Excellent discussion document.
The stream and rule models can potentially be merged. One can use declarative
rules to configure an assembly of streamed proxylets. There are two sorts of
configuration possible - ordering and conditional execution.
The ordering rules are used at configuration time to create an ordered pipeline
of streamed proxylet Transform Modules (TM). They express dependencies between
different proxylets, leading to a partial ordering that the host proxy can then
resolve into a final ordering. This ordering is dependent upon the required
proxy
functions but largely independent of specific request/response messages. This
ordering can also be achieved by attaching modules to specific processing phases
as you suggest. However, a rule-based approach is more general and more openly
extendible that a phased processing model. In the case of the modular proxy I am
currently working on I want the set of proxylets to be (partially) under the
control of individual users so the added flexibility of ordering constraints,
even simple "must run before" type rules, are useful.
Having created a TM pipeline then the actual proxylet actions are still going to
depend on the details of each individual request/response. This could simply be
buried in the Proxylet code (or script) by it choosing to do a no-op transform
if
it decides it is not needed. However, using declarative trigger rules to
pre-screen the proxy to decide whether it should run on this request/response is
useful. It allows the host proxy (and a user or administrator inspecting the
proxy configuration) to reason about what actions are performed in what
circumstances. This may allow optimizations or consistency checks which are not
possible if the behaviour is only buried in the TM code.
Regards,
Dave Reynolds
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