A System Design Expert’s Guide to Content Delivery Networks.

A Content Delivery Network (CDN) is a distributed network of servers strategically located across various geographical locations to deliver web content, such as images, videos, and other static assets, more efficiently to users. The primary purpose of a CDN is to reduce latency and improve the overall performance of web applications by serving content from the server nearest to the user. CDNs can also help improve reliability, availability, and security of web applications.

What will we cover?

· How CDNs work?
∘ Key terminology and concepts
· Benefits of using a CDN
· CDN Architecture
∘ Points of Presence (PoPs) and Edge Servers
∘ CDN Routing and Request Handling
∘ Caching Mechanisms
· CDN Network Topologies

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How CDNs work?

When a user requests content from a web application, the request is routed to the nearest CDN server (also known as an edge server) based on factors such as network latency and server load. The edge server then checks if the requested content is already cached. If it is, the content is served directly from the cache; otherwise, the edge server fetches the content from the origin server, caches it, and serves it to the user. Subsequent requests for the same content can then be served from the cache, reducing latency and offloading traffic from the origin server.

Key terminology and concepts

1. Point of Presence (PoP): A PoP is a physical location where CDN servers are deployed, typically in data centers distributed across various geographical locations. PoPs are strategically placed close to end-users to minimize latency and improve content delivery performance.

2. Edge Server: An edge server is a CDN server located at a PoP, responsible for caching and delivering content to end-users. These servers store cached copies of the content, reducing the need to fetch data from the origin server.

3. Origin Server: The origin server is the primary server where the original content is stored. CDNs fetch content from the origin server and cache it on edge servers for faster delivery to end-users.

4. Cache Warming: Cache warming is the process of preloading content into the edge server’s cache before it is requested by users, ensuring that the content is available for fast delivery when it is needed.

5. Time to Live (TTL) : TTL is a value that determines how long a piece of content should be stored in the cache before it is considered stale and needs to be refreshed from the origin server.

6. Anycast: Anycast is a network routing technique used by CDNs to direct user requests to the nearest available edge server, based on the lowest latency or the shortest network path.

7. Content Invalidation: Content invalidation is the process of removing or updating cached content when the original content on the origin server changes, ensuring that end-users receive the most up-to-date version of the content.

8. Cache Purging: Cache purging is the process of forcibly removing content from the edge server’s cache, usually triggered manually or automatically when specific conditions are met.

Benefits of using a CDN

CDNs play a crucial role in enhancing the performance, reliability, and security of modern web applications. By serving content from geographically distributed edge servers, CDNs can provide users with a fast and seamless experience, while reducing load on origin servers and protecting against security threats. Here are the top benefits of using CDNs:

1. Reduced latency: By serving content from geographically distributed edge servers, CDNs reduce the time it takes for content to travel from the server to the user, resulting in faster page load times and improved user experience.

2. Improved performance: CDNs can offload static content delivery from the origin server, freeing up resources for dynamic content generation and reducing server load. This can lead to improved overall performance for web applications.

3. Enhanced reliability and availability: With multiple edge servers in different locations, CDNs can provide built-in redundancy and fault tolerance. If one server becomes unavailable, requests can be automatically rerouted to another server, ensuring continuous content delivery.

4. Scalability: CDNs can handle sudden traffic spikes and large volumes of concurrent requests, making it easier to scale web applications to handle growing traffic demands.

5. Security: Many CDNs offer additional security features, such as DDoS protection, Web Application Firewalls (WAF), and SSL/TLS termination at the edge, helping to safeguard web applications from various security threats.

CDN Architecture

Points of Presence (PoPs) and Edge Servers

A Point of Presence (PoP) is a physical location containing a group of edge servers within the CDN’s distributed network. PoPs are strategically situated across various geographical regions to minimize the latency experienced by users when requesting content. Each PoP typically consists of multiple edge servers to provide redundancy, fault tolerance, and load balancing.

Edge servers are the servers within a PoP that store cached content and serve it to users. When a user makes a request for content, the request is directed to the nearest edge server, which either serves the content from its cache or fetches it from the origin server and then caches it for future requests. By serving content from the geographically nearest server, CDNs can significantly reduce latency and improve the overall user experience.

CDN Routing and Request Handling

CDN routing is the process of directing user requests to the most suitable edge server. Routing decisions are typically based on factors such as network latency, server load, and the user’s geographical location. Various techniques can be employed to determine the optimal edge server for handling a request, including:

• Anycast Routing: In anycast routing, multiple edge servers share a single IP address. When a user sends a request to that IP address, the network’s routing system directs the request to the nearest edge server based on network latency or the number of hops. This approach helps ensure that requests are automatically routed to the most appropriate server.
• DNS-based Routing: With DNS-based routing, when a user requests content, the CDN’s DNS server responds with the IP address of the most suitable edge server. This approach can take into account factors such as geographical proximity and server load to select the best edge server for handling the request.
• GeoIP-based Routing: In this approach, the user’s geographical location is determined based on their IP address. The request is then directed to the nearest edge server in terms of geographical distance, which often corresponds to lower network latency.

Caching Mechanisms

Caching is a crucial component of CDN architecture. Edge servers cache content to reduce latency and offload traffic from the origin server. Various caching mechanisms can be employed to determine what content is stored, when it is updated, and when it should be removed from the cache. Some common caching mechanisms include:

• Time-to-Live (TTL): TTL is a value set by the origin server that determines how long a piece of content should be stored in the cache before it is considered stale and needs to be fetched again from the origin server.
• Cache Invalidation: Cache invalidation is the process of removing content from the cache before its TTL expires. This is typically done when content is updated or deleted on the origin server and needs to be reflected immediately in the CDN.
• Cache Control Headers: Cache control headers are used by the origin server to provide instructions to the CDN regarding caching behavior. These headers can dictate the cacheability of content, its TTL, and other caching-related settings.

CDN Network Topologies

CDN network topologies describe the structure and organization of the CDN’s distributed network. Different topologies can be employed to optimize content delivery based on factors such as performance, reliability, and cost. Some common CDN network topologies include:

• Flat Topology: In a flat topology, all edge servers in the CDN are directly connected to the origin server. This approach can be effective for smaller CDNs, but may not scale well as the network grows.
• Hierarchical Topology: In a hierarchical topology, edge servers are organized into multiple tiers, with each tier being responsible for serving content to the tier below it. This approach can improve scalability by distributing the load among multiple levels of servers and reducing the number of direct connections to the origin server.
• Mesh Topology: In a mesh topology, edge servers are interconnected, allowing them to share content and load with each other. This approach can enhance the redundancy and fault tolerance of the CDN, as well as improve content delivery performance by reducing the need to fetch content from the origin server.
• Hybrid Topology: A hybrid topology combines elements from various topologies to create an optimized CDN architecture tailored to specific needs. For example, a CDN could use a hierarchical structure for serving static content, while employing a mesh topology for dynamic content delivery.

Summary

CDN architecture involves the strategic placement of PoPs and edge servers, efficient routing and request handling mechanisms, effective caching strategies, and the appropriate selection of network topologies to optimize content delivery. By considering these factors, CDNs can provide significant improvements in latency, performance, reliability, and security for web applications.

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