The Time-to-Live Based Consistency Mechanism:

Understanding Performance Issues and Their Impact
  • Edith Cohen
  • Haim Kaplan
Part of the Web Information Systems Engineering and Internet Technologies Book Series book series (WISE, volume 2)


The Web is a large distributed database were copies of objects are replicated and used in multiple places. The dominant consistency mechanism deployed for HTTP (Hyper Text Transfer Protocol) and DNS (Domain Name Service) records is Time-to-Live (TTL) based weak consistency. Each object has a lifetimeduration assigned to it by its origin server. A copy of the object fetched from its origin server is received with maximum time-to-live (TTL) that equals its lifetime duration. Cached copies have shorter TTLs since the age (elapsed time since fetched from the origin) is deducted from the objects lifetime duration.

A request served by a cache constitutes a hit if the cache has a fresh copy of the object. Otherwise, the request is considered a miss and is propagated to another server. With HTTP, expired cached copies need to be validated, and if they turned out to be not modified, we refer to the request as a freshness miss.

We study how cache performance is affected by TTL-based consistency. Since cache misses induce user-perceived latency, a cache can reduce user perceived latency by refreshing its copies of popular objects proactively, before they are requested. For hierarchical caches, the number of cache misses depends in subtle ways on the age of the copies the cache receives. Thus, fresh copies obtained through another cache are less effective than fresh copies received from an authoritative server.


TTL Time To Live Cache Web caching Domain Name System 


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Copyright information

© Springer Science+Business Media, Inc. 2005

Authors and Affiliations

  • Edith Cohen
    • 1
  • Haim Kaplan
    • 2
  1. 1.AT&T Labs-ResearchFlorham ParkUSA
  2. 2.Tel-Aviv UniversityTel AvivIsrael

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