Monday, January 08, 2007

Pat Helland Discovers REST - the hard way!

Pat Helland has an interesting paper on scalability and transactions called: Life beyond Distributed Transactions: an Apostate’s Opinion. First, it raises questions about our use of Paxos Commit to solve the co-allocation problem in HARC. However, we don't expect co-allocation to ever involve more than a few resources and we do not expect a heavy demand on the service, also we are quite loose about serializability.

The more interesting aspect of the paper is the similarity of the ideas in it and the concepts in REST. The paper discusses "infinite scalability", yet never mentions the Web, which must be the best example of infinite scalability in a distributed application (Fielding uses the term anarchic scalability which I prefer as it sounds even more daunting). I guess this reflects Pat's background as a transaction guru; he is interested in scaling out the systems he knows.

The paper introduces the concept of "entities" which are identified by keys and to which messages are sent. This maps to resources and URIs, the issues of primary keys and secondary keys wrt to scalability can be handled more elegantly and transparently using DNS trickery and re-directs. The paper also talks about the usefulness of the concept of recognising that some messages are idempotent, for example reads - hello GET and PUT! He misses the usefulness of caching though, I am sure he might like it.

Also the seperation of the application into two layers, with only the lower layer needing to be scale aware maps to the Web. The developer writes the scale agnostic layer that interfaces with the scale aware layer through the "Scale Agnostic Programming Abstraction".
This means he does not have to worry about scaling issues and can leave them to the developer of the scale aware layer. On the Web, a Web developer can use a Yahoo API (the Scale Agnostic Programming Abstraction, which is of course RESTful), to write applciations which have no idea about the scale of Yahoo or how the internals of Yahoo works.


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