Idempotency keys for safe retries in Rest API - Step-by-Step Execution

Concept Flow - Idempotency keys for safe retries

Client sends request with Idempotency Key

↓

Server checks if Key seen before?

Yes→Return stored response

No↓

Process request normally

↓

Store response with Idempotency Key

↓

Send response to client

The client sends a request with a unique key. The server checks if it already processed this key. If yes, it returns the saved response. If no, it processes and saves the response for future retries.

Execution Sample

Rest API

POST /payment
Headers: Idempotency-Key: abc123

Server:
if key in store:
  return stored_response
else:
  process payment
  store response
  return response

This code shows how a server handles a payment request with an idempotency key to avoid duplicate processing.

Execution Table

Step	Action	Idempotency Key	Key Seen?	Server Action	Response Sent
1	Receive request	abc123	No	Process payment, store response	Payment processed
2	Receive retry request	abc123	Yes	Return stored response	Payment processed
3	Receive new request	xyz789	No	Process payment, store response	Payment processed
4	Receive retry request	xyz789	Yes	Return stored response	Payment processed
5	Receive request	abc123	Yes	Return stored response	Payment processed

💡 Requests with seen keys return stored response to avoid duplicate processing.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
Idempotency Store	{}	{"abc123": "Payment processed"}	{"abc123": "Payment processed"}	{"abc123": "Payment processed", "xyz789": "Payment processed"}	{"abc123": "Payment processed", "xyz789": "Payment processed"}	{"abc123": "Payment processed", "xyz789": "Payment processed"}

Key Moments - 3 Insights

Why does the server return the stored response instead of processing the request again?

Because the idempotency key was seen before (see Step 2 in execution_table), the server avoids duplicate processing by returning the saved response.

What happens if the client sends a request with a new idempotency key?

The server processes the request normally and stores the response with the new key (see Step 3 in execution_table).

Can the client safely retry the same request multiple times?

Yes, retries with the same idempotency key return the stored response without reprocessing (see Steps 2, 4, and 5).

Visual Quiz - 3 Questions

Look at the execution_table, what does the server do at Step 2 when it receives the retry request with key 'abc123'?

A Ignore the request

B Process payment again and store new response

C Return stored response without processing

D Generate a new idempotency key

💡 Hint

Check the 'Server Action' column at Step 2 in the execution_table.

At which step does the server first store the response for key 'xyz789'?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Idempotency Key' and 'Key Seen?' columns in execution_table.

If the client sends a request with a new key 'newkey', what will the server do according to the flow?

A Return an error because key is unknown

B Process the request and store the response

C Return the last stored response

D Ignore the idempotency key and process normally

💡 Hint

Refer to the concept_flow where new keys lead to processing and storing the response.

Concept Snapshot

Idempotency keys are unique IDs sent with requests.
Server checks if key was seen before.
If yes, returns stored response to avoid duplicates.
If no, processes request and saves response.
This ensures safe retries without side effects.

Full Transcript

Idempotency keys help servers handle repeated requests safely. When a client sends a request with a unique key, the server checks if it has processed that key before. If it has, the server returns the saved response instead of processing again. If not, it processes the request, stores the response with the key, and returns it. This way, clients can retry requests without causing duplicate actions. The execution table shows steps where requests with keys are processed or returned from storage. The variable tracker shows how the server's store of keys and responses grows. Key moments clarify why stored responses are returned and how new keys are handled. The quiz tests understanding of these steps and behaviors.