Imported from stellar/stellar-core (.claude/skills/subsystem-summary-of-herder/SKILL.md). Install upstream with npx skills add stellar/stellar-core --skill subsystem-summary-of-herder. Copyright stays with the author.

Herder Subsystem Technical Summary

Overview

The Herder subsystem drives the Stellar Consensus Protocol (SCP) for stellar-core. It is responsible for collecting transactions from the network, proposing transaction sets for consensus, validating SCP messages, managing protocol upgrades, and delivering externalized ledger values to the LedgerManager for application. All core Herder logic runs on the main thread.

Key Classes and Data Structures

Herder (Abstract Interface)

• File: Herder.h
• Pure virtual interface defining the public API for the subsystem.
• Defines constants: TARGET_LEDGER_CLOSE_TIME_BEFORE_PROTOCOL_VERSION_23_MS (5s), MAX_SCP_TIMEOUT_SECONDS (240s), CONSENSUS_STUCK_TIMEOUT_SECONDS (35s), OUT_OF_SYNC_RECOVERY_TIMER (10s), LEDGER_VALIDITY_BRACKET (100 slots), MAX_TIME_SLIP_SECONDS (60s), TX_SET_GC_DELAY (1 min).
• Defines state machine: HERDER_BOOTING_STATE → HERDER_SYNCING_STATE → HERDER_TRACKING_NETWORK_STATE.
• Defines EnvelopeStatus: DISCARDED, SKIPPED_SELF, PROCESSED, FETCHING, READY.

HerderImpl (Core Implementation)

• File: HerderImpl.h, HerderImpl.cpp (~2679 lines)
• Concrete implementation of Herder. Owns all major sub-components:
  • mTransactionQueue (ClassicTransactionQueue) — classic transaction queue.
  • mSorobanTransactionQueue (unique_ptr<SorobanTransactionQueue>) — created lazily at protocol ≥ 20.
  • mPendingEnvelopes (PendingEnvelopes) — manages SCP envelope fetching/staging.
  • mUpgrades (Upgrades) — protocol upgrade management.
  • mHerderSCPDriver (HerderSCPDriver) — SCP driver implementation.
  • mLastQuorumMapIntersectionState (shared_ptr<QuorumMapIntersectionState>) — quorum intersection analysis state.
• Tracks consensus via ConsensusData struct (mTrackingSCP): mConsensusIndex and mConsensusCloseTime.
• State transitions: BOOTING → SYNCING ↔ TRACKING. Transition to BOOTING from TRACKING/SYNCING is disallowed.

HerderSCPDriver (SCP Driver)

• File: HerderSCPDriver.h, HerderSCPDriver.cpp (~1517 lines)
• Implements SCPDriver interface. Bridges between SCP core and Herder.
• Owns mSCP (SCP instance), mSCPTimers (per-slot timer map), mSCPExecutionTimes (timing data per slot), mTxSetValidCache (RandomEvictionCache for tx set validity).
• Tracks mMissingNodes and mDeadNodes for dead node detection.
• Key inner struct SCPTiming: records nomination start, prepare start, timeout counts, and externalize timing.

PendingEnvelopes

• File: PendingEnvelopes.h, PendingEnvelopes.cpp (~1013 lines)
• Manages the lifecycle of SCP envelopes from receipt to SCP processing.
• Per-slot state tracked via SlotEnvelopes struct:
  • mDiscardedEnvelopes, mProcessedEnvelopes (sets of SCPEnvelope).
  • mFetchingEnvelopes (map from envelope → timestamp).
  • mReadyEnvelopes (vector of SCPEnvelopeWrapperPtr).
  • mReceivedCost (cost tracking per validator NodeID).
• Caches: mQsetCache and mTxSetCache (both RandomEvictionCache), plus mKnownQSets/mKnownTxSets (weak reference maps).
• Owns mTxSetFetcher and mQuorumSetFetcher (ItemFetcher instances) — request missing data from peers.
• Owns mQuorumTracker (QuorumTracker) — maintains transitive quorum set.

