Async FilesystemStore #3931

joostjager · 2025-07-15T13:12:41Z

Try out for an async store with eventually consistent writes. It is just using tokio's spawn_blocking, because that is what tokio::fs would otherwise do as well. Using tokio::fs would make it complicated to reuse the sync code.

ldk-node try out: lightningdevkit/ldk-node@main...joostjager:ldk-node:async-fsstore

ldk-reviews-bot · 2025-07-15T13:12:44Z

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tnull · 2025-07-15T13:49:38Z

lightning-persister/src/fs_store_async.rs

+		let this = Arc::clone(&self.inner);
+
+		Box::pin(async move {
+			tokio::task::spawn_blocking(move || {


Mhh, so I'm not sure if spawning blocking tasks for every IO call is the way to go (see for example https://docs.rs/tokio/latest/tokio/fs/index.html#tuning-your-file-io: "To get good performance with file IO on Tokio, it is recommended to batch your operations into as few spawn_blocking calls as possible."). Maybe there are other designs that we should at least consider before moving forward with this approach. For example, we could create a dedicated pool of longer-lived worker task(s) that process a queue?

If we use spawn_blocking, can we give the user control over which runtime this exactly will be spawned on? Also, rather than just doing wrapping, should we be using tokio::fs?

Mhh, so I'm not sure if spawning blocking tasks for every IO call is the way to go (see for example https://docs.rs/tokio/latest/tokio/fs/index.html#tuning-your-file-io: "To get good performance with file IO on Tokio, it is recommended to batch your operations into as few spawn_blocking calls as possible.").

If we should batch operations, I think the current approach is better than using tokio::fs? Because it already batches the various operations inside kvstoresync::write.

Further batching probably needs to happen at a higher level in LDK, and might be a bigger change. Not sure if that is worth it just for FIlesystemStore, especially when that store is not the preferred store for real world usage?

For example, we could create a dedicated pool of longer-lived worker task(s) that process a queue?

Isn't Tokio doing that already when a task is spawned?

If we use spawn_blocking, can we give the user control over which runtime this exactly will be spawned on? Also, rather than just doing wrapping, should we be using tokio::fs?

With tokio::fs, the current runtime is used. I'd think that that is then also sufficient if we spawn ourselves, without a need to specifiy which runtime exactly?

More generally, I think the main purpose of this PR is to show how an async kvstore could be implemented, and to have something for testing potentially. Additionally if there are users that really want to use this type of store in production, they could. But I don't think it is something to spend too much time on. A remote database is probably the more important target to design for.

tnull · 2025-07-15T13:50:24Z

lightning/src/util/persist.rs

 }

 /// Provides additional interface methods that are required for [`KVStore`]-to-[`KVStore`]
 /// data migration.
-pub trait MigratableKVStore: KVStore {
+pub trait MigratableKVStore: KVStoreSync {


How will we solve this for an KVStore?

I think this comment belongs in #3905?

We might not need to solve it now, as long as we still require a sync implementation alongside an async one? If we support async-only kvstores, then we can create an async version of this trait?

lightning-persister/src/fs_store.rs

joostjager · 2025-07-15T15:38:14Z

Removed garbage collector, because we need to keep the last written version.

codecov · 2025-07-23T19:56:31Z

Codecov Report

❌ Patch coverage is 94.53125% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.94%. Comparing base (8fd7cbf) to head (05a4bb4).

Files with missing lines	Patch %	Lines
lightning-persister/src/fs_store.rs	94.53%	3 Missing and 4 partials ⚠️

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@@           Coverage Diff            @@
##             main    #3931    +/-   ##
========================================
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  Lines      123880   123991   +111     
  Branches   123880   123991   +111     
========================================
+ Hits       110169   110279   +110     
+ Misses      11260    11259     -1     
- Partials     2451     2453     +2

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lightning-persister/src/fs_store_async.rs

joostjager · 2025-07-25T13:48:12Z

Updated code to not use an async wrapper, but conditionally expose the async KVStore trait on FilesystemStore.

I didn't yet update the ldk-node branch using this PR, because it seems many other things broke in main again.

ldk-reviews-bot · 2025-07-28T00:00:57Z

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tnull

Approach ACK I think, but I find it really concerning that we'd double our memory footprint while writing here. I don't think that is really acceptable if the async KVStore is going to be our default. I think we'll need to find a solution before moving on.

This also needs a rebase now that #3799 landed.

tnull · 2025-07-29T13:19:02Z

lightning-persister/src/fs_store.rs

+		let this = Arc::clone(&self.inner);
+
+		// Obtain a version number to retain the call sequence.
+		let version = self.version_counter.fetch_add(1, Ordering::SeqCst);


Very, very unlikely, but should we still panic if we'd ever hit u64::MAX to avoid we'd ever silently start skipping all writes? Also, I'm not sure we really need SeqCst here.

Relaxed and added overflow panic.

tnull · 2025-07-29T13:40:45Z

lightning-persister/src/fs_store.rs

+		let primary_namespace = primary_namespace.to_string();
+		let secondary_namespace = secondary_namespace.to_string();
+		let key = key.to_string();
+		let buf = buf.to_vec();


Ugh, all these reallocations are not great, in particular not for buf, which we then give as a reference to write_version. So we'll then hold the full encoded data in memory at least twice. If we don't find a way to deal with the references, I wonder if we should switch the async version of KVStore to take owned primary_namespace, secondary_namespace, key, buf args.

I think the problem is that those args should then have static lifetime, because it isn't clear when they will be used by the runtime? Open to suggestions how to do that.

Agreed that copy is not ideal. Not sure if it actually matters if we also need to hit the disk with the same data.

ldk-reviews-bot · 2025-07-29T13:42:47Z

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joostjager · 2025-07-29T14:57:41Z

Rebased. Only diff is the counter overflow change

joostjager changed the title ~~Async fsstore~~ Async FilesystemStore Jul 15, 2025

joostjager force-pushed the async-fsstore branch 4 times, most recently from 29b8bcf to 81ad668 Compare July 15, 2025 13:40

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joostjager added this to Weekly Goals Jul 17, 2025

joostjager self-assigned this Jul 17, 2025

joostjager mentioned this pull request Jul 17, 2025

Async Persistence TODOs #3052

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joostjager marked this pull request as ready for review July 24, 2025 08:32

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Add async implementation of FilesystemStore

05a4bb4

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Async FilesystemStore #3931

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