Interview Q&A / AWS & Cloud

1. Can you use Spot Instances inside an Auto Scaling group, and how do you do it safely?

   Yes. A mixed instances policy lets an ASG combine On-Demand and Spot across several instance types. Keep a small On-Demand base, spread Spot across types and AZs, and make the app handle the two-minute interruption notice.

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2. How does cross-account access work with IAM roles and AWS STS?

   The target account creates a role with a trust policy naming the other account, and permissions for what the role can do. A user or service in the other account calls STS AssumeRole and receives temporary credentials for that role.

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3. How many read capacity units does a DynamoDB read use for strongly and eventually consistent reads?

   One RCU covers one strongly consistent read per second of an item up to 4 KB, or two eventually consistent reads. Larger items are rounded up to the next 4 KB.

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4. What is a service control policy in AWS Organizations?

   A service control policy sets the maximum permissions for accounts in an organization or organizational unit. It never grants permissions by itself; it only limits what IAM policies in those accounts can allow, even for the root user of the member account.

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5. What is a VPC endpoint and what is the difference between gateway and interface endpoints?

   A VPC endpoint lets resources in your VPC reach AWS services privately without an internet gateway or NAT. Gateway endpoints, for S3 and DynamoDB, are route-table entries and free. Interface endpoints use PrivateLink with a network interface and are charged.

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6. What is envelope encryption in AWS KMS and why is it used?

   Envelope encryption encrypts data with a data key, then encrypts that data key with a KMS key. KMS directly handles only small payloads (up to 4 KB), so large data is encrypted locally with the data key, and only the small data key goes to KMS.

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7. What is RDS Proxy and why would you use it with Lambda?

   RDS Proxy sits between your application and the database and pools connections. It stops many short-lived Lambda functions from opening thousands of connections that exhaust the database, and it speeds up failover handling.

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8. What is the difference between a global secondary index and a local secondary index in DynamoDB?

   A GSI has its own partition and sort key and can be added any time, with its own throughput. An LSI keeps the table's partition key with a different sort key, must be created with the table, and shares its capacity.

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9. What is the difference between reserved and provisioned concurrency in Lambda?

   Reserved concurrency sets aside part of your account's concurrency for one function and also caps it. Provisioned concurrency keeps a number of environments pre-initialised so invocations avoid cold starts, and it is billed for the time it is kept ready.

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