TransactionQueue (Base) / ClassicTransactionQueue / SorobanTransactionQueue

• File: TransactionQueue.h, TransactionQueue.cpp (~1416 lines)
• Stores pending transactions per source account (AccountStates map).
• AccountState: mTotalFees, mAge, mTransaction (optional TimestampedTx).
• BannedTransactions: deque of UnorderedSet<Hash> for ban tracking.
• Owns mTxQueueLimiter (unique_ptr<TxQueueLimiter>) for resource-based eviction.
• ClassicTransactionQueue: supports DEX arbitrage flood damping via mArbitrageFloodDamping.
• SorobanTransactionQueue: supports resetAndRebuild() triggered on config upgrades; has separate flood period (FLOOD_SOROBAN_TX_PERIOD_MS).
• AddResult codes: PENDING, DUPLICATE, ERROR, TRY_AGAIN_LATER, FILTERED.
• Key lifecycle: tryAdd() → removeApplied() / ban() → shift() (age-out) → rebroadcast().

TxSetXDRFrame / ApplicableTxSetFrame / TxSetPhaseFrame

• File: TxSetFrame.h, TxSetFrame.cpp
• TxSetXDRFrame: immutable wrapper around raw XDR (TransactionSet or GeneralizedTransactionSet). Safe for overlay exchange without validation. Created via makeFromWire(), makeEmpty(), makeFromHistoryTransactions().
• ApplicableTxSetFrame: validated/interpreted tx set ready for application. Created from TxSetXDRFrame::prepareForApply() or makeTxSetFromTransactions(). Contains TxSetPhaseFrame per phase.
• TxSetPhaseFrame: represents one phase (CLASSIC or SOROBAN). May be sequential (flat TxFrameList) or parallel (TxStageFrameList with stages → clusters → txs). Has an InclusionFeeMap for per-tx base fees. Provides iterator for ordered traversal.
• Parallel structure types: TxClusterFrame (list of txs), TxStageFrame (list of clusters), TxStageFrameList (list of stages).

SurgePricingUtils

• File: SurgePricingUtils.h, SurgePricingUtils.cpp
• SurgePricingLaneConfig (abstract): defines lane assignment, per-lane resource limits.
  • DexLimitingLaneConfig: lane 0 = generic, lane 1 = DEX-limited.
  • SorobanGenericLaneConfig: single generic lane for Soroban multi-dimensional resources.
• SurgePricingPriorityQueue: priority queue for transactions sorted by fee rate. Supports add, erase, canFitWithEviction, visitTopTxs, popTopTxs. Used by both tx queue limiter and tx set building.

TxQueueLimiter

• File: TxQueueLimiter.h, TxQueueLimiter.cpp
• Enforces resource limits on the transaction queue using two SurgePricingPriorityQueues:
  • mTxs: lowest-fee-first ordering for eviction decisions.
  • mTxsToFlood: highest-fee-first ordering for flood priority.
• Tracks mLaneEvictedInclusionFee (max evicted fee per lane).
• canAddTx() determines if a tx can fit, returning eviction candidates.

TxSetUtils

• File: TxSetUtils.h, TxSetUtils.cpp
• Static utilities: sortTxsInHashOrder, buildAccountTxQueues, getInvalidTxList, trimInvalid.
• AccountTransactionQueue: helper deque for per-account tx ordering.

ParallelTxSetBuilder

• File: ParallelTxSetBuilder.h, ParallelTxSetBuilder.cpp
• buildSurgePricedParallelSorobanPhase(): builds parallel processing stages from Soroban txs. Uses surge pricing to fill stages/clusters within network config limits.

