How Do CDNs Work?
In a traditional network setup without a CDN, a user’s request to access a website’s content travels all the way to the website’s central server. This can result in higher latency, especially if the user and the server are far apart geographically.
CDNs change this by caching, or storing, a website’s content at multiple points, known as points of presence (PoPs), distributed around the world. When a user sends a request for content, the CDN redirects that request from the originating site’s server to the nearest PoP, which then delivers the cached content. In essence, CDNs take a load off the original server and reduce the distance a request has to travel, leading to faster, more reliable content delivery.
Key Components of a CDN
CDNs are more than just a simple network of servers. They comprise several key components:
- Edge Servers: These servers are located at the ‘edge’ of different geographical locations, serving as the entry point for a user’s request to the CDN.
- PoPs: Points of Presence are physical locations that host clusters of edge servers. A single CDN can have dozens, hundreds, or even thousands of PoPs worldwide.
- Caching: The process of storing data temporarily on the CDN’s edge servers, reducing the need to fetch it from the original source every time.
- Load Balancers: These tools distribute traffic across multiple servers to optimise resource use and ensure high availability and reliability.
Building Blocks of a CDN
Building a CDN involves several key components and steps:
1. Points of Presence (PoPs):
PoPs are the backbone of a CDN. Each PoP houses several ‘edge servers’, strategically placed in data centers around the world. When a user makes a request to a website using a CDN, the PoP closest to the user responds to the request, minimising latency.
Creating a PoP requires careful planning. It involves:
- Selecting an optimal location that can serve a significant number of end users. For instance, a company might decide to place a PoP in New York to cater to the massive user base on the East Coast of the USA.
- Collaborating with data center providers to lease space for servers. Negotiating contracts, understanding bandwidth provisions, and ensuring physical security measures are all part of this step.
- Deploying edge servers in the leased space, ensuring they are correctly configured and networked.
2. Edge Servers:
An edge server is a server located closest to the end user. These servers are responsible for delivering web content to users, and they serve as the CDN’s backbone.
The primary role of edge servers is to store, or ‘cache’, web content. When a user makes a request, it’s directed to the closest edge server, which delivers the cached content, reducing the load on the origin server and providing a faster user experience.
The process of setting up an edge server involves:
- Selecting the right hardware specifications considering storage, processing power, and network capabilities.
- Configuring the server to interact with the CDN’s network and the wider internet. This includes installing and setting up appropriate software, such as Nginx or Apache Traffic Server, to facilitate caching and content delivery.
- Regularly maintaining and updating these servers to ensure optimal performance and security.
3. Routing and DNS Resolution:
Once the PoPs and edge servers are set up, the next task is routing user requests to the nearest edge server. This is done via DNS resolution, a process where the user’s browser converts a URL into an IP address.
In the context of CDNs, when a request is made to a CDN-enabled site, the DNS resolver doesn’t return the IP address of the origin server. Instead, it communicates with the CDN’s DNS network, which uses geolocation data and real-time internet conditions to determine the closest, most optimal edge server, and returns its IP address to the user’s browser.
- Implementing Anycast routing, a popular method used by CDNs. Here, all the CDN’s edge servers share the same IP address. When a request is made, network routers direct the request to the nearest edge server with that IP address.
- Setting up a robust DNS network that can accurately geo-locate users and perform load balancing, ensuring no single server becomes overwhelmed.
4. Caching and Content Delivery:
The final piece of the puzzle is determining how content is stored and delivered.
When an edge server receives a request for content it doesn’t have cached, it will make a request to the origin server to fetch the content. This content is then stored on the edge server, ready for delivery to any subsequent requests.
Crucial steps here include:
- Implementing caching rules, like time-to-live (TTL), to determine how long content is stored before a new copy is fetched from the origin server.
- Optimising content delivery, for instance, using GZIP compression to reduce the size of the data being transferred.
- Implementing smart cache purging policies when the content needs to be updated or invalidated.