Pagination with SDK in DynamoDB - Step-by-Step Execution

Concept Flow - Pagination with SDK

Start Query

↓

Fetch First Page

↓

Check for LastEvaluatedKey

↓

Store LastEvaluatedKey

↓

Use LastEvaluatedKey to Fetch Next Page

↩Back to Check for LastEvaluatedKey

The SDK fetches data page by page. After each fetch, it checks if more data exists using LastEvaluatedKey. If yes, it uses that key to get the next page.

Execution Sample

DynamoDB

const params = {
  TableName: 'Users',
  Limit: 2
};

const data = await dynamodb.scan(params);
// Use data.LastEvaluatedKey for next page

This code fetches the first 2 items from the 'Users' table and prepares to fetch the next page if available.

Execution Table

Step	Action	Parameters	Result	Next Step
1	Call scan with Limit=2, no ExclusiveStartKey	{TableName:'Users', Limit:2}	Returns 2 items, LastEvaluatedKey present	Check LastEvaluatedKey
2	LastEvaluatedKey exists?	LastEvaluatedKey != null	Yes	Prepare next scan with ExclusiveStartKey
3	Call scan with Limit=2, ExclusiveStartKey=LastEvaluatedKey	{TableName:'Users', Limit:2, ExclusiveStartKey: key}	Returns next 2 items, LastEvaluatedKey present	Check LastEvaluatedKey
4	LastEvaluatedKey exists?	LastEvaluatedKey != null	Yes	Prepare next scan with ExclusiveStartKey
5	Call scan with Limit=2, ExclusiveStartKey=LastEvaluatedKey	{TableName:'Users', Limit:2, ExclusiveStartKey: key}	Returns last items, no LastEvaluatedKey	End Pagination
6	LastEvaluatedKey exists?	LastEvaluatedKey == null	No	Stop fetching pages

💡 No LastEvaluatedKey means all pages fetched; pagination ends.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	Final
LastEvaluatedKey	null	key1	key2	null	null
FetchedItemsCount	0	2	4	6	6

Key Moments - 2 Insights

Why do we need LastEvaluatedKey to fetch the next page?

LastEvaluatedKey tells us where the last fetch stopped. Without it, we can't continue from the right place. See execution_table rows 2 and 4 where we check for it.

What happens if LastEvaluatedKey is null after a scan?

It means there are no more pages to fetch. The pagination ends. This is shown in execution_table row 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of LastEvaluatedKey after Step 3?

A key2

B key1

C null

D undefined

💡 Hint

Check variable_tracker column 'After Step 3' for LastEvaluatedKey value.

At which step does the pagination end according to the execution_table?

A Step 4

B Step 5

C Step 6

D Step 2

💡 Hint

Look for the step where LastEvaluatedKey is null and pagination stops.

If we increase Limit to 3, how would the FetchedItemsCount change after Step 1?

A It would be 2

B It would be 3

C It would be 6

D It would be 0

💡 Hint

Limit controls how many items are fetched per scan, see execution_sample and variable_tracker.

Concept Snapshot

Pagination with SDK:
- Use Limit to set page size.
- After each fetch, check LastEvaluatedKey.
- If present, use it as ExclusiveStartKey for next fetch.
- Repeat until LastEvaluatedKey is null.
- This fetches data page by page efficiently.

Full Transcript

Pagination with SDK in DynamoDB works by fetching data in pages. We start by calling scan with a Limit to get a small set of items. The response may include LastEvaluatedKey, which marks where to continue next. If LastEvaluatedKey exists, we use it as ExclusiveStartKey in the next scan call to get the next page. This repeats until LastEvaluatedKey is null, meaning no more data. The execution table shows each step: fetching pages, checking keys, and stopping. Variables like LastEvaluatedKey and FetchedItemsCount track progress. This method helps handle large data sets without loading all at once.

Practice

1. What does the Limit parameter do when paginating results in DynamoDB SDK?

easy

A. It specifies the starting point for the next page.

B. It sets the maximum number of items returned in one page.

C. It filters items based on a condition.

D. It sorts the results in ascending order.

Solution

1. Step 1: Understand the role of Limit in pagination

   The Limit parameter controls how many items DynamoDB returns in a single request, effectively setting the page size.

2. Step 2: Differentiate from other parameters

   ExclusiveStartKey sets the starting point for the next page, not Limit. Filtering and sorting are handled differently.

