Feature Request: Introduce Custom Pathfinding in SimNode

[carlaKC]

Describe desired feature

Right now sim-node simply uses LDK's default pathfinding to find routes for payments that are dispatched in the mocked network.

To vastly expand the use of sim-ln as a library for pathfinding-related use cases, we should provide a way for calling code to provide custom routes instead.

This could be implemented a few ways:

1. Add an interceptor similar to Feature Request: Introduce Forward Interceptors to SimNode #255 that sends proposed route and gets back modified one
2. Provide custom pathfinding in on creation of mocked nodes

Part of this issue will be to spend some time figuring out the best way to implement this.

Use case for feature

This opens up a lot of doors for A/B testing different pathfinding strategies.

Would you like to contribute code for this feature?

I'll review design proposal + code, unlikely to have time to do the code myself!

[sangbida]

Could I give this one a crack?

[carlaKC]

Sure! Would be useful to post some design thoughts in here first so we can discuss the approach!

[sangbida]

So, I was thinking there are a few approaches. Currently, find_payment_route inside sim_node.rs calls LDK's find_route. I think in any case this should still be the default option.

• Option 1: We add a PathFinder trait, like so:

pub trait PathFinder: Send + Sync {
    fn find_route(
        &self,
        source: &PublicKey,
        dest: PublicKey,
        amount_msat: u64,
        pathfinding_graph: &NetworkGraph<&WrappedLog>,
        scorer: &ProbabilisticScorer<Arc<NetworkGraph<&WrappedLog>>, &WrappedLog>,
    ) -> Result<Route, SimulationError>;
}

SimNode takes in a pathfinder argument like below and we can provide a DefaultPathFinder that calls LDK's find_route. So SimNode will be able to work with any implementation of PathFinder. The PathFinder implentation will require a find_route method.

struct SimNode<'a, T: SimNetwork, P: PathFinder> {
    info: NodeInfo,
    network: Arc<Mutex<T>>,
    in_flight: HashMap<PaymentHash, Receiver<Result<PaymentResult, LightningError>>>,
    pathfinding_graph: Arc<NetworkGraph<&'a WrappedLog>>,
    scorer: ProbabilisticScorer<Arc<NetworkGraph<&'a WrappedLog>>, &'a WrappedLog>,
    pathfinder: P,
}

• Option 2: We still keep the PathFinder trait from Option 1 and have a DefaultPathFinder, we create an implementation of a PathFinder with a RouteInterceptor

I think there maybe a solution that uses dynamic traits, I'm still learning about those so will come back with more options once I have a better understanding.

[carlaKC]

I think that I lean to towards option 1 - just adding a trait and creating SimNode with whatever PathFinder the user chooses (custom or default impl which uses LDK find_route).

I think it would be good to get some input from folks who would use a feature like this to see what would be useful (I don't have firsthand use for it rn, so can't give much insight).

[sangbida]

I'll start working on the first approach!

[sangbida]

Hey hey,
I've started working on this one :) I've added a generic PathFinder to Simnode but it seem like I need to thread this generic implentation through parsing.rs and main.rs because this function ln_node_from_graph (which now requires the Pathfinder) is nested in functions in those files.

It's probably okay to thread it along, it just looks messy? It will probably also make the PR a little messier to read? I was wondering if that was okay? or if there is a nicer way to do this that I'm missing?

[carlaKC]

It's probably okay to thread it along, it just looks messy?

Coming to peace with the ugly look of generics errywhere is part of the 🦀 journey :')

Sounds like the right approach to me, so don't worry about how it looks!

[sangbida]

Thanks @carlaKC, this PR is taking a bit longer but I'm hoping to get it done by the end of next week.

[carlaKC]

Appreciate the update 🙏 🙏

[sangbida]

I was wondering if there was a preference on how this custom Pathfinder will be passed into the main function? I was thinking it could be a path to the Pathfinder as a CLI argument? In my draft PR I've just added the DefaultPathFinder to main.

[carlaKC]

I was wondering if there was a preference on how this custom Pathfinder will be passed into the main function? I was thinking it could be a path to the Pathfinder as a CLI argument?

I think that we can just add it to the constructor and allow people to provide it there? Primary use case would be for folks using sim-ln as a library, so they can just use it like that.

In my draft PR I've just added the DefaultPathFinder to main.

👍