A Method of
Mediators for Building Web Interfaces of CORBA Distributed Enterprise
Applications
R. Schevchenko, A. Doroshenko
Institute of Software Systems
National Academy of Sciences of Ukraine
Glushkov prosp. 40, 03187 Kiev, Ukraine
Email: dor@isofts.kiev.ua,
rssh@gradsoft.com.ua
Abstract: A method of building Web mediated interface for CORBA based enterprise
distributed applications is proposed. It consists in adding an extra software
layer to conventional 3-tiered architecture to realize HTTP to CORBA requests
transformation. A time cost model developed by authors for CORBA distributed
applications is used and gives a new insight on enhancing performance and
availability of applications. A comparison with other web-based system
architectures as well as an analysis of emerging changes are made.
1 Introduction
Rapid development of computer networks,
introduction of technologies Intranet and universal distribution Internet
services caused essential shifts in basic paradigms for enterprise software
systems design. These shifts can be described as transition from exploiting some
fixed set of programs, when each of which is a workplace for performing a fixed
set of functionality for some business process, to continuously developing
information environment of enterprise where elements of this information
framework can be repeatedly reused by different business processes in different
fields of enterprise activity. This trend of further integration of information
services and exploiting reusable business processes leads to development of new
classes of middleware that can influence significantly user interfaces to
transform this domain into a self-established software layer with the similar
transparency properties as business processes have. Particularly some needs are
felt in creating end-user interfaces and separation of user interfaces into yet
another, presentation layer of software systems where objects of presentation
might have the same properties of location invariance and semantic load as
those of layer of business objects. [Mf96,SLJ94]. Among other desirable
properties are seen ever growing demands on performance characteristics and
easy programming interfaces of enterprise distributed applications.
CORBA (Common Object Request Broker
Architecture) standard determines software architecture of distributed objects
and interaction between them in heterogeneous networks [OMG99]. It gives a way
of organization of the distributed computation having a number of properties
attractive for a designer such as precise object model, separation of object
description from its implementation and call transparency. A number of Apache
[LL97] originated projects are known to integrate Web and application servers
that are at the same time database independent. We follow this way by introducing
mediator into Apache based HTTP server, named ModCbroker [Sh001], that is
integrated with CORBA application server. The purpose of this paper is to
describe a mediator based method for building flexible high performance
distributed CORBA applications suitable for practical use in
industrial-strength software. The work relies on our experience of research and
development of a CORBA based enterprise distributed software system [Sh002]. We
use some insights on methods of performance optimization of distributed
programs in a new 4-tiered architecture for Internet based distributed
applications augmented with the mediator which enhances the conventional
3-tiered by means of various methods of minimization of objects interaction
time.
2 Existing techniques
for Web interface of front-ends
Conventional process of building distributed
software using CORBA implies interfaces of distributed system to be specified
in object model of IDL language. These specifications are then transformed into
definitions of chosen programming language
in accordance with standard mapping of IDL into this language, (for
example, there exist now standards of IDL mapping to C, C++, Java, Ada95,
Cobol, Python, and others). In compliance with these definitions server and
client side of program objects are implemented and from programmers point of
view client's access to remote object looks the same as access to local object
which respective definitions. ORB libraries provide coding and decoding of
requests to remote objects and transition with IIOP protocol of those requests
over the net.
Nowadays there exist many technologies for
building Web interface of front-ends to distributed applications. To choose a
method of integration software system with Web server is one of the main tasks
that must solved while building of Internet/Intranet system. In this section we
will take up main peculiarities of existing technologies and then compare with
mediator based architectures.
СGI interface [CR99,DC96]. This is first method of building
Web applications, born at the same time
WWW was created. From technical point of view CGI interface is based on
starting an external process by Web server, passing to this process request
parameters via command line, environment variables and standard input. Result
of request are returned from standard output of the external process. The main
(and only) merit of CGI interface is its simplicity. May be as result of such
simplicity this technology is still of wide usage. But it has a number of drawbacks, and among them: 1)
spawning of external process on each request to web server that causes
excessive expenditure of computation resources and bad scaling of resulting
applications; 2) limited ability of requests interaction: each request is
handled independently from all others and links between different forms and
state variables must be coded via extra parameters, so structure of CGI
application is naturally mapped to task only for stateless cases; 3) low level
of interface specifications: CGI program must decode MIME-coded request data
itself; 4) lack of integration with software environment of distributed
software. So CGI interfaces can be recommended only for externally simple tasks
which have no scaling requirements.
Fast-CGI [Br99]. This is extension of CGI specifications,
which define C API for embedding of CGI-like modules into Web server. This
method eliminates the main disadvantage of CGI – spawning of external process
on each request. The reverse of the medal is the fact that WWW server and user
application share the same address space, so failure of one user application
can cause failure of Web server. As for other features Fast CGI is not differ
from CGI.