Upgrades / ConfigUpgradeSetFrame

• File: Upgrades.h, Upgrades.cpp
• Upgrades::UpgradeParameters: scheduled upgrade config (protocol version, base fee, max tx set size, base reserve, flags, Soroban config key, nomination timeout limit, expiration).
• createUpgradesFor(): creates upgrade steps based on LCL and current time.
• isValid() / isValidForApply(): validates upgrade steps.
• removeUpgrades(): strips applied upgrades from pending set.
• ConfigUpgradeSetFrame: wraps a ConfigUpgradeSet retrieved from ledger via ConfigUpgradeSetKey. Validates XDR hash match and applies Soroban network config changes.

LedgerCloseData

• File: LedgerCloseData.h, LedgerCloseData.cpp
• Value object carrying: mLedgerSeq, mTxSet (TxSetXDRFrameConstPtr), mValue (StellarValue), optional mExpectedLedgerHash.
• Passed from Herder to LedgerManager::valueExternalized().

QuorumTracker

• File: QuorumTracker.h, QuorumTracker.cpp
• Tracks the transitive quorum graph rooted at the local node.
• NodeInfo: mQuorumSet, mDistance (from local node), mClosestValidators (set of local qset nodes shortest-distance to this node).
• QuorumMap = UnorderedMap<NodeID, NodeInfo>.
• expand(): incrementally adds a node; returns false if quorum set conflicts (triggers rebuild()).
• rebuild(): reconstructs entire quorum from a lookup function.

QuorumIntersectionChecker / QuorumIntersectionCheckerImpl

• File: QuorumIntersectionChecker.h, QuorumIntersectionCheckerImpl.h (547 lines), QuorumIntersectionCheckerImpl.cpp
• V1 checker: C++ implementation based on Lachowski's algorithm (branch-and-bound enumeration of minimal quorums, checking for non-intersecting pairs). Runs on a background thread with mInterruptFlag for cooperative cancellation.
• Key refinements: complement checking, set contraction to maximal quorums, bottom-up enumeration, half-space pruning, SCC-based pruning.
• getIntersectionCriticalGroups(): finds node groups whose removal breaks quorum intersection.

RustQuorumCheckerAdaptor

• File: RustQuorumCheckerAdaptor.h, RustQuorumCheckerAdaptor.cpp
• V2 checker: runs quorum intersection analysis via Rust SAT solver in a separate process (critical for memory safety — Rust allocator aborts on OOM).
• runQuorumIntersectionCheckAsync(): serializes quorum map to JSON, spawns stellar-core --check-quorum-intersection subprocess, reads results from output JSON.
• networkEnjoysQuorumIntersection(): in-process Rust entry point (used by subprocess).
• QuorumCheckerMetrics: tracks success/failure/abort counts, timing, memory usage.
• QuorumMapIntersectionState: shared state between main thread and background analysis — tracks last check ledger, hash, recalculating flag, results.

HerderPersistence / HerderPersistenceImpl

• File: HerderPersistence.h, HerderPersistenceImpl.h, HerderPersistenceImpl.cpp
• Persists SCP history (envelopes + quorum map) to database per ledger.
• saveSCPHistory(): stores envelopes and quorum sets.
• Static helpers: copySCPHistoryToStream(), getNodeQuorumSet(), getQuorumSet().

HerderUtils

• File: HerderUtils.h, HerderUtils.cpp
• Utility functions: toStellarValue(), getTxSetHashes(), getStellarValues(), toShortString(), toQuorumIntersectionMap(), parseQuorumMapFromJson().

FilteredEntries

• File: FilteredEntries.h
• Constants for ledger keys to filter from Soroban transaction queue (production network only). Currently empty (KEYS_TO_FILTER_P24_COUNT = 0).

Key Control Flows

Startup Flow

1. HerderImpl::start() initializes mMaxTxSize, sets up Soroban queue if protocol ≥ 20, validates flow control config.
2. Sets tracking state from LCL. If not genesis and not FORCE_SCP, restores SCP state from database via restoreSCPState() and restores upgrades via restoreUpgrades().
3. Starts mTxSetGarbageCollectTimer and mCheckForDeadNodesTimer.
4. If FORCE_SCP, calls bootstrap() which forces join via mHerderSCPDriver.bootstrap() + setupTriggerNextLedger().

