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Q241. What is the difference between Singleton and Prototype Bean scope in Spring ?

Ans. Prototype scope - A new object is created each time it is injected/looked up. It will use new SomeClass() each time.

Singleton scope - It is the default scope. The same object is returned each time it is injected/looked up. Here it will instantiate one instance of SomeClass and then return it each time.

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Q242. What is the difference between uninitialized values for primitive types and Wrapper classes ?

Ans. When we create reference for primitive type, it's memory is allocated. So even if we don't assign any value to it, the default value is initialized.

int x; // default value 0 initialized

But for object references, it's different as references hold nothing till an object is assigned.

Object obj; // contains null

So all primitive types when declared , contains their respective default values on the basis of their type whereas all wrapper class References contains null irrespective of their types.

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Q243. What is the use of Object class ?

Ans. 1. Default definition for some of the methods , like equals, hashcode etc that gets carried to all objects even if you don't define anything. But that default definition is kind of like assigning 0 to integer, it just provide a safe state , nothing much for comparison.

2. Places where you have no idea about what object you may receive and just want to perform out of 8 basic methods of object class, something like printing their string representation ( defined by their toString method ), or equality ( using their equals method )

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Q244. What is cloud computing ?

Cloud Computing

Ans. Cloud Computing is a way of using IT infrastructure with following traits -

1. On Demand Infrastructure
2. Broad Network Access
3. Resource Pooling
4. Rapid Elasticity
5. Measured Service

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Q245. What are the services provided by AWS ?

Amazon Web Services (AWS)

Ans. AWS provides various types of services like Computing ( EC2 , Lambda , EKS ), Storage ( S3, Glacier ), Database ( RDS , DynamoDB ), Streaming ( Kinesis ), Queue ( SQS ), Security and Access ( IAM ) etc.

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Q246. What are the problems one could face while working with serverless technologies like AWS Lambda ?

Amazon Web Services (AWS)

Ans. Warm up time for certain technologies like Java ( JVM Warmup )

Performance

Size of Deployable ( Bigger size packaging should go in S3 )

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Q247. How Java 9 would help resolve NoClassDefFoundError through it's modularity ?

Ans. Java 9 would provide an error at compile time if there are different modules with the same package mapped to the same class loader and hence would provide a compile time check for this issue.

NoClassDefFoundError exists because of ambiguity at realtime because of multiple versions of same package / class exist. As Java 9 would provide a compile time check for it through clearly defined dependencies and exports and a check on duplicate packages being loaded by single class loader, it will fix the problem.

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Q248. What is the difference between subject and observable ?

Ans. Subject has state, it keeps a list of observers. On the other hand, an Observable is really just a function that sets up observation. While Subjects are Observables, Subjects also implement an Observer interface.

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Q249. What are the disadvantages of Design Patterns ?

Ans. 1. Design patterns are abstraction heavy and hence sometimes creates memory and performance overheads.

2. Sometimes they create unnecessary complexity without much added advantage.

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Q250. Differences between class not found exception and noclassdef found error ?

Ans. ClassNotFoundException is an exception that occurs when we try to load a class at run time using Class.forName() or loadClass() methods and mentioned classes are not found in the classpath.

NoClassDefFoundError is an error that occurs when a particular class is present at compile time, but was missing at run time.

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Q251. serialization and deserialization of singleton class

Ans. The problem with serialized singleton class is that whenever we deserialize it, it will create a new instance of the class. To overcome this scenario all we need to do is to provide the implementation of readResolve() method.

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Q252. What is the cost of using Cloud Formation ?

Amazon Web Services (AWS)

Ans. Cloud Formation does not have any additional cost but you are charged for the underlying resources it builds.

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Q253. How does AWS Lambda handle failure during event processing?

Amazon Web Services (AWS)

Ans. In AWS Lambda we can run a function in synchronous or asynchronous mode. In synchronous mode, if AWS Lambda function fails, then it will just give an exception to the calling application. In asynchronous mode, if AWS Lambda function fails then it will retry the same function at least 3 times.If AWS Lambda is running in response to an event in Amazon DynamoDB or Amazon Kinesis, then the event will be retried till the Lambda function succeeds or the data expires. In DynamoDB or Kinesis, AWS maintains data for at least 24 hours.

