1. Broker Pattern Pattern-Oriented Software Architecture (POSA 1)
F. Buschmann, Regine Meunier, H. Rohnert, P. Sommerlad, M. Stal

2. ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
Basic Pattern Format
Name and summary description
Example
Context
Problem
Solution
Structure
CRC cards
Scenarios and Sequence diagrams
Implementation
Example Resolved
Variants
Known Uses
Consequences
Related Patterns and Credits
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado

3. ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
Basic Pattern Format
Name and summary description
Example
Context
Problem, including forces
Solution
Structure
CRC cards
Scenarios and Sequence diagrams
Implementation
Example Resolved
Variants
Known Uses
Consequences: benefits and liabilities
Related Patterns and Credits
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado

4. Summary Description of Broker Pattern
ECEN 5053 SW Eng of Distributed Systems
Summary Description of Broker Pattern
9/19/2006
Structure distributed sw systems
with decoupled components
that interact by remote service invocations
Responsible for
coordinating communication
transmitting results
transmitting exceptions
Suppose you have a City Information System for a WAN
Some computers host:
1 or more services to maintain information about events, restaurants, hotels, historical monuments, public transportation
Computer terminals connected to the network
Tourists can retrieve information using a browser.
Front-end sw supports online retrieval of info from appropriate servers and its display on the screen
Data is distributed across the network; not all maintained in the terminals.
Expectations:
The system changes and grows continuously so individual services are decoupled from each other
Terminal sw should be able to access services without having to know their location
Allows us to move, replicate, or migrate services
One (not good enough) solution:
Install a separate network to connect all terminals and servers – an intranet system.
Disadvantages:
Not every information provider wants to connect to a closed intranet.
Available services should also be accessible from all over the world.
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
University of Colorado, Boulder

5. Broker Pattern: Context
“Your environment is a distributed and possibly heterogeneous system with independent cooperating components.”
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado

6. Broker Pattern: Problem
Build a complex sw system as a set of decoupled, interoperating components rather than a monolith.
Greater flexibility, maintainability, changeability
Partitioning into independent components makes system distributable and scalable.
Require a means of inter-process communication
If components themselves handle communication, result has several dependencies and limitations
System depends on which comm mechanism used
Clients need to know location of servers
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado

7. Broker Pattern: Problem - 2
ECEN 5053 SW Eng of Distributed Systems
Broker Pattern: Problem - 2
9/19/2006
Need services for adding, removing, exchanging, activating, and locating components
Must not depend on system-specific details to guarantee portability and interoperability
An object that uses an object should only see the interface offered by the object – know nothing about implementation.
Previous slide is about inter-process communication.
The above are additional services needed.
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
University of Colorado, Boulder

8. Broker Pattern: Problem - 3
ECEN 5053 SW Eng of Distributed Systems
Broker Pattern: Problem - 3
9/19/2006
Broker pattern balances the following forces
Components access others’ services via remote, location-transparent service invocations.
Need to exchange, add, or remove components at run-time
Architecture should hide system-specific and implementation-specific details from users of components and services.
Previous slide is about inter-process communication.
The above are additional services needed.
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
University of Colorado, Boulder

9. Broker Pattern: Solution
ECEN 5053 SW Eng of Distributed Systems
Broker Pattern: Solution
9/19/2006
Introduce a broker component to achieve better decoupling of clients and servers
Servers: register themselves with the broker and make their services available to clients through method interfaces.
Clients: access the functionality of servers by sending requests via the broker
A broker’s tasks:
Locating the appropriate server and forwarding a request to that server
Transmitting results and exceptions back to the client
Application accesses distributed services by sending message calls to the appropriate object.
Does not need to focus on low-level inter-process communication.
Broker architecture is flexible
Allows dynamic change, addition, deletion, and relocation
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
University of Colorado, Boulder

10. Broker Pattern: Solution -- 2
ECEN 5053 SW Eng of Distributed Systems
Broker Pattern: Solution -- 2
9/19/2006
Reduces the complexity involved in developing distributed applications.
Introduces object model where distributed services are encapsulated within objects.
Broker systems offer a path to the integration of two core technologies:
Distribution
Object technology
Extend object models from single applications to dist’d applications made of decoupled components that can
run on heterogeneous machines and
written in different programming languages.
Application accesses distributed services by sending message calls to the appropriate object.
Does not need to focus on low-level inter-process communication.
Broker architecture is flexible
Allows dynamic change, addition, deletion, and relocation
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
University of Colorado, Boulder

11. Broker Pattern: Structure
ECEN 5053 SW Eng of Distributed Systems
Broker Pattern: Structure
9/19/2006
Participating components
Clients
Servers
Brokers
Bridges
Client-side proxies
Server-side proxies
6 pages on “structure”
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
University of Colorado, Boulder

12. Server-side interaction
ECEN 5053 SW Eng of Distributed Systems
9/19/2006
9 pages spelling out structure for 3 scenarios
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
University of Colorado, Boulder

13. ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
Client-Server interaction via Broker
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado

14. ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
Object relationships
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado

15. Broker pattern: Implementation
ECEN 5053 SW Eng of Distributed Systems
Broker pattern: Implementation
9/19/2006
Define an object model or use an existing one
Decide which kind of component-interoperability the system should offer
Specify the APIs the broker component provides for collaborating with clients and servers
Use proxy objects to hide implementation details from clients and servers
Design the broker component in parallel with steps 3 and 4
broken down into nine steps
Develop IDL compilers 4.
On client side, a local proxy represents the remote server object called by the client.
On the server side, a proxy is used for playing the role of the client.
Proxy objects:
package procedure calls into messages and forward the messages to the local broker component
they receive responses and exceptions from the local broker and pass them to the calling client
Whenever you implement interoperability by providing an IDL, you need to build an IDL compiler for every programming language you support.
The IDL compiler translate the server interface definitions to programming language code.
When many programming languages are in use, it is best to develop the compiler as a framework that allows the developer to add his own code generators.
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
University of Colorado, Boulder

16. Broker pattern: Known Uses
CORBA
Microsoft’s OLE
WWW – Hypertext browsers such as Mosaic and Netscape act as brokers and WWW servers play the role of service providers
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado

17. Broker Pattern: Consequences
ECEN 5053 SW Eng of Distributed Systems
Broker Pattern: Consequences
9/19/2006
Benefits
Location transparency
Changeability and extensibility of components
Portability of a Broker system
Interoperability between different Broker systems
Reusability
Liabilities
Restricted efficiency
Lower fault tolerance (server fails, broker fails, ...)
Testing and debugging
Liabilities – testing and debugging –
A client app developed from tested services is more robust and easier itself to test
Debugging and testing a broker system is a tedious job because of the many components involved
9/19/2006
ECEN 5053 SW Eng of Dist Sys, Univ of Colorado
University of Colorado, Boulder