HTTP: The Definitive Guide

Cache Processing Steps

Modern commercial proxy caches are quite complicated. They are built to be very high-performance and to support advanced features of HTTP and other technologies. But, despite some subtle details, the basic workings of a web cache are mostly simple. A basic cache-processing sequence for an HTTP GET message consists of seven steps (illustrated in Figure 7-11):

1. Receiving—Cache reads the arriving request message from the network.

2. Parsing—Cache parses the message, extracting the URL and headers.

3. Lookup—Cache checks if a local copy is available and, if not, fetches a copy (and stores it locally).

4. Freshness check—Cache checks if cached copy is fresh enough and, if not, asks server for any updates.

5. Response creation—Cache makes a response message with the new headers and cached body.

6. Sending—Cache sends the response back to the client over the network.

7. Logging—Optionally, cache creates a log file entry describing the transaction.

Figure 7-11. Processing a fresh cache hit

Step 1: Receiving

In Step 1, the cache detects activity on a network connection and reads the incoming data. High-performance caches read data simultaneously from multiple incoming connections and begin processing the transaction before the entire message has arrived.

Step 2: Parsing

Next, the cache parses the request message into pieces and places the header parts in easy-to-manipulate data structures. ...