lnd.xprv/autopilot/prefattach.go
2018-09-05 20:10:37 -07:00

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package autopilot
import (
"bytes"
"fmt"
prand "math/rand"
"time"
"github.com/btcsuite/btcd/btcec"
"github.com/btcsuite/btcutil"
)
// ConstrainedPrefAttachment is an implementation of the AttachmentHeuristic
// interface that implement a constrained non-linear preferential attachment
// heuristic. This means that given a threshold to allocate to automatic
// channel establishment, the heuristic will attempt to favor connecting to
// nodes which already have a set amount of links, selected by sampling from a
// power law distribution. The attachment ins non-linear in that it favors
// nodes with a higher in-degree but less so that regular linear preferential
// attachment. As a result, this creates smaller and less clusters than regular
// linear preferential attachment.
//
// TODO(roasbeef): BA, with k=-3
type ConstrainedPrefAttachment struct {
minChanSize btcutil.Amount
maxChanSize btcutil.Amount
chanLimit uint16
threshold float64
}
// NewConstrainedPrefAttachment creates a new instance of a
// ConstrainedPrefAttachment heuristics given bounds on allowed channel sizes,
// and an allocation amount which is interpreted as a percentage of funds that
// is to be committed to channels at all times.
func NewConstrainedPrefAttachment(minChanSize, maxChanSize btcutil.Amount,
chanLimit uint16, allocation float64) *ConstrainedPrefAttachment {
prand.Seed(time.Now().Unix())
return &ConstrainedPrefAttachment{
minChanSize: minChanSize,
chanLimit: chanLimit,
maxChanSize: maxChanSize,
threshold: allocation,
}
}
// A compile time assertion to ensure ConstrainedPrefAttachment meets the
// AttachmentHeuristic interface.
var _ AttachmentHeuristic = (*ConstrainedPrefAttachment)(nil)
// NeedMoreChans is a predicate that should return true if, given the passed
// parameters, and its internal state, more channels should be opened within
// the channel graph. If the heuristic decides that we do indeed need more
// channels, then the second argument returned will represent the amount of
// additional funds to be used towards creating channels.
//
// NOTE: This is a part of the AttachmentHeuristic interface.
func (p *ConstrainedPrefAttachment) NeedMoreChans(channels []Channel,
funds btcutil.Amount) (btcutil.Amount, uint32, bool) {
// If we're already over our maximum allowed number of channels, then
// we'll instruct the controller not to create any more channels.
if len(channels) >= int(p.chanLimit) {
return 0, 0, false
}
// The number of additional channels that should be opened is the
// difference between the channel limit, and the number of channels we
// already have open.
numAdditionalChans := uint32(p.chanLimit) - uint32(len(channels))
// First, we'll tally up the total amount of funds that are currently
// present within the set of active channels.
var totalChanAllocation btcutil.Amount
for _, channel := range channels {
totalChanAllocation += channel.Capacity
}
// With this value known, we'll now compute the total amount of fund
// allocated across regular utxo's and channel utxo's.
totalFunds := funds + totalChanAllocation
// Once the total amount has been computed, we then calculate the
// fraction of funds currently allocated to channels.
fundsFraction := float64(totalChanAllocation) / float64(totalFunds)
// If this fraction is below our threshold, then we'll return true, to
// indicate the controller should call Select to obtain a candidate set
// of channels to attempt to open.
needMore := fundsFraction < p.threshold
if !needMore {
return 0, 0, false
}
// Now that we know we need more funds, we'll compute the amount of
// additional funds we should allocate towards channels.
targetAllocation := btcutil.Amount(float64(totalFunds) * p.threshold)
fundsAvailable := targetAllocation - totalChanAllocation
return fundsAvailable, numAdditionalChans, true
}
// NodeID is a simple type that holds an EC public key serialized in compressed
// format.
type NodeID [33]byte
// NewNodeID creates a new nodeID from a passed public key.
func NewNodeID(pub *btcec.PublicKey) NodeID {
var n NodeID
copy(n[:], pub.SerializeCompressed())
return n
}
// shuffleCandidates shuffles the set of candidate nodes for preferential
// attachment in order to break any ordering already enforced by the sorted
// order of the public key for each node. To shuffle the set of candidates, we
// use a version of the FisherYates shuffle algorithm.
func shuffleCandidates(candidates []Node) []Node {
shuffledNodes := make([]Node, len(candidates))
perm := prand.Perm(len(candidates))
for i, v := range perm {
shuffledNodes[v] = candidates[i]
}
return shuffledNodes
}
// Select returns a candidate set of attachment directives that should be
// executed based on the current internal state, the state of the channel
// graph, the set of nodes we should exclude, and the amount of funds
// available. The heuristic employed by this method is one that attempts to
// promote a scale-free network globally, via local attachment preferences for
// new nodes joining the network with an amount of available funds to be
// allocated to channels. Specifically, we consider the degree of each node
// (and the flow in/out of the node available via its open channels) and
// utilize the BarabásiAlbert model to drive our recommended attachment
// heuristics. If implemented globally for each new participant, this results
// in a channel graph that is scale-free and follows a power law distribution
// with k=-3.
