21.Which UML diagrams you usually use for design ?

UseCase Diagram.
Component Diagram for High level Design.
Class Diagram.
Sequence Diagram for low level design.

22.How do you coordinate and communicate with the team developers ?

We as a team of developers , testers , analyst , lead and architect sit close to each other.
Most of the time I would just jump to their seat and talk to them ( if required ).
We have daily stand up where we discuss things that needs team attention.

23.What kind of software architecture your organization follow ?

We have multi tier architecture with multiple layers.
We have series of web servers and applications in application tier.
infrastructure libraries at middle tier.
Database servers at the lower tier.
We are using Oracle as Database, ESB ( Enterprise service Bus ) for asynchronous communication and Rest Web Services.

24.Difference between Proxy and Adapter Design Patterns?

Adapter object has a different input than the real subject where as Proxy object has the same input as the real subject.
Proxy object is such that it should be placed as it is in place of the real subject.

25.Difference between Adapter and Facade ?

The Difference between these patterns in only the intent.
Adapter is used because the objects in current form cannot communicate where as in Facade
though the objects can communicate
A Facade object is placed between the client and subject to simplify the interface.

26.Difference between Builder and Composite ?

Builder is a creational Design Pattern whereas Composite is a structural design pattern.
Composite creates Parent-Child relations between your objects while Builder is used to create group of objects of predefined types.

27.Difference between Factory and Strategy Design Pattern ?

Factory is a creational design pattern whereas Strategy is behavioral design pattern.
Factory revolves around the creation of object at runtime whereas Strategy or Policy revolves around the decision at runtime.

28.Shall we use abstract classes or Interfaces in Policy / Strategy Design Pattern ?

Strategy deals only with decision making at runtime so Interfaces should be used.
You can check more: www.javatechnologycenter.com and click on java design pattern

29.What Do You Mean By High Availability?

Having better service capacity with high availability and low latency is mission critical for almost all businesses.
Availability means the ability of the application user to access the system,If a user cannot access the application, it is assumed unavailable.
High Availability means the application will be available, without interruption.
Achieving high availability for a application is not always a easy task. Using redundant server nodes with clustering is a common way to achieve higher level of availability in web applications.
Availability is commonly expressed as a percentage of uptime in a given year.

30.What Is Scalability?

Scalability is the ability of a system, network, or process to handle a growing amount of load by adding more resources. The adding of resource can be done in two ways

• Scaling Up

• Scaling Out

31.What Is A Cluster?

A cluster is group of computer machines that can individually run a software. Clusters are typically utilized to achieve high availability for a server software.

Clustering is used in many types of servers for high availability.

• App Server Cluster

• Database Server Cluster

32.Why Do You Need Clustering?

Clustering is needed for achieving high availability for a server software. The main purpose of clustering is to achieve 100% availability or a zero down time in service.

A typical server software can be running on one computer machine and it can serve as long as there is no hardware failure or some other failure.

By creating a cluster of more than one machine, we can reduce the chances of our service going un-available in case one of the machine fails.

Doing clustering does not always guarantee that service will be 100% available since there can still be a chance that all the machine in a cluster fail at the same time. However it in not very likely in case you have many machines and they are located at different location or supported by their own resources.

33.What Is Middle Tier Clustering?

Middle tier clustering is just a cluster that is used for service the middle tier in a application. This is popular since many clients may be using middle tier and a lot of heavy load may also be served by middle tier that requires it be to highly available.

Failure of middle tier can cause multiple clients and systems to fail, therefore its one of the approaches to do clustering at the middle tier of a application.

In java world, it is really common to have EJB server clusters that are used by many clients. In general any application that has a business logic that can be shared across multiple client can use a middle tier cluster for high availability.

34.What Is Load Balancing?

Load balancing is simple technique for distributing workloads across multiple machines or clusters.
The most common and simple load balancing algorithm is Round Robin. In this type of load balancing the request is divided in circular order ensuring all machines get equal number of requests and no single machine is overloaded or underloaded.

The Purpose of load balancing is to

• Optimize resource usage (Avoid overload and under-load of any machines.)
• Achieve Maximum Throughput
• Minimize response time

35.What Is Sticky Session (session Affinity) Load Balancing? What Do You Mean By 'session Affinity'?

Sticky session or a session affinity technique another popular load balancing technique that requires a user session to be always served by a allocated machine.

36.Why Sticky Session?

In a load balanced server application where user information is stored in session it will be required to keep the session data available to all machines. This can be avoided by always serving a particular user session request from one machine.

37.How It Is Done?

The machine is associated with a session as soon as the session is created. All the requests in a particular session are always redirected to the associated machine. This ensures the user data is only at one machine and load is also shared.

In Java world, this is typically done by using jsessionid cookie. The cookie is sent to the client for the first request and every subsequent request by client must be containing that same cookie to identify the session.

38.What Are The Issues With Sticky Session?

There are few issues that you may face with this approach

• The client browser may not support cookies, and your load balancer will not be able to identify if a request belongs to a session. This may cause strange behavior for the users who use no cookie based browsers.
• In case one of the machine fails or goes down, the user information (served by that machine) will be lost and there will be no way to recover user session.

39.What Is IP Address Affinity Technique For Load Balancing?

IP address affinity is another popular way to do load balancing. In this approach, the client IP address is associated with a server node. All requests from a client IP address are served by one server node.

This approach can be really easy to implement since IP address is always available in a HTTP request header and no additional settings need to be performed.

This type of load balancing can be useful if you clients are likely to have disabled cookies.

However there is a down side of this approach. If many of your users are behind a NATed IP address then all of them will end up using the same server node. This may cause uneven load on your server nodes.

NATed IP address is really common, in fact anytime you are browsing from a office network its likely that you and all your coworkers are using same NATed IP address.

40.What Is Fail Over?

Fail over means switching to another machine when one of the machine fails.
Fail over is a important technique in achieving high availability.
Typically a load balancer is configured to fail over to another machine when the main machine fails.

To achieve least down time, most load balancer support a feature of heart beat check. This ensures that target machine is responding. As soon as a hear beat signal fails, load balancer stops sending request to that machine and redirects to other machines or cluster.