chainselection

package
v0.43.0 Latest Latest
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 Go to latest Published: May 9, 2026 License: Apache-2.0 Imports: 19 Imported by: 0

Documentation

Overview

Package chainselection implements Ouroboros Praos multi-peer chain selection. It tracks the tip reported by each connected peer and picks the best candidate chain to follow using the standard Praos rules: longer chain wins, then density within the security window, then VRF tie-break.

Genesis selection mode can be enabled at startup to prefer observed density during bootstrap before automatically falling back to Praos once the local tip is close enough to the best observed peer tip.

ChainSelector is the top-level type. It consumes PeerTipUpdateEvent events from chainsync, compares peer tips against the local tip, and emits ChainSwitchEvent on the EventBus when the selected peer changes. The node's Run loop listens for those events and repoints the active chainsync stream at the new best peer.

This package does not perform any block validation; it only selects which peer's chain to follow. Validation happens downstream in the ledger package after blocks are fetched from the selected peer.

Index

Constants

View Source 
const (
	PeerTipUpdateEventType  event.EventType = "chainselection.peer_tip_update"
	PeerActivityEventType   event.EventType = "chainselection.peer_activity"
	PeerRollbackEventType   event.EventType = "chainselection.peer_rollback"
	ChainSwitchEventType    event.EventType = "chainselection.chain_switch"
	ChainSelectionEventType event.EventType = "chainselection.selection"
	PeerEvictedEventType    event.EventType = "chainselection.peer_evicted"
)
View Source 
const (

	// DefaultMaxTrackedPeers is the maximum number of peers tracked by
	// the ChainSelector. When a new peer is added and the limit is reached,
	// the least-recently-updated peer is evicted. This bounds memory usage
	// and CPU cost of chain selection, preventing Sybil-based resource
	// exhaustion.
	DefaultMaxTrackedPeers = 200
)
View Source 
const VRFOutputSize = 64

VRFOutputSize is the expected size of a VRF output in bytes.

Variables

This section is empty.

Functions

func GenesisWindowSlotsForParams added in v0.37.0 

func GenesisWindowSlotsForParams(
	securityParam uint64,
	activeSlotsCoeff float64,
) uint64

GenesisWindowSlotsForParams returns the Genesis density window in slots. Shelley-style networks use 3k/f, where k is the security parameter and f is the active slot coefficient.

func GetVRFOutput 

func GetVRFOutput(header ledger.BlockHeader) []byte

GetVRFOutput extracts the VRF output from a block header. Returns nil if the header type is not supported or doesn't contain VRF data.

The VRF output is used for tie-breaking in chain selection when two chains have equal block numbers and equal density. The chain with the LOWER VRF output (lexicographically) wins.

func IsBetterChain 

func IsBetterChain(newTip, currentTip ochainsync.Tip) bool

IsBetterChain returns true if newTip represents a better chain than currentTip according to Ouroboros Praos rules.

func IsBetterChainWithDensity 

func IsBetterChainWithDensity(
	newTip, currentTip ochainsync.Tip,
	blocksInWindowNew, blocksInWindowCurrent uint64,
) bool

IsBetterChainWithDensity returns true if newTip represents a better chain than currentTip according to Ouroboros Praos density-based rules.

func IsBetterChainWithVRF 

func IsBetterChainWithVRF(
	newTip, currentTip ochainsync.Tip,
	blocksInWindowNew, blocksInWindowCurrent uint64,
	vrfOutputNew, vrfOutputCurrent []byte,
) bool

IsBetterChainWithVRF returns true if newTip represents a better chain than currentTip according to the complete Ouroboros Praos rules including VRF.

func IsSignificantlyBetter 

func IsSignificantlyBetter(
	newTip, currentTip ochainsync.Tip,
	minBlockDiff uint64,
) bool

IsSignificantlyBetter returns true if newTip is better than currentTip by at least the specified minimum block difference. This can be used to avoid frequent chain switches for marginal improvements.

Types

type ChainComparisonResult 

type ChainComparisonResult int

ChainComparisonResult indicates the result of comparing two chains. 

const (
	ChainEqual             ChainComparisonResult = 0
	ChainABetter           ChainComparisonResult = 1
	ChainBBetter           ChainComparisonResult = -1
	ChainComparisonUnknown ChainComparisonResult = 2
)

func CompareChains 

func CompareChains(tipA, tipB ochainsync.Tip) ChainComparisonResult

CompareChains compares two chain tips according to Ouroboros Praos rules:

  1. Higher block number wins (longer chain)
  2. At equal block number, lower slot wins (denser chain)
  3. At equal block number AND equal slot (a slot battle between two pools' competing forges), lower block hash wins.