//
// NOTE: This is a part of the AttachmentHeuristic interface.
func (p *ConstrainedPrefAttachment) Select(self *btcec.PublicKey, g ChannelGraph,
fundsAvailable btcutil.Amount, numNewChans uint32,
skipNodes map[NodeID]struct{}) ([]AttachmentDirective, error) {
// TODO(roasbeef): rename?
var directives []AttachmentDirective
if fundsAvailable < p.minChanSize {
return directives, nil
}
selfPubBytes := self.SerializeCompressed()
// We'll continue our attachment loop until we've exhausted the current
// amount of available funds.
visited := make(map[NodeID]struct{})
for i := uint32(0); i < numNewChans; i++ {
// selectionSlice will be used to randomly select a node
// according to a power law distribution. For each connected
// edge, we'll add an instance of the node to this slice. Thus,
// for a given node, the probability that we'll attach to it
// is: k_i / sum(k_j), where k_i is the degree of the target
// node, and k_j is the degree of all other nodes i != j. This
// implements the classic BarabásiAlbert model for
// preferential attachment.
var selectionSlice []Node
// For each node, and each channel that the node has, we'll add
// an instance of that node to the selection slice above.
// This'll slice where the frequency of each node is equivalent
// to the number of channels that connect to it.
//
// TODO(roasbeef): add noise to make adversarially resistant?
if err := g.ForEachNode(func(node Node) error {
nID := NodeID(node.PubKey())
// Once a node has already been attached to, we'll
// ensure that it isn't factored into any further
// decisions within this round.
if _, ok := visited[nID]; ok {
return nil
}
// If we come across ourselves, them we'll continue in
// order to avoid attempting to make a channel with
// ourselves.
if bytes.Equal(nID[:], selfPubBytes) {
return nil
}
// Additionally, if this node is in the backlist, then
// we'll skip it.
if _, ok := skipNodes[nID]; ok {
return nil
}
// For initial bootstrap purposes, if a node doesn't
// have any channels, then we'll ensure that it has at
// least one item in the selection slice.
//
// TODO(roasbeef): make conditional?
selectionSlice = append(selectionSlice, node)
// For each active channel the node has, we'll add an
// additional channel to the selection slice to
// increase their weight.
if err := node.ForEachChannel(func(channel ChannelEdge) error {
selectionSlice = append(selectionSlice, node)
return nil
}); err != nil {
return err
}
return nil
}); err != nil {
return nil, err
}
// If no nodes at all were accumulated, then we'll exit early
// as there are no eligible candidates.
if len(selectionSlice) == 0 {
break
}
// Given our selection slice, we'll now generate a random index
// into this slice. The node we select will be recommended by
// us to create a channel to.
candidates := shuffleCandidates(selectionSlice)
selectedIndex := prand.Int31n(int32(len(candidates)))
selectedNode := candidates[selectedIndex]
// TODO(roasbeef): cap on num channels to same participant?
// With the node selected, we'll add this (node, amount) tuple
// to out set of recommended directives.
pubBytes := selectedNode.PubKey()
pub, err := btcec.ParsePubKey(pubBytes[:], btcec.S256())
if err != nil {
return nil, err
}
directives = append(directives, AttachmentDirective{
// TODO(roasbeef): need curve?
NodeKey: &btcec.PublicKey{
X: pub.X,
Y: pub.Y,
},
NodeID: NewNodeID(pub),
Addrs: selectedNode.Addrs(),
})
// With the node selected, we'll add it to the set of visited
// nodes to avoid attaching to it again.
visited[NodeID(pubBytes)] = struct{}{}
}
numSelectedNodes := int64(len(directives))
switch {
// If we have enough available funds to distribute the maximum channel
// size for each of the selected peers to attach to, then we'll
// allocate the maximum amount to each peer.
case int64(fundsAvailable) >= numSelectedNodes*int64(p.maxChanSize):
for i := 0; i < int(numSelectedNodes); i++ {
directives[i].ChanAmt = p.maxChanSize
}
return directives, nil
// Otherwise, we'll greedily allocate our funds to the channels
// successively until we run out of available funds, or can't create a
// channel above the min channel size.
case int64(fundsAvailable) < numSelectedNodes*int64(p.maxChanSize):
i := 0
for fundsAvailable > p.minChanSize {
// We'll attempt to allocate the max channel size
// initially. If we don't have enough funds to do this,
// then we'll allocate the remainder of the funds
// available to the channel.
delta := p.maxChanSize
if fundsAvailable-delta < 0 {
delta = fundsAvailable
}
directives[i].ChanAmt = delta
fundsAvailable -= delta
i++
}
// We'll slice the initial set of directives to properly
// reflect the amount of funds we were able to allocate.
return directives[:i:i], nil
default:
return nil, fmt.Errorf("err")
}
}