kevy_persist/

aof.rs

//! Append-only command log. Split out from `lib.rs` to keep that file
//! under the 500-LOC house rule; the snapshot writer/reader stays there.

use std::fs::{File, OpenOptions};
use std::io::{self, BufWriter, Seek, SeekFrom, Write};
use std::path::{Path, PathBuf};
use std::time::{Duration, Instant};

use kevy_resp::ArgvView;
use kevy_store::Store;

use crate::{
    dump_store_to_aof, estimate_multibulk_bytes, write_multibulk,
};

/// 9-byte file-format header written at the start of every kevy-managed
/// AOF as of v1.2.0. `replay_aof` strips it before parsing RESP, so
/// non-kevy bytes accidentally written into the AOF path (e.g. a deploy
/// pipeline redirecting shell stderr into the file) get the same loud
/// rejection as any other corrupt frame. Pre-1.2 AOFs (no magic) still
/// replay — the parser only consumes the magic if it sees it.
pub(crate) const AOF_MAGIC: &[u8; 9] = b"KEVYAOF1\n";

/// When to fsync the AOF to disk.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Fsync {
    /// fsync after every write — safest, slowest.
    Always,
    /// fsync at most once per second (call [`Aof::maybe_sync`] periodically).
    EverySec,
    /// Never fsync explicitly; leave it to the OS.
    No,
}

/// An append-only command log. Each write command is appended as a RESP
/// multi-bulk frame; [`crate::replay_aof`] re-applies them on startup.
///
/// Durability model (paired with snapshots): a snapshot taken at T0 plus
/// the AOF of writes in (T0, now] reconstructs the current state. `SAVE`
/// writes the snapshot then [`Aof::truncate`]s the log, so replay never
/// double-applies.
///
/// Sizes (`size_bytes`, `size_at_last_rewrite`) drive auto-trigger of
/// [`Aof::rewrite_from`] (BGREWRITEAOF) via the
/// `auto_aof_rewrite_percentage` + `auto_aof_rewrite_min_size` knobs in
/// `kevy_config`.
pub struct Aof {
    file: BufWriter<File>,
    path: PathBuf,
    fsync: Fsync,
    dirty: bool,
    last_sync: Instant,
    /// Estimated bytes currently in the AOF file (existing + appended since
    /// open). Maintained without fstat() syscalls per append.
    size_bytes: u64,
    /// File size right after the most recent [`Self::rewrite_from`] (or
    /// `Self::open` if never rewritten). Anchor for `auto_aof_rewrite_*`.
    size_at_last_rewrite: u64,
    /// Total rewrites successfully completed since open. Surfaced via INFO.
    rewrites_total: u64,
}

/// Result of an [`Aof::rewrite_from`] call. Surfaced by `BGREWRITEAOF` /
/// `INFO persistence`.
#[derive(Debug, Clone, Copy)]
pub struct RewriteStats {
    /// Keys dumped into the new AOF.
    pub keys: u64,
    /// New AOF size in bytes.
    pub bytes: u64,
}

impl Aof {
    /// Open (creating if needed) `path` for appending. New files get the
    /// 9-byte [`AOF_MAGIC`] header so replays can identify the file as
    /// kevy-managed. Pre-existing files (legacy bare-RESP or already-
    /// magic'd) are left untouched.
    pub fn open(path: &Path, fsync: Fsync) -> io::Result<Self> {
        let mut file = OpenOptions::new().create(true).append(true).open(path)?;
        let mut size = file.metadata().map(|m| m.len()).unwrap_or(0);
        if size == 0 {
            // Fresh file: stamp the magic header so the replayer can
            // distinguish kevy-written AOFs from accidental writes.
            file.write_all(AOF_MAGIC)?;
            file.sync_data()?;
            size = AOF_MAGIC.len() as u64;
        }
        Ok(Aof {
            file: BufWriter::new(file),
            path: path.to_path_buf(),
            fsync,
            dirty: false,
            last_sync: Instant::now(),
            size_bytes: size,
            size_at_last_rewrite: size,
            rewrites_total: 0,
        })
    }

    /// Append one command, applying the fsync policy.
    pub fn append<A: ArgvView + ?Sized>(&mut self, args: &A) -> io::Result<()> {
        write_multibulk(&mut self.file, args)?;
        self.size_bytes = self
            .size_bytes
            .saturating_add(estimate_multibulk_bytes(args));
        match self.fsync {
            Fsync::Always => {
                self.file.flush()?;
                self.file.get_ref().sync_data()?;
            }
            Fsync::EverySec | Fsync::No => self.dirty = true,
        }
        Ok(())
    }

