Solana Head Detection: slotSubscribe

[tonatoz]

Solana Head Detection: slotSubscribe vs blockSubscribe

Summary

This PR replaces the expensive blockSubscribe WebSocket subscription with the lightweight slotSubscribe approach for Solana head detection. The change reduces WebSocket bandwidth by 95% while maintaining the same real-time performance.

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Problem

The blockSubscribe API has significant drawbacks:

• High bandwidth: ~1KB per notification (full block data)
• Unstable API: Requires --rpc-pubsub-enable-block-subscription node flag
• Limited support: Not available on most RPC providers (premium tier only)

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Solution

Use slotSubscribe with throttled getBlockHeight HTTP calls:

• ~50 bytes per notification (only slot/parent/root numbers)
• Stable API: No special flags required
• Universal support: Works with all Solana RPC providers
• Throttled HTTP calls: Every 5 slots (~20% of slot notifications)

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Benchmark Results

Synthetic Benchmark (100 slots)

Metric	blockSubscribe	slotSubscribe
WS messages	100	100
HTTP calls	0	20
Head updates	100	100
Processing time	3728ms	124ms

Real Production Test (Solana Mainnet, ~11 minutes each)

Metric	blockSubscribe	slotSubscribe
Test duration	663 sec	682 sec
WS messages	1677	1723
HTTP calls	3	370
Head updates	1678	1726
Errors	0	0
WS rate	2.53/sec	2.53/sec
HTTP rate	0.00/sec	0.54/sec

Daily Extrapolation (24 hours)

Metric	blockSubscribe	slotSubscribe	Winner
WS payload/day	~217 MB	~10.8 MB	slotSubscribe (95% less)
HTTP calls/day	~3	~46,656	blockSubscribe
HTTP bandwidth/day	~0	~4.7 MB	blockSubscribe
Total bandwidth/day	~217 MB	~15.5 MB	slotSubscribe (93% less)
API stability	Unstable	Stable	slotSubscribe
Provider compatibility	Limited	Universal	slotSubscribe

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Implementation Details

slotSubscribe Notification Format

{"slot": 112301554, "parent": 112301553, "root": 112301500}

~50 bytes per notification

Throttling Logic

• Check actual block height via HTTP every 5 slots
• Between checks, use cached height value
• On HTTP error, fallback to cached height or use slot as height

Synthetic Hash

Since slotSubscribe doesn't provide block hash, we generate a deterministic synthetic hash from slot:

val syntheticHash = BlockId.from(ByteBuffer.allocate(32).putLong(slot).array())

This ensures ForkChoice deduplication works correctly.

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Conclusion

slotSubscribe is the recommended approach because:

1. 93% less total bandwidth (15.5 MB vs 217 MB per day)
2. Stable API - no special node flags required
3. Universal provider support - works with all Solana RPC providers
4. Identical head update rate - same real-time performance
5. Zero errors in production testing

The trade-off of ~46K HTTP calls/day is acceptable:

• Each call is lightweight (~100 bytes response)
• Total HTTP bandwidth is only ~4.7 MB/day
• HTTP calls are throttled (every 5 slots, not every slot)

[Copilot]

Pull request overview

This PR migrates Solana head detection from the expensive blockSubscribe WebSocket API to the lightweight slotSubscribe API. The change significantly reduces bandwidth usage (by ~93% according to benchmarks) while maintaining real-time performance through throttled HTTP calls to retrieve actual block heights.

Changes:

• Replaced blockSubscribe/blockUnsubscribe with slotSubscribe/slotUnsubscribe for WebSocket subscriptions
• Implemented throttled HTTP calls (every 5 slots) to retrieve actual block heights via getBlockHeight
• Introduced synthetic hash generation based on slot numbers for ForkChoice deduplication
• Added caching mechanism for block heights per upstream to minimize HTTP calls
• Simplified data models: replaced SolanaWrapper/SolanaContext/SolanaResult with SolanaSlotNotification

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 10 comments.

File	Description
src/main/kotlin/io/emeraldpay/dshackle/upstream/solana/SolanaChainSpecific.kt	Core implementation of slotSubscribe-based head detection with throttled HTTP calls, synthetic hash generation, and per-upstream caching
src/test/kotlin/io/emeraldpay/dshackle/upstream/solana/SolanaChainSpecificTest.kt	Comprehensive test coverage for slot parsing, throttling logic, synthetic hashes, and error handling scenarios

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