Rule 3 is the deterministic tiebreak the chainsync handler needs to pick a side at a slot battle. Without it, two competing forges at the same slot+length collapse to ChainEqual, the handler refuses to switch in either direction, and the local chain permanently keeps whichever block was adopted first. The losing block's VRF output then keeps folding into the local evolving nonce, and the drift compounds across epochs until eta0 disagreement breaks header verification at the next epoch boundary.

Block hash is a deterministic function of every input the full Praos select-view comparator (chain length → slot → self-issued → issuer hash → VRF output → opcert counter) considers, so two peers independently applying "lower hash wins" to the same pair of competing tips arrive at the same decision and the chains converge.

Returns:

  • ChainABetter (1) if tipA represents a better chain
  • ChainBBetter (-1) if tipB represents a better chain
  • ChainEqual (0) only when both tips refer to the same block

func CompareChainsWithDensity 

func CompareChainsWithDensity(
	tipA, tipB ochainsync.Tip,
	blocksInWindowA, blocksInWindowB uint64,
) ChainComparisonResult

CompareChainsWithDensity compares two chain tips using density-based comparison according to Ouroboros Praos rules: 1. Higher block number wins (longer chain) 2. At equal block number, higher density wins (more blocks in 3k/f window) 3. At equal density, lower slot wins (tie-breaker)

The blocksInWindowA and blocksInWindowB parameters represent the number of blocks in the stability window (3k/f slots) for each chain tip. These values should be computed by counting blocks within the window ending at each tip.

Returns:

  • ChainABetter (1) if tipA represents a better chain
  • ChainBBetter (-1) if tipB represents a better chain
  • ChainEqual (0) if they are equal

func CompareChainsWithVRF 

func CompareChainsWithVRF(
	tipA, tipB ochainsync.Tip,
	blocksInWindowA, blocksInWindowB uint64,
	vrfOutputA, vrfOutputB []byte,
) ChainComparisonResult

CompareChainsWithVRF compares two chain tips using the complete Ouroboros Praos chain selection algorithm including VRF tie-breaking: 1. Higher block number wins (longer chain) 2. At equal block number, higher density wins (more blocks in 3k/f window) 3. At equal density, lower VRF output wins (deterministic tie-breaker)

The vrfOutputA and vrfOutputB parameters are the VRF outputs from the tip blocks of each chain. These should be extracted using GetVRFOutput() from the block headers. If VRF outputs are nil, falls back to slot-based comparison.

Returns:

  • ChainABetter (1) if tipA represents a better chain
  • ChainBBetter (-1) if tipB represents a better chain
  • ChainEqual (0) if they are equal

func CompareHeaders 

func CompareHeaders(headerA, headerB ledger.BlockHeader) ChainComparisonResult

CompareHeaders compares two block headers for chain selection tie-breaking using their VRF outputs. This is a convenience wrapper around GetVRFOutput and CompareVRFOutputs.

func CompareVRFOutputs 

func CompareVRFOutputs(vrfA, vrfB []byte) ChainComparisonResult

CompareVRFOutputs compares two VRF outputs for chain selection tie-breaking. Returns:

  • ChainABetter (1) if vrfA is lower (A wins)
  • ChainBBetter (-1) if vrfB is lower (B wins)
  • ChainEqual (0) if equal, either is nil, or either has invalid length

Per Ouroboros Praos: the chain with the LOWER VRF output wins the tie-break. Both outputs must be exactly VRFOutputSize (64) bytes; outputs of any other length are treated the same as nil to prevent truncated values from winning.

type ChainSelectionEvent 

type ChainSelectionEvent struct {
	BestConnectionId ouroboros.ConnectionId
	BestTip          ochainsync.Tip
	PeerCount        int
	SwitchOccurred   bool
}

ChainSelectionEvent is published when chain selection evaluation completes.

type ChainSelector 

type ChainSelector struct {
	// contains filtered or unexported fields
}

ChainSelector tracks chain tips from multiple peers and selects the best chain according to Ouroboros Praos rules.

func NewChainSelector 

func NewChainSelector(cfg ChainSelectorConfig) *ChainSelector

NewChainSelector creates a new ChainSelector with the given configuration.

func (*ChainSelector) EvaluateAndSwitch 

func (cs *ChainSelector) EvaluateAndSwitch() bool

EvaluateAndSwitch evaluates all peer tips and switches to the best chain if it differs from the current best. Returns true if a switch occurred.

func (*ChainSelector) GenesisWindowSlots added in v0.37.0 

func (cs *ChainSelector) GenesisWindowSlots() uint64

GenesisWindowSlots returns the slot window used for Genesis density checks.

