Code Mobility and Session State

Code mobility as provided for by Request Based Distributed Computing RBDC (see backgrounder) is key for delivering On-Demand computing, Distributed Computing (e.g. SETI@home) and Rich Internet Applications.

RBDC enables the mobility of code that gets its input from http sources (url, request body, cookie, and passwordless GETs). This post looks into whether session state can be made mobile as well.

How can code that relies on session state be made mobile?

In a typical scenario, http servers associate session state with client request streams through the use of a server-unique session-id that is preserved between accesses via a cookie. An example of this is PHP’s handling of sessions. Under this scheme the server held session state prevents the code being mobile. The code is not mobile because the session state is only available in the server that generated the client’s requested resource.

Using RBDC, code that relies on session state can be made securely mobile. Here is one way.

[[Since writing this post, I have realised that the mechanism described here is the same mechanism that makes Google Reader Public Pages both globally available and private.]]

Firstly, the server stores the session state using a globally-unique-id (GUID) insead of server-unique-id as the key. The key is preserved between requests in the client cookie as is now done. Then the server makes the session state publically available at a well known URL. For example, an xml serialised version of the state could be GET and POSTable at a URL like The GUID used is sufficiently long to prevent guessing and therefore session state will be securely and globally available.

With session state stored in such a secure and globally available fashion, code that requires session state may also be mobile.


  1. […] have similar session-state mechanisms. See here for one proposal about state within […]

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