A Whitepaper
Edition 0.0.7, 2002-06-03
Copyright © 2002 Free Software Foundation
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU Free Documentation License".
In 2-tier systems, the application logic lies either in the front end or in the database (via triggers and stored procedures). The main purpose of the Application Server is to pull the application logic out of both of them and serve as a middle layer that abstracts the logic (called business rules) from the user interface as well as from the database backend.
Apart from (of course) fulfilling the purpose, we have defined several additional, ethical as well as technical, goals. The following list is sorted by priority:
GEAS allows definition of data entities (for example name and address of a customer) and of program code to perform on such entities (for example how to build the address line from country, zipcode and city).
The combination of a data entity with all code functions that can be performed on the entity is called a business object.
GEAS lets the user define classes of business objects. The class definitions describe both the data elements (called attributes) and the available functions (called methods) of the business object.
The specific incarnation of a business object (for example a specific customer) is called a business object instance.
GEAS will provide the following attribute types:
A method is code performed on an object. Methods can have parameters of any attribute type (string, number, reference, etc.). Every method has the paramter self, that is the object instance to operate on, as a parameter.
Modules define namespaces for classes, attributes, and methods. When module A defines a class and some attribute and methods for the class, module B can extend the class with new attributes. Another module C can independently extend the class, without taking care about not using the same attribute names as module B, because all modules have their own namespace.
Note: This section is absolutely subject to discussion. We are looking for a good and understandable syntax for fully qualified names.
Class, attribute and method names can be preceded by a module name to override the current module context. In this case, the module name is separated by a colon (:).
Compound attribute names can be followed with a dot (.) and a member name of the compound.
Reference attribute names can be followed with a dot (.) and a attribute name of the referenced object to form an indirect attribute.
Example:
Module "cust" defines a class "customer". Module "sales" defines a class "invoice_head" and a class "invoice_item". Module "base" defines a class "item".
Then, cust:customer
is the fully referenced class name for the
customer class, and sales:invoice_head
is the fully referenced class
name for the invoice_head class.
Now, let module "cust" define the attributes "name" and "address" for the customer class, where address is a compound attribute consisting of "street" and "city".
The following are now valid attribute references of a customer object:
name
or cust:name
is a base attribute.
address
or cust:address
is a compound attribute.
address.street
or cust:address.street
is a compound member
attribute.
Now, the module "sales" extends the customer class by a attribute "last_invoice" which is a reference to an invoice_head object.
sales:last_invoice
is a reference containing an invoice_head object.
If module "sales" defines the attribute "number" and "items" in "invoice_header", then
sales:last_invoice.number
is an indirect attribute and
sales:last_invoice.items
is an indirect attribute, which is a list
attribute.
And if yet another module "acct" extends the invoice_header class by a attribute "paid", then
sales:last_invoice.acct:paid
could be a boolean attribute that tells you
whether the customer has paid his last invoice or not, and you would access
it just as easy as the "name" attribute. However, this attribute would only be
available if all three modules "cust", "sales", and "acct" are installed.
Triggers are methods that are automatically called upon occurance of defined events, for example on every change of a specific attribute, or before a commit of a changed object.
Triggers are always methods of the object where the event occurs. Because every module can extend any class with a method of arbitary name, calling of triggers could be automated by method name.
Example:
The cust module, which defines the customer object, defines a method
"OnChangeName", whose fully qualified name is of course
cust:OnChangeName
.
The sales module could extend the customer class by a method and also call
this method "OnChangeName", because the fully qualified name of this method
will be sales:OnChangeName
and therefore different from the other
method.
If the attribute "Name" is changed in a customer object, GEAS would call both methods because both are named "OnChangeName". The order of the method calls would be unpredictable.
For easy understanding, here is the basic way the Application Server will provide data:
GEDI will provide an API that allows creation and extension of tables, reading of data and updating, adding and deleting of records in a table, all without SQL.
Eventually, GEDI will support creating and extending tables "on the fly", which means that reading a recordset with attributes that don't exist in the database will automagically add the missing columns to the table. This will of course be parameterized.
GEDI operates strictly on a table/row interface and doesn't know anything about object, attributes and methods.
GEDI actually consists of two mostly independent parts, the Database Adapter and the SQL generator.
The Database Adapter is completely backend dependent and provides an abstraction of the different APIs of the various database systems.
The SQL generator generates valid SQL statements from requests that come in through the table/row interface. As most SQL databases share a major part of the SQL syntax, this module can be mostly backend independent, which means that the drivers for the different databases share a lot of code. However, there are differences in SQL syntax, and so parts of this code has to be backend specific. An object oriented programming language that provides inheritance could be very helpful in implementing this.
GEMA will abstract the calls to business methods written in the different languages.
Python will be the only language to support here for probably a long time, so in the first versions, GEMA will be more of a stub than of a real module.
GEOR will hold all the business object definitions: what attributes the object consists of, what methods exist, which triggers should be called on what event, and so on.
The object definitions could be stored in .gcd
files, in XML files,
or in the database.
It should be possible to change object definitions and bring them into effect without restarting the server. This might be very hard to achieve for ways of storage other than database.
This is the main part of the Application Server.
GOAT uses GEDI and GEMA to fulfill requests directed at business objects, after it has checked the validity of the request against GEOR.
Security will be implemented here, as a request can simply be rejected if the user doesn't have the necessary permissions.
Security implementation will also select or reject based on the users authorizations to any regular query.
Example: if the division president uses a form to request all sales orders, GOAT will query the database and return only the object data that represents the divisions sales orders.
Form level (view) security will not be enforced by GOAT.
GOAT will also provide object transparency. Meaning that there will not necessarily be a direct relationship between business objects and tables.
While accessing data from the database (via GEDI), GOAT will also automagically check for defined trigger methods, and call them (via GEMA).
To be defined
The Remote Protocol Adapter is used to export the functionality of the Application Server over the net, using a variety of RPC mechanisms.
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