JESS
The JESS rules engine is a programming library written in Java.
It uses the JESS language to define rules, which are
subsequently run on the JESS engine. The JESS language evolved
itself from the CLIPS expert system, which was initially
developed by NASA. The language is very similar to the LISP
programming language in syntax and structure.
A JESS rules engine has two main parts, the rules and the
knowledge base. The rules are the "if else" conditions
representing a problem domain that a rule engine program needs
to evaluate. The knowledge base is the actual data on which the
rules will be applied. The knowledge defines the states of the
problem domain. JESS will apply the rules on the knowledge base
to get the desired results.
The JESS language allows the user to interact directly with the
JESS API. JESS language does not need any compilation and can be
used interactively to execute commands on the JESS API. Moreover
the language is flexible enough to allow the programmer to add
new commands written in Java. The JESS language can be
customized to suit the needs of a specific application.
JESS provides the flexibility to be used as an embedded API, a
GUI tool, servlets and other Java code. It can also work on the
command line executing command line JESS commands and returning
the results. For those who want the ability to include JESS into
their proprietary framework, JESS allows itself to be controlled
entirely by Java code without any JESS language extensions
included in the code.
The JESS rule engine uses the RETE algorithm to evaluate its
rules. The RETE algorithm is a popular rule evaluation algorithm
used by many players in the industry. The algorithm is known to
be efficient even when the numbers of rules are increased.
Conclusion
The Java Rules Engine API only provides guidelines on how to
implement the Runtime API and the Administrator API. It misses
out on the language to be used for creating rules and actions.
The rules language forms a major part of a rules Engine
application. All the business rules need to be defined in some
language and stored in RuleExecutionSets so that Rules engines
can load them and process them. As long as there is no common
specification on how rules should be defined, there will be
numerous flavors of rules languages in the market. The problem
with multiple rules languages is the compatibility between
different rule engine implementations. A common Rules Engine API
might alleviate the problems of proprietary APIs but the
absence of a common rule language will still handicap the
interoperability between rule engine implementations.
Most of the popular rule engines in the market have their own
rule languages. There is some effort in the industry to come up
with a common rule language. Efforts like Rule Markup language
(RuleML), Simple Rule Markup language (SRML) are underway in the
industry, but there still is no common standard on which all the
rule engine vendors agree. It might be some time before a
consensus language to define rules is seen in the market. So
until then we will have to live with multiple rule engines and
their flavors of the rule languages. As the Rules engine API
gains acceptance applications server providers might offer JDBC
like support for multiple vendor implementations of the API,
then just like multiple database support provided by the
application server, multiple rule engine support can be provided
within the same application server. As of now the vendor
implementations of rule engines can be accessed using JNDI calls
but the process of registration has to be done manually.
Sun may have done its part of the job by trying to standardize
the Java API for rules engine, but the open question of the
standards on the rule language would bog down any efforts to
make interoperable rule engines.
Resources
Java Rules Engine JSR info
JESS Rule Engine
Benoy Jose is a web
developer with over six years of experience in J2EE and
Microsoft technologies. He is a Sun Certified programmer and
enjoys writing technical and non-technical articles for various
magazines.
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