Cold start and premium plan in Azure - Step-by-Step Execution

Process Flow - Cold start and premium plan

User sends request

↓

Check if function app is warm

↓

Cold start triggered

↓

Allocate resources

↓

Load function code

↓

Function ready

↓

Execute function

↓

Return response

When a request comes, the system checks if the function app is ready. If not, a cold start happens, causing delay. Premium plans keep apps warm to avoid cold starts.

Execution Sample

Azure

Request -> Check warm -> Cold start? -> Load resources -> Execute -> Response

This flow shows how a function app handles a request, with cold start causing delay if app is not warm.

Process Table

Step	Action	Condition	Result	Delay (ms)
1	Receive request	N/A	Start processing	0
2	Check if app is warm	App is cold	Trigger cold start	0
3	Allocate resources	Cold start ongoing	Resources allocated	500
4	Load function code	Resources ready	Code loaded	300
5	Function ready	Code loaded	Execute function	0
6	Execute function	Function ready	Response generated	100
7	Return response	Response ready	Request complete	0
8	Next request	App now warm	Execute immediately	0

💡 Execution stops after response is returned; subsequent requests avoid cold start if app is warm.

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 7	After Step 8
App Warm State	False	False	True	True	True
Resources Allocated	False	False	True	True	True
Function Code Loaded	False	False	True	True	True
Response Ready	False	False	False	True	True

Key Moments - 3 Insights

Why does the first request take longer than the next ones?

Because the app is cold at first (see Step 2 in execution_table), it triggers a cold start which allocates resources and loads code, causing delay. Later requests find the app warm and execute faster (Step 8).

What does 'app warm' mean in this context?

'App warm' means the function app has resources allocated and code loaded, ready to execute immediately without delay. This state is reached after Step 4 and maintained after Step 7.

How does the premium plan affect cold starts?

Premium plans keep the app warm continuously, so cold starts are avoided. This means requests execute quickly every time, like at Step 8.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the function code loaded?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Check the 'Action' and 'Result' columns in execution_table rows.

According to variable_tracker, when does the app warm state change to True?

A After Step 2

B After Step 4

C After Step 7

D After Step 8

💡 Hint

Look at the 'App Warm State' row in variable_tracker.

If the app is on a premium plan, which step's delay is eliminated?

A Step 3 - Allocate resources

B Step 6 - Execute function

C Step 2 - Check if app is warm

D Step 7 - Return response

💡 Hint

Premium plans keep resources allocated, so cold start steps like resource allocation are skipped.

Concept Snapshot

Cold start happens when a function app is not ready, causing delay to allocate resources and load code.
Premium plans keep apps warm to avoid cold starts.
Warm apps respond faster as they skip cold start steps.
Cold start delay includes resource allocation and code loading.
Understanding cold start helps optimize app performance and cost.

Full Transcript

When a user sends a request to an Azure function app, the system first checks if the app is warm. If the app is cold, a cold start is triggered, which means the system allocates resources and loads the function code before executing the function. This causes a delay for the first request. After the app is warm, subsequent requests execute immediately without delay. Premium plans keep the app warm continuously, avoiding cold starts and improving response times. The execution table shows each step with associated delays, and the variable tracker shows how the app's warm state and resources change over time.

Practice

1. What is the main reason for a cold start in Azure Functions?

easy

A. The app runs continuously without any breaks

B. The app starts from zero after a period of inactivity causing delay

C. The app uses a premium plan to stay warm

D. The app is deployed in multiple regions

Solution

1. Step 1: Understand cold start concept

   Cold start happens when an app is idle and then starts fresh, causing delay.

2. Step 2: Identify cause of delay

   The delay is due to the app starting from zero after inactivity, not continuous running or premium plans.

3. Final Answer:

   The app starts from zero after a period of inactivity causing delay -> Option B

4. Quick Check:

   Cold start = delay after inactivity [OK]

Hint: Cold start means delay after app inactivity [OK]

Common Mistakes:

• Confusing cold start with continuous running
• Thinking premium plans cause cold start
• Assuming multi-region deployment causes cold start

2. Which Azure plan helps avoid cold start delays by keeping apps warm?

easy

A. Consumption plan

B. Free plan

C. Basic plan

D. Premium plan

Solution

1. Step 1: Identify plan types

   Azure offers different plans: Consumption, Premium, Basic, Free.

2. Step 2: Match plan to cold start behavior

   Premium plan keeps apps warm to avoid cold start delays, unlike Consumption or Free plans.

3. Final Answer:

   Premium plan -> Option D

4. Quick Check:

   Premium plan = keeps apps warm [OK]

Hint: Premium plan avoids cold start by keeping app warm [OK]

Common Mistakes:

• Choosing Consumption plan which allows cold starts
• Confusing Basic plan with Premium
• Assuming Free plan prevents cold start

3. Consider this scenario: An Azure Function app on a Consumption plan is idle for hours. What happens when a new request arrives?

medium

A. The app experiences a cold start delay before responding

B. The app responds instantly with no delay

C. The app crashes due to inactivity

D. The app automatically switches to Premium plan

Solution

1. Step 1: Analyze Consumption plan behavior

   Consumption plan apps scale down to zero when idle, causing cold start on new requests.

2. Step 2: Predict app response on new request

   When a new request arrives after inactivity, the app must start fresh, causing delay.

3. Final Answer:

   The app experiences a cold start delay before responding -> Option A

4. Quick Check:

   Consumption plan + idle = cold start delay [OK]

Hint: Consumption plan apps cold start after idle [OK]

Common Mistakes:

• Assuming instant response on Consumption plan
• Thinking app crashes after inactivity
• Believing app auto-switches plans

4. You deployed an Azure Function on a Premium plan but still notice cold start delays. What is the most likely cause?

medium

A. The app is configured to scale to zero during inactivity

B. The app is using the Consumption plan instead

C. The app has too many instances running

D. The app is deployed in multiple regions

Solution

1. Step 1: Understand Premium plan behavior

   Premium plan keeps apps warm and avoids scaling to zero by default.

2. Step 2: Identify misconfiguration

   If cold start occurs, likely the app is configured to scale to zero despite Premium plan, causing cold start.

3. Final Answer:

   The app is configured to scale to zero during inactivity -> Option A

4. Quick Check:

   Scaling to zero causes cold start even on Premium [OK]

Hint: Check if scaling to zero is enabled on Premium plan [OK]

Common Mistakes:

• Assuming Premium plan never cold starts regardless of config
• Confusing Consumption plan usage
• Thinking multiple regions cause cold start

5. You want an Azure Function app to respond instantly even after long idle periods. Which combination best achieves this?

hard

A. Use Consumption plan with manual warm-up triggers

B. Use Free plan with auto-scaling enabled

C. Use Premium plan with Always On enabled

D. Use Basic plan with cold start optimization

Solution

1. Step 1: Identify plan for instant response

   Premium plan supports Always On feature to keep app warm and avoid cold start.

2. Step 2: Evaluate other options

   Consumption plan needs manual warm-up and may still cold start; Free and Basic plans lack Always On.

3. Final Answer:

   Use Premium plan with Always On enabled -> Option C

4. Quick Check:

   Premium + Always On = no cold start [OK]

Hint: Enable Always On in Premium plan for instant response [OK]

Common Mistakes:

• Relying on Consumption plan for instant response
• Assuming Free plan supports Always On
• Thinking Basic plan has cold start optimization