302 lines
10 KiB
Go
302 lines
10 KiB
Go
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 Fisher–Yates 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 Barabási–Albert 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 Barabási–Albert 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")
|
||
}
|
||
}
|