3. Final Answer:

   It sets the maximum number of items returned in one page. -> Option B

4. Quick Check:

   Limit = page size [OK]

Hint: Limit controls page size, not start or filter [OK]

Common Mistakes:

• Confusing Limit with ExclusiveStartKey
• Thinking Limit filters data
• Assuming Limit sorts results

2. Which of the following is the correct way to continue fetching the next page of results in DynamoDB SDK?

easy

A. Set StartKey to zero.

B. Use Limit with the last item key.

C. Set ExclusiveStartKey to the last evaluated key from previous response.

D. Use NextPageToken parameter.

Solution

1. Step 1: Identify the parameter for pagination continuation

   DynamoDB uses ExclusiveStartKey to tell the query where to start for the next page.

2. Step 2: Verify other options

   Limit sets page size, StartKey is not a valid parameter, and NextPageToken is not used in DynamoDB SDK.

3. Final Answer:

   Set ExclusiveStartKey to the last evaluated key from previous response. -> Option C

4. Quick Check:

   Use ExclusiveStartKey for next page [OK]

Hint: Use ExclusiveStartKey to continue pagination [OK]

Common Mistakes:

• Using Limit to continue pages
• Using invalid parameter StartKey
• Assuming NextPageToken exists in DynamoDB

3. Given the following DynamoDB SDK code snippet, what will be the output if LastEvaluatedKey is not empty?

const params = { TableName: 'Books', Limit: 2 };
const data = await dynamoDbClient.scan(params);
console.log(data.Items.length);
console.log(data.LastEvaluatedKey !== undefined);

medium

A. Error due to missing ExclusiveStartKey

B. All items and false

C. 2 and false

D. 2 and true

Solution

1. Step 1: Understand Limit effect on Items returned

   With Limit: 2, the scan returns at most 2 items, so data.Items.length will be 2.

2. Step 2: Check LastEvaluatedKey presence

   If LastEvaluatedKey is not empty, it means more pages exist, so data.LastEvaluatedKey !== undefined is true.

3. Final Answer:

   2 and true -> Option D

4. Quick Check:

   Limit = 2 items, LastEvaluatedKey present = true [OK]

Hint: Limit caps items; LastEvaluatedKey true means more pages [OK]

Common Mistakes:

• Assuming all items returned ignoring Limit
• Thinking LastEvaluatedKey is false when present
• Expecting error without ExclusiveStartKey

4. You wrote this code to paginate DynamoDB scan results but it returns only the first page repeatedly:

let params = { TableName: 'Users', Limit: 3 };
let data;
do {
  data = await dynamoDbClient.scan(params);
  console.log(data.Items);
} while (data.LastEvaluatedKey);

What is the main issue?

medium

A. Missing update of ExclusiveStartKey in params inside the loop.

B. Limit should be increased to fetch all items.

C. The loop condition should check data.Items instead.

D. The scan method does not support pagination.

Solution

1. Step 1: Analyze loop behavior

   The code calls scan repeatedly but never updates ExclusiveStartKey, so it fetches the same first page each time.

2. Step 2: Identify correct fix

   To get next pages, params.ExclusiveStartKey must be set to data.LastEvaluatedKey inside the loop.

3. Final Answer:

   Missing update of ExclusiveStartKey in params inside the loop. -> Option A

4. Quick Check:

   Update ExclusiveStartKey to paginate [OK]

Hint: Update ExclusiveStartKey each loop to get next page [OK]

Common Mistakes:

• Not updating ExclusiveStartKey inside loop
• Increasing Limit instead of paginating
• Wrong loop condition on Items
• Thinking scan can't paginate

5. You want to fetch all items from a large DynamoDB table using pagination with the SDK. Which approach correctly handles this scenario?

hard

A. Use a loop that calls scan with Limit, updating ExclusiveStartKey each time until LastEvaluatedKey is undefined.

B. Call scan once with a very large Limit to get all items at once.

C. Use scan without Limit and stop when Items array is empty.

D. Call scan repeatedly without ExclusiveStartKey until Items length is zero.

Solution

1. Step 1: Understand DynamoDB pagination for large tables

   Large tables require multiple scan calls with a Limit to avoid timeouts and memory issues.

2. Step 2: Correctly implement pagination loop

   Each scan call should use ExclusiveStartKey from the previous LastEvaluatedKey until it is undefined, indicating no more pages.

3. Final Answer:

   Use a loop that calls scan with Limit, updating ExclusiveStartKey each time until LastEvaluatedKey is undefined. -> Option A

4. Quick Check:

   Loop with Limit and ExclusiveStartKey until done [OK]

Hint: Loop with Limit and ExclusiveStartKey until no LastEvaluatedKey [OK]

Common Mistakes:

• Using huge Limit causing timeouts
• Ignoring ExclusiveStartKey for next pages
• Stopping on empty Items instead of LastEvaluatedKey
• Calling scan repeatedly without pagination