• What is Adaptive Leases?
• How do I install it?
• Download Squid2.2 enhanced with leases
• Download Apache.1.3.6 enhanced with leases
• Download Lease Server1.1
• Documentation

Adaptive leases is a cache consistency mechanism for guaranteeing strong consistency in the world wide web. Most existing web proxies provide only weak consistency guarantees and hence can return stale data. Strong consistency mechanisms such as server invalidation and poll-on-every-read incur either a large state space overhead or a large control message overhead. Adaptive leases provide a way to balance these tradeoffs. The basic mechanism works as follows:

Server:

• Returns a lease for an object along with the requested object
• Notifies all proxies with valid leases about modifications to the object

• Serves objects from local cache as long as the lease is valid
• Renews an expired leases the next time the object is read
• Invalidates objects in cache when notified by server

A prototype implementation of the adaptive leases is available using the Squid web proxy, the Apache web server and a Lease Server. The implementation has the following features:

• Squid can be configured to allow for coexistence of TTL based weak mechanism and adaptive leases based strong consistency
• Apache Server allows for the lease server to be located at a remote location.
• Lease Server allows for the following lease policies:
• Age-based leases
• Renewal Frequency-based leases
• Fixed-Based leases

The current implementation for adaptive leases is available here.

People

Adaptive Leases was been developed at the Laboratory for Advanced System Software (University of Massachusetts) The following people were involved in this project.

• Venkata Duvvuri
• Prashant Shenoy
• Renu Tewari

Adaptive Leases is based on the following research publications:

[1] Venkata Duvvuri, Prashant Shenoy and Renu Tewari Adaptive Leases: A Strong Consistency Mechanism for the World Wide Web Technical Report TR99-41, Department of Computer Science, University of Massachusetts at Amherst, June 1999

[2] Venkata Duvvuri.Adaptive Leases: A Strong Consistency Mechanism for the World Wide Web. Masters Thesis Department of Computer Sciences, University of Massachusetts, Amherst, June 1999

Download Software

The implementation consists of

• Squid2.2.Leases (1.04MB gzip'd)
• Apache1.3.6.Leases (1.25MB gzip'd)
• Lease Server1.1 (20KB gzip'd)

The Leases server is freely downloadable under the Gnu General Public License. Our lease-enhanced versions of Apache and Squid can be downloaded under the same licensing terms as the original versions of the software.

Release Documentation

Release notes and installation instructions are available here