Flowee the Hub — API documentation


Transaction Monitor Service

This service, with service-id 20 is a subscription service. A connected peer can subscribe to a list of TXIDs and as long as the connection stays open the Hub will monitor incoming transactions and blocks for matches, which it will report by sending messages to the peer.

The service ID for this service is 20.



We use a TXID tag which is a sha256, send as a 32-byte bytearray. subscriptions and detection happens on the transaction-id of the entire transaction.




These messages are initiated by the service and don’t require a reply. The service sends a message for each transaction that matches one of the subscribed ids. One message is sent to the client if accepted into mempool, another when accepted in a block.

TransactionFound on initial subscription (or) when matched in mempool

On initial subscribe we check the mempool for a hit but also we keep monitoring the mempool as the connection stays open. If there is a mempool match the below message is sent.

TransactionFound on block-mined

When a block is mined and a match is included, this message is sent. Notice that multiple TxId / OffsetInBlock pairs can be included in the message if there are multiple subscriptions.


These messages are initiated by the service and don’t require a reply. The service sends a message for each double spend that touches one of the transactions.

A double spend notification is useful for unconfimed transactions because the network will only ever mine one of the two. What gets mined does not have to be the first transaction you have seen. (more), this message serves as a warning to be aware of the issue. Receiving parties might want to wait for a confirmation before they release the goods.

DoubleSpendFound based on transactions

When a node itself detects a double spend it will have both transactions available and you will be able to get the full transaction that double spends one in the mempool. API users can naturally use the livetx service to fetch that transaction too.

DoubleSpendFound based on double-spend-proof

Peers exchange double spend proof messages so even if the node you are connected to didn’t observe two conflicting transactions we still get a notification of such an event happening.

A double spend proof holds cryptographic proof that the sender created two conflicting transactions spending the same money.