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5. CDNs and Other Cloud-Based Systems Many systems now sit behind remote cloud-based services, meaning that any requests made to your server are routed via these systems before hitting it. CDNs The most common example of these systems are CDNs (content delivery networks). These are systems that sit outside your infra‐ structure, handling traffic before it hits your servers to provide glob‐ ally distributed caching of content. CDNs are part of any best-practice setup for a high-usage website, providing higher-speed distribution of data as well as lowering over‐ head of your servers. The way they work is conceptually simple: when a user makes a request for a resource from your system, the DNS resolution is resolved to the point of presence within the CDN infrastructure that has the least latency and load. The user then makes the request to that server. If the server has a cached copy of the resource the user is requesting, it returns it; if it doesn't, or if the version it has has expired, then it requests a copy from your server and caches it for future requests. If the CDN has a cached copy, then the latency for that request is much lower; if not, then the connection between the CDN and the origin server is optimized so that the longer-distance part of the request is completed faster than if the request was made directly by the end user. Other Systems There are many other examples of systems that can sit in front of yours, including: DDoS protection Protects your system from being affected by a DoS (Denial of Ser‐ vice) attack. Web application firewall Provides protection against some standard security exploits, such as cross-site scripting or SQL injection. 5. CDNs and Other Cloud-Based Systems | 9

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