Compute routes backward #797

araspitzu · Jan 2, 2019

Compute the routes backward from the target and with the correct channels cost, nodes that are 'neighbor' to the local node (direct channel) will have a routing cost of 0.

araspitzu · Jan 2, 2019

I chose to have an explicit parameter on the graph construction and manipulation functions, alternatively we can have a separate function with reverse in the name i.e "addEdgeReverse", please let me know what do you think of it.

sstone · Jan 2, 2019

Isn't it possible to use the same graph (i.e. not need for the additional reverse parameter) and just compute the route backwards internally ? It should be just an implementation choice that is not even exposed in the API ?

araspitzu · Jan 2, 2019

Sure it's possible, especially if we want to support only the graph in reverse order.

araspitzu · Jan 3, 2019

Latest commit hid the "reverse" parameter and now the graph and shortestPath method work only in reverse mode (i left some comment in the docs to clarify the graph operations as it might be a little confusing)

sstone · Jan 4, 2019

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sstone · Jan 4, 2019

eclair-core/src/main/scala/fr/acinq/eclair/router/Router.scala

@@ -791,7 +791,7 @@ object Router {
  def findRoute(g: DirectedGraph, localNodeId: PublicKey, targetNodeId: PublicKey, amountMsat: Long, extraEdges: Set[GraphEdge] = Set.empty, ignoredEdges: Set[ChannelDesc] = Set.empty): Try[Seq[Hop]] = Try {
    if (localNodeId == targetNodeId) throw CannotRouteToSelf

-    Graph.shortestPath(g, localNodeId, targetNodeId, amountMsat, ignoredEdges, extraEdges) match {
+    Graph.shortestPath(g, targetNodeId, localNodeId, amountMsat, ignoredEdges.map(reverseDesc), extraEdges.map(edge => edge.copy(desc = reverseDesc(edge.desc)))) match {


I think we should keep the method and parameters as they were, we are still computing a shortest path from the source to the destination, it's just that internally we compute it backwards

sstone · Jan 4, 2019

sstone reviewed Jan 4, 2019

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eclair-core/src/main/scala/fr/acinq/eclair/router/Graph.scala

pm47 · Jan 8, 2019

pm47 reviewed Jan 8, 2019

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What is the overall performance impact in terms of compute time?

pm47 · Jan 8, 2019

eclair-core/src/main/scala/fr/acinq/eclair/router/Graph.scala

-            // note: the default value here will never be used, as there is always an entry for the current in the 'cost' map
-            val newMinimumKnownCost = cost.get(current.key) + edgeWeightByAmount(edge, amountMsat)
+          // test for ignored edges
+          if (!(edge.update.htlcMaximumMsat.exists(_ < newMinimumKnownCost) ||


is that the most optimal order for the testing conditions? Given that the first to return true will return immediately

done - optimized in 1151af9

pm47 · Jan 8, 2019

eclair-core/src/main/scala/fr/acinq/eclair/router/Graph.scala

+    * @param isNeighborTarget true if the receiving vertex of this edge is the target node (source in a reversed graph), which has cost 0
+    * @return the new amount updated with the necessary fees for this edge
+    */
+  private def edgeWeight(edge: GraphEdge, amountWithFees: Long, isNeighborTarget: Boolean): Long = isNeighborTarget match {


would an if be more optimal here?

Depends on which one you consider more readable, i've used 'match' for testing conditions and this is consistent to it.

eclair-core/src/main/scala/fr/acinq/eclair/router/Graph.scala

araspitzu · Jan 9, 2019

Performance impact is not significant, i just benchmarked it and the graph initialization is still around 2.4s with 13k channels (measurements not taken on the mobile phone). On the other hand we're now computing the correct routes thanks to the backward search and incremental edge cost.

Benchmark                                   Mode  Cnt  Score   Error  Units
GraphBenchmark.findPath                     avgt       0.002           s/op
GraphBenchmark.findPathWithExtraChannels    avgt       0.003           s/op
GraphBenchmark.findPathWithIgnoredChannels  avgt       0.019           s/op
GraphBenchmark.routerLoadingTime            avgt       2.401           s/op

araspitzu · Jan 9, 2019

Todo: update the wiki as soon as this is merged

sstone · Jan 11, 2019

sstone approved these changes Jan 11, 2019

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araspitzu · Jan 11, 2019

wiki updated

Use non reversed graph in GraphSpec

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3a74df7

araspitzu added some commits Jan 2, 2019

Always use the graph reversed

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8eabbf5

Clarify graph operations in the docs

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4ff7575

Define reverse function on ChannelDesc

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sstone merged commit fd3ff91 into master Jan 11, 2019
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Compute routes backward #797

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