Ledger Close / Trigger Flow

1. setupTriggerNextLedger(): computes next trigger time from last ballot start + expected close time. Sets mTriggerTimer to call triggerNextLedger().
2. triggerNextLedger(ledgerSeqToTrigger): gathers txs from both queues, computes close time, calls makeTxSetFromTransactions() to build proposed set with surge pricing, creates StellarValue, calls mHerderSCPDriver.nominate().
3. SCP runs nomination → prepare → commit → externalize.
4. HerderSCPDriver::valueExternalized(): cancels SCP timers, records metrics, calls mHerder.valueExternalized().
5. HerderImpl::valueExternalized(): records close time drift, dumps SCP info if slow, calls processExternalized() then newSlotExternalized(), then quorum intersection check.
6. processExternalized(): saves SCP history, removes applied upgrades, creates LedgerCloseData, calls mLedgerManager.valueExternalized().
7. lastClosedLedgerIncreased() (callback from LedgerManager after apply): updates tx queue (updateTransactionQueue()), handles upgrades (maybeHandleUpgrade()), calls setupTriggerNextLedger() if latest.

SCP Envelope Processing Flow

1. recvSCPEnvelope(): validates close time, checks ledger range, verifies signature, passes to mPendingEnvelopes.recvSCPEnvelope().
2. PendingEnvelopes::recvSCPEnvelope(): checks if envelope is already processed/discarded, starts fetching missing tx sets and quorum sets via ItemFetcher.
3. When all dependencies are fetched, envelope becomes READY, added to mReadyEnvelopes.
4. processSCPQueue() / processSCPQueueUpToIndex(): pops ready envelopes and feeds them to SCP::receiveEnvelope().

Transaction Queue Flow

1. recvTransaction(): routes to classic or Soroban queue based on tx->isSoroban(). Enforces 1-tx-per-source-account-per-ledger across both queues.
2. TransactionQueue::tryAdd(): calls canAdd() which checks validity, sequence numbers, fee sufficiency via TxQueueLimiter::canAddTx(). May evict lower-fee txs.
3. updateTransactionQueue(): after ledger close, calls removeApplied() + shift() (age accounts) + ban() invalid txs + rebroadcast().
4. SorobanTransactionQueue::resetAndRebuild(): triggered on config upgrade; extracts all txs, clears state, re-adds with new limits.

Out-of-Sync Recovery

1. trackingHeartBeat(): resets mTrackingTimer (35s). If it fires, calls herderOutOfSync().
2. herderOutOfSync(): transitions to SYNCING, starts mOutOfSyncTimer (10s periodic).
3. outOfSyncRecovery(): purges old slots, rebroadcasts own messages, calls getMoreSCPState() to ask 2 random peers for SCP messages.

Quorum Intersection Checking

• V1 (checkAndMaybeReanalyzeQuorumMap): hashes current quorum map, if changed spawns background thread running C++ QuorumIntersectionChecker. Supports cooperative interruption.
• V2 (checkAndMaybeReanalyzeQuorumMapV2): serializes quorum map to JSON, spawns out-of-process stellar-core --check-quorum-intersection running Rust SAT solver. Results posted back to main thread.
• Controlled by USE_QUORUM_INTERSECTION_CHECKER_V2 config flag.

