AppSync with DynamoDB (GraphQL) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this GraphQL query with DynamoDB resolver?

Given a DynamoDB table Books with items having id and title, and this GraphQL query:

{ getBook(id: "123") { id title } }

Assuming the item with id = "123" exists with title "Learn GraphQL", what is the expected output?

A {"data": {"getBook": null}}

B {"data": {"getBook": {"id": "123", "title": "Learn GraphQL"}}}

C {"errors": [{"message": "Item not found"}]}

D {"data": {"getBook": {"id": "123"}}}

💡 Hint

Think about what the resolver returns when the item exists.

The resolver fetches the item by id and returns all requested fields. Since the item exists, the data is returned with id and title.

📝 Syntax

intermediate

Which VTL snippet correctly retrieves an item by id in DynamoDB resolver?

In an AppSync DynamoDB resolver, you want to get an item by id from the Books table. Which Velocity Template Language (VTL) snippet is syntactically correct?

A

{
  "version": "2018-05-29",
  "operation": "GetItem",
  "key": {
    "id": { "S": "$ctx.args.id" }
  }
}

B

{
  "version": "2018-05-29",
  "operation": "GetItem",
  "key": {
    "id": "$ctx.args.id"
  }
}

C

{
  "version": "2018-05-29",
  "operation": "Query",
  "key": {
    "id": { "S": "$ctx.args.id" }
  }
}

D

{
  "version": "2018-05-29",
  "operation": "GetItem",
  "key": {
    "id": { "N": "$ctx.args.id" }
  }
}

💡 Hint

Remember DynamoDB expects attribute types in the key.

Option A correctly specifies the operation as GetItem and the key with attribute type S (string) for id. Option A misses the attribute type, C uses Query instead of GetItem, and A uses N (number) which is incorrect if id is a string.

❓ optimization

advanced

How to optimize a DynamoDB resolver for filtering items by a non-key attribute?

You want to filter Books by author attribute, which is not a key. Which approach optimizes performance in AppSync with DynamoDB?

A Use BatchGetItem operation with <code>author</code> values.

B Use a Scan operation with a filter expression on <code>author</code>.

C Create a Global Secondary Index (GSI) on <code>author</code> and use Query operation on the GSI.

D Use GetItem operation with <code>author</code> as key.

💡 Hint

Think about how DynamoDB handles queries on non-key attributes efficiently.

Scan reads the entire table and is slow. GetItem requires key attributes, which author is not. BatchGetItem requires keys too. Creating a GSI on author allows efficient Query operations.

🔧 Debug

advanced

Why does this AppSync resolver return null for an existing item?

Given this VTL request mapping template:

{
  "version": "2018-05-29",
  "operation": "GetItem",
  "key": {
    "id": { "S": "$ctx.args.bookId" }
  }
}

And the GraphQL query:

{ getBook(id: "abc123") { id title } }

The item with id = "abc123" exists, but the response is null. What is the likely cause?

A The request mapping uses <code>$ctx.args.bookId</code> but the query argument is <code>id</code>.

B The DynamoDB table does not have an item with <code>id = "abc123"</code>.

C The response mapping template is missing.

D The operation should be Query instead of GetItem.

💡 Hint

Check if the argument names match between query and resolver.

The resolver expects bookId argument but the query provides id. This mismatch causes the key to be empty, so DynamoDB returns no item, resulting in null.

🧠 Conceptual

expert

What is a key benefit of using DynamoDB with AppSync for real-time GraphQL APIs?

Choose the best explanation of why DynamoDB is a good choice for AppSync real-time GraphQL APIs.

A DynamoDB stores data in XML format which AppSync parses efficiently for real-time APIs.

B DynamoDB supports flexible relational joins which AppSync uses for real-time updates.

C DynamoDB automatically generates GraphQL schemas for AppSync without configuration.

D DynamoDB's single-digit millisecond latency and seamless integration with AppSync enable fast real-time data access.

💡 Hint

Think about performance and integration benefits.

DynamoDB offers very low latency and integrates natively with AppSync, making it ideal for real-time GraphQL APIs. It does not support relational joins or automatic schema generation, nor does it store XML data.