The Extensible Services Switch (ESS) as a Heterogenous Execution Environment
for Enterprise Integration

Chris Tomlinson, Phil Cannata, Greg Lavender, Greg Meredith, Derrell Woelk

Abstract:

The Carnot project in the Enterprise Integration Division of MCC has
developed an extremely flexible means of supporting the integration of
a variety of heterogeneous components in a distributed computational
setting.  We term this means an Extensible Services Switch or ESS.
The basic idea is that of a software version of a communication
front-end on a traditional mainframe.  In our case the ESS is used as
an application layer front-end for applications and/or resources at a
node of a distributed system.  The ESS implements access to communications
resources such as TCP/IP; OSI upper layers over TCP/IP, TP4, and  X25;
and as necessary, DCE etc.

The basic technology of the ESS is a high-performance implementation
of the Actor model [Agha 86], enhanced with mechanisms for inheritance
and reflection.  The Actor model provides the ESS with ultra-lightweight
threads that are coordinated w.r.t. interference via the replacement
behavior semantics.  This makes it quite natural to express control and
data flows of nested and distributed tasks that comprise a logical
unit of work in an enterprise.

The switch is completely interrupt driven and incorporates sophisticated
support for the dynamic definition of storage structures and type-checked
access to procedures generated by foreign language processors and linked
dynamically into the switch environment.  These features make it possible
to rapidly integrate new facilities into the switch and to configure switches
at runtime in the field.

The majority of our experience with ESS technology has been in two areas:
1) OSI application and presentation layer support, and 2) so-called
`tree spaces' for distributed/heterogenous workflow support.  In
workshop III we will focus on upper layer OSI support.

We contend that the most effective approach to heterogeneous execution
environment support for Enterprise Integration is to build directly on
and enhance the OSI upper layers (6 and 7).  To this end we use the ESS
as an experimental model of a relatively ideal realization of the
application layer framework.  An ESS is fully capable of multiplexing
and demultiplexing data and control flows for a multiplicity of
service elements, application entities, and application processes.
Some of these may be implemented directly in the switch, others may be
implemented via the integration of library procedures, and still
others may be supported via access to external O/S processes.

For example, the ESS has been configured to support the SQL Access
Group definition of the Remote Database Access protocol (SAG/RDA).  In
this application there may a variety of database resources accessible
locally through an ESS and in this case the `application entity
qualifier' is used to select the appropriate advertised database
service at a switch.  An interesting aspect of OSI upper layer support
that is prominent in this application is the implementation of ASN.1
within the ESS.  The developer is provided with class generators that
build actor classes corresponding to each ASN.1 type defined in an
application layer protocol.  This allows the developer to work
entirely in terms of the ASN.1 specification (including support for
conversion to and from ASN.1 value notation).  The ESS also includes
generators that work from the Remote Operation Notation making it
trivial to generate so-called client agents that access service
elements defined via ROSE.  Future directions include enhancing the
ASN.1 support in the ESS to directly generate the tables used by the
ISODE 7.0 supplied encoder/decoders, direct support for arbitrary
macro notations, and experimentation with the new approaches to syntax
extension in ASN.1.  Of particular interest is support for the CMIS
GDMO and enhancements.