Functions with Cosmos DB integration in Azure - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Azure Function?

An Azure Function is a small piece of code that runs in the cloud when triggered by an event, like a timer or a database change. It helps automate tasks without managing servers.

beginner

How does Azure Functions integrate with Cosmos DB?

Azure Functions can automatically run when data changes in Cosmos DB or can read/write data to Cosmos DB using bindings, making it easy to connect code with the database without extra setup.

intermediate

What is an input binding in Azure Functions for Cosmos DB?

An input binding lets the function read data from Cosmos DB automatically when it runs, so the code can use that data without writing database connection code.

intermediate

What is an output binding in Azure Functions for Cosmos DB?

An output binding lets the function write data to Cosmos DB automatically after it runs, so the code just sends data to the binding and Azure handles saving it.

intermediate

Why use Cosmos DB trigger in Azure Functions?

A Cosmos DB trigger runs your function automatically when data changes in a Cosmos DB container, like when a new item is added. This helps react to data changes instantly without polling.

What does an Azure Function with a Cosmos DB output binding do?

A Writes data to Cosmos DB automatically

B Reads data from Cosmos DB automatically

C Triggers when Cosmos DB data changes

D Manages Cosmos DB server settings

An output binding sends data from the function to Cosmos DB automatically.

Which Azure Function binding triggers the function when Cosmos DB data changes?

A Input binding

B Output binding

C Cosmos DB trigger

D Timer trigger

The Cosmos DB trigger runs the function when data changes in the database.

What is the main benefit of using bindings in Azure Functions with Cosmos DB?

A Automatic scaling of Cosmos DB

B No need to write database connection code

C Improved security for Cosmos DB

D Faster database queries

Bindings simplify code by handling database connections automatically.

Which of these is NOT a typical trigger for Azure Functions?

A Timer schedule

B Cosmos DB data change

C HTTP request

D Manual server restart

Manual server restart is not a trigger for Azure Functions.

If you want your function to process new documents added to Cosmos DB, which should you use?

A Cosmos DB trigger

B Cosmos DB input binding

C Cosmos DB output binding

D HTTP trigger

The Cosmos DB trigger runs the function when new documents are added.

Explain how Azure Functions can interact with Cosmos DB using bindings and triggers.

Think about how the function gets data and how it sends data back.

Describe a real-life scenario where using a Cosmos DB trigger in an Azure Function is helpful.

Imagine a store that wants to act right away when a customer places an order.

Practice

1. What is the main benefit of using Azure Functions with Cosmos DB integration?

easy

A. Automatically run code when data changes in Cosmos DB

B. Manually trigger code only through HTTP requests

C. Store large files directly in Cosmos DB

D. Replace Cosmos DB with Azure Blob Storage

Solution

1. Step 1: Understand Azure Functions with Cosmos DB

   Azure Functions can be triggered automatically by changes in Cosmos DB data.

2. Step 2: Identify the main benefit

   This automatic trigger saves resources by running code only when needed, without manual calls.

3. Final Answer:

   Automatically run code when data changes in Cosmos DB -> Option A

4. Quick Check:

   Functions trigger on data changes = A [OK]

Hint: Functions run on data change events automatically [OK]

Common Mistakes:

• Thinking functions run only on HTTP triggers
• Confusing Cosmos DB with file storage
• Assuming manual triggers are required

2. Which of the following is the correct binding direction for a Cosmos DB input binding in an Azure Function?

easy

A. direction: "out"

B. direction: "both"

C. direction: "trigger"

D. direction: "in"

Solution

1. Step 1: Recall binding directions

   Input bindings receive data into the function, so their direction is "in".

2. Step 2: Match binding direction for Cosmos DB input

   Cosmos DB input binding must have direction set to "in" to read data.

3. Final Answer:

   direction: "in" -> Option D

4. Quick Check:

   Input binding direction = in [OK]

Hint: Input bindings always use direction "in" [OK]

Common Mistakes:

• Using "out" for input bindings
• Confusing trigger with input binding
• Using invalid directions like "both"

3. Given this Azure Function code snippet triggered by Cosmos DB changes, what will be logged if a new document with id "123" is added?

module.exports = async function (context, documents) {
  if (!!documents && documents.length > 0) {
    context.log(`Document id: ${documents[0].id}`);
  }
};

medium

A. No output logged

B. Document id: undefined

C. Document id: 123

D. Error: documents is not defined

Solution

1. Step 1: Understand the trigger input

   The function receives an array 'documents' with changed documents; the first document has id "123".

2. Step 2: Analyze the logging statement

   The code logs the id of the first document, which is "123".

3. Final Answer:

   Document id: 123 -> Option C

4. Quick Check:

   documents[0].id = 123 logged [OK]

Hint: documents array holds changed items; access first with documents[0] [OK]

Common Mistakes:

• Assuming documents is undefined
• Logging without checking documents length
• Confusing document id property

4. You wrote an Azure Function triggered by Cosmos DB changes, but it never runs when documents change. Which is the most likely cause?

medium

A. The function code has a syntax error

B. The function.json binding has incorrect connection string name

C. The Cosmos DB container is empty

D. The function app is stopped

Solution

1. Step 1: Check trigger configuration

   If the connection string name in function.json is wrong, the function won't connect to Cosmos DB changes.

2. Step 2: Consider other causes

   Syntax errors cause failures but usually show errors; empty container still triggers on inserts; stopped app won't run but question implies function exists.

3. Final Answer:

   The function.json binding has incorrect connection string name -> Option B

4. Quick Check:

   Wrong connection string stops trigger [OK]

Hint: Check connection string name in function.json first [OK]

Common Mistakes:

• Ignoring binding configuration errors
• Assuming empty container prevents triggers
• Not verifying function app status

5. You want to create an Azure Function that writes a summary document to Cosmos DB whenever multiple documents are added. Which binding setup should you use?

hard

A. Use Cosmos DB trigger for input and Cosmos DB output binding for summary document

B. Use HTTP trigger and Cosmos DB input binding only

C. Use Cosmos DB input binding only, no trigger

D. Use Timer trigger and Cosmos DB output binding only

Solution

1. Step 1: Identify trigger for reacting to data changes

   Cosmos DB trigger runs the function automatically when documents change.

2. Step 2: Use output binding to write summary

   Output binding lets the function write a new summary document back to Cosmos DB.

3. Final Answer:

   Use Cosmos DB trigger for input and Cosmos DB output binding for summary document -> Option A

4. Quick Check:

   Trigger input + output binding for writing = C [OK]

Hint: Trigger on changes, output binding to write summary [OK]

Common Mistakes:

• Using HTTP trigger instead of Cosmos DB trigger
• Missing output binding for writing data
• Using timer trigger without data event