Server Side Scripting: Neowebscript [NWS00],
PHP[RG00][PHP01].
These are methods based on immersion of programming language interpreter into
Web server. Instead of static web pages Web server contains page templates with
scripting language sentences embedded. Efficiency of such scheme is higher than
CGI but lower than using C API. This technology literally move all merits and
drawbacks of script languages into appropriate area of WWW programming: easy of
usage and short learning curve make them irreplaceable in case of relatively small projects. This technique is
recommended in cases of problems which can be solved by few pages. For large
systems traditional limitations of scripting languages begin to tell on. Yet
another drawback of this technique is low level of integration with computation
environment – usually for script languages only database interfaces can be
imported from external world.
Using Web server API. All widespread Web
servers provide C API which allow developers to insert generation of web pages
into server plugins. [Cl98] Advantages of this method consists in efficiency
and ability of fine integration of WWW server with computational environment.
Drawbacks are low level of programming interface, locking to WWW server
platform and some deterioration of reliability
(for reasons, the same as in Fast-CGI CASE). This technique is recommended to
use only in cases when there exist strict requirements of productivity (for example – passing video
information).
JSP [HC98][Ch97]. JSP – is the
open standard which defines high-level model of Web process request handling
inside of so-called Java Servlet Engine. It's one of the most developed methods
of web interface building. Only disadvantages are locking to Java language and
work of virtual Java machine in one address space with Web server.
Using of specialized Web
servers managing server-side scripting engine which import CORBA API. This direction appeared not so long ago
and now is considered to be one of
priority directions for standardization by W3 consortium. The best known
implementations are ILU/NG [JSJ98], and CorbaScript [MGG97]. Merits of such systems are flexibility and
natural integration with Web frameworks. Drawbacks are relative complexity of
tuning and programming. Also such systems are based on dynamic interface
invocation (DII), which cause low productivity
Using of specialized Web
servers which exports CORBA API.
ModCbroker-based systems belong to this class of software. Note that we
avoid using of DII by fixing CORBA API for requests processing, so performance
problems of previous methods is eliminated. A fee is decreasing of flexibility
and necessity of direct programming Web interfaces.
Database integrated
application servers. (Examples are Oracle
Application Server [BNJ98] and Sybase Jaguar [Sy99]). Many suppliers of
databases begin to provide their own mechanisms for integration database data
into Web. In most cases this is a combination of procedural extension of SQL and tools for embedding data views
into web pages. The advantages of the technology is easy integration with databases
and existence of RAD tools. From other side, ability of integration with other
software is limited and cost is usually expensive. So, this tools can be
recommended in cases when a customer uses principle “all from one hand” and
future development of system does not envisage connection of Web application with other data sources.
Use of client-side Java
applets now is not in wide use as
technique of building generic Web applications. This is caused by
relative inefficiency of 3-tiered architecture in low speed networks and lack
of standardized Java environment in browsers as consequence of competitive
activity between Microsoft and Sun.. But this technique can be successfully
used in special cases where browser environment is controlled and information
passing can be efficiently organized with a help of apriori knowledge of data
domain. One of areas where such technique can be successful is using applets
with combination of server engine for representation of geographical
information.
3. Cost Model for Object
Interaction
General prerequisites of CORBA object requests
broker interactions where GIOP (General InterORB Protocol) is a protocol for
data transfer
between brokers [OMG99]. This architecture
establishes following limitations on
client-server interactions:
·
interaction establishes permanent
connection between the server and client while request processing;
·
method invocation is synchronous, that is once client's thread has
executed remote method invocation it is blocked until of the reply;
asynchronous computation can be founded on
existence of another (parallel) thread communicating a server thread by means of
synchronous method invocation;
·
implementation of the remote method may require sending some
context needed for correct execution of the method;
·
stages of request processing and their order are predefined.
Our analysis of time
costs of request brokers shows that performance of a broker is mainly dependent
on following functions underlying stages of request processing.
1. Marshalling
(demarshalling) function M(x) (Dm(x)) which implement coding (decoding)
stages in request x processing. These functions are almost additive in
space, M(x|y)=M(x)|M(y)|pad(x,y),
where x|y is concatenation, pad(x,y) is a quantity of
bytes aligning y after x, and almost linear in time TM(x|y)=TM(x)+TM(y)+d(x,y), where TM(x) is a time for coding x, and d(x,y) is negligible with respect to TM(x).
Notice that the size of according GIOP sequence |M(x)|=KSM|x|
in bytes can be considered as proportional to size of request |x| with a
coefficient KSM. Time for coding and decoding are considered
as nearly equal.
2. Search objects function. The main parameter
this function is dependent on is amount of objects supported in the system. So time cost for
invocation this function can be designated TFO(o,No),
where No is a size of object table in the system.
