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AWS Interview Questions
* NAT stands for Network Address Translation.

* If you want your EC2 instance in a private subnet can access the internet, this can be achieved only when it can communicate to the internet. However, we do not want to make a subnet public as we want to maintain the degree of control. To overcome the problem, we need to create either NAT Gateways or NAT Instances.

* In real time, NAT Gateways are highly used than NAT instances as NAT instances are an individual EC2 instances, and NAT Gateways are highly available across multiple availability zones, and they are not on a single EC2 instance.
You can control the security to your VPC in two ways:
 
Security Groups : It acts as a virtual firewall for associated EC2 instances that control both inbound and outbound traffic at the instance level. 

Network access control lists (NACL) : It acts as a firewall for associated subnets that control both inbound and outbound traffic at the subnet level. 
Following are the different database types in RDS :
 
Amazon Aurora
It is a database engine developed in RDS. Aurora database can run only on AWS infrastructure not like MySQL database which can be installed on any local device. It is a MySQL compatible relational database engine that combines the speed and availability of traditional databases with the open source databases. 

Postgre SQL
         * PostgreSQL is an open source relational database for many developers and startups.
         * It is easy to set up, operate, and can also scale PostgreSQL deployments in the cloud.
         * You can also scale PostgreSQL deployments in minutes with cost-efficient.
         * PostgreSQL database manages time-consuming administrative tasks such as PostgreSQL software installation, storage management, and backups for disaster recovery.
 
MySQL
* It is an open source relational database.
* It is easy to set up, operate, and can also scale MySQL deployments in the cloud.
* By using Amazon RDS, you can deploy scalable MySQL servers in minutes with cost-efficient.

MariaDB
* It is an open source relational database created by the developers of MySQL.
* It is easy to set up, operate, and can also scale MariaDB server deployments in the cloud.
* By using Amazon RDS, you can deploy scalable MariaDB servers in minutes with cost-efficient.
* It frees you from managing administrative tasks such as backups, software patching, monitoring, scaling and replication.

Oracle
* It is a relational database developed by Oracle.
* It is easy to set up, operate, and can also scale Oracle database deployments in the cloud.
* You can deploy multiple editions of Oracle in minutes with cost-efficient.
* It frees you from managing administrative tasks such as backups, software patching, monitoring, scaling and replication.
* You can run Oracle under two different licensing models: "License Included" and "Bring Your Own License (BYOL)". In License Included service model, you do need have to purchase the Oracle license separately as it is already licensed by AWS. In this model, pricing starts at $0.04 per hour. If you already have purchased the Oracle license, then you can use the BYOL model to run Oracle databases in Amazon RDS with pricing starts at $0.025 per hour.

SQL Server
* SQL Server is a relational database developed by Microsoft.
* It is easy to set up, operate, and can also scale SQL Server deployments in the cloud.
* You can deploy multiple editions of SQL Server in minutes with cost-efficient.
* It frees you from managing administrative tasks such as backups, software patching, monitoring, scaling and replication.
RTO (Recovery Time Objective) refers to the maximum waiting time for resumption of AWS services/operations during an outage/disaster. Due to unexpected failure, firms have to wait for the recovery process, and the maximum waiting time for an organization is defined as the RTO. When an organization starts using AWS, they have to set their RTO, which can also be called a metric. It defines the time firms can wait during disaster recovery of applications and business processes on AWS. Organizations calculate their RTO as part of their BIA (Business Impact Analysis).
 
Like RTO, RPO (Recovery Point Objective) is also a business metric calculated by a business as part of its BIA. RPO defines the amount of data a firm can afford to lose during an outage or disaster. It is measured in a particular time frame within the recovery period. RPO also defines the frequency of data backup in a firm/organization. For example, if a firm uses AWS services and its RPO is 3 hours, then it implies that all its data/disk volumes will be backed up every three hours.
Multi-AZ RDS is helpful to make a replica of the production database to be available in other availability zones. They come handy in case of disaster recovery and primary database shutdown, to have a complete set of database as a backup.
The processor state control has 2 states, namely :
 
The C State : Represents the sleep state. Varies from c0 to c6, where c6 is the deepest sleep state for a processor.

The P State :
Represents the performance state. Varies from p0 to p15, where p15 is the lowest possible frequency.

A processor has multiple cores, and each of them requires thermal headroom for gaining a boost in performance. Hence, the temperature needs to be kept at an optimal level so that the cores can perform at their highest.
 
When a core is put into the sleep state then it results in a reduction of the overall temperature of the processor. This gives an opportunity to other cores for giving out a better performance. Hence, a strategy can be devised by properly putting some cores to sleep and others in a performance state to get an overall performance boost from the processor.
 
Instances like the c4.8xlarge allow customizing the C and P states for customizing the processor performance according to the workload.
While the c4.8xlarge instance will be preferred for the master machine, the i2.large instance seems fit for the slave machine. Another way is to launch the Amazon EMR instance that automatically configures the servers.
 
Hence, you need not deal with manually configuring the instance and installing Hadoop cluster while using Amazon EMR instance. Simply dump the data to be processed in S3. EMR picks it up from there, processes the same, and then dumps it back into S3.
A Stateful Firewall is the one that maintains the state of the rules defined. It requires you to define only inbound rules. Based on the inbound rules defined, it automatically allows the outbound traffic to flow. 
 
On the other hand, a Stateless Firewall requires you to explicitly define rules for inbound as well as outbound traffic. 
 
For example, if you allow inbound traffic from Port 80, a Stateful Firewall will allow outbound traffic to Port 80, but a Stateless Firewall will not do so.
An Administrator User will be similar to the owner of the AWS Resources. He can create, delete, modify or view the resources and also grant permissions to other users for the AWS Resources.
 
A Power User Access provides Administrator Access without the capability to manage the users and permissions. In other words, a user with Power User Access can create, delete, modify or see the resources, but he cannot grant permissions to other users.
An Instance Store Volume is temporary storage that is used to store the temporary data required by an instance to function. The data is available as long as the instance is running. As soon as the instance is turned off, the Instance Store Volume gets removed and the data gets deleted.
 
On the other hand, an EBS Volume represents a persistent storage disk. The data stored in an EBS Volume will be available even after the instance is turned off.