• In apache 2.2, the cached directory contain two files: one with header and one with data (to be delivered to the client). It is more faster to extract the modified date by reading a shorter header file and it is easier to read the document (the .data file) and attache the proper header to the client. In apache 1.3, the caching info and the content are saved in the same file.
  For example, http://cs.uccs.edu/~cs591/apache/proxy/cache/P3B/TV@/rwE/
  
• Question 2.1. Why we put cached documents in directories of different levels as opposed to put them all in a single cache directory?
  Ans: By organizing cached document in directories of different level, each file search operation in a directory level will be quicker. For a proxy server with large set of cached documents, the file search will be much faster on average compared with puting all cached document in a single directory.
• Question 2.2. Why the cached document is saved with a file name generated by applying hash on the url?
  Ans: Without using the hash of the url as the file name. Since all urls will start with http://<domain name>/,
  The first cache level directory will contain the same directory name, e.g., for CacheDirLevels=2, only directory with "ht" will be there.
  
• Question 2.3. One of the HTTP headers in the cached document file is the last modified. How the cache server uses the value of this header? (Hint: http://cs.uccs.edu/~cs522/hw/hwF2004/hw.htm#hw1 exercise 2.4; http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13).
  Ans: The proxy server will extract the modified date and create a If-Modified-Since header to be appended in the relayed http request sent to the original web site. If the returned http response is 304 (not modified), then the cached document is returned to the web browser. Note that for the response with code 304, the original web site does not have to send the content of the document. Therefore it is much faster. Normally, the route between proxy server and the original web site goes through Internet with much smaller bandwidth than the route between the proxy server and the local web browser (local connection much faster).

Exercise 4. Evaluate Web Server Performance using httperf.