yo_kv/

walk.rs

//! Walking the keyspace: `SCAN`, `KEYS` and `RANDOMKEY`.
//!
//! Three commands that all want the same thing, which is to look at keys the
//! caller has not named, and that want it in three different shapes. `KEYS`
//! wants every key now and does not care what it costs. `SCAN` wants a bounded
//! bite and a number it can come back with. `RANDOMKEY` wants one key and does
//! not want to look at the others to find it.
//!
//! # A walk reaps what it walks past
//!
//! A key past its deadline is skipped, and then it is deleted once the walk has
//! finished. It cannot be deleted while the walk is running, because the walk
//! holds a shared borrow of the index for as long as it is handing out borrowed
//! keys, so the dead names go in a list and the deletes happen after. The list
//! is empty in the ordinary case and only ever holds keys that were already
//! dead, so a database with nothing expiring in it allocates nothing.
//!
//! This matters because there is no active expiry cycle yet, so a key nobody
//! reads again is a key nobody collects. Without the reap, `SET k v PX 50` and
//! a wait would leave `DBSIZE` answering one more than Redis answers, which is
//! the difference a side by side run against 8.10.1 actually turned up. Redis
//! collects that key from its own cycle within a hundred milliseconds, and a
//! walk finding it is the closest thing we have until the cycle lands in M5.
//!
//! The cost is that these three take `&mut self` rather than `&self`, which is
//! what every other read in this crate already takes for the same reason.
//!
//! # `COUNT` is a floor
//!
//! [`Keyspace::scan`] stops at the first bucket boundary past the budget, so a
//! `COUNT 10` can come back with fifteen keys, and a `MATCH` that rejects all
//! of them can come back with none and a cursor that is not zero. That is
//! Redis's behaviour exactly, and a client that treats an empty batch as the
//! end of the scan is broken against Redis too.
//!
//! The filtering happens in the caller's closure and the budget is counted
//! before it, so a `MATCH` that matches nothing still walks the whole keyspace
//! a bucket at a time rather than in one unbounded call.

use yo_index::Cursor as KeyCursor;

use crate::keyspace::Keyspace;
use crate::value::{self, Kind};

/// How many random buckets to try before giving up and walking the whole thing.
///
/// A bucket holds fourteen entries and the index keeps the table loaded, so the
/// first try finds a key in almost every database anyone has. The tries only
/// come into play when the keyspace is tiny or has just had most of it deleted,
/// and the walk behind them is what stops those cases from being wrong rather
/// than slow.
const TRIES: usize = 64;

impl Keyspace {
    /// A batch of keys, and where the next batch starts.
    ///
    /// This is `SCAN`. `budget` is `COUNT`, `ty` is `TYPE`, and `MATCH` belongs
    /// to the caller because a glob is a wire concern and this is not the wire.
    ///
    /// The cursor is opaque to the client and is not opaque here: it names a
    /// place in the keyspace rather than a place in memory, which is what lets
    /// it survive the index doubling between two calls. The reasoning is in
    /// [`yo_index::Cursor`].
    ///
    /// A key that is there for the whole scan comes back at least once. A key
    /// added or removed partway through may or may not, and any key may come
    /// back twice. That is Redis's contract and a client written against Redis
    /// already copes with all three.
    pub fn scan(
        &mut self,
        from: KeyCursor,
        budget: usize,
        ty: Option<Kind>,
        mut out: impl FnMut(&[u8]),
    ) -> KeyCursor {
        let now = self.clock.now_ms();
        let mut dead = Vec::new();
        let next = self.map.scan(from, budget, |key, rec| {
            if value::is_expired(rec, now) {
                dead.push(key.to_vec());
                return;
            }
            if ty.is_some_and(|want| value::kind(rec) != want) {
                return;
            }
            out(key);
        });
        self.reap_all(dead);
        next
    }

    /// Every key in the database, once each.
    ///
    /// This is `KEYS`, and it is the command whose reputation is deserved: it
    /// visits every bucket in the index before it answers anything, and a
    /// database of ten million keys is ten million calls to `out` with the
    /// shard doing nothing else. It is here because tooling needs it and
    /// because `SCAN` is the answer for everything else.
    ///
    /// One walk with an unbounded budget rather than a loop over [`Keyspace::scan`],
    /// which is the same walk without the chance of a duplicate, because nothing
    /// can split the index while this is running.
    pub fn keys(&mut self, out: impl FnMut(&[u8])) {
        self.scan(KeyCursor::START, usize::MAX, None, out);
    }

