From: Tobias DiPasquale <anany@...>
Date: 2001-12-09T05:28:07+09:00
Subject: [ruby-talk:27975] Re: App server for Ruby?

Todd,


The term you are thinking of is a "Web application server", or "Internet 
application server". A prefix qualifier is necessary to truly discern 
the nature of the kind of app server you are talking about. If no 
qualifier is used, then truly, an OS *would* be an app server. But, 
lemme give it a shot:

<definition>
An app server such as you are referring to necessarily relies on the 
definition of the N-tier application architecture; that is what they all 
have in common. Here is its definition:

'N-tier application architecture provides a model for developers so they 
can create a flexible and reusable application. Because an application 
is broken down into tiers, developers only have to modify or add a 
specific layer, rather than rewriting the entire application over if 
they decide to upgrade or change technologies. In the term "N-tier," "N 
" implies any number (like 2-tier, or 4-tier or any number of tiers used 
in your architecture). Application architectures are part of Layer 7 of 
the OSI reference model.'

Given this, we can now delineate the common services found in 
application servers that are distributed as such today:

1. They provide network services to allow for resolving object locations 
and determining/invoking the methods of said objects. This functionality 
relies on the use of interfaces to export methods to remote clients. 
These are invariably implemented via various RPC technologies.

2. They provide the capacity to persist objects over time, most often 
through the interaction with a DBMS, but also via XML or flat files.

3. They provide access control mechanisms for objects contained within, 
allowing object and container alike to manage access to services.

4. They provide the ability to allow objects to partake in transactions, 
either on a single or multiple resources. These resources may be located 
locally or remotely to the server itself.

These commonalities apply to all "application servers" distributed as 
such today. Systems in this classification include all full 
implementations of the EJB specification, all full implementations of 
the CORBA ORB specification and implementors of the Open Group's DCE, 
such as Microsoft's COM+. The ability to exploit these features relies 
on the use of a specific API, which is usually vendor- or 
specification-dependent.
</definition>

The above is an attempt to clarify what the term "application server" 
means in context today, based on research I have done on component 
transaction monitors/application servers over the past four months.

Also note that this definition would not cover such things as Tomcat, 
IIS, mod_ruby or the like, by themselves.

Hope this helps, Todd.

-- 
Tobias DiPasquale
Solaris System Administrator
Electrical and Computer Engineering Dept.
Villanova University
mailto: anany@ece.vill.edu
tel: 610-519-5109