@JonathanSilverblood 16 July 2020 edited

electrum-cash: broadcasting transactions

Introduction **Electrum-cash** is a javascript/typescript library for communicating with electrum servers. https://www.npmjs.com/package/electrum-cash/library This example demonstrates how to broadcast a raw transaction and how to get notified of when the transaction gets included in a block, using `v1.4.3` of the electrum protocol. *To keep the example simple and clear, the code to set up and close a connection is omitted.* Broadcasting a transaction To broadcast a raw transaction, send a `blockchain.transaction.broadcast` **request** with the raw transaction hex as the first parameter: // The raw transaction we want to broadcast. const transactionHex = '01000000017616e3106ca8e7a92eed190319fea34327edfb0991714444a5cc757000cde21a000000006a473044022007b6e2e864f9807602b9937d25b7df7ffdcd1ff559dad0cc130e9174bfea5cd8022070ae24a132f9420090d878defb17b78db7f15ced5ea4090c2f423d65f0d70d6c412102c94e85aa74899fb9378079e42c0e225c7481689c7fd7b3accd567ddafb53c6acfeffffff026db40600000000001976a914ebdeb6430f3d16a9c6758d6c0d7a400c8e6bbee488acdc120b00000000001976a9142c805fc822ea73b954040d787a8ea69e8d8caf5c88acf5d30900'; // Broadcast the transaction. const transactionHash = await electrum.request('blockchain.transaction.broadcast', transactionHex); // Log the transaction hash. console.log(transactionHash); *Example output:* 6df81e9abee5702d9c8b0ad941e0c0d663855d2186162fb5e86136d99904c48c Checking transaction confirmations To look up transaction confirmations, send a `blockchain.transaction.get` **request** with the transaction hash as the first parameter and set verbose flag as the second parameter to **true**. // The transaction we want to look up. const transactionHash = '6df81e9abee5702d9c8b0ad941e0c0d663855d2186162fb5e86136d99904c48c'; // Request verbose transaction details which includes the number of confirmations. const { confirmations } = await electrum.request('blockchain.transaction.get', transactionHash, true); // Log the transaction confirmations. console.log(confirmations); *Example output when the transaction is still in the mempool:* undefined *Example output when the transaction has 4 confirmations:* 4 Listening for transaction block inclusion To get notification when your transaction is included in a block, **subscribe** to new block notifications with `blockchain.headers.subscribe`, then request the transaction status. Remember to **unsubscribe** when you no longer need the information. **Disclaimer**: the unsubscribe feature will be included in the v1.2.0 release of electrum-cash https://gitlab.com/GeneralProtocols/electrum-cash/library/-/issues/7 https://gitlab.com/GeneralProtocols/electrum-cash/library/-/issues?milestone_title=v1.2.0 // Create a function that will handle confirmation updates. const handleConfirmationUpdates = async function(callbackFunction, transactionHash) { // Request verbose transaction details which includes the number of confirmations. const { confirmations } = await electrum.request('blockchain.transaction.get', transactionHash, true); // If the transaction now have confirmations.. if(confirmations) { // Log that the transaction was confirmed. console.log(`Transaction '${transactionHash}' now have ${confirmations} confirmations.`); // Unsubscribe from further updates. // NOTE: we bind the transaction to ensure that it uses the same name as when we subscribed. electrum.unsubscribe(callbackFunction.bind(this), `blockchain.headers.subscribe`); } } // The transaction we want to look up. const transactionHash = '6df81e9abee5702d9c8b0ad941e0c0d663855d2186162fb5e86136d99904c48c'; // Subscribe to notifications for new blocks // NOTE: we bind the transaction to provide the transaction hash to it. await electrum.subscribe(handleConfirmationUpdates.bind(this, handleConfirmationUpdates, transactionHash), `blockchain.headers.subscribe`); **Example output when the transaction gets included in a block:** Transaction '6df81e9abee5702d9c8b0ad941e0c0d663855d2186162fb5e86136d99904c48c' now have 1 confirmations. Conclusion Broadcasting a transaction with `electrum-cash` is easy, but tracking when the transaction gets confirmed involves tracking different related data which can cause problems if you naively unsubscribe while having multiple active subscriptions. I have reached out to the backend developers to try and coordinate an update of the protocol to provide `blockchain.transaction.subscribe`, a subscription call that sends notifications when the status of a transaction changes which would simplify this process. *Issues requesting the new feature:* Electrs: https://github.com/BitcoinUnlimited/ElectrsCash/issues/86 Fulcrum: https://github.com/cculianu/Fulcrum/issues/32