How can I know if I trigger the create transaction API isn't this a duplicate call?

[BinMunawir]

Hi,
If I trigger the API with the IK for the first time, I'll get a 200 status code. If I retrigger the request, I'll receive the same response as the first time. which means the API is idempotent.
However, I have a business case to know that if my request is the first or a duplicate?

my business case is this:

my client needs out to send 500 USD
Before I send the 500USD to the rails, I need to deduct the amount from their wallet
if I had duplicate calls to the ledger, I would have to handle it from my side, which is very complicated to do
Basically, similar to HTTP status codes 200 for the first request, and 201 for any subsequent ones

I searched Blnk ledger, when a duplicate call happened, it shows this error "duplicate reference" error

you might argue, we shouldn't return an error, but atleast this is very much safer and easy to handle than have a response which I wouldn't know if it's a duplicate or not

[aalloiteau]

Hi @BinMunawir

Add Idempotency-Key in the header of the request; the system will skip your request processing and return a success directly should you execute the same request twice

The feature is applied on endpoints which write to the database :

• Create transaction
• Update metadata (account or transaction)
• Revert transaction

When requesting the service with a IK, if the IK has already been used on another request terminated with success (Status code 2XX), the same response will be returned, plus a header named "Idempotency-Hit" with the value "true".

Given a IK, if the request does not match the originated request, either if the path or the body does not match, the service will responds with a 400 status code. The scope of the idempotency keys is bound to a specific ledger. Therefore a same IK can be used on two differents ledgers without triggering any alert.

Other option -> use reference field in the body of a transaction creation. if this reference already exists, an error will be returned.

[BinMunawir]

to simplify my question, how can I differentiate between first-request and subsequent-request when I create a transaction from the API?

thanks @aalloiteau for your reply, two points I have

• plus a header named "Idempotency-Hit" with the value "true". I didn't see any response header with that field name
• use reference field in the body of a transaction creation. if this reference already exists, an error will be returned this is working thanks

[aalloiteau]

Hi @BinMunawir ,
Sorry for the confusion, the Idempotency-Hit was in ledger v1.
Let me get back to you shortly on this one, it could be interesting to re-introduce it for use-cases like yours indeed.

[aalloiteau]

Hi @BinMunawir , for your information Idempotency-Hit was added back here released in v2.3.6