experiment: batch verification

[ozgb]

Overview

Experimental / draft. Batch ZK-proof verification for Midnight transactions, so the expensive aggregate crypto runs once per block/batch instead of once per transaction. Everything is gated behind node config and defaults OFF, so with the flags unset the node behaves as today.

Two ingress points feed a process-global proof cache that downstream get_verified_transaction reads to skip (defer) the ZK crypto:

• Block import (batch_verify_block_import): received blocks are batch-verified up front (fail-fast; a bad proof rejects the block). (landed earlier on this branch)
• Mempool (batch_verify_mempool): external submissions are batched natively via a custom MidnightChainApi + bounded queue + blocking worker pool.

This PR adds the two pieces that make the mempool path real and safe to enable:

• Isolation fallback (ledger): when an aggregate batch fails, each transaction is re-verified individually to isolate the offender(s) — caching false for bad proofs and warming caches for good ones — instead of panicking. Extracts a shared warm_verified_tx helper.
• Mempool ingress (node): MidnightChainApi wraps the stock FullChainApi, delegating everything except validate_transaction; external Midnight send_mn_transaction extrinsics are queued and drained in batches by N blocking workers, which warm the caches and build the same validity tag the runtime would (with_tag_prefix("Midnight").longevity(600).and_provides(tx_hash)). Invalid/unavailable cases delegate to the runtime for the authoritative result.

The transaction pool is now always the SingleState BasicPool<MidnightChainApi> (batch verification requires the single-state ChainApi seam). Pool limits are reconstructed from the CLI (--pool-limit/--pool-kbytes/--tx-ban-seconds); --pool-type is intentionally not applied.

⚙️ Configuration (new MidnightCfg fields, for perf testing)

All new fields live in node/src/cfg/midnight_cfg/mod.rs, default OFF/unset, and are optional in res/cfg presets — existing presets need no changes to keep today's behavior.

Field	Type	Default	Meaning
batch_verify_block_import	bool	false	Enable batch ZK-proof verification for received blocks at block import. Fail-fast: a bad proof anywhere in the batch rejects the block (no fallback).
batch_verify_mempool	bool	false	Enable batch verification on the mempool ingress path (external send_mn_transaction submissions). A bad proof is isolated per-tx, not panicked.
batch_verify_max_batch_size	usize	64	M — max transactions verified together in one aggregate crypto call. Upper bound on amortization; also an upper bound on per-batch latency, since every tx in a batch waits for the whole aggregate call to finish.
batch_verify_target_batch_size	usize	16	k_target — queue depth (mempool path) that triggers an immediate dispatch, without waiting for the age timeout. Lower = lower latency but smaller/less-amortized batches; higher = better amortization but more latency under light load.
batch_verify_max_age_ms	u64	50	tau — max time (ms) the oldest queued tx waits before a partial batch dispatches anyway (mempool path). Caps worst-case added latency when traffic is too light to hit k_target.
batch_verify_workers	usize	4	N — number of blocking worker tasks draining the mempool batch queue concurrently. Each runs one aggregate crypto call at a time, so more workers only help up to available CPU cores. Mempool-only; block import is single-threaded per block.
batch_verify_queue_capacity	usize	4096	Bounded mempool queue capacity. Submissions beyond this are shed immediately with a pool ImmediatelyDropped error instead of being queued.

Tuning notes (see scripts/tests/batch-verify-perf/FINDINGS.md for the block-import harness, its methodology, and open caveats):

• Batching only pays off once a batch carries many proof-txs — at batch sizes ≤5 the aggregate call's fixed cost dominates and there's ~nothing to amortize (measured on the block-import path). batch_verify_max_batch_size / batch_verify_target_batch_size are the levers to explore the favourable regime; workload needs enough proof-txs per block/window to actually fill a batch (see the FINDINGS.md note on the DUST-output ceiling that limited the current harness).
• batch_verify_target_batch_size and batch_verify_max_age_ms jointly set the mempool latency/throughput trade-off: raising either trades added tail latency for larger, more efficient batches.
• batch_verify_workers is CPU-bound — sizing it above available cores just adds contention, not throughput.
• batch_verify_queue_capacity is a burst-absorption/shedding valve, not a throughput knob: too small drops legitimate traffic under burst (watch queue_rejected_total), too large just grows tail latency for queued txs.
• Useful metrics while tuning: midnight_batch_verify_batch_size (histogram), midnight_batch_verify_duration_seconds (per-batch aggregate crypto time, both ingress paths), midnight_batch_verify_batches_total{outcome=...}, midnight_batch_verify_txs_total, midnight_batch_verify_queue_depth, midnight_batch_verify_queue_rejected_total, midnight_batch_verify_dispatch_reason_total{trigger=...} (k_target vs tau), midnight_batch_verify_fallback_total (mempool-only).
• New just targets for the block-import A/B harness: just batch-verify-perf-prime <NODE_IMAGE> <TOOLKIT_IMAGE> (prime + archive a proof-heavy chain, once) and just batch-verify-perf-bench <NODE_IMAGE> (A/B benchmark off vs on against the primed archive) — see scripts/tests/batch-verify-perf/README.md.

🗹 TODO before merging

• Ready
• Integration validation: with both flags on in a dev preset, sync a chain and author blocks; confirm received blocks import via the wrapper, mempool/authored txs warm the proof cache (no downstream inline-verify error logs), and a bad-proof submission is isolated (not a panic)
• Decide whether to honor/warn on --pool-type fork-aware (currently ignored; SingleState is forced)
  • Fine as-is - this is just an experiment, not to be merged
• Bump node minor version if this graduates from experiment to a shipped feature
  • It will not be shipped by this PR

📌 Submission Checklist

• All commits are signed off (git commit -s) for the DCO
• Changes are backward-compatible (both flags default off; note: the pool is now always SingleState, so a --pool-type fork-aware run now builds SingleState)
• Pull request description explains why the change is needed
• Self-reviewed the diff
• I have included a change file, or skipped for this reason: experimental/draft branch
• If the changes introduce a new feature, I have bumped the node minor version
• Update documentation (if relevant)
• Updated AGENTS.md if build commands, architecture, or workflows changed
• No new todos introduced (the previous todo!() fallback is now implemented)

🧪 Testing Evidence

• cargo check -p midnight-node-ledger -p midnight-node — clean

• cargo fmt --check — clean

• cargo clippy — no new findings (only pre-existing, workspace-allowed lints on touched files)

• Unit tests:

  • ledger: fallback_isolates_bad_proof_and_caches_outcomes (mixed good/bad batch isolates the offender, caches true/false correctly, correct per-tx results) — passes on all 3 ledger versions
  • node: 7 tests for the worker pool — dispatch at k_target and tau, bounded-queue shedding (ImmediatelyDropped), oneshot resolution, invalid/unavailable → delegate, and native validity-tag shape

• Additional tests are provided (if possible)

🔱 Fork Strategy

• Node Client Update

Runtime/pallets are unchanged; the ledger crate is invoked natively by the node. No metadata or genesis rebuild required.

Links

N/A (experimental)

🤖 Generated with Claude Code

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⚠️ Warnings

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