AppSync with DynamoDB (GraphQL) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a GraphQL query that fetches all items from a DynamoDB table.

DynamoDB

query ListItems {
  listItems [1] {
    items {
      id
      name
    }
  }
}

Drag options to blanks, or click blank then click option'

A()

B{filter: {}}

C(limit: 10)

D(id: "123")

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting parentheses causes syntax errors.

Passing unnecessary arguments when none are required.

The listItems query requires parentheses even if no arguments are passed, so () is correct.

2fill in blank

medium

Complete the mutation to add a new item with id and name fields.

DynamoDB

mutation AddItem {
  createItem(input: [1]) {
    id
    name
  }
}

Drag options to blanks, or click blank then click option'

A"id: 1, name: Item1"

B{id: "1", name: "Item1"}

C[id, name]

Did, name

Attempts:

3 left

💡 Hint

Common Mistakes

Using strings instead of objects for input.

Using arrays or comma-separated values without braces.

The input argument must be an object with field names and values, so {id: "1", name: "Item1"} is correct.

3fill in blank

hard

Fix the error in the resolver mapping template to get an item by id.

DynamoDB

{
  "version": "2018-05-29",
  "operation": "[1]",
  "key": {
    "id": $util.dynamodb.toDynamoDBJson($ctx.args.id)
  }
}

Drag options to blanks, or click blank then click option'

APutItem

BScan

CQuery

DGetItem

Attempts:

3 left

💡 Hint

Common Mistakes

Using PutItem instead of GetItem causes wrong operation.

Using Query without specifying key conditions.

To get a single item by key, the operation must be GetItem.

4fill in blank

hard

Fill both blanks to create a resolver that queries items with a filter expression.

DynamoDB

{
  "version": "2018-05-29",
  "operation": "[1]",
  "filter": {
    "expression": "[2]",
    "expressionValues": {
      ":val": $util.dynamodb.toDynamoDBJson($ctx.args.filterValue)
    }
  }
}

Drag options to blanks, or click blank then click option'

AScan

BQuery

Cid = :val

Dname = :val

Attempts:

3 left

💡 Hint

Common Mistakes

Using Query without key condition causes errors.

Incorrect filter expression syntax.

Scan operation supports filter expressions like name = :val to filter items.

5fill in blank

hard

Fill all three blanks to define a mutation resolver that updates an item attribute.

DynamoDB

{
  "version": "2018-05-29",
  "operation": "[1]",
  "key": {
    "id": $util.dynamodb.toDynamoDBJson($ctx.args.id)
  },
  "update": {
    "expression": "set [2] = :val",
    "expressionValues": {
      ":val": $util.dynamodb.toDynamoDBJson($ctx.args.newValue)
    }
  },
  "condition": "[3]"
}

Drag options to blanks, or click blank then click option'

AUpdateItem

BattributeName

Cattribute_exists(id)

DPutItem

Attempts:

3 left

💡 Hint

Common Mistakes

Using PutItem overwrites the whole item.

Missing condition can cause errors if item does not exist.

UpdateItem is the operation to update attributes. The expression sets the attributeName to a new value. The condition ensures the item exists.

Practice

1. What is the main benefit of using AWS AppSync with DynamoDB in an application?

easy

A. It allows apps to request only the data they need using GraphQL.

B. It automatically creates SQL tables for data storage.

C. It requires manual scaling of the database.

D. It replaces the need for any backend code.

Solution

1. Step 1: Understand AppSync's role

   AppSync uses GraphQL to let apps ask for exactly the data they need, avoiding over-fetching.

2. Step 2: Understand DynamoDB's role

   DynamoDB stores data quickly and scales automatically, but does not create SQL tables or replace backend code entirely.

3. Final Answer:

   It allows apps to request only the data they need using GraphQL. -> Option A

4. Quick Check:

   AppSync + GraphQL = precise data requests [OK]

Hint: Remember: AppSync uses GraphQL for precise data fetching [OK]

Common Mistakes:

• Thinking DynamoDB creates SQL tables
• Believing manual scaling is needed
• Assuming AppSync removes all backend code

2. Which of the following is the correct way to define a DynamoDB data source in an AWS AppSync GraphQL schema?

easy

A. type Query { getItem(id: ID!): Item @aws_dynamodb(tableName: MyTable) }

B. type Query { getItem(id: ID!): Item @aws_lambda(functionName: "MyFunction") }

C. type Query { getItem(id: ID!): Item @aws_dynamodb(tableName: "MyTable", region: "us-east-1") }

D. type Query { getItem(id: ID!): Item @aws_s3(bucketName: "MyBucket") }

Solution

1. Step 1: Identify the correct directive for DynamoDB

   The @aws_dynamodb directive is used to link a DynamoDB table as a data source in AppSync.

