Problems of Expiration-Based Coherence

1) users must wait for expiration checks to occur

• the user gets no information while the check is being performed
• the check may go all the way to the original site and take tens of seconds
• even a "stale" page could contain requested information
• even when the check returns not modified, the page can actually be stale

2) When not satisfied with staleness, users can only RELOAD

• may take very long
• goes to the original site even though a cache along the way may store requested information

3) Expiration mechanism provides no hard guarantee on staleness

• Expires header is rare
• Both default-expiry and LM-factor can accumulate staleness

4) Users can't specify their staleness tolerance

• cache admin sets both default expiration and LM-factor
• users may often have to press RELOAD or wait unnecessarily long