Idempotency-Key in our API: repeating a purchase, payment or ticket without doing it twice

The short answer: if the connection drops in the middle of a request, you cannot tell whether it was carried out. With the Idempotency-Key header you can repeat the same request without any risk of registering, charging or opening twice: for 24 hours, the same value with the same body returns the stored answer and nothing is done again.

How to use it

Generate a unique value for each operation, such as a UUID, and send it with a POST request. It can be up to 255 visible characters. Reuse the same value only when you repeat the same operation.

curl https://api.mozout.com/v1/invoices/3127/pay -X POST \
-H "Authorization: Bearer $RLX_API_KEY" \
-H "Idempotency-Key: 0f8fad5b-d9cb-469f-a165-70867728950e"

What happens in each case

Situation	Answer
First request with this value	The operation runs normally.
Same value and same body	The stored answer comes back, with the Idempotent-Replayed: true header. Nothing is done again.
Same value and a different body	409 idempotency_conflict
Same value while the first request is still running	409 idempotency_conflict

The value stays reserved per key for 24 hours.

Where it is required

Operation	What it prevents
POST /v1/domains/register, /renew, /transfer	Registering, renewing or transferring the same domain twice
POST /v1/extras/purchase	Buying the same certificate or SiteLock twice
POST /v1/invoices/{id}/pay	Charging the same invoice twice
POST /v1/tickets	Opening the same ticket twice
POST /v1/servers/{id}/reinstall, /restore	Reinstalling or restoring the server twice
POST /v1/sms/messages	Sending the same SMS twice

For ticket replies, POST /v1/tickets/{id}/replies, it is recommended, so that a retry does not post the reply twice.

After a failure

1	No answer arrived (timeout, dropped connection): repeat the request with the same value.

2	You got a 409 because the first is still running: wait a moment and retry with the same value.

3	You got a 409 because the body differs: if it really is a different operation, use a new value.

Where the key does not protect you

Password requests and cPanel login: for mailbox, database and FTP passwords, and for the cPanel login link, the Idempotency-Key has no effect: repeating the request runs it again, because the API never stores the password or the link.

Special case	What happens
VPS root password	Retrying with the same key does not generate another password and returns password as null. The password only appears in the first answer: save it straight away.
Hosting with a 502	If the request had already reached the server, the same key returns that 502 without running it again. Check the result and, to try again, use a new key.

Generate the key before the first attempt and store it with the operation (for example, on the order row in your database). That way, even if your program restarts halfway, the retry uses the same value. The general idea, for any API, is in timeouts, retries and idempotency.

Is your program going to buy or pay through the API? Test your retries in Sandbox before you use a Production key.

See the API page

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