What is IaaS, Paas & SaaS?

These are three main Cloud Computing Stack : Infrastructure as a Service, Platform as a Service and Software as a Service.

• SaaS applications are designed for end-users, accessible over the web.
• PaaS are set of tools and services to help developers design, develop, build & deploy application quickly.
• IaaS serves the need of storage, hardware, servers and networking components.

These cloud computing stacks can provide : 

• any Service on-demand.
• any Platform on-demand.
• large Infrastructure on-demand.

Elasticity of cloud computing brings scalability & accessibility to the applications. 

Examples : Amazon AWS (EC2), Google Cloud (Gmail), Microsoft Azure(Sky Drive) etc.

Java RMI : Remote Method Invocation

Remote Method Invocation (RMI) : It allows an object running in one Java virtual machine (say a client machine)  to invoke methods on an object running in another Java virtual machine (a Server machine).

• Server accepts tasks from clients, runs the tasks, and returns any results. The server code consists of an interface and a class. The interface defines the methods that can be invoked from the client. 
• RMI interface extends the interface java.rmi.Remote, and each method declares java.rmi.RemoteException in its throws clause. 
• Server register its remote objects with RMI's simple naming facility, the RMI registry.
• Client program obtains a stub for the registry on the server's host, looks up the remote object's stub by name in the registry, and then invokes method on the remote object using the stub.
• A Serializable object can be passed to-and-fro Client-Server.
• Source files can be compiled like :
  
  javac -d destDir RMIInterface.java RMIInterfaceImpl.java Client.java

RMI stub : In simple, its a proxy or surrogate which helps in managing invocation of remote objects.

Agile Scrum Methodology

What is Agile : Able to move Quickly or Easily 

In Software Industry or Product Development, it says team should divide their task in Iteration and rapidly start working once design (most IMP. spend as much time you have to make it better) is ready.

The Role Of Scrum

Scrum has three fundamental roles: Product Owner, Scrum Master, and Team Member.

• Product Owner is responsible for communicating the vision of the product and creates a prioritized wish list called a product backlog. 
• Scrum Master acts as a liaison between the Product Owner and the team and keeps the team focused on its goal. S/He meets team each day in Daily Scrum to assess its progress.
• Team Members are responsible for determining how tasks will be accomplished. They can select any work of their choice which they commit to finish.

More Scrum Terminologies 

1. Sprint Planning
2. Sprint Goal
3. Sprint Backlog
4. Sprint Burndown
5. Sprint Review

Agile Scrum Benefits

1. Shorter Delivery Cycles
2. Customer Involvement via feedback 
3. Self Organizing Team
4. Adaptable to Change

Scrum Cycle repeats at end of every Sprint with newer Goal and prioritized tasks.

Things to remember : In Map Reduce

Q 1. What is IdentityMapper?
A - An empty Mapper which directly writes key/value to the output.
         Mapper<K,V,K,V>
Q 2. What is InverseMapper?
A - A Mapper which swaps the <Key,Value> to <Value,Key>.
         Mapper<K,V,V,K>
Q 3. What is IdentityReducer?
A - It performs no reduction, directly writes key/value to the output.
         Reducer<K,V,K,V>
Q 4. What is Partitioner?
A - It runs after completion of Map Jobs. A custom Partitioner can be implemented to decide which key/value should go to which Reducer.
In Map-Reduce model, unique key 'K' with all its Iterable<V> should go to same Reducer.
Q 5. What are the uses of Combiner?
A - It helps in performing local aggregation on Map jobs output to reduce the ammount of data sent to any Reducer.
Q 6. Where Map outputs are stored?
A - Intermediate or Grouped Map output are stored in Sequence File(can be gzipped) on HDFS cluster.
Q 7. How to set number of mapper & reducer?
A - JobConf class object is used to set number of mapper and reducer.
JobConf is present in package org.apache.hadoop.mapred and extends org.apache.hadoop.conf.Configuration
public void setNumMapTasks(int n);// sets number of mapper Job
public void setNumReduceTasks(int n);// sets number of reducer Job
Q 8. What is ChainMapper?
A - It allows to use multiple Mapper class in single Map task.
Output of one mapper is passed to another mapper and so on.
Each Mappper get executed in chain.
Q 9. What is RegexMapper?
A - A Mapper that extracts text matching a regular expression.

Things to remember : In Core JAVA

Q 1. Can you tell, which Algorithm is used by HashMap/HashTable?
A - HashMap internally uses bucket to store key-value pair. When a key is passed to HashMap, it is not used as 'key' as it is! It gets converted to another HashKey using HashCode(). When same HashKey is generated for multiple key(s) (ie: Collision in HashMap/HashTable), It(another key-value pair) get stored in same bucket as next item( Each bucket is a Linked List, It can contain multiple key-value pair).
HashMap can take a 'initial Capacity' & 'load Factor' in its constructor. 
initial Capacity : number of bucket get created at the time of initialization. 
load Factor : number of buckets get increased when Items cross this load factor.
HashTable is a synchronized version of HashMap. But HashMap gives performance bonus as object is not accessed by multiple Threads. 
Q 2. Name some way of Inter Process Communication(IPC)?
A - These are :

1. Socket
2. Message Queue
3. Pipe
4. Signal
5. File
6. Remote Method Invocation (RMI)
7. Shared Memory
8. SOAP, REST, Thrift, XML, JSON

Q 3. What is Mutual Exclusion?

A - Mutual Exclusion in OS (Mutex) is a collection of techniques/algorithms for sharing resources so that concurrent uses do not conflict and cause unwanted interactions. One of the most commonly used techniques for mutual exclusion is the semaphore.
Q 4. What are Abstraction and Encapsulation?
A - Abstraction : Hiding away unimportant details of an object, focuses on outside view.
      Encapsulation : It is defined as the process of wrapping up the data members and member functions together into a single unit.

What is : Hadoop Sequence File?

Hadoop Sequence File : These are flat files consisting of binary Key-Value Pair. It can store any key-value pair as byte arrays.

3 Types : 

UnCompressed

Record-Compressed

Block-Compressed.

Need for Sequential File : Hadoop is meant for processing BigData. It has 64 MB default block size on any cluster. Which mean any file with size lesser than 64 MB will eventually occupy 64 MB physical space on disk storage.

      In practical, Applications deal with files with fewer KB. So, It is advantageous to keep number of such small file in sequential Key-Value pair, which allows programmers to run similar logic on each file found in a block with help of Map-Reduce Jobs.

    -org.apache.hadoop.io.SequenceFile Class provides Read, Write methods. It also grants provision for 'Sorting' of SequenceFile Keys.

Thank YOU

How to : choose between DOM, SAX or XMLStreamWriter

What is XML : I call it a "language for Internet", It help applications to communicate  seamlessly. At the same time it's in human readable format too. Any XML file typically contains 'Elements' and 'Attributes', which are also called XML 'Node'. 
(Element, CData, Comment, Attribute, Entity, Text are few examples of Node Type).
There are number of API(s) available to work with XML, and each has its positive and negative aspects.

W3C DOM : 

good for - random read and  write with XML nodes.
not suitable for - larger memory footprint, performance.

SAX :

good for : faster read access, it's lightweight.
not suitable for - writing/creating XML nodes.

XMLStreamWriter :

good for - streaming out XML while building it, useful in web services handling larger files.
not suitable for - random read or write, It's sequential, one-way, cursor like implementation. 


All what you need is to choose your API closest to your need. XMLStreamWriter is a good for all purpose. Most effective for mobile devices.

Thank YOU