IAM policy for DynamoDB - Time & Space Complexity

Time Complexity: IAM policy for DynamoDB

O(n)

Understanding Time Complexity

We want to understand how the time it takes to check permissions grows when using an IAM policy for DynamoDB.

Specifically, how does the number of permission checks change as the policy or requests grow?

Scenario Under Consideration

Analyze the time complexity of evaluating this IAM policy for DynamoDB access.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": ["dynamodb:GetItem", "dynamodb:PutItem"],
      "Resource": "arn:aws:dynamodb:region:account-id:table/ExampleTable"
    }
  ]
}
    

This policy allows GetItem and PutItem actions on a specific DynamoDB table.

Identify Repeating Operations

When a request is made, AWS checks the policy statements to decide if the action is allowed.

• Primary operation: Evaluating each statement in the policy against the request.
• How many times: Once per request, checking all statements until a match is found.

How Execution Grows With Input

As the number of statements in the policy grows, the time to check permissions grows too.

Input Size (n statements)	Approx. Permission Checks
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The number of checks grows directly with the number of statements.

Final Time Complexity

Time Complexity: O(n)

This means the permission check time grows linearly with the number of policy statements.

Common Mistake

[X] Wrong: "Permission checks happen instantly no matter how big the policy is."

[OK] Correct: Each statement must be checked in order, so bigger policies take more time.

Interview Connect

Understanding how permission checks scale helps you design policies that keep your system fast and secure.

Self-Check

"What if the policy uses wildcards (*) in actions or resources? How would that affect the time complexity?"

Practice

1. What does the Action element specify in an IAM policy for DynamoDB?

easy

A. The allowed or denied DynamoDB operations

B. The AWS region where the table is located

C. The encryption method used by the table

D. The billing mode of the DynamoDB table

Solution

1. Step 1: Understand the role of Action in IAM policies

   The Action element defines which DynamoDB operations (like read, write) are allowed or denied.

2. Step 2: Differentiate from other policy elements

   Other elements like Resource specify the table, but Action controls what can be done.

3. Final Answer:

   The allowed or denied DynamoDB operations -> Option A

4. Quick Check:

   Action = Allowed/Deny operations [OK]

Hint: Action controls what operations are allowed or denied [OK]

Common Mistakes:

• Confusing Action with Resource
• Thinking Action specifies table location
• Mixing Action with encryption or billing settings

2. Which of the following is the correct syntax to specify a DynamoDB table resource ARN in an IAM policy?

easy

A. "Resource": "arn:aws:s3:::mybucket"

B. "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/MyTable"

C. "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table"

D. "Resource": "arn:aws:dynamodb:us-east-1:123456789012:database/MyTable"

Solution

1. Step 1: Identify correct ARN format for DynamoDB table

   The ARN format for a DynamoDB table is: arn:aws:dynamodb:<region>::table/.

2. Step 2: Compare options to the correct format

   "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/MyTable" matches the correct format exactly. "Resource": "arn:aws:s3:::mybucket" is for S3, C misses table name, D uses wrong resource type.

3. Final Answer:

   "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/MyTable" -> Option B

4. Quick Check:

   Resource ARN for DynamoDB table = "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/MyTable" [OK]

Hint: DynamoDB table ARN ends with 'table/YourTableName' [OK]

Common Mistakes:

• Using S3 ARN format instead of DynamoDB
• Omitting the table name in ARN
• Using 'database' instead of 'table' in ARN

3. Given this IAM policy snippet, what DynamoDB actions are allowed?

{
  "Effect": "Allow",
  "Action": ["dynamodb:GetItem", "dynamodb:Query"],
  "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/Orders"
}

medium

A. Full access to all DynamoDB tables

B. Writing and deleting items in the Orders table

C. Only reading items from the Orders table

D. No access because the policy is incomplete

Solution

1. Step 1: Analyze the Action list in the policy

   The policy allows only "dynamodb:GetItem" and "dynamodb:Query", which are read operations.

2. Step 2: Check the Resource and Effect

   The resource is the Orders table only, and Effect is Allow, so only reading from Orders is allowed.

3. Final Answer:

   Only reading items from the Orders table -> Option C

4. Quick Check:

   Allowed actions = GetItem and Query (read) [OK]

Hint: GetItem and Query mean read-only access [OK]

Common Mistakes:

• Assuming write or delete permissions are included
• Thinking policy applies to all tables
• Ignoring the Effect value

4. You wrote this IAM policy but it does not allow any access to your DynamoDB table. What is the error?

{
  "Effect": "Allow",
  "Action": "dynamodb:PutItem",
  "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table"
}

medium

A. Resource ARN is incomplete; missing table name

B. Action should be an array, not a string

C. Effect should be Deny to allow access

D. Policy is missing a Condition block

Solution

1. Step 1: Check the Resource ARN format

   The ARN ends with 'table' but does not specify the table name, so it does not point to any specific table.

2. Step 2: Understand impact of incomplete ARN

   Without a full ARN including the table name, the policy does not grant access to any resource.

3. Final Answer:

   Resource ARN is incomplete; missing table name -> Option A

4. Quick Check:

   Resource ARN must include table name [OK]

Hint: Resource ARN must specify full table name [OK]

Common Mistakes:

• Thinking Action must be an array always
• Confusing Effect Allow with Deny
• Assuming Condition is required for access

5. You want to create an IAM policy that allows a user to read from any DynamoDB table in your account but only write to a specific table named 'Inventory'. Which policy setup is correct?

hard

A. { "Effect": "Allow", "Action": ["dynamodb:PutItem"], "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/*" }, { "Effect": "Allow", "Action": ["dynamodb:GetItem", "dynamodb:Query"], "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/Inventory" }

B. { "Effect": "Allow", "Action": ["dynamodb:GetItem", "dynamodb:Query", "dynamodb:PutItem"], "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/*" }

C. { "Effect": "Allow", "Action": "dynamodb:*", "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/Inventory" }

D. { "Effect": "Allow", "Action": ["dynamodb:GetItem", "dynamodb:Query"], "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/*" }, { "Effect": "Allow", "Action": ["dynamodb:PutItem"], "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/Inventory" }

Solution

1. Step 1: Identify read permissions scope

   Read actions (GetItem, Query) should apply to all tables, so Resource uses wildcard '*'.

2. Step 2: Identify write permissions scope

   Write action (PutItem) is limited to the 'Inventory' table only.

3. Step 3: Match policy statements to requirements

   { "Effect": "Allow", "Action": ["dynamodb:GetItem", "dynamodb:Query"], "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/*" }, { "Effect": "Allow", "Action": ["dynamodb:PutItem"], "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/Inventory" } correctly separates read and write permissions with proper Resource ARNs.

4. Final Answer:

   Separate statements: read on all tables, write only on Inventory -> Option D

5. Quick Check:

   Read = all tables, Write = Inventory only [OK]

Hint: Use separate statements for different resource scopes [OK]

Common Mistakes:

• Granting write access to all tables
• Mixing read and write on wrong resources
• Using wildcard for write when not intended