Transactions signed by the same private key (i.e., with the same address) are submitted and handled in an order using nonce. Usually, nonce is a non-negative integer value, starting from 0, that stands for the number of transactions accepted to blocks so far. Whenever a transaction is submitted its nonce is check in the following way:
If the nonce value is equal to (the number of transactions with the same address in blocks) + 1, it's accepted
Otherwise, it's kept in pending mode with a predefined timeout value until the condition met.
We call this type of nonce strictly ordered nonce. This is enough for typical human-generated transactions. For more use cases like machine-generated transactions, we support two more types: loosely ordered nonce and unordered nonce.
Transactions with loosely ordered nonce are ordered using timestamp:
If the timestamp is larger than the last timestamp, it's accepted
Otherwise, it' rejected
Transactions with unordered nonce is always accepted unless a transaction with the same transaction hash is already in blocks.
Three different types of nonce can be compared as follows:
When To Use
Single or multiple clients, transactions are not aligned with each other.
Machine-generated txs with different generation time and submission time
Multiple clients, transactions are aligned with time
Machine-generated txs with multiple clients