8000 [locking] Get a revision while a batch is active by xinifinity · Pull Request #17 · ava-labs/firewood · GitHub
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[locking] Get a revision while a batch is active #17

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Merged
merged 2 commits into from
Apr 14, 2023
Merged

[locking] Get a revision while a batch is active #17

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[locking] Get a revision while a batch is active
Apr 13, 2023
a376d4d
Address review comment
Apr 13, 2023
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24 changes: 24 additions & 0 deletions firewood/examples/rev.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -21,10 +21,34 @@ fn main() {
"{}",
hex::encode(*db.get_revision(1, None).unwrap().kv_root_hash().unwrap())
);
let root_hash = *db.get_revision(1, None).unwrap().kv_root_hash().unwrap();
println!(
"{}",
hex::encode(*db.get_revision(2, None).unwrap().kv_root_hash().unwrap())
);
let write = db.new_writebatch().kv_insert("k", vec![b'v']).unwrap();

// Get a revision while a batch is active.
println!(
"{}",
hex::encode(*db.get_revision(1, None).unwrap().kv_root_hash().unwrap())
);
assert_eq!(
root_hash,
*db.get_revision(1, None).unwrap().kv_root_hash().unwrap()
);

// Read the uncommitted value while the batch is still active.
let val = db.kv_get("k").unwrap();
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@hexfusion hexfusion Apr 13, 2023 •
edited
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My expectation here would be that kv_get would return no value as the key has not been committed. db.kv_get() should return against the latest revision. In this case its returning against an uncommitted batch which makes me think a mutation has taken place on disk.

ref.

firewood/firewood/src/db.rs

Lines 710 to 717 in 8d46653

/// Get a value in the kv store associated with a particular key.
pub fn kv_get<K: AsRef<[u8]>>(&self, key: K) -> Result<Vec<u8>, DBError> {
self.inner
.lock()
.latest
.kv_get(key)
.ok_or(DBError::KeyNotFound)
}

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I see your point, for the current user who open the batch and want to read the uncommitted changes. It is desired the user can read it. But the other concurrent user shouldn't read it.

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i think this will be further complicated by the API changes. I think we should proceed with improving the concurrency of this now though; this seems to be heading the right way for now.

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yeah, talked to @hexfusion offline, with this change, there is still something needs to be improved that DB should not has APIs that would expose the staging changes. And user should directly interact with a DBRev to write/read their uncommitted changes. I will have a follow up PR for it. But agree would want to merge this as incremental changes.

assert_eq!("v".as_bytes().to_vec(), val);

write.commit();
println!(
"{}",
hex::encode(*db.get_revision(1, None).unwrap().kv_root_hash().unwrap())
);
let val = db.kv_get("k").unwrap();
assert_eq!("v".as_bytes().to_vec(), val);
}
{
let db = DB::new("rev_db", &cfg.truncate(false).build()).unwrap();
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