The Mathematics of Caching: Cache Hit Ratio (CHR) and Bandwidth Volume
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Caching is one of the most effective strategies for scaling web applications and reducing hosting costs. A Content Delivery Network (CDN) deploys a global network of edge servers to intercept user requests. If the requested file is cached at the edge (a cache hit), it is returned immediately to the user. If the file is missing (a cache miss), the request is forwarded to the origin server, which serves the file and caches it at the edge for future requests.
The primary efficiency metric of a CDN is its Cache Hit Ratio (CHR), also known as the Cache Hit Rate. It is the percentage of all incoming requests that are successfully served from the cache: $$\text{CHR} = \frac{N_{\text{hits}}}{N_{\text{total}}} \times 100$$ where \(N_{\text{hits}}\) is the number of cache hits and \(N_{\text{total}}\) is the total number of requests. The total bandwidth served in gigabytes is calculated as: $$\text{Total Bandwidth (GB)} = \frac{N_{\text{views}} \times S_{\text{page}}}{1024 \times 1024}$$ where \(N_{\text{views}}\) is the monthly page views, and \(S_{\text{page}}\) is the average page weight in kilobytes (KB).
To optimize your overall network egress strategy, you can calculate data transfer fees using the cloud egress cost calculator or track performance implications using the API latency SLA calculator. Improving your cache hit rate directly lowers your origin server resource requirements.
The storage of cached assets at the edge is controlled by Time-To-Live (TTL) policies defined in the `Cache-Control` header. A high TTL (e.g., 1 year for static assets like images and CSS) maximizes your CHR. In contrast, dynamic resources (like API endpoints) require lower TTLs or active revalidation, which can lower your CHR. Sizing your caching layers to cache static assets aggressively while utilizing edge computed rules for dynamic routing helps developers maximize system performance.
Let's calculate the cost comparison for 10 million monthly page views with an average page weight of 1,200 KB. The total bandwidth served is $11,444.09\text{ GB}$ (11.44 TB). If your origin egress rate is $0.08 per GB and your CDN has a CHR of 85% with an egress rate of $0.02 per GB and a request price of $0.50 per million: The cost without a CDN is $11,444.09 × 0.08 = $915.53. The cost with a CDN is CDN egress ($11,444.09 × 0.02 = $228.88) plus origin egress on misses ($1,716.61 × 0.08 = $137.33) plus request costs ($5.00), totaling $371.21. This yields a net monthly savings of $544.32, or a 59.5% reduction in network bills.