There are hits and hits. We consider four hit categories or classes:

• Disk hits: Hits that were resolved by reading content from disk. Only 200 replies are considered.
• Memory hits: Hits that were resolved by sending content from memory. The requested document was in the hot memory buffer and, thus, no disk activity was necessary. Only 200 replies are considered.
• IMS hits: The client sent an If-Modified-Since request. The object was in the cache and still fresh. This corresponds to the TCP_IMS_HIT action in Squid. Note: The cache may have more recent data than the client, so Squid may send the full body with the new content. It is still a hit from Squid point of view.
• 304 hits: IMS hits (see above) with 304 reply code. Note that the 304 code guarantees that the object was not read from disk and its content was not sent to a client.
• Negative hits: Hits with reply code other than 200 or 304.

To illustrate relative importance of each class, we plot the percentage of all hits a class represents.