1. Architecture styles Pipes and filters Object-oriented design Implicit invocation Layering Repositories

2. Pipes and filters Two components: –Pipe  streams of data for input and output –Filter  transforms data from input to output Filters are independent –Not aware of the existence or functions of the system’s other filters

3. Pipes and filters

4. Advantages –Designers can understand the entire system’s effects on inputs and output as the composition of filters –Since any two filters can be linked together, the filters can be reused easily on other systems –System evolution is simple, because new filters can be added and old filters removed with relative ease –Because of filter independence, designers can simulate system behaviour and analyse system properties (such as throughput) –Allow concurrent execution of filters

5. Pipes and filters Disadvantages –Encourage batch processing, which is not good for handling interactive applications –When two data streams are related, the system must maintain a correspondence between them –Independence of filters means that some filters may duplicate preparatory functions that are performed by filters, thereby affecting performance and making code quite complex

6. Object-Oriented Design This design builds components around abstract data types It has two important characteristics: –Object must preserve the integrity of the data representation –Data representation must be hidden from other objects (encapsulation)

7. Object-Oriented Design Advantages –Encapsulation makes it easy to change the implementation without greatly disturbing the rest of the system –Combining object access routines with the data manipulated by them, encourages designers to decompose the underlying problem into a collection of interacting agents Disadvantages –One object must know the identity of the other objects in order for them to interact

8. Implicit Invocation This design is event-driven and based on the notion of broadcasting –Instead of invoking a procedure directly, a component announces that one or more events have taken place –Other components can associate a procedure with those events  registering a procedure –System invokes all such registered procedures Data exchanged must be done through a shared data in a repository In general, when a component or system announces an event, it does not know which components will be affected by the event –For this reason there is usually some explicit invocation as well

9. Implicit Invocation Advantage –Especially useful for reusing design component from the other systems Disadvantage –Lack of assurance that a component will respond to an event The dependence on the context and sequence of events makes it very difficult to test the system and check for correctness

10. Layering Layers are hierarchical Each layer provides services to the one outside it and acts as a client to the layer inside it –In some systems, each layer has to access some or all of the other layers –In other systems, a given layer has access only to adjacent layers Design includes the protocols that explain how each pair of layers will interact

11. Layering

12. Advantages –Each layer can be considered to be an increasing level of abstraction, and the designers can use the layers to decompose a problem into a sequence of more abstract steps –Because there are restrictions on which layers interact with other layers, it is relatively easy to add or modify a layer as the need arises. Usually such changes affect only the two adjacent layers

13. Layering Disadvantages –Not easy to structure a system in layers, as multiple layers of abstraction are not always evident when we examine a set of requirements –Even when a layered design is created, the system performance may suffer from the extra coordination among the layers

14. Repositories Has two types of components –A central data store –A collection of components that operate on it to store, retrieve, and update information The challenge in this design, lies in deciding how the two types of components will interact Traditional database  the transactions, in the form of an input stream, trigger process execution Blackboard  the central store controls the triggering of the process

15. Repositories

16. Advantage –Its openness as data representation is often made available to several vendors so that they can build tolls to access the repository Disadvantage –Shared data must be in a form acceptable to all knowledge sources, even if the knowledge sources themselves are radically different