func (*ChainSelector) GetAllPeerTips 

func (cs *ChainSelector) GetAllPeerTips() map[ouroboros.ConnectionId]*PeerChainTip

GetAllPeerTips returns a deep copy of all tracked peer tips.

func (*ChainSelector) GetBestPeer 

func (cs *ChainSelector) GetBestPeer() *ouroboros.ConnectionId

GetBestPeer returns the connection ID of the peer with the best chain, or nil if no suitable peer is available.

func (*ChainSelector) GetPeerTip 

func (cs *ChainSelector) GetPeerTip(
	connId ouroboros.ConnectionId,
) *PeerChainTip

GetPeerTip returns a deep copy of the chain tip for a specific peer. Returns nil if the peer is not tracked.

func (*ChainSelector) HandlePeerActivityEvent added in v0.27.7 

func (cs *ChainSelector) HandlePeerActivityEvent(evt event.Event)

HandlePeerActivityEvent refreshes a peer's liveness on non-tip protocol activity such as keepalive responses.

func (*ChainSelector) HandlePeerTipUpdateEvent 

func (cs *ChainSelector) HandlePeerTipUpdateEvent(evt event.Event)

HandlePeerTipUpdateEvent handles PeerTipUpdateEvent from the event bus.

func (*ChainSelector) PeerCount 

func (cs *ChainSelector) PeerCount() int

PeerCount returns the number of peers being tracked.

func (*ChainSelector) RemovePeer 

func (cs *ChainSelector) RemovePeer(connId ouroboros.ConnectionId)

RemovePeer removes a peer from tracking.

func (*ChainSelector) SelectBestChain 

func (cs *ChainSelector) SelectBestChain() *ouroboros.ConnectionId

SelectBestChain evaluates all peer tips and returns the connection ID of the peer with the best chain.

func (*ChainSelector) SelectionMode added in v0.37.0 

func (cs *ChainSelector) SelectionMode() SelectionMode

SelectionMode returns the selector's current mode.

func (*ChainSelector) SetLocalTip 

func (cs *ChainSelector) SetLocalTip(tip ochainsync.Tip)

SetLocalTip updates the local chain tip for comparison.

func (*ChainSelector) SetSecurityParam 

func (cs *ChainSelector) SetSecurityParam(k uint64)

SetSecurityParam updates the security parameter (k) dynamically. This allows the selector to use protocol parameters for density-based comparison.

func (*ChainSelector) Start 

func (cs *ChainSelector) Start(ctx context.Context) error

Start begins the chain selector's background evaluation loop and subscribes to relevant events.

func (*ChainSelector) Stop 

func (cs *ChainSelector) Stop()

Stop stops the chain selector.

func (*ChainSelector) TouchPeerActivity added in v0.27.7 

func (cs *ChainSelector) TouchPeerActivity(connId ouroboros.ConnectionId)

func (*ChainSelector) UpdatePeerTip 

func (cs *ChainSelector) UpdatePeerTip(
	connId ouroboros.ConnectionId,
	tip ochainsync.Tip,
	vrfOutput []byte,
) bool

UpdatePeerTip updates the chain tip for a specific peer and triggers evaluation if needed. The vrfOutput parameter is the VRF output from the tip block header, used for tie-breaking when chains have equal block number and slot.

Returns true if the tip was accepted, false if it was rejected as implausible. A tip is considered implausible if it claims a block number more than securityParam (k) blocks ahead of a reference point. For known peers, the reference is the peer's own previous tip; for new peers, the reference is the best known peer tip. This avoids rejecting legitimate peers during sync (where the local tip is far behind).

type ChainSelectorConfig 

type ChainSelectorConfig struct {
	Logger             *slog.Logger
	EventBus           *event.EventBus
	EvaluationInterval time.Duration
	StaleTipThreshold  time.Duration
	MinSwitchBlockDiff uint64
	SecurityParam      uint64
	GenesisMode        bool
	GenesisWindowSlots uint64
	ConnectionLive     func(ouroboros.ConnectionId) bool
	ConnectionEligible func(ouroboros.ConnectionId) bool
	ConnectionPriority func(ouroboros.ConnectionId) int
	MaxTrackedPeers    int // 0 means use DefaultMaxTrackedPeers
	// BlockfetchLatency returns the EWMA first-block latency for a
	// connection and whether any samples exist. Used as a tiebreaker
	// when two peers share the same SelectionTip and VRF output.
	BlockfetchLatency func(ouroboros.ConnectionId) (time.Duration, bool)
}

ChainSelectorConfig holds configuration for the ChainSelector.

type ChainSwitchEvent 

type ChainSwitchEvent struct {
	PreviousConnectionId ouroboros.ConnectionId
	NewConnectionId      ouroboros.ConnectionId
	NewTip               ochainsync.Tip
	PreviousTip          ochainsync.Tip
	ComparisonResult     ChainComparisonResult
	BlockDifference      int64
}

ChainSwitchEvent is published when the chain selector decides to switch to a different peer's chain.

