Hi,
I've been studying the code base and evaluating which approach is the
best to take. At present, I will try to clarify the framework
architecture I am using as 'evangelising' might actually be quite
effective. Firstly, there is nothing new to learn, the architecture is
exactly the same as the internet. Applications are networks of machines
and each machine on the network is a component of one or more
applications. Each machine/component specialises in doing one small job
very well, for example you could have a network of machines/components
that do something like the following.
Machine 1's job is to listen on a port for new client connections, when
a new client comes along, it is accepted and the resulting socket is
contextualised. A context is basically an object that maintains state,
such as application/session/request or whatever is relevant. This
context/state is then dispatched to Machine 2.
Machine 2 will now alter the context state by reading bytes from the
socket and buffering them in some suitable form before dispatching to
Machine 3.
Machine 3 is responsible for taking the buffered content and parsing
into some sort object model, like an email. If an error occurs this
machine will forward it to an exception handling machine which
implements the exception policy of the system (Machine 4?). Otherwise,
it is sent to the next machine (Machine 5?) which does some sort of
matching or something
And so on.
Now if we want to do some logging at any time, we just put a logging
machine in between Machine 1 and 2 or between 2 and 3 to check socket
state, buffer state or some other combination of properties.
To relate this somewhat to James, you are currently using Guice to wire
components together and.this is equivalent to defining a network. With
the design I am following there is not really a configuration as such,
they are basically replaced with policy statements and change logs,
which are a kind of configuration. Its fair to say James would be a
network of machines defined by a policy statement. These policy
statements can be large or small and applied to new or already running
systems. Newer policy statements always supersede older ones (unless a
prior policy forbids it). James already has a fairly substantial default
policy statement like thing, its configuration files. However, to
understand the benefits of policy statements, instead of configurations,
it is worth looking at a small example.
In this example I have an installation of James, it does not do any
logging at present but I want to do some. And this is what I want to do:
For the next two days, between the hours of the 3-5 pm, if a client
using the SMTP service invokes the Expn command and has a IP address
matching some address log some information to this file in this format
and if its between 3.45pm and 4.00pm send me an email with such and such
information.
This policy is expressed (but doesn't have to be) as an XML document.
The document is evaluated and the necessary machines are added to the
network to implement the policy. How the policy is defined and
implemented is system independent. The new machines added to the network
that implement the policy usually simply fit in between two existing
machines and do stuff like filtering, logging, security and anything
that makes logical sense (putting a logging machine that records
information about a http request in an smtp service doesn't make logical
sense). Here, Java is used to create very small, highly robust units of
functionality with XML policy statements (closer to natural language and
easy to learn for users) stating how the machines should interact. The
sophistication of policy statements is increased as the size and
collective ability of a community of machines/components in a given
installation grows.
The current mailets and matchers are sort of ideal for use in this
system as they can (in principle) be easily wrapped/adapted or what not
to become a network machine. A machine here is simply an object with a
single method (I've been thinking about using the reflection api's to
extract individual methods from existing James classes and assign them
to individual machines but its not straight forward as the existing
methods in James classes tend to mix concerns together, like application
logic and writing to an output in a single method)
I hope this is of interest, declarative logging is really easy to
implement and requires no learning, only a subtle shift in approach.
James could be implemented as a guice module and a policy statement, in
fact it would probably would be fairly easy to generate guice modules
from a policy statement. Its not a direct translation but they are not
dissimilar in nature. Also I am actually thinking of employing guice
myself to do a few things, I am not in competition with guice. I am not
in competition with spring either because its got a load of stuff with
it mine doesn't. That said, Platformed is not 'just another framework',
it has features (an architecture) that make it distinct.
I'm thinking about a re-factoring strategy but I am still learning James
under the hood. Ideally I would like to convince you of the benefits,
they are fairly obvious.
Regards,
Simon
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