and marks server DOWN, blocking traffic to a working server.

Circuit breaking in load balancers prevents which failure mode?

Circuit breaking isolates slow/failing servers, preventing request queues from building up and causing cascading failures through the system.

What is the primary reason someone might use sticky sessions?

Sticky sessions ensure users reach the same server where their in-memory session state is stored. Without them, each request would need session lookups.

How does sticky session load balancing use cookies?

The LB sets a cookie like SERVERID=s2, and subsequent requests include this cookie. The LB reads it to route back to the same server.

What happens in a sticky session system when the pinned server goes down?

When a pinned server fails, the user's session state is lost because it was only in that server's memory. Sticky sessions offer no failover protection.

How do sticky sessions defeat the benefits of load balancing?

Sticky sessions ignore actual server load. If a 'heavy' user is pinned to Server 1, it becomes overloaded while others sit idle.

Which approach to avoiding sticky sessions stores session data in a place all servers can access?

Centralized session stores like Redis let any server fetch any user's session. Servers become interchangeable without sticky sessions.

What is the key advantage of using JWT tokens instead of server-side sessions?

JWTs carry user claims inside the token itself, cryptographically signed. Any server can validate the signature without needing to store the session.

What is a trade-off of centralized session stores like Redis?

Centralized stores avoid sticky sessions but require serializing sessions to Redis and deserializing on every request, adding latency compared to in-memory access.

What is stateless service design?

Stateless design means each server contains no user-specific memory. Combined with centralized session storage or JWTs, any server can serve any request.

Load balancing — System Design Concept

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7 sections

~28 min total

33 quick quizzes

22 SD challenges linked

0 of 7 done·~24 min left

Concepts›Load balancing›What is Load balancing?

0 / 7

7 sections~28 min

What is Load balancing?

Distributes incoming traffic across multiple servers to prevent overload and ensure availability

Read

~4 min

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Why Load Balancing Matters

Load balancing prevents single-server bottlenecks, enables high availability and scalability

ReadQuiz

~4 min

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Round-Robin, Least-Connections, IP Hash Algorithms

Three core algorithms for distributing requests: Round-Robin cycles through servers, Least-Connections picks the least busy, IP Hash routes by client address

ReadQuizCode

~4 min

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Layer 4 vs Layer 7 Load Balancing

Layer 4 routes by IP and port only, Layer 7 inspects HTTP content for smart content-based routing decisions

ReadQuizCode

~4 min

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Health Checks and Circuit Breaking at the LB Level

Health checks verify server readiness; circuit breaking prevents cascading failures by isolating unhealthy backends

ReadQuizCode

~4 min

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Sticky Sessions — When Needed and How to Avoid Them

Sticky sessions pin users to single servers but create bottlenecks; stateless architectures with shared stores are preferred

ReadQuizCode

~4 min

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Practice test

33 questions

~11 min

Section 1 of 7Read

What is Load balancing?

Distributes incoming traffic across multiple servers to prevent overload and ensure availability

~4 min read

Load balancing is the process of distributing incoming network traffic across multiple servers so that no single server becomes overwhelmed. Think of it like a traffic cop standing at a busy intersection, directing cars to different lanes so that traffic flows smoothly and no single lane gets jammed. Without load balancing, one server might crash under heavy load while others sit idle — a waste of resources and a poor experience for users.

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