How does Cardano execute real time transactions if block time is 20 seconds?

Question

New to the crypto space, please correct me if I misunderstood the meaning of block time.

When a cardano node submits a transaction, it goes to a local mempool and submitted on the blockchain for verification. Let's assume this part is instantaneous from the wallet node.

The official docs state that blocks are produced every 20 seconds. In other words, even if the validator nodes receive the transaction immediately, they will wait upto another 20 seconds, and then create a new block to broadcast.

This implies even a low assurance for block will take atleast 20 seconds. In my experience with cardano wallets (before the dexes were launched), it rarely took more than 10 seconds to get a high assurance for transaction.

I also read on the forums that 20 seconds block time is configurable, and can't be enforced on a node. So are we relying on SPOs to use a low block creation time interval to solve real time transactions in the long run?

Answer 1

Block time is 20 seconds in average. Sometimes we have blocks at shorter intervals, sometimes much longer. I saw sometimes sequences of 4 or 5 blocks at one second interval from each other. And sometimes even 2 minutes. And sometimes we have 2 or even stake pools which are minting a block in the same second, and only one will be adopted by the blockchain (this is called a slot battle). And when a stake pool mints a block without receiving the previous minted block (if it was minted in the second before, and the propagation time is more than 1 second), a height battle will occur, and only one of the 2 blocks will be adopted.

SPOs cannot choose when to mint a block. This is decided randomly, based on the VRF key of each stake pool, and is known 1.5 days before each epoch starts for the next epoch. In about 2 hours, I will know how many slots to mint blocks my stake pool has allocated for epoch 318 (exact date and time, in seconds, because one slot is one second). The average interval of 20 seconds between block is given by a parameter, "density", which is not 0.05 (5%). This means 5 blocks in average for every 100 seconds. This is a parameter that can be adjusted, but I don't think it will be adjusted soon.

If your transactions were confirmed until now in less than 10 seconds, you were just "lucky". Now the blockchain is pretty busy, and it will probably take a few blocks until a transaction is confirmed, even more if you are using a light wallet.