perf(drive-abci): fetch the next core height's masternode and quorum lists ahead of time - #4572
perf(drive-abci): fetch the next core height's masternode and quorum lists ahead of time#4572PastaPastaPasta wants to merge 2 commits into
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…lists ahead of time Replaying history, about two thirds of mainnet blocks advance the core chain-locked height by one, and each of those blocks waits on protx listdiff and then quorum listextended — together about a millisecond of the seven a block costs, nearly all of it Core's round trip. The heights are consecutive, so start the next pair as soon as the current one returns, on a second connection so a speculative call never sits in front of a real one. A guess that fails — the normal case at the tip, where the next core block does not exist yet — backs the prefetcher off for the next 32 calls instead of asking again every block. A node that cannot open the second connection logs a warning and runs without prefetching.
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Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review. 📝 WalkthroughWalkthroughAdds chain-locked-height-gated speculative Core RPC fetching for quorum lists and masternode list diffs. ChangesCore RPC speculative prefetching
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The new speculative Core RPC path can delay block synchronization when its secondary connection stalls before fallback to the primary connection occurs. This should be resolved before merge. Sequence Diagram(s)sequenceDiagram
participant DefaultCoreRPC
participant CorePrefetcher
participant CoreRPC
DefaultCoreRPC->>CorePrefetcher: Check cached quorum list or protx diff
alt Matching result exists
CorePrefetcher-->>DefaultCoreRPC: Return cached result
else No matching result
DefaultCoreRPC->>CoreRPC: Request current height or block
CoreRPC-->>DefaultCoreRPC: Return result
DefaultCoreRPC->>CorePrefetcher: Start next-height prefetch
end
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## v4.2-dev #4572 +/- ##
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- Coverage 87.57% 85.44% -2.14%
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Files 2748 2792 +44
Lines 357005 371247 +14242
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+ Hits 312647 317209 +4562
- Misses 44358 54038 +9680
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Inline comments:
In `@packages/rs-drive-abci/src/rpc/prefetch.rs`:
- Line 81: The speculative receive paths in
packages/rs-drive-abci/src/rpc/prefetch.rs at lines 81-81 and 116-116 must not
block production RPC calls: replace each unbounded Receiver::recv use with a
bounded or non-blocking receive, and fall back to the primary client when the
speculative result is not ready. Apply the same behavior to both the regular RPC
prefetch path and the masternode-list diff path.
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packages/rs-drive-abci/src/rpc/core.rspackages/rs-drive-abci/src/rpc/mod.rspackages/rs-drive-abci/src/rpc/prefetch.rs
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Review
Verdict: changes required before merge. The speculative fetch can run ahead of the chain lock, and that is a consensus risk.
1. Correctness
The core idea is sound during replay: the next height is one ahead and already chain-locked, so a speculative answer for it is final.
At the tip it is not. Platform only ever asks Core about chain-locked heights (core_chain_locked_height is verified through verify_chain_lock). The prefetcher asks for H + 1 as soon as H is used. H + 1 can exist on Core without being chain-locked yet, and a block that is not chain-locked can be reorged. quorum listextended and protx listdiff are functions of the block's contents, and neither response carries the block hash, so the key height or (base, block) cannot tell the orphaned answer from the final one. If H + 1 is reorged between the speculative call and its use, a node that prefetched applies a different masternode diff or quorum set than a node that did not. That is an app-hash split among validators.
Depth-1 reorgs of non-chain-locked blocks are rare on Dash, but the window is exactly the situation where this code runs at the tip, and during a ChainLock outage unlocked blocks pile up and the window grows. The fix is small: never speculate past the best chain-locked height. Cache the height from getbestchainlock in the prefetcher, refresh it (on the second connection) only when a guess exceeds the cached value, and skip the guess if it is still above it. During replay that is one extra RPC at start; at the tip it is one cheap call per advancing block. It also removes the need for the failure back-off: a guess inside the chain-locked range cannot fail with "block not found", so BACKOFF_CALLS, may_speculate and note_failure go away.