Timers and Periodic Tasks

Ownership Relationships

HerderImpl
├── mTransactionQueue (ClassicTransactionQueue, owned)
│   └── mTxQueueLimiter (TxQueueLimiter, unique_ptr)
│       ├── mTxs (SurgePricingPriorityQueue, unique_ptr) — eviction ordering
│       └── mTxsToFlood (SurgePricingPriorityQueue, unique_ptr) — flood ordering
├── mSorobanTransactionQueue (SorobanTransactionQueue, unique_ptr, created at protocol ≥ 20)
│   └── mTxQueueLimiter (same structure as above)
├── mPendingEnvelopes (PendingEnvelopes, owned)
│   ├── mEnvelopes (map<uint64, SlotEnvelopes>)
│   ├── mQsetCache / mKnownQSets — quorum set caches
│   ├── mTxSetCache / mKnownTxSets — tx set caches
│   ├── mTxSetFetcher (ItemFetcher) — fetches missing tx sets from peers
│   ├── mQuorumSetFetcher (ItemFetcher) — fetches missing quorum sets from peers
│   └── mQuorumTracker (QuorumTracker) — transitive quorum state
├── mUpgrades (Upgrades, owned)
├── mHerderSCPDriver (HerderSCPDriver, owned)
│   ├── mSCP (SCP instance, owned)
│   ├── mSCPTimers (map<slotIndex, map<timerID, VirtualTimer>>)
│   └── mTxSetValidCache (RandomEvictionCache)
├── mLastQuorumMapIntersectionState (shared_ptr<QuorumMapIntersectionState>)
└── Various VirtualTimers (mTriggerTimer, mTrackingTimer, mOutOfSyncTimer, etc.)

Key Data Flows

Transaction Ingestion → Consensus

Network/HTTP → recvTransaction() → TransactionQueue::tryAdd()
    → TxQueueLimiter::canAddTx() [surge pricing check]
    → AccountStates update
    → broadcastTx() [periodic via mBroadcastTimer]

triggerNextLedger():
    TransactionQueue::getTransactions() → makeTxSetFromTransactions()
    → SurgePricingPriorityQueue::getMostTopTxsWithinLimits() [for each phase]
    → TxSetXDRFrame + ApplicableTxSetFrame
    → HerderSCPDriver::nominate()

SCP Message Processing

Network → recvSCPEnvelope()
    → checkCloseTime() + verifyEnvelope()
    → PendingEnvelopes::recvSCPEnvelope()
        → startFetch() [tx sets, quorum sets via ItemFetcher]
        → [when fetched] envelopeReady() → mReadyEnvelopes
    → processSCPQueue()
        → PendingEnvelopes::pop() → SCP::receiveEnvelope()
        → [on externalize] HerderSCPDriver::valueExternalized()
            → HerderImpl::valueExternalized()
                → processExternalized() → LedgerCloseData → LedgerManager

Ledger Close Feedback

LedgerManager::valueExternalized()
    → [applies ledger]
    → Herder::lastClosedLedgerIncreased()
        → maybeSetupSorobanQueue()
        → maybeHandleUpgrade() [overlay flow control sizing]
        → updateTransactionQueue() [removeApplied, shift, ban invalid, rebroadcast]
        → setupTriggerNextLedger() [next round]

Quorum Health Monitoring

valueExternalized() → checkAndMaybeReanalyzeQuorumMap[V2]()
    → getQmapHash(currentQuorum)
    → [if changed] spawn background analysis
        V1: background thread with QuorumIntersectionChecker
        V2: subprocess with Rust SAT solver
    → results posted to main thread → QuorumMapIntersectionState
    → exposed via getJsonTransitiveQuorumIntersectionInfo()

Important Constants

• TRANSACTION_QUEUE_TIMEOUT_LEDGERS = 4 — max age before tx eviction.
• TRANSACTION_QUEUE_BAN_LEDGERS = 10 — ban duration for rejected txs.
• TXSETVALID_CACHE_SIZE = 1000 — tx set validity cache entries.
• QSET_CACHE_SIZE = 10000, TXSET_CACHE_SIZE = 10000 — pending envelope caches.
• CLOSE_TIME_DRIFT_LEDGER_WINDOW_SIZE = 120 — window for clock drift detection.
• CLOSE_TIME_DRIFT_SECONDS_THRESHOLD = 10 — drift warning threshold.
• FEE_MULTIPLIER = 10 — fee multiplier for replace-by-fee.
• MAX_TIMEOUT_MS = 1,800,000 (30 min) — maximum SCP timer timeout.