Improve routing efficiency by utilizing source_pub_key argument in QueryRoutes

[jgwtt]

If the FROM channel is specified on the command line, the script should probably call GetRoutes while specifying the source_pub_key argument as the first node in the route (the node on the other end of the FROM channel). Currently, GetRoutes tries to find N routes indiscriminately and the check to match the FROM channel happens after routes are returned. This is inefficient and thus it takes longer to find the suitable route and could result in higher chances of routing failure.

I observed that often the 15/30/.. routes returned by the RPC call do not match the FROM channel and the rebalancing fails for that reason

[C-Otto]

The source node where the request route should originated from. If empty, self is assumed.

That means, when using source_pub_key, that we have to extend the first hop to that node, and modify every hop due to fees etc.

[jgwtt]

Maybe I am missing something but here is how I picture it.

Suppose a valid rebalancing chain is A->B->...->Z->A and you specify -f AB -t AZ on the command line. I think currently you request routes from A to Z and try to match the first hop to the -f argument (channel AB). However, if you specified source_pub_key=B, won't it find routes from B to Z (you can also blacklist AZ channel if that's a problem) thus finding only the rebalancing routes satisfying the given -f and -t arguments?

[C-Otto]

You're exactly right. However, the route returned is only going from B to Z, and we need to add the first hop from A to B. This requires us to update the fees along the route, similar to the route extension as it currently happens for the last hop.

[jgwtt]

If it's similar, it should not be hard to implement, do I understand you correctly?

I often see failure to find a route among 45-60 returned routes just because the first hop does not match -f arg

It may become even bigger issue when they depreciate num_routes_to_request argument

[wamde]

@jgwtt the GetRoutes call you are mentioning is not a lnd call, is it? Either way, lnd's QueryRoutes now has a source_pub_key parameter, which was just recently added: lightningnetwork/lnd#2572

We should use that when the -f parameter is used, you are right.
Do you want to give it a shot?

[jgwtt]

@wamde let me see if I can get this going

[manreo]

You do need to update the https://github.com/lightningnetwork/lnd/tree/master/lnrpc the rebalance-lnd uses

[C-Otto]

In the current master I just ignore all channels but the one specified by -f. This is a bit hacked, but it works and I don't see the advantage of having more complicated logic as described above.