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