2. Step 2: Check syntax correctness

   type Query { getItem(id: ID!): Item @aws_dynamodb(tableName: "MyTable", region: "us-east-1") } correctly includes the tableName and region parameters, which are valid for specifying the DynamoDB data source.

3. Final Answer:

   type Query { getItem(id: ID!): Item @aws_dynamodb(tableName: "MyTable", region: "us-east-1") } -> Option C

4. Quick Check:

   @aws_dynamodb with tableName and region is correct syntax [OK]

Hint: Look for @aws_dynamodb with tableName and region parameters [OK]

Common Mistakes:

• Using @aws_lambda instead of @aws_dynamodb for DynamoDB
• Forgetting quotes around tableName value
• Using @aws_s3 which is unrelated to DynamoDB

3. Given this GraphQL query in AppSync linked to DynamoDB:

query GetBook { getBook(id: "123") { id title author } }

And the DynamoDB table has an item:

{"id": "123", "title": "Learn DynamoDB", "author": "Jane"}

What will be the output of this query?

medium

A. {"data": {"getBook": {"id": "123", "title": "Learn DynamoDB", "author": "Jane"}}}

B. {"data": {"getBook": null}}

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

D. {"data": {"getBook": {"id": "123", "title": null, "author": null}}}

Solution

1. Step 1: Understand the query and data

   The query requests the item with id "123" and fields id, title, author. The table has this exact item.

2. Step 2: Predict the query result

   Since the item exists, the query returns all requested fields with their values.

3. Final Answer:

   {"data": {"getBook": {"id": "123", "title": "Learn DynamoDB", "author": "Jane"}}} -> Option A

4. Quick Check:

   Existing item returns full data [OK]

Hint: If item exists, query returns requested fields with values [OK]

Common Mistakes:

• Assuming null fields if item exists
• Expecting errors when item is present
• Confusing null result with missing item

4. You wrote this resolver mapping template for a DynamoDB getItem operation in AppSync:

{
  "version": "2017-02-28",
  "operation": "GetItem",
  "key": {
    "id": $util.dynamodb.toDynamoDBJson($ctx.args.id)
  }
}

But your query always returns null. What is the likely mistake?

medium

A. The $util.dynamodb.toDynamoDBJson function is not needed.

B. The operation should be "Query" instead of "GetItem".

C. The version "2017-02-28" is outdated and causes errors.

D. The key attribute name "id" does not match the DynamoDB table's primary key name.

Solution

1. Step 1: Check key attribute name

   The key attribute name in the resolver must exactly match the DynamoDB table's primary key name.

2. Step 2: Understand why null is returned

   If the key name is wrong, DynamoDB cannot find the item, so it returns null.

3. Final Answer:

   The key attribute name "id" does not match the DynamoDB table's primary key name. -> Option D

4. Quick Check:

   Key name mismatch causes null result [OK]

Hint: Match key names exactly between resolver and table [OK]

Common Mistakes:

• Using wrong operation type for getItem
• Ignoring key name case sensitivity
• Removing $util.dynamodb.toDynamoDBJson causing syntax errors

5. You want to create a real-time chat app using AppSync and DynamoDB. Which approach best ensures that clients receive new messages instantly without polling?

hard

A. Have clients poll DynamoDB every few seconds for new messages.

B. Use AppSync subscriptions linked to DynamoDB streams to push updates to clients.

C. Use a Lambda function to write messages to DynamoDB and notify clients via email.

D. Store messages in DynamoDB and refresh the UI only when the app restarts.

Solution

1. Step 1: Understand real-time updates in AppSync

   AppSync supports GraphQL subscriptions that can push data to clients when data changes.

2. Step 2: Connect DynamoDB streams to subscriptions

   DynamoDB streams capture table changes and can trigger AppSync subscriptions to notify clients instantly.

3. Step 3: Evaluate other options

   Polling wastes resources, email notifications are slow and unrelated, and UI refresh on restart is not real-time.

4. Final Answer:

   Use AppSync subscriptions linked to DynamoDB streams to push updates to clients. -> Option B

5. Quick Check:

   Subscriptions + DynamoDB streams = instant updates [OK]

Hint: Use subscriptions with DynamoDB streams for real-time data [OK]

Common Mistakes:

• Relying on polling instead of subscriptions
• Using email for real-time notifications
• Ignoring DynamoDB streams integration