What is sender_id param in send_deferred function?

Question

I tried to implement repeated execution of particular action and set this param as _self of calling contract, but the transaction gets executed only once in this case. Then i tried to set it as time() and it worked, so has it to be unique?

Answer 1

The sender_id is just a value set by the contract that sends the deferred transaction so that later operations can refer to the same transaction. The semantics are:

• sending a new transaction with the same sender_id as one in-flight will replace the in-flight transaction
• canceling a transaction can be achieved by passing that sender_id into cancel_deferred

So, if you want to have N transactions in-flight at the same time they need N unique ID's