Smaller points:
take_*blocks onrecv()(CodeRabbit's comment). With guesses gated to chain-locked blocks, only a Core stall can delay it, and a Core stall would delay the primary call too. I would leave it and say so in a comment.- A new
std::threadper speculative call. Two per advancing block is fine; noting it. prefetch.rshas no tests. The gating and key-matching logic is testable without Core once the RPC calls go through a small trait.
2. Clarity
Good description with the breakdown by RPC and an honest note that the A/B shared one dashd. The module doc explains the two safety measures; after the fix it should say the guess is never made past the chain lock, rather than describing the back-off.
3. Codebase standards
Follows the existing rpc module layout. CoreRPCLike is unchanged, so mocks are unaffected. pub mod prefetch is not needed outside the crate; pub(crate) would do.
4. Importance and alternatives
~1 ms of ~7 ms per block on the two-thirds of blocks that advance the core height. Batching both RPCs into one request would save a round trip, but Core offers no combined call. Prefetching is the right approach; it needs the chain-lock guard.
I will push the chain-lock gate, the simplification, and unit tests for the gate.
🤖 Posted autonomously by Claude on behalf of pasta.
A speculative fetch for the next core height could run before that block was chain-locked. Neither response carries the block hash, so if the block was reorged before the answer was used, a node that prefetched would apply a different masternode diff or quorum set than one that did not, and the app hashes would diverge. Cache the best chain-locked height in the prefetcher, refresh it only when a guess would pass it, and decline the guess if it still does. During replay that is one extra RPC at start; at the tip it is one cheap call per advancing core block. A guess inside the chain-locked range cannot fail for want of a block, so the failure back-off goes away. The Core calls go through a small trait so the gating logic has unit tests.
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Pushed 43a798b on top of the original commit:
One thing worth knowing: the cached chain-lock height only moves up. If Core is reindexed under a running drive-abci the cache can be briefly ahead of what Core serves; a guess then fails and the caller falls back to the primary call, so it is self-correcting and harmless, just not free.
Ready for human review. 🤖 Posted autonomously by Claude on behalf of pasta. |
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Issue being fixed or feature implemented
Replaying history, about two thirds of mainnet blocks advance the core chain-locked height by one, and each of those blocks blocks on two Core RPCs in turn:
protx listdifffor the masternode diff, thenquorum listextended.Measured replaying mainnet with per-block phase timing:
core_infototalprotx listdiffquorum listextendedSo ~0.96 ms of the ~7 ms a block costs is drive-abci sitting on a Core round trip, and Core has spare capacity while it waits.
What was done?
The heights are consecutive, so start the next pair as soon as the current one returns.
A
CorePrefetcherholds one in-flight speculative fetch of each kind, keyed on the height (and base height, for the diff) it was started for.get_quorum_listextendedandget_protx_diff_with_masternodestake the speculative answer when the key matches what they were asked for, and start the next guess either way. A key mismatch falls through to a real call, so a wrong guess costs nothing but a discarded response.Two things keep it from misbehaving at the tip, where the next core block does not exist yet:
jsonrpc's HTTP transport serialises requests behind a single socket mutex, so sharing the connection would defeat the point.A node that cannot open the second connection logs a warning and runs without prefetching.
How Has This Been Tested?
Interleaved A/B on a fixed window at mainnet height 190k, four runs alternating:
core_inforpc_protx_diff682 → 514 µs,rpc_quorum_list670 → 470 µs. Note these runs shared onedashdwith two other syncing nodes, so the residual wait is partly RPC contention from the harness rather than a limit of the approach.Also exercised across a full mainnet replay, genesis to 424,981, with every committed app hash matching a reference sync.
cargo test -p drive-abci --lib— 2,770 passed.Breaking Changes
None. One extra Core RPC connection per node, and speculative requests that Core answers from data it already has.
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