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Q254. What does AMI include ?

Amazon Web Services (AWS)

Ans. A template for the instance.

Launch permission decisions

A block device mapping, when an instance is launched it determines the volumes to attach to that instances.

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Q255. Why is cloud infrastructure so powerful ?

Cloud Computing

Ans. Because it gives the ability to provision infrastructure on Demand with No contracts and strings attached.

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Q256. Which region is the latest and most popular in AWS ? What are the advantages and disadvantages of using this region ?

Amazon Web Services (AWS)

Ans. US-East Virginia

This is the oldest and most popular region. All new features are rolled first in this region and hence they are first available there. The downside is that because of experimentation with new services, unavailability time is quite high.

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Q257. What are the advantage of having Edge Locations ?

Amazon Web Services (AWS)

Ans. It saves Network traffic and facilitates faster delivery by hosting bulk media and cached data near to client.

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Q258. What does effect , action and resource means with respect to AWS IAM policy ?

Amazon Web Services (AWS)

Ans. effect means whether the effect of this Policy is to allow or deny, action means the type of action ( read , write ) and service, and resource is the resource where this should apply.

For example - If we want to have a Policy of providing read access on S3 ,
effect will be "allow"
action will be "s3:GetObject"
resource will be "arn:aws:s3:::examplebucket/*"

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Q259. Within AWS S3, Why new files are available immediately for read operation but not the updates ?

Amazon Web Services (AWS)

Ans. Because new files will be fed from where ever they are first available but AWS have no way to identify which is the most updated version among the various regions. The updated version can only be assured once the replication to all availability zones is complete.

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Q260. What kind of Access Policy should be kept on S3 Buckets and Policy ?

Amazon Web Services (AWS)

Ans. We should only allow access that's needed. So the security Policy should be as restrictive as possible.

Buckets and Files should never be made public unless until they are needed it to be like that.

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Q261. How can we protect from accidental deletes in S3 ?

Amazon Web Services (AWS)

Ans. Versioning ( will create Delete marker )
MFA Delete ( authentication upon deletion )
Cross Region Replication ( Replicates aren't deleted )

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Q262. How can we restrict CloudFront access to a particular Geography ?

Amazon Web Services (AWS)

Ans. We can select the appropriate Price Class if that suits us.

We can do Geo Targeting by black listing / white listing certain geographies.

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Q263. What is the use case for "On Demand" EC2 ?

Amazon Web Services (AWS)

Ans. If the computation required is Spiky and unpredictable.

If it's first time use.

If the lifespan of the application isn't known.

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Q264. What is the use case for "Spot" EC2 plan ?

Amazon Web Services (AWS)

Ans. If the usage is non continuous and have flexible start and End Time. For ex - Analysis , Research , mining etc.

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Q265. What are the different price options available for EC2 ?

Amazon Web Services (AWS)

Ans. On Demand
Reserved
Spot
Dedicated Host

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Q266. What EC2 configuration would you use if you have to use it for Bitcoin mining ?

Amazon Web Services (AWS)

Ans. Keeping Low cost for processing , i would choose

Spot Price Plan
F1 or P3 Instance Type
Cold Hdd (SCI ) EBS Volume

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Q267. Which AWS services would you use if you have find a cheapest solution for an analytics application ?

Amazon Web Services (AWS)

Ans. Computation

Spot Price Plan
F1 or P3 Instance Type
Cold Hdd (SCI ) EBS Volume

Storage

S3-IA

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Q268. Can a EC2 instance have multiple Security Groups ? Can a single security group be assigned to multiple Ec2 instances ? If either of them is true , doesn't it result in conflict ?

Amazon Web Services (AWS)

Ans. We can assign multiple security groups to single instance and we can use single security group with multiple instances.

It never results in conflict as security groups only have allow rules and no deny rules. Multiple security groups for an instance results in union of all rules.

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Q269. Can we deny access for particular IP using Security Groups ?

Amazon Web Services (AWS)

Ans. No, We can do that through ACL

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Q270. Difference between S3 and EFS ?

Amazon Web Services (AWS)

Ans. S3 is object based whereas EFS is block based.

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