    /// Drop the dead keys a walk went past, now that the walk has let go.
    ///
    /// The count goes up by one per key, the same as a lazy reap on an ordinary
    /// read, because `INFO stats` reports one number for both and Redis counts
    /// its active cycle into it too.
    fn reap_all(&mut self, dead: Vec<Vec<u8>>) {
        for key in dead {
            self.reaped(&key);
        }
    }

    /// One key, chosen at random, or `None` if the database is empty.
    ///
    /// This is `RANDOMKEY`. It picks a random position in the index and takes a
    /// key from the bucket that lands in, which is a constant number of loads
    /// and does not depend on how many keys there are.
    ///
    /// Uniform within the bucket and only roughly uniform across the keyspace,
    /// since a bucket holding two keys and a bucket holding twelve are equally
    /// likely to be landed on. Redis's is biased the same way and for the same
    /// reason. What it is not is skewed towards any particular key, which is
    /// what matters for the thing `RANDOMKEY` is actually used for, which is
    /// sampling a live database to see what is in it.
    ///
    /// The answer borrows the database's scratch buffer, so it is good until the
    /// next call and the caller copies it if it wants to keep it. That is what
    /// takes the allocation off the command: sampling is a thing callers do in a
    /// loop, and a key name is a handful of bytes that used to cost a malloc and
    /// a free every time round.
    pub fn random_key(&mut self) -> Option<&[u8]> {
        if self.map.is_empty() {
            return None;
        }
        let mut found = false;
        for _ in 0..TRIES {
            let from = KeyCursor::from_raw(self.rng.next_u64());
            if self.sample(from, 0) {
                found = true;
                break;
            }
        }
        // Every bucket that was tried was empty or held nothing but dead keys.
        // A full walk is the only answer left that can tell an unlucky run of
        // tries from a database whose keys have all expired.
        if !found {
            found = self.sample(KeyCursor::START, usize::MAX);
        }
        // Written out rather than returned from inside the loop because the
        // answer borrows `self` and the next turn of the loop wants it back.
        found.then_some(self.scratch.as_slice())
    }

    /// Walk from `from` and leave one key in the scratch buffer, `true` if there
    /// was one.
    ///
    /// Reservoir sampling, which is the version that needs one pass and one
    /// slot of memory. Taking the first key instead would answer the same key
    /// every time for as long as the bucket held still.
    ///
    /// The slot is the scratch buffer, taken out for the walk and put back
    /// after it, because the closure has to hold it while `self.map` is
    /// borrowed by the scan.
    fn sample(&mut self, from: KeyCursor, budget: usize) -> bool {
        let now = self.clock.now_ms();
        // Named separately so the closure borrows the counter and not the whole
        // keyspace, which the walk is holding.
        let rng = &mut self.rng;
        let mut buf = std::mem::take(&mut self.scratch);
        let mut seen = 0usize;
        let mut found = false;
        let mut dead = Vec::new();
        self.map.scan(from, budget, |key, rec| {
            if value::is_expired(rec, now) {
                dead.push(key.to_vec());
                return;
            }
            seen += 1;
            if rng.below(seen) == 0 {
                buf.clear();
                buf.extend_from_slice(key);
                found = true;
            }
        });
        self.scratch = buf;
        self.reap_all(dead);
        found
    }
}

#[cfg(test)]
mod tests {
    use std::collections::HashSet;

    use super::*;
    use crate::Clock;

    fn db() -> Keyspace {
        Keyspace::with_clock(Clock::fixed(1_000_000))
    }

    fn put(d: &mut Keyspace, key: &[u8]) {
        d.set_plain(key, b"v").expect("room for a record");
    }

    fn keys_of(db: &mut Keyspace) -> HashSet<Vec<u8>> {
        let mut out = HashSet::new();
        db.keys(|k| {
            out.insert(k.to_vec());
        });
        out
    }

    #[test]
    fn an_empty_database_has_nothing_to_walk() {
        let mut db = db();
        assert!(keys_of(&mut db).is_empty());
        assert_eq!(db.random_key(), None);
        assert!(db.scan(KeyCursor::START, 10, None, |_| {}).is_end());
    }

    #[test]
    fn a_scan_comes_back_with_every_key_once() {
        let mut db = db();
        for i in 0..2_000u32 {
            put(&mut db, format!("k{i}").as_bytes());
        }

        let mut seen: Vec<Vec<u8>> = Vec::new();
        let mut at = KeyCursor::START;
        loop {
            at = db.scan(at, 10, None, |k| seen.push(k.to_vec()));
            if at.is_end() {
                break;
            }
        }

        let unique: HashSet<Vec<u8>> = seen.iter().cloned().collect();
        assert_eq!(unique.len(), 2_000);
        assert_eq!(seen.len(), 2_000, "a quiet scan returned a key twice");
        assert_eq!(unique, keys_of(&mut db));
    }

