Part 2. Setup Cache Server

• Question 1. Compare the access log file of cache server with that of ws2. What is difference between the url field of access record? It is longer. Why browsers send http requests with different formats for cache server vs. normal web server?
  
  Ans: The ws2 access log shows the entries with the format as follows:

  128.198.16.100 - - [31/Jan/2012:21:52:30 -0700] "GET / HTTP/1.1" 200 112
  

  The cache server access log shows the entries as follows:

  128.198.60.197 - - [31/Jan/2012:23:26:05 -0700] "GET http://ciast.uccs.edu/ HTTP/1.0" 200 2005

  The browser sent in a full qualified url as the first parameter of the GET command in a http request, when it is configured to use a proxy. It knows that the cache server need the domain name of the target web server to establish TCP connection there.
  
• Question 2. Cache server after receiving the http request, will check the local cache to see if there is a document with the same url. If there is a cached document, they will send a http request with If-Modified-Since header to the original web server. Where they get the value of the If-Modified-Since header?
  
  Ans: Each cached document is saved with two files: one with .header and the other with .data file extension. The file with .header extension contains the If-Modified-Since header and value similar to the one below (Note that the first line, which contains 8bit non-printable characters, is not show here):

  Server: Apache/2.2.3 (CentOS)
  Last-Modified: Thu, 20 Jan 2011 00:35:51 GMT
  ETag: "8e4950-786c-50ca67c0"
  Accept-Ranges: bytes
  Content-Length: 30828
  Content-Type: image/jpeg
  
  Accept: image/png, image/svg+xml, image/*;q=0.8, */*;q=0.5
  Referer: http://cs.uccs.edu/~cs526/
  Accept-Language: en-US
  User-Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)
  Accept-Encoding: gzip, deflate
  Host: cs.uccs.edu
  If-Modified-Since: Thu, 20 Jan 2011 00:35:51 GMT
  If-None-Match: "1a9c034-786c-50ca67c0"
  Proxy-Connection: Keep-Alive
  Cookie: __utma=63958315.1885387007.1298263914.1298263914.1309316248.2
      

• Question 3. From time to time, a client browser will also send the http request with the If-Modified Since header. When will that be?
  Ans: It is when there is a cached document on the local cache directory of the client browser.
  
• Question 4. Akamai Edge Side Include tries to improve the above cache server processing. Explain how they achieve that.
  Ans: The most important feature of Akamai Edge Side Include (ESI) is to remove the need for querying the original web server using requests with If-Modified Science header. It achieves this by prefetching the fragments of a web page or requiring the original web server to push the new updates to the cache server. Related article: https://www.varnish-cache.org/trac/wiki/ESIfeatures
• Question 5. Assume that we use default CacheDirLevel of 3 and default CacheDirLength of 1, then with the original url as file path name, all cached documents will be saved in the same directory. With the hash value of url as path name, the cached documents will spread out hopefully evenly among the subdirectories.
  • a. If the original url is used as file path name, how many filename matching operations are needed on average, given 4096 doucments?
    Ans: Since every url will comes in with http(s)://...., The first three characters, h, t, t, will be used to create three subdirectories in the cache directory. A single leave directory will contain all 4096 documents. On average, it will take 4096/2=2048 filename matching operations in the leave directory. One filename matching iperation on each of the top directories. Therefore on average it requires 2051 filename matching operations. It is not very efficient.
  • b. If the original url is used as file path name, what are names of those three sudirectories?
    Ans: h, t, and t.

Exercise 4. Evaluate Web Server Performance using httperf.