---
title: CDN Architecture and Edge Caching
description: Comprehensive technical documentation on CDN architecture, edge caching, origin shielding, cache invalidation, geographic routing, and anycast.
keywords: 
  - CDN
  - content delivery network
  - edge caching
  - origin shielding
  - cache invalidation
  - geographic routing
  - anycast
category: networking
tags:
  - CDN
  - edge computing
  - caching
  - load balancing
  - networking
---

## CDN Architecture

A Content Delivery Network (CDN) is a geographically distributed network of servers and data centers designed to efficiently deliver content to users around the world. The core components of a CDN architecture include:

1. **Edge Servers**: These are the servers closest to the end-user, responsible for serving content directly from the CDN's cache. Edge servers are strategically placed in various locations to minimize the distance and latency between the user and the content.

2. **Origin Server**: The origin server is the primary source of the content, typically hosted by the content provider. It is the authoritative source for all the content that the CDN serves.

3. **Load Balancers**: Load balancers are responsible for directing user requests to the appropriate edge server, taking into account factors like geographic location, server load, and content availability.

4. **Cache Management System**: The cache management system is responsible for managing the content stored on the edge servers, including caching, invalidation, and refresh policies.

5. **Monitoring and Reporting**: CDNs typically include monitoring and reporting capabilities to track performance metrics, usage patterns, and any issues or failures within the network.

The general flow of a CDN-delivered request is as follows:

1. The user requests content from the CDN-enabled domain.
2. The load balancer directs the request to the most appropriate edge server, based on factors like the user's location, server load, and content availability.
3. The edge server checks its cache for the requested content. If the content is found in the cache and is still valid, it is served directly to the user.
4. If the content is not found in the cache or is expired, the edge server forwards the request to the origin server.
5. The origin server provides the content, which is then cached on the edge server for future requests.
6. The content is delivered to the user from the edge server.

This architecture allows CDNs to efficiently distribute content and reduce the load on the origin server, resulting in faster content delivery and improved user experience.

## Edge Caching

Edge caching is a fundamental aspect of CDN architecture, where content is stored on the edge servers closest to the end-users. This caching mechanism allows the CDN to serve content directly from the edge, reducing the latency and load on the origin server.

The process of edge caching involves the following steps:

1. **Content Ingestion**: When new content is published or updated on the origin server, the CDN's cache management system detects the changes and initiates the process of ingesting the content into the edge servers.

2. **Content Placement**: The CDN's load balancing and routing algorithms determine the optimal edge servers to cache the content, based on factors such as user demand, geographic location, and server capacity.

3. **Cache Expiration and Invalidation**: The CDN sets cache expiration policies for the content, determining how long the content should be stored on the edge servers before it needs to be refreshed from the origin. When the content becomes stale or is updated on the origin, the cache management system invalidates the cached content, ensuring that users receive the latest version.

4. **Cache Serving**: When a user requests content, the CDN's load balancer directs the request to the nearest edge server that has the requested content cached. The edge server then serves the content directly to the user, reducing latency and offloading the origin server.

Edge caching can be configured with various policies to optimize content delivery. Some common caching policies include:

- **Time-to-Live (TTL)**: The duration for which the cached content remains valid before it needs to be refreshed.
- **Capacity-based Eviction**: Cached content is evicted from the edge server when the server's storage capacity is full, based on factors like the content's popularity and last access time.
- **Purge-on-Publish**: When new content is published or updated on the origin, the CDN immediately invalidates the cached content, ensuring that users always receive the latest version.

Edge caching is a crucial component of CDN architecture, as it enables faster content delivery, reduced origin server load, and improved user experience.

## Origin Shielding

Origin shielding is a CDN feature that helps to further offload the origin server by introducing an additional layer of caching between the edge servers and the origin. The main components of origin shielding are:

1. **Shield Servers**: These are specialized edge servers that are located closer to the origin server, acting as a buffer between the origin and the distributed edge servers.

2. **Cache Hierarchy**: The cache hierarchy in a CDN with origin shielding follows this structure:
   - Edge Servers: Closest to the end-users, serving content directly from the edge cache.
   - Shield Servers: Located between the edge servers and the origin, serving as a secondary cache.
   - Origin Server: The authoritative source of the content.

The origin shielding process works as follows:

1. When a user requests content, the request is directed to the nearest edge server, as in the standard CDN architecture.
2. The edge server checks its cache for the requested content. If the content is found and valid, it is served to the user.
3. If the content is not found in the edge server's cache or is expired, the request is forwarded to the shield server.
4. The shield server checks its cache for the requested content. If the content is found and valid, it is served to the edge server, which then caches it and serves it to the user.
5. If the content is not found in the shield server's cache or is expired, the request is forwarded to the origin server.
6. The origin server provides the content, which is then cached on the shield server and the edge server for future requests.

The key benefits of origin shielding include:

- **Reduced Origin Server Load**: By caching content on the shield servers, the origin server is shielded from a significant amount of traffic, reducing the load on the origin.
- **Improved Cache Hit Ratio**: The additional cache layer provided by the shield servers increases the overall cache hit ratio, further reducing the load on the origin server.
- **Increased Redundancy**: If an edge server experiences a failure or high load, the request can be served from the shield server, ensuring a more reliable and redundant content delivery.

