Best way to monitor HD wallet balance using only xpub (watch mode)

[sulliwane]

Hello,

I was wondering what is the easiest way to monitor a wallet whole balance having only the wallet xpub key (implementing a simple watch service) without having to maintain my own bitcoin node.

Any recommendation on this, using bcoin?

Many thanks!

[pinheadmz]

You can run a pruned node or an SPV node. (Soon we will have Neutrino client available as well!)

Pruned node: bcoin --prune Only keeps the last 288 blocks on disk, so some operations are limited like big wallet rescans, but most API calls still work. Still needs to sync the entire blockchain. And because rescans are impaired, if you are importing an xpub with transaction history already on chain, you need to be sure you create your wallet BEFORE the node gets to those transactions. Otherwise you will not be able to discover them, because the blocks will no longer be on disk.

SPV node: bcoin --spv Only stores block headers, not full blocks. It's a compromise for security and privacy because you rely on other full nodes on the network to validate the chain and serve your wallet data, but there is still a good deal of verification. Mainnet SPV sync takes a fraction of the time (like on the scale of an hour, as opposed to multiple days).

Once you have a node running you can create a watch-only wallet and import your xpub.
See http://bcoin.io/api-docs/#create-a-wallet

bwallet-cli mkwallet --id=<wallet name> --account-key=<xpub...> --watch-only=true

Then you can query the wallet any time (lots more at https://bcoin.io/api-docs)

bwallet-cli --id=<wallet name> history
bwallet-cli --id=<wallet name> balance

...etc

Let me know if this helps. If you really want to run a wallet without any network at all, the wallet is a separate module and can be run without a node, but you wouldn't be able to get its balance or tx history, just generate addresses basically.

[sulliwane]

Ok got it. I will consider --prune and --spv options! Thank you very much for you feedback on this.

[tynes]

There are socket events that are also useful for this, so that you can get live updates of when you receive a transaction

[sulliwane]

Thanks for your anwsers. @pinheadmz I'm wondering if when running in --prune mode I will have to rescan when in the case you described above (adding wallet after its transactions).

Also, how slower is prune compared to spv ? Thanks !

[pinheadmz]

In prune mode you can only rescan as many blocks as you have on disk, which is only two days worth. Actually now it is a dangerous unchecked situation, I have a PR to return an error: #532

Prune takes as long as Full - it must download and verify the entire blockchain. SPV is very fast (less than hour depending on your machine)

If you want to describe more the application you are building here or on slack, I might have more ideas for you

[JoePotentier]

Hey, I'm creating a watch-only wallet with a full node.

If I'm only given an xpub and the user of this wallet is generating receiving addresses in their private wallet. Will bcoin be able to detect transactions that are outside of the "account depth" of the wallet?

I want to use the sockets API to be alerted when transaction occur in the wallet. But my impression is that we will have to create some mechanism that guarantees that any receiving address the end-user has generated is within the account depth index. Is this correct?

[pinheadmz]

@JoePotentier For each account, bcoin has a lookahead value of 10:

bcoin/lib/wallet/account.js

Line 54 in f1abba5

this.lookahead = 10;

So if the user generates more than 10 addresses without receiving any transactions then yes, potentially the watch only wallet will fall behind. If the user actually receives TXs on the blockchain within that window, your watch-only wallet will be able to keep up.

If you think the user might end up large gaps in the BIP32 account, you may want to pre-derive more addresses on your side.