    /// Flush+fsync if the `EverySec` window has elapsed. Call once per loop tick.
    pub fn maybe_sync(&mut self) -> io::Result<()> {
        if matches!(self.fsync, Fsync::EverySec)
            && self.dirty
            && self.last_sync.elapsed() >= Duration::from_secs(1)
        {
            self.file.flush()?;
            self.file.get_ref().sync_data()?;
            self.dirty = false;
            self.last_sync = Instant::now();
        }
        Ok(())
    }

    /// Empty the log (after a snapshot has captured the full state). The
    /// post-truncate file keeps the [`AOF_MAGIC`] header so replays of
    /// the freshly-trimmed log still identify as kevy-managed.
    pub fn truncate(&mut self) -> io::Result<()> {
        self.file.flush()?;
        let f = self.file.get_mut();
        f.set_len(0)?;
        f.seek(SeekFrom::Start(0))?; // harmless under O_APPEND; keeps len/pos coherent
        f.write_all(AOF_MAGIC)?;
        f.sync_all()?;
        self.dirty = false;
        self.size_bytes = AOF_MAGIC.len() as u64;
        self.size_at_last_rewrite = AOF_MAGIC.len() as u64;
        Ok(())
    }

    /// Estimated current AOF size in bytes (file content as of last append).
    #[inline]
    pub fn size_bytes(&self) -> u64 {
        self.size_bytes
    }

    /// AOF size at the most recent rewrite (or open). Auto-trigger compares
    /// `(size_bytes - size_at_last_rewrite) * 100 / size_at_last_rewrite` to
    /// the `auto_aof_rewrite_percentage` knob.
    #[inline]
    pub fn size_at_last_rewrite(&self) -> u64 {
        self.size_at_last_rewrite
    }

    /// Successful rewrite count since `Self::open`. Surfaced in INFO.
    #[inline]
    pub fn rewrites_total(&self) -> u64 {
        self.rewrites_total
    }

    /// BGREWRITEAOF: rebuild a compact AOF from `store`'s current state and
    /// atomically swap it in.
    ///
    /// **v1.0 is synchronous** — the calling shard blocks for the rewrite's
    /// duration. Each shard owns its own AOF, so the shards' rewrites
    /// proceed independently; per-shard blocking matches Redis's `BGSAVE`
    /// cost in a typical single-key-per-shard workload. Concurrent
    /// (rewrite-during-writes) incrementalisation is a v1.x perf item.
    ///
    /// Writes to a `<path>.rewrite` temp file with fsync, then `rename(2)`s
    /// it over the live AOF. The append handle is reopened against the new
    /// file before this call returns, so subsequent `append` calls land in
    /// the rewritten log.
    pub fn rewrite_from(&mut self, store: &Store) -> io::Result<RewriteStats> {
        // Flush any pending writes to the OLD file first so the snapshot
        // accounts for everything the caller intended to durabilise.
        self.file.flush()?;

        let tmp = rewrite_tmp_path(&self.path);
        let (keys, bytes) = dump_store_to_aof(&tmp, store)?;

        // Atomic replacement. After this, the OLD file descriptor in
        // `self.file` is open against an unlinked inode; new writes would
        // go nowhere visible. Reopen against the new path.
        std::fs::rename(&tmp, &self.path)?;
        let f = OpenOptions::new().append(true).open(&self.path)?;
        self.file = BufWriter::new(f);
        self.size_bytes = bytes;
        self.size_at_last_rewrite = bytes;
        self.dirty = false;
        self.rewrites_total = self.rewrites_total.saturating_add(1);
        Ok(RewriteStats { keys, bytes })
    }
}

/// `<aof>.rewrite` — same-directory temp path so `rename(2)` stays atomic.
fn rewrite_tmp_path(path: &Path) -> PathBuf {
    let mut p = path.to_path_buf();
    let new_name = match path.file_name() {
        Some(n) => {
            let mut s = n.to_os_string();
            s.push(".rewrite");
            s
        }
        None => std::ffi::OsString::from("aof.rewrite"),
    };
    p.set_file_name(new_name);
    p
}