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.