Support partial mempool synchronization #168
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Prior to this change, the local electrs mempool was only updated with new transactions once we were able to get a complete snapshot of every transaction in bitcoind's mempool, which could require multiple rounds of attempts if any transactions are replaced (or otherwise evicted) between querying for the mempool txids and querying for the transactions themselves. PR romanz#89 made each round more efficient, but obtaining a complete snapshot can still potentially take many rounds the more RBF is used, with each round taking longer the larger the mempool gets (just listing the mempool txids to find the delta becomes costly). With this change, the local mempool is instead updated with whatever transactions we are able to get, without waiting for a fully consistent snapshot - which should improve performance and reduce latency. Making partial updates requires some extra care on the electrs side to ensure mempool consistency (i.e., no double-spends or missing input txos), since we can no longer fully rely on bitcoind validating consistency for us. Specifically: - If any transactions were RBF'd and gone by the time we queried for them, we explicitly check for transactions that depend on the missing transactions (or their descendants) and remove them too. - Mempool evictions must be processed first before any additions, even if the replacing transactions aren't available. This ensures double-spend conflicts are not possible.
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This could definitely use some tests, but testing the edge-cases here is pretty tricky (in particular, the case where we manage to get the child tx before the parent is RBF'd, but then miss the parent). Still thinking on ways to approach it. |
Randy808
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Sep 30, 2025
Randy808
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Sep 30, 2025
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tack |
philippem
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Oct 9, 2025
RCasatta
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Oct 10, 2025
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| let new_txids = bitcoind_txids | ||
| .iter() | ||
| .filter(|&txid| !indexed_txids.contains(txid)) | ||
| .collect::<Vec<_>>(); |
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nit: shouldn't this be another difference() ?
just mentioning it is usually pretty fast, I am doing similar logic in another repo 
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RCasatta
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Oct 10, 2025
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Prior to this change, the local electrs mempool was only updated with new transactions once we were able to get a complete snapshot of every transaction in bitcoind's mempool, which could require multiple rounds of attempts if any transactions are replaced (or otherwise evicted) between querying for the mempool txids and querying for the transactions themselves.
PR #89 made each round more efficient, but obtaining a complete snapshot can still potentially take many rounds the more RBF is used, with each round taking longer the larger the mempool gets (just listing the mempool txids to find the delta becomes costly).
With this change, the local mempool is instead updated with whatever transactions we are able to get, without waiting for a fully consistent snapshot - which should improve performance and reduce latency.
Making partial updates requires some extra care on the electrs side to ensure mempool consistency (i.e., no double-spends or missing input txos), since we can no longer fully rely on bitcoind validating consistency for us. Specifically:
If any mempool transactions became unavailable (replaced) by the time we queried for them, we explicitly check for transactions that depend on the missing transactions (or their descendants) and remove them too.
Mempool evictions are processed first before any additions, even if the replacing transactions aren't available. This ensures double-spend conflicts are not possible. (technically this was already the case, but now it became more critical)