3. Search methods function.
Searches in table of methods with time cost designated as TFm(m,Nm(o)), where m is method and Nm(o)
is a size of table of remote methods of the object.
4. Activation object and
invocation method function, estimated on time as TI(o,m) which includes time for servant invocation
and, if needed, its thread initialisation.
5.
Network data
transfer function. 
,
where Ks is mean value for a byte transfer time.
So let there is a program code y=o.m(x) with propagating a context c, where m is the method of remote
object o with input parameter x and output y. To find
estimation of time cost for this basic piece of code of distributed
applications we need to define a number of time cost model constituents that
are characterised in terms of functions introduced above:
1) if designate req(o,m,x,c)
a function of sending appropriate request for o.m(x)operation then coding
request time can be estimated as
;
2) transferring request
time:
3) decoding request time:
4) time of search object in active
object map, object activation, method invocation and evaluation of request:
5) time for reply transfer:
6) time for reply decoding :
Summarising these time estimations, we can
deduce the following timing cost model of CORBA remote method invocation Ty=o.m(x)
as:
4. Basics and features of
mediator based architecture
The essence of suggested mediator based
architecture is to exploit CORBA architecture for business logic layer and to use Web based architecture for
presentation layer. Interaction scheme of software layer for systems build up
with such a HTTP/CORBA mediator looks as follow:
So mediator provides CORBA API for handling of
users requests from one side and acts as Web server for HTTP requests handling
from other side. Our implementation of such generic mediator component is
ModCbroker developed by GradSoft (Kiev, Ukraine) [Sh001].
What differs Intranet and Internet
applications is the cost of network data transfer –10-100Mbs for LAN and 1-10
Kbs. for Internet: Thus a good design of Internet application implies
minimisation of network data transfer time TS, while for
Intranet application the time of invocation TI can be more
critical. One of consequence for making decisions in architecture design is to
insert additional software layer for collecting data passed in large pieces of
information in order to speed up integral performance of Internet application:
Such Internet case architecture with four layers (Database, Logic, Server
Front-End, Client) can be more efficient than traditional 3-tiered one
consisting of Database, Server and Clients.
Suppose that for collecting data in a
single chunk it is needed to process N requests with method invocations
of approximately equal time complexity. So for standard 3-tiered architecture
we have following assessment of time evaluation:
Now consider the extra layer of logic
where we have heterogeneous medium with transfer factor for external medium
(Internet) and internal (LAN) as K1 and K+1
respectively. If WWW Servlet executes all invocations in LAN environment collecting all needed parameters with
additional information and sending it to remote browser in a single chunk. then
we obtain:
where 
is a size of
additional information added by servlet and 
is an overhead due to servlet invocation. The difference will
be:
Considering that 
and the difference is
of three orders of magnitude we can conclude LAN expenses is usually negligible
in respect to the time of network
transfer. So the benefit T –T* is surely can be significant if the size
of additional data is not enormous |z| < (|x|+|y|)*N and if time of
operation is determined mostly by time of network data transfer.
Work of ModCBroker as any other application server
engine is to translate incoming http
requests to CORBA IIOP requests and provide to application developer
programming interface (API) for input of http requests parameters and output of
results to WWW browser of end-user. ModCBroker is the Apache module and is
aimed at efficient building Web interfaces of CORBA based applications. Its
mission is transparent translation of HTTP queries to requests to CORBA
servers by means of conventional
NameService without bindings to local
computer. In bird's-eye view process of building WWW application with help of ModCbroker
can be described by following steps:
- Application
developer implements CORBA interfaces HTTP::Servlet and HTTP::Handler.
- Incoming
http requests are authorized by HTTP::Servlet and then, if authorization
was successful, Servlet creates Handler and pass it to ModCBroker.
- Then
ModCBroker calls handle method of HTTP::Handler where parameters are
information about request and object of HTTPStream type.
- HTTPStream
provides API for interaction with client browser.
Detailed
API description with examples of usage can be found in [Sh001].
Main features of developed HTTP/CORBA mediators
consist in following. First, installation of mediator on chosen Web servers
makes them fully transparent and callable by name. This means independence of
service from its location and needs just registering servlets in common
namespace in accordance with standard scheme of building CORBA applications.
Adding or removing web servers and changes of external configurations of our
system does not violate functionality of servlets. Second, program complex
obtains property of independence from physical interface, and we receive
possibility to use component design principles. That is we have standard
interfaces of presentation of different entities glued with each other via HTML
references and interface components can be reused without tedious procedure of re-arrangement of application. And third, a
paradigm of building complex as set of standard components can extended to
building of user interfaces. As a remarkable example we can note component of report
generator build as interface of general kind between UAKGQuery[Sh01,SD01] and
ModCbroker.
