A Whitepaper
Edition 0.0.11, 2004-02-24
Copyright © 2002-2004 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:
(Note that not all features are yet implemented)
GNUe Appserver 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.
Appserver lets the user define classes of business objects. The class definitions describe both the data elements (called properties) and the available functions (called procedures) of the business object.
Procedures may also be bound to a specific event, that is, they are called automatically as soon as the event happens. These procedures are called bound procedures.
GNUe Appserver will provide the following property types:
A procedure is code performed on an object. Procedures can have parameters of any attribute type (string, number, reference, etc.). Every method is passed a parameter, self, that identifies the object to operate on.
Modules define namespaces for classes, properties, and procedures. When module A defines a class with various properties and procedures, module B can extend the class with new properties. Another module, C, can independently extend the same class, without fear of conflicting with module B. All modules have their own namespace.
Class, property, and procedure names can be preceded by a module name to override the current module context. In this case, the module name is separated by an underscore (_). This results in the rule that neither a module name nor a class, property or procedure name may contain an underscore.
Property and procedure names are separated from the object references by a dot (.).
Example:
Module "cust" defines a class "customer". Module "sales" defines a class "invoiceHead" and a class "invoiceItem". Module "base" defines a class "item".
Then, cust_customer is the fully referenced class name for the
customer class, and sales_invoiceHead is the fully referenced class
name for the invoiceHead class.
Now, let module "cust" define the properties "name" and "address" for the customer class, where address is a compound property consisting of "street" and "city".
The following are now valid property references of a customer object:
name or cust_name is a base property.
address or cust_address is a compound property.
address.street or cust_address.street is a compound member
property.
Now, the module "sales" extends the customer class by a property "lastInvoice" which is a reference to an invoiceHead object.
sales_lastInvoice is a reference containing an invoice_head object.
If module "sales" defines the attribute "number" and "items" in "invoiceHeader", then
sales_lastInvoice.number is the number of the last invoice of the
customer. We could call that an indirect property.
sales_lastInvoice.items is an indirect list property.
And if yet another module, "acct", extends the invoice_header class by a property "paid", then
sales_lastInvoice.acct_paid could be a boolean property 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.
Bound procedures are procedures that are automatically called upon occurance of defined events; for example, on every change of a specific property or before a commit of a changed object.
Bound procedures are always procedures of the object where the event occurs. Because every module can extend any class with a procedure of arbitary name, calling of bound procedures could be automated by method name.
Example:
The cust module, which defines the customer class, defines a procedure
"OnChangeName", whose fully qualified name is of course
cust_OnChangeName.
The sales module could extend the customer class by a procedure and also call
this it "OnChangeName", because the fully qualified name of this procedure
will be sales_OnChangeName and therefore different from the other
procedure.
If the property "Name" is changed in a customer object, Appserver would call both procedures because both are named "OnChangeName". The order of the calls would be unpredictable.
For easy understanding, here is the basic way the Application Server will provide data:
The Data Interface provides 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.
The Data Interface operates strictly on a table/row interface and doesn't know anything about objects, properties and procedures.
The Data Interface caches data read from the database backend as well as data modified through the RPC API. This does not only reduce the traffic between appserver and the database backend. Because the cache is shared between all cursors of a connection, updates done with cursor A become visible in cursor B immediately without re-issuing a query.
The Data Interface used in GNUe Appserver uses GNUe Common's datasources system to communicate with the database.
The Code Adapter abstracts the calls to business methods written in the different languages.
Python is the only language supported for the near future.
The Code Adapter, or at least parts of it, will reside in GNUe Common.
The Class Repository holds all the business object definitions: what properties the object consists of, what procedures exist, which bound procedures should be called on what event, and so on.
The class definitions are stored in the database and are accessible like normal business objects. Consequently, the Class Repository accesses the definitions via the Python Language Interface.
The Authentication Interface will provide a means to authenticate a user on connection to the Appserver, and to check access rights for an already connected user.
For the sake of flexibility and independence from operating system and database backend, access rights per user will probably be stored in the database and be accessible via system business objects.
This is the central part of the Application Server.
The Object Server uses the Data Interface and the Code Interface to fulfill requests directed at business objects, after it has checked the validity of the request against the Class Repository and the Authentication Interface.
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, Appserver 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 Appserver.
The Object Server 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 the Data Interface, the Object Server will also automagically check for defined bound procedures, and call them via the Code Interface.
The Remote Protocol Interface is used to export the functionality of the Application Server over the net, using a variety of RPC mechanisms.
The Language Interface makes business objects easily accessible for code written in supported languages. Each language has it's own Language Interface.
For example, the Python Language Interface (which is the first and currently only one implemented) lets you access business objects as if they are normal Python objects. Procedures written in Python use this interface to access business objects in a rather straightforward way. Other parts of the Application Server also use the Language Interface to access system objects like the ones defining classes or access rights.
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