Lambda concurrency and throttling in AWS - Step-by-Step Execution

Process Flow - Lambda concurrency and throttling

Incoming Lambda Requests

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Check Current Concurrency

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Below Limit

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Invoke Lambda

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Process

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Complete Execution

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Decrease Concurrency Count

Lambda checks how many functions run at once. If below limit, it runs. If at limit, it stops new runs and returns an error.

Execution Sample

AWS

1. Set concurrency limit = 3
2. Receive 5 requests quickly
3. Invoke Lambda if concurrency < 3
4. Throttle if concurrency >= 3

Simulates 5 quick Lambda calls with a concurrency limit of 3 to show which run and which get throttled.

Process Table

Step	Incoming Request #	Current Concurrency	Condition (Concurrency < Limit?)	Action	Result
1	1	0	0 < 3 (True)	Invoke Lambda	Request 1 runs, concurrency = 1
2	2	1	1 < 3 (True)	Invoke Lambda	Request 2 runs, concurrency = 2
3	3	2	2 < 3 (True)	Invoke Lambda	Request 3 runs, concurrency = 3
4	4	3	3 < 3 (False)	Throttle Request	Request 4 throttled, concurrency = 3
5	5	3	3 < 3 (False)	Throttle Request	Request 5 throttled, concurrency = 3
6	Request 1 completes	3	-	Decrease concurrency	Concurrency = 2
7	Request 4 retry	2	2 < 3 (True)	Invoke Lambda	Request 4 runs, concurrency = 3

💡 Requests 4 and 5 are throttled because concurrency reached the limit of 3.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
concurrency	0	1	2	3	3	3	2	3
request_status	none	running(1)	running(1,2)	running(1,2,3)	throttled(4)	throttled(5)	running(2,3)	running(2,3,4)

Key Moments - 2 Insights

Why are requests 4 and 5 throttled even though they arrived after requests 1, 2, and 3?

Because concurrency was already at the limit 3 when requests 4 and 5 arrived (see steps 4 and 5 in execution_table), Lambda cannot run more than 3 at once.

What happens to concurrency when a Lambda request finishes?

Concurrency decreases by 1 (step 6), allowing new requests to run if they retry (step 7).

Visual Quiz - 3 Questions

Look at the execution table, what is the concurrency value after step 3?

A 3

B 2

C 1

D 0

💡 Hint

Check the 'Current Concurrency' column at step 3 in the execution_table.

At which step does the first request get throttled?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for the first 'Throttle Request' action in the execution_table.

If the concurrency limit was increased to 5, what would happen at step 5?

A Request 4 would be throttled

B Request 5 would be throttled

C Request 5 would run

D No requests would run

💡 Hint

Compare concurrency and limit at step 5 in the execution_table and imagine the limit is 5 instead of 3.

Concept Snapshot

Lambda concurrency limits how many functions run at once.
If concurrency reaches the limit, new requests are throttled.
When a function finishes, concurrency decreases.
Throttled requests can retry later.
This protects backend resources from overload.

Full Transcript

This visual execution shows how AWS Lambda handles concurrency and throttling. When Lambda receives requests, it checks how many functions are running. If the number is below the concurrency limit, Lambda runs the function and increases concurrency. If the limit is reached, Lambda throttles new requests, returning an error. When a running function finishes, concurrency decreases, allowing throttled requests to retry and run. This process ensures Lambda does not overload resources by running too many functions at once.