Q. Explain architectural Model of Distributed System with neat diagram. (December 17) (10 Marks)
Ans - There are four different architectural styles, plus the hybrid architecture, when it comes to distributed systems. The basic idea is to organize logically different components, and distribute those computers over the various machines.
- Layered Architecture
- Object Based Architecture
- Data-centered Architecture
- Event Based Architecture
- Hybrid Architecture
Layered Architecture
The layered architecture separates layers of components from each other, giving it a much more modular approach. A well known example for this is the OSI model that incorporates a layered architecture when interacting with each of the components. Each interaction is sequential where a layer will contact the adjacent layer and this process continues, until the request is been catered to. But in certain cases, the implementation can be made so that some layers will be skipped, which is called cross-layer coordination. Through cross-layer coordination, one can obtain better results due to performance increase.
The layers on the bottom provide a service to the layers on the top. The request flows from top to bottom, whereas the response is sent from bottom to top. The advantage of using this approach is that, the calls always follow a predefined path, and that each layer can be easily replaced or modified without affecting the entire architecture. The following image is the basic idea of a layered architecture style.
Object Based Architecture
This architecture style is based on loosely coupled arrangement of objects. This has no specific architecture like layers. Like in layers, this does not have a sequential set of steps that needs to be carried out for a given call. Each of the components are referred to as objects, where each object can interact with other objects through a given connector or interface. These are much more direct where all the different components can interact directly with other components through a direct method call.
As shown in the above image, communication between object happen as method invocations. These are generally called Remote Procedure Calls (RPC). Some popular examples are Java RMI, Web Services and REST API Calls. This has the following properties.
- This architecture style is less structured.
- component = object
- connector = RPC or RMI
When decoupling these processes in space, people wanted the components to be anonymous and replaceable. And the synchronization process needed to be asynchronous, which has led to Data Centered Architectures and Event Based Architectures.
Data Centered Architecture
As the title suggests, this architecture is based on a data center, where the primary communication happens via a central data repository. This common repository can be either active or passive. This is more like a producer consumer problem. The producers produce items to a common data store, and the consumers can request data from it. This common repository, could even be a simple database. But the idea is that, the communication between objects happening through this shared common storage. This supports different components (or objects) by providing a persistent storage space for those components (such as a MySQL database). All the information related to the nodes in the system are stored in this persistent storage. In event-based architectures, data is only sent and received by those components who have already subscribed.
Some popular examples are distributed file systems, producer consumer, and web based data services.
Event Based Architecture
The entire communication in this kind of a system happens through events. When an event is generated, it will be sent to the bus system. With this, everyone else will be notified telling that such an event has occurred. So, if anyone is interested, that node can pull the event from the bus and use it. Sometimes these events could be data, or even URLs to resources. So the receiver can access whatever the information is given in the event and process accordingly. processes communicate through the propagation of events.
These events occasionally carry data. An advantage in this architectural style is that, components are loosely coupled. So it is easy to add, remove and modify components in the system. Some examples are, publisher - subscriber system, Enterprise Services Bus (ESB) and akka.io.
One major advantage is that, these heterogeneous components can contact the bus, through any communication protocol. But an ESB or a specific bus, has the capability to handle any type of incoming request and process accordingly.
This architectural style is based on the publisher-subscriber architecture. Between each node there is no direct communication or coordination. Instead, objects which are subscribed to the service communicate through the event bus.
The event based architecture supports, several communication styles.
- Publisher-subscriber
- Broadcast
- Point-to-Point
The major advantages of this architecture is that the Components are decoupled in space - loosely coupled.
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