Fields:

  • PreviousConnectionId: The connection ID of the peer we were following.
  • NewConnectionId: The connection ID of the peer we are now following.
  • NewTip: The chain tip of the new peer.
  • PreviousTip: The chain tip of the previous peer at the time of the switch.
  • ComparisonResult: Why the new chain is better than the previous chain.
  • BlockDifference: NewTip.BlockNumber - PreviousTip.BlockNumber.

type PeerActivityEvent added in v0.27.7 

type PeerActivityEvent struct {
	ConnectionId ouroboros.ConnectionId
}

PeerActivityEvent is published when a peer has recent protocol activity (for example, a keepalive response) without a tip change. This refreshes selector liveness for healthy but temporarily quiet peers.

type PeerChainTip 

type PeerChainTip struct {
	ConnectionId ouroboros.ConnectionId
	Tip          ochainsync.Tip
	ObservedTip  ochainsync.Tip
	VRFOutput    []byte // VRF output from tip block for tie-breaking
	LastUpdated  time.Time
	// contains filtered or unexported fields
}

PeerChainTip tracks the chain tip reported by a specific peer.

func NewPeerChainTip 

func NewPeerChainTip(
	connId ouroboros.ConnectionId,
	tip ochainsync.Tip,
	vrfOutput []byte,
) *PeerChainTip

NewPeerChainTip creates a new PeerChainTip with the given connection ID, tip, and VRF output.

func (*PeerChainTip) ApplyRollback added in v0.37.0 

func (p *PeerChainTip) ApplyRollback(
	point ocommon.Point,
	tip ochainsync.Tip,
)

ApplyRollback trims observed history at the rollback point and refreshes the peer tip to the chainsync tip reported with the rollback.

func (*PeerChainTip) IsStale 

func (p *PeerChainTip) IsStale(threshold time.Duration) bool

IsStale returns true if the peer's tip hasn't been updated within the given duration.

func (*PeerChainTip) SelectionTip added in v0.27.7 

func (p *PeerChainTip) SelectionTip() ochainsync.Tip

SelectionTip returns the best locally observed frontier for this peer. When available, prefer the latest block the peer has actually delivered to us over its remote advertised tip. This avoids switching to peers whose far-end tip is high while their chainsync cursor is still lagging.

func (*PeerChainTip) Touch added in v0.27.7 

func (p *PeerChainTip) Touch()

Touch marks the peer as recently active without changing its advertised tip.

func (*PeerChainTip) UpdateTip 

func (p *PeerChainTip) UpdateTip(tip ochainsync.Tip, vrfOutput []byte)

UpdateTip updates the peer's chain tip, VRF output, and last updated timestamp.

func (*PeerChainTip) UpdateTipWithObserved added in v0.27.7 

func (p *PeerChainTip) UpdateTipWithObserved(
	tip ochainsync.Tip,
	observedTip ochainsync.Tip,
	vrfOutput []byte,
)

UpdateTipWithObserved updates both the remote advertised tip and the latest locally observed frontier for the peer.

type PeerEvictedEvent added in v0.22.0 

type PeerEvictedEvent struct {
	ConnectionId ouroboros.ConnectionId
}

PeerEvictedEvent is published when a tracked peer is evicted from the chain selector to make room for a new peer. Subscribers (e.g. connection manager) can use this to close the evicted peer's connection.

type PeerRollbackEvent added in v0.37.0 

type PeerRollbackEvent struct {
	ConnectionId ouroboros.ConnectionId
	Point        ocommon.Point
	Tip          ochainsync.Tip
}

PeerRollbackEvent is published when an ingress-eligible chainsync peer reports a rollback. Point is the rollback point; Tip is the peer's current chainsync tip after the rollback.

type PeerTipUpdateEvent 

type PeerTipUpdateEvent struct {
	ConnectionId ouroboros.ConnectionId
	Tip          ochainsync.Tip
	ObservedTip  ochainsync.Tip
	VRFOutput    []byte // VRF output from observed block header for tie-breaking
}

PeerTipUpdateEvent is published when a peer's chain tip is updated via chainsync roll forward.

type SelectionMode added in v0.37.0 

type SelectionMode uint8

SelectionMode describes the chain-selection strategy currently in use. 

const (
	SelectionModePraos SelectionMode = iota
	SelectionModeGenesis
)

func (SelectionMode) String added in v0.37.0 

func (m SelectionMode) String() string

Source Files

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