feat(exex): write ahead log (#10995)

crates/exex/exex/Cargo.toml

@@ -16,7 +16,8 @@ workspace = true
 reth-chainspec.workspace = true
 reth-config.workspace = true
 reth-evm.workspace = true
-reth-exