The GEAS Whitepaper

The GNU Enterprise Application Server

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".


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Node:Introduction, Next:, Previous:Top, Up:Top

Introduction

Purpose

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.

Additional goals

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:

Freedom
GEAS must be GPL and must be built with truly free tools.
Stability
GEAS must be reasonable stable. For a mission-critical application in a business, reasonable stable means very stable.
Security
GEAS must be reasonable secure.
Maintainability
The code base of GEAS itself must be and remain maintainable by the GEAS development team, and the code must be clear enough to allow interested programmers to adapt it, fix bugs or even take over maintenance of a part of GEAS.
Configurability
GEAS must be configurable and reconfigurable dynamically, centrally, without programming skills, without downtime, and in seperate "layers" for various levels of specification.
Performance
GEAS must perform reasonably with large quantities of users and/or data.
Database Independence
GEAS must be able to use a number of database systems as backend.
Portability
GEAS must run on multiple operating systems and architectures.
Communication Independence
GEAS must be able to use a number of communication means to communicate with the front end (CORBA, XML-RPC...).
Language Independence
GEAS must be able to deal with business methods written in different languages.


Node:Features, Next:, Previous:Introduction, Up:Top

Features

Business Objects

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.

Attributes

GEAS will provide the following attribute types:

Basic attributes
contain most of the actual information, in the basic attribute types string, number, datetime, and boolean. Examples could contain customer name, item price, invoice date, and invoice payment status.
Compound attributes
are a means for combining attributes that logically belong together and appear repeatedly in the same combination. Compound attributes can be built from attributes of any type, not only of basic attributes. An example could be a monetary value consisting of the amount and the currency.
Reference attributes
point to another object and declare a relation between two objects. The value of a reference attribut is another object. Examples include the customer of an invoice or the preferred vendor of an item.
List attributes
point to lists of objects and declare a relation between these objects. The value of a list attributes is a list of objects of a defined class, where all objects in the list will be of the same class. Examples include all line items of an invoice, or all contact persons of a customer.
Calculated attributes
contain information that is generated by GEAS out of other attributes. Calculated attributes are generally read-only. An example could be the total value of an invoice item (calculated from price * pieces) or the total value of an invoice (calculated from all total item values).
Indirect attributes
are attributes of a referenced object and can be accessed as if they were attributes of the current object. Examples could include the name of the customer of the invoice, or the name of the preferred vendor of the item of an invoice line item.

Methods

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

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.

Qualified class, attribute and method names

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

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.


Node:Implementation, Next:, Previous:Features, Up:Top

Implementation


Node:Building Blocks, Next:, Previous:Implementation, Up:Implementation

The basic building blocks


Data Interface:
abstracts database access from a specific database API and from SQL.
Methods Adapter:
abstracts calls to methods from the language and the specific API.
Object Repository:
holds and provides the definition of objects.
Object Access Translator:
translates all requests to the business objects into appropriate database transactions and method calls, by using the other building blocks.
Security Adapter:
handles security
Remote Protocol Adapter:

Theory of Operation

For easy understanding, here is the basic way the Application Server will provide data:

  1. The client requests an object from the app server through the Remote Protocol Adapter.
  2. The Application Server checks if the requested class name is valid (according to the given definitions, i.e. via XML) by looking up the class definition in the Object Repository.
  3. If it's valid, the Object Access Translator translates a class name into a table name. This will include adding a prefix to enable different application modules to have their own namespace.
  4. Then the Application Server passes the database request to the Data Interface to actually get the data.


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GNU Enterprise Data Interface (GEDI)

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.


Node:Methods Adapter, Next:, Previous:Data Interface, Up:Implementation

GNU Enterprise Methods Adapter (GEMA)

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.


Node:Object Repository, Next:, Previous:Methods Adapter, Up:Implementation

GNU Enterprise Object Repository (GEOR)

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.


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GNUe Object Access Translator (GOAT)

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).


Node:Security Adapter, Next:, Previous:Object Access Translator, Up:Implementation

Security Adapter

To be defined


Node:Remote Protocol Adapter, Next:, Previous:Security Adapter, Up:Implementation

Remote Protocol Adapter

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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