Client-Server Architecture
Client-server is the most fundamental architectural pattern. Understanding it is the starting point for all system design discussions.
What Client-Server Architecture Actually Means
Client-server architecture divides a system into two roles: clients (request services) and servers (provide services). The client initiates communication; the server listens, processes, and responds. This is the foundation of web applications, mobile apps, and most networked systems.
When to Use It (and When Not To)
Client-server is the most fundamental architectural pattern. Understanding it is the starting point for all system design discussions. Web browsers are clients; web servers are servers; databases are servers to the application and clients to the storage layer.
The Architecture
A user opens a browser (client) and types a URL. The browser sends an HTTP request to the web server. The server processes the request — queries the database, applies business logic — and returns an HTTP response (HTML, JSON). The browser renders the response for the user.
Key Principles
- Separation of concerns: Clients handle presentation/UI; servers handle business logic and data.
- Stateless communication: Each request is independent. The server does not remember previous requests (stateless HTTP).
- Scalability: Servers can be scaled horizontally (add more servers) to handle more clients.
- Thin client vs Thick client: Thin clients (browsers) rely on the server for processing. Thick clients (native apps) do more work locally.
- Multi-tier: Most systems use multiple server tiers — web server, application server, database server.
Who Does This Well
Every web application uses client-server architecture — the browser is the client, the backend is the server.
Mobile apps are thick clients that communicate with backend servers via REST or GraphQL APIs.
Email is client-server: email clients (Outlook, Gmail) communicate with email servers (SMTP, IMAP).
The Hard Parts Nobody Talks About
- Not separating concerns — putting business logic in the client
- Not planning for server scalability from the start
- Ignoring network unreliability between client and server
The Tradeoffs
- Centralization: Server is a potential bottleneck and SPOF. Can be mitigated with load balancing and replication.
- Latency: Every interaction requires a network round trip to the server.
- Offline capability: Clients depend on server availability. Offline-first approaches (PWAs) mitigate this.
Interview Angles
- Describe client-server architecture.
- What are the advantages of client-server over peer-to-peer?
- What is a three-tier architecture?
Keep Learning
The Real-World Incident That Made This Famous
Understanding Client Server became critical after multiple high-profile production incidents at major tech companies. When systems handle millions of users, even small misunderstandings about Client Server can lead to cascading failures that cost millions in lost revenue and erode user trust. Companies like Netflix, Google, Amazon, and Meta have all invested heavily in mastering Client Server because they learned the hard way that ignoring it leads to outages.
The key lesson from these incidents: Client Server is not just a theoretical concept — it is a practical skill that separates engineers who build resilient systems from those who build fragile ones.
How Senior Engineers Think About This
Senior engineers approach Client Server differently from textbook definitions. Instead of memorizing rules, they build mental models. They ask: "What problem does Client Server solve? When does it fail? What are the alternatives?" This problem-first thinking leads to better design decisions because every system has unique constraints.
When evaluating Client Server in a system design context, experienced engineers consider the failure modes first. What happens when this component goes down? How does the system degrade? Is the degradation graceful or catastrophic? These questions reveal more about your understanding than any textbook definition.
Common Interview Mistakes
Mistake 1: Giving a textbook definition without context. Interviewers want to see you connect Client Server to real systems and real problems.
Mistake 2: Not discussing trade-offs. Every design decision involving Client Server has trade-offs. Discuss what you gain and what you give up.
Mistake 3: Overcomplicating the solution. Start with the simplest approach to Client Server that meets the requirements, then add complexity only when justified.
Production Checklist
- Define clear metrics for measuring the effectiveness of your Client Server implementation
- Set up monitoring and alerting that specifically tracks Client Server-related failures
- Document your Client Server design decisions in Architecture Decision Records (ADRs)
- Test failure scenarios related to Client Server in staging before production deployment
- Review and update your Client Server implementation quarterly as system requirements evolve
- Train new team members on the specific Client Server patterns used in your system
Read the original source | Content from System-Design-Overview
Practical Implementation for .NET Developers
In a .NET application, you would typically implement this pattern using the following approach:
ASP.NET Core setup: Create a service class that encapsulates the logic, register it with dependency injection, and inject it into your controllers or minimal API endpoints. The built-in DI container handles lifecycle management.
Entity Framework Core: For database interactions, EF Core provides the ORM layer. Use migrations for schema management and raw SQL for performance-critical queries. Consider Dapper for read-heavy paths where EF Core's overhead matters.
Azure integration: If deploying to Azure, leverage managed services — Azure Cache for Redis, Azure SQL, Azure Service Bus, Azure Cosmos DB. These eliminate operational overhead and provide built-in monitoring through Application Insights.
Testing: Use xUnit with Testcontainers for integration tests that spin up real databases in Docker. Mock external dependencies with NSubstitute. The WebApplicationFactory class lets you test your entire HTTP pipeline in-process.
Monitoring: Add Application Insights telemetry to track request latency, dependency calls, and custom metrics. Use structured logging with Serilog to make production debugging possible:
Log.Information("Processing order {OrderId} for {CustomerId}", orderId, customerId);
This gives you searchable, structured logs in Azure Monitor or Seq.