When is a pending transaction technically confirmed?

Question

I'd like to understand the steps that lead to an eventual confirmation of a transaction. I understand that there are several child transactions required and that the individual weights of each of these need to add up to a certain threshold.

However, what is that threshold? Is it dynamic? And what happens when that threshold is reached? Is it at that point that the node syncs with all the neighbour nodes? Or when does the syncing happen?

Or is a transaction only confirmed when it's available on a certain minimum number of nodes (similar to BTC)?

Answer 1

The validity of a transaction (let's call it target-tx) can be evaluated with this algorithm:

• random selection of 100 new transaction
• for each of them: count 1 if there is a path to target-tx (i.e. if this random transaction indirectly—or directly—validates target-tx)

• once you have done this evaluation for all 100 transactions, the number you have is a indication of the validity of target-tx:

  • less than 50% --> not validated (yet)
  • more than 50% --> fair chance to be valid
  • 99% --> you can be sure it is valid.

The percentage to reach to consider a transaction valid is up to you.

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I forgot to mention one important point. Today, the network is still quite small and so there is a special actor run by the IOTA foundation: the Coordinator.

The Coordinator is issuing a milestone transaction every minute, and there is a rule stating that "Any transaction validated directly or indirectly by a milestone transaction can be considered 'confirmed'".

I assume that tangle explorers use that rule.