Also important is possibility of automatic
adaptation of conventional WWW techniques of load distribution, fault tolerance
and caching to CORBA-based distributed applications. Indeed method of load
distribution on Web servers consists in
providing multiple DNS records for each of Web servers set with IP
addresses laying in different areas of routing. This allows fault tolerant work
of Web server during breakdown of one
of communication links, on one side, and distribution of workload –on the other
one. Applying this technique to mediator based systems we automatically receive
fault-tolerance property relative to fail of one of communication channels.
Load balancing can not be achieved directly in this way if all mediators refers
to the same servlet, but combining this approach with standard CORBA methods of
load-balancing [OMG01, OMG00] we can receive sought result.
Methods of request acceleration
known as reverse caching consist in placing few caching proxy servers before
actual WWW servers. For case of Web
application, not just static web pages, this method can be applied only for
read-only requests of slowly changed data (such as dictionaries or OLAP timestamp
based data). It is interesting that this method of acceleration is similar to
“memoizing” optimization well-known in functional programming. Note that
non-caching reverse proxies can be used for load-balancing.
Yet another feature of Web interface is a
reducing cost of users education as they work with already familiar web
interface. Of course, technique of using Web interfaces have not only merits
but also some known shortcomings: low interactiveness and high computational
workload on server side.
Costs for security systems support in
conventional 3-tiered architecture is also significantly higher because
transferring information through non-protected channel like Internet imply
transferring communication commands for subsystems of lower level such as
databases. So in this case it is needed either to protect the whole traffic of
application by encryption of transport level commands (what is very expensive)
or to embed into protocols some security subsystems what complicates design
process.
So we come to decision to use mediator based
solutions in geographically distributed systems and Internet. For local
networks it is better to use combined approach and apply traditional model for
the cases: when high interactiveness of user interface is required, when
complex graphical images must be built for each client, and when intensive
computation on limited set of data must be performed for each client session.
Note that in case of low speed communication channels such as Internet
interactiveness of mediator based application can be higher than
interactiveness of traditional client-server architectures as time of response
of Web application in low speed channels appeared to be less than that of
traditional client-server application. This fact is causes by difference
between data transfer speed in Intranet and Internet: for LAN we usually have
10-100Mb/c, for WAN – 1-100Kb/c. Meidator
can be used as collector for LAN messages: it receive request from WAN, then form presentation layer, doing sequence
of Intranet calls, then pass data to remote Internet client in one big
chunk. The more formal mathematical
model, based on analysing of physical processes can be found in [Sh002, SD01].
This justifies the fact why non-fast Java first broadly used at client side in
mediated 4-tiered architectures now is being exploited mostly at server side.
From our point of view the main value of
ModCbroker architecture is not just technical details of possible performance
increasing, but in reflecting of general evolution of enterprise information
systems from set of applications driven by information flow of fixed business
processes to set of services not bound
to specific business processes, but applicable to wide set of use-cases in specific
business domain and driven by general business rules, not processes. Other examples of such evolution are using
directory service in HR and CRM software and using enterprise-wide access
control service. Web presentation service is just next step in this process. Existence of flexible server-side web presentation service in
open framework of software components will
fill gap between speed of changes in today-s business environments
and speed of development process for custom solutions.
5. Conclusion
HTTP/CORBA mediators architecture are deployed in few ModCbrocker based software systems, some of them are in production during more than a year. During this time some bottleneck of this model became visible, some directions to enlarge functionality are understood. Some problems of current implementation and ways to it’s solving can be described as follows.
1. Low level of HTML handling.
Servlet
passes to client’s browser low-level code on HTML language. In large projects
it is preferable to have some API for interaction with client based on
semantics entities. This problem can be solved by supplying with mod_cbrocker
library of high level semantics based presentation primitives. But this
library must provide local non-CORBA
API for productivity reasons, so language independence would be lost.
Lack of automatic session management. Current
ModCbroker API is request-oriented. So, application programmer must keep
session management “by hand”. This problem also can be solved by providing
specialised library.
Future workability of CORBA AMI [OMG011]. There
exist possibilities for increasing ModCbroker productivity by using
asynchronous processing of interaction between CORBA application server and WWW
interface server. CORBA AMI (Asynchronous Message Invocation) feature,
introduced in CORBA-2.4 can be used in perspective. Why not now - AMI is yet not supported in most object
request brokers available in today’s market. Note that in spite of evident
inefficiency of current model of execution in all real applications deployed by
now system productivity was not limited
by speed of mediator execution.
So we show that technique of building Web
front-ends to software systems using CORBA/HTTP mediators can be useful and
efficient for Intranet and Internet based environments. We have intention to
develop a client library to provide high level session based model for web
interfaces.
Note that our implementation of ModCbroker is
available for free download in source form from site http://www.gradsoft.kiev.ua with
documentation package and license, which allow use of ModCbroker without fee
for non-commercial purpose.
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