Origin shielding is a common optimization technique used in CDN architectures to enhance the efficiency and scalability of the content delivery process.

## Cache Invalidation

Cache invalidation is a critical component of CDN architecture, responsible for ensuring that users receive the latest version of content and that the cached content on the edge servers remains up-to-date.

There are several cache invalidation mechanisms used in CDNs:

1. **Time-to-Live (TTL)**: Each piece of cached content is assigned a TTL, which determines the duration for which the content remains valid in the cache. When the TTL expires, the cached content is considered stale and is automatically invalidated.

2. **Purge-on-Publish**: Whenever new content is published or existing content is updated on the origin server, the CDN's cache management system immediately invalidates the corresponding cached content on the edge servers, ensuring that users receive the latest version.

3. **Explicit Invalidation**: Content providers can manually trigger the invalidation of specific cached content by sending a cache invalidation request to the CDN. This is useful for time-sensitive content or content that needs to be updated immediately.

4. **Wildcards**: CDNs often support the use of wildcards in cache invalidation requests, allowing content providers to invalidate multiple related pieces of content with a single request. For example, `/images/*` would invalidate all cached content in the `/images/` directory.

The cache invalidation process typically involves the following steps:

1. The content provider triggers a cache invalidation request, either automatically (e.g., on content update) or manually.
2. The CDN's cache management system receives the invalidation request and identifies the affected cached content.
3. The cache management system sends invalidation commands to the relevant edge servers, instructing them to remove the specified content from their caches.
4. The edge servers acknowledge the invalidation commands and remove the cached content from their local storage.
5. Subsequent requests for the invalidated content will result in a cache miss, causing the edge servers to fetch the latest version from the origin server.

Effective cache invalidation is crucial for ensuring that users always receive the most up-to-date content, and for maintaining the overall reliability and consistency of the CDN-powered content delivery system.

## Geographic Routing

Geographic routing, also known as geo-routing, is a key feature of CDN architecture that enables efficient content delivery based on the user's geographic location. The goal of geographic routing is to direct user requests to the edge server that is physically closest to the user, thereby minimizing latency and improving the overall user experience.

The process of geographic routing involves the following components:

1. **DNS-based Routing**: CDNs typically use a DNS-based routing mechanism to direct user requests to the appropriate edge server. When a user requests content from a CDN-enabled domain, the CDN's DNS server resolves the domain name to the IP address of the edge server that is closest to the user based on their geographic location.

2. **IP Geolocation**: CDNs rely on IP geolocation databases to determine the approximate geographic location of the user based on their IP address. This information is used by the load balancing and routing algorithms to direct the request to the nearest edge server.

3. **Load Balancing**: In addition to geographic location, CDNs also consider the load on the edge servers when directing user requests. The load balancing algorithms ensure that requests are distributed across the edge servers, preventing any single server from becoming overloaded.

4. **Dynamic Routing**: CDNs can also use dynamic routing algorithms that constantly monitor the performance and availability of the edge servers, adjusting the routing decisions in real-time to ensure the best possible user experience.

The geographic routing process works as follows:

1. A user requests content from a CDN-enabled domain.
2. The user's DNS request is directed to the CDN's authoritative DNS server.
3. The CDN's DNS server uses the user's IP address to determine their approximate geographic location.
4. The DNS server selects the optimal edge server based on the user's location, server load, and other factors.
5. The DNS server returns the IP address of the selected edge server to the user's client.
6. The user's client then connects directly to the selected edge server to retrieve the requested content.

Geographic routing is a crucial feature of CDN architecture, as it enables content to be delivered from the edge server closest to the user, reducing latency and improving the overall user experience.

## Anycast

Anycast is a networking technology used in CDN architecture to optimize content delivery and improve reliability. In an anycast network, a single IP address is advertised from multiple locations, allowing user requests to be routed to the "closest" or "best" available server.

The key components of an anycast-based CDN architecture are:

1. **Anycast Routing**: The CDN's edge servers are configured with the same anycast IP address, which is then advertised to the global routing infrastructure. When a user requests content, the network automatically routes the request to the nearest or most appropriate edge server that is advertising the anycast IP address.

2. **Load Balancing**: Anycast routing is combined with load balancing algorithms to ensure that the content requests are distributed evenly across the available edge servers. This helps to prevent any single server from becoming overloaded.

3. **Failover and Redundancy**: If an edge server becomes unavailable or experiences high load, the network can automatically route requests to the next closest edge server advertising the same anycast IP address. This provides a high degree of redundancy and failover capability within the CDN.

The benefits of using anycast in a CDN architecture include:

- **Improved Latency**: By routing requests to the nearest edge server, anycast can significantly reduce the latency experienced by users, improving the overall user experience.

- **Increased Reliability**: The failover and redundancy capabilities provided by anycast ensure that the CDN can continue to serve content even if individual edge servers or data centers experience issues.

- **Simplified Configuration**: Anycast eliminates the need for complex load balancing and DNS-based routing algorithms, as the network infrastructure handles the routing automatically.

- **Scalability**: As new edge servers are added to the CDN, they can be configured with the same anycast IP address, allowing the network to scale easily without the need for substantial changes to the underlying architecture.

Anycast is a powerful technology that is widely used in modern CDN architectures to optimize content delivery, improve reliability, and simplify the overall network infrastructure.