Transaction Routing

We all know that transactions are ‘added’ to the chain - but how do they get there?

Hopefully by the end of this article, the image below should make total sense.

Step 1: Transaction creator/author

The journey starts with the author of the transaction - who creates the transaction object (basically list of commands) - and signs them with their private key.

Basically, they prepare the payload that looks like this:

#![allow(unused)]
fn main() {
pub struct SignedTransaction {
    pub transaction: Transaction,
    pub signature: Signature,
}
}

With such a payload, they can go ahead and send it as a JSON-RPC request to ANY node in the system (they can choose between using ‘sync’ or ‘async’ options).

From now on, they’ll also be able to query the status of the transaction - by using the hash of this object.

Fun fact: the Transaction object also contains some fields to prevent attacks: like nonce to prevent replay attack, and block_hash to limit the validity of the transaction (it must be added within transaction_validity_period (defined in genesis) blocks of block_hash).

Step 2: Inside the node

Our transaction has made it to a node in the system - but most of the nodes are not validators - which means that they cannot mutate the chain.

That’s why the node has to forward it to someone who can - the upcoming validator.

The node, roughly, does the following steps:

• verify transaction’s metadata - check signatures etc. (we want to make sure that we don’t forward bogus data)
• forward it to the ‘upcoming’ validator - currently we pick the validators that would be a chunk creator in +2, +3, +4 and +8 blocks (this is controlled by TX_ROUTING_HEIGHT_HORIZON) - and send the transaction to all of them.

Step 3: En-route to validator/producer

Great, the node knows to send (forward) the transaction to the validator, but how does the routing work? How do we know which peer is hosting a validator?

Each validator is regularly (every config.ttl_account_id_router/2 seconds == 30 minutes in production) broadcasting so called AnnounceAccount, which is basically a pair of (account_id, peer_id), to the whole network. This way each node knows which peer_id to send the message to.

Then it asks the routing table about the shortest path to the peer, and sends the ForwardTx message to the peer.

Step 4: Chunk producer

When a validator receives such a forwarded transaction, it double-checks that it is about to produce the block, and if so, it adds the transaction to the mempool (TransactionPool) for this shard, where it waits to be picked up when the chunk is produced.

What happens afterwards will be covered in future episodes/articles.

Additional notes:

Transaction being added multiple times

But such an approach means, that we’re forwarding the same transaction to multiple validators (currently 4) - so can it be added multiple times?

No. Remember that a transaction has a concrete hash which is used as a global identifier. If the validator sees that the transaction is present in the chain, it removes it from its local mempool.

Can transaction get lost?

Yes - they can and they do. Sometimes a node doesn’t have a path to a given validator or it didn’t receive an AnnouceAccount for it, so it doesn’t know where to forward the message. And if this happens to all 4 validators that we try to send to, then the message can be silently dropped.

We’re working on adding some monitoring to see how often this happens.