    #[test]
    fn a_scan_can_ask_for_one_type() {
        let mut db = db();
        put(&mut db, b"s");
        db.sadd(b"members", [b"a".as_slice()].into_iter())
            .expect("a fresh key");
        db.hset(b"h", [(b"f".as_slice(), b"v".as_slice())].into_iter())
            .expect("a fresh key");

        for (want, name) in [
            (Kind::String, "s"),
            (Kind::Set, "members"),
            (Kind::Hash, "h"),
        ] {
            let mut seen = Vec::new();
            let mut at = KeyCursor::START;
            loop {
                at = db.scan(at, 100, Some(want), |k| seen.push(k.to_vec()));
                if at.is_end() {
                    break;
                }
            }
            assert_eq!(seen, vec![name.as_bytes().to_vec()], "type {want:?}");
        }
    }

    #[test]
    fn a_key_past_its_deadline_is_collected_by_the_walk() {
        let mut db = db();
        put(&mut db, b"alive");
        put(&mut db, b"dead");
        assert!(db.set_expiry(b"dead", Some(1_000_500)));

        db.clock().advance(1_000);
        let before = db.expired_keys();
        assert_eq!(keys_of(&mut db), HashSet::from([b"alive".to_vec()]));
        // Gone from the map and counted, so `DBSIZE` after a walk answers what
        // Redis answers after its active cycle has been round.
        assert_eq!(db.len(), 1);
        assert_eq!(db.expired_keys(), before + 1);

        // And the walk is the only thing that touched it, so a second walk has
        // nothing left to collect and does not count it twice.
        assert_eq!(keys_of(&mut db), HashSet::from([b"alive".to_vec()]));
        assert_eq!(db.expired_keys(), before + 1);
    }

    #[test]
    fn a_random_key_is_a_key_that_is_there() {
        let mut db = db();
        for i in 0..500u32 {
            put(&mut db, format!("k{i}").as_bytes());
        }

        let all = keys_of(&mut db);
        let mut picked = HashSet::new();
        for _ in 0..200 {
            // Copied out, because the answer borrows the buffer the next draw
            // writes into.
            let k = db.random_key().expect("the database is not empty").to_vec();
            assert!(
                all.contains(&k),
                "randomkey answered a key that is not there"
            );
            picked.insert(k);
        }
        // Not a distribution test, just a check that it is not answering the
        // same key every time, which is what a walk that always takes the first
        // hit would do.
        assert!(
            picked.len() > 10,
            "only {} distinct keys in 200 draws",
            picked.len()
        );
    }

    /// `RANDOMKEY` used to hand back an owned key, which is a malloc and a free
    /// on a command whose whole job is to be called in a loop.
    #[test]
    fn randomkey_does_not_allocate() {
        let mut db = db();
        for i in 0..500u32 {
            put(&mut db, format!("k{i}").as_bytes());
        }
        // Nothing here has expired, so the walk never has a key to reap and the
        // only thing left that could allocate is the answer itself.
        let (_, allocs) = crate::tally::counted(|| {
            for _ in 0..200 {
                assert!(db.random_key().is_some(), "the database is not empty");
            }
        });
        assert_eq!(
            allocs, 0,
            "randomkey allocated {allocs} times in two hundred"
        );
    }

    #[test]
    fn the_last_key_left_is_the_one_randomkey_finds() {
        let mut db = db();
        for i in 0..5_000u32 {
            put(&mut db, format!("k{i}").as_bytes());
        }
        for i in 0..5_000u32 {
            if i != 4_242 {
                db.del(format!("k{i}").as_bytes());
            }
        }

        // One key in a directory that grew to hold five thousand, so every
        // random try misses and the fallback walk is what answers.
        assert_eq!(db.random_key(), Some(&b"k4242"[..]));
    }

    #[test]
    fn a_scan_survives_the_keyspace_growing_underneath_it() {
        let mut db = db();
        for i in 0..2_000u32 {
            put(&mut db, format!("k{i}").as_bytes());
        }

        let mut seen: HashSet<Vec<u8>> = HashSet::new();
        let mut at = KeyCursor::START;
        let mut added = 2_000u32;
        loop {
            at = db.scan(at, 8, None, |k| {
                seen.insert(k.to_vec());
            });
            if at.is_end() {
                break;
            }
            for _ in 0..64 {
                put(&mut db, format!("k{added}").as_bytes());
                added += 1;
            }
        }

        for i in 0..2_000u32 {
            let k = format!("k{i}").into_bytes();
            assert!(
                seen.contains(&k),
                "k{i} was there throughout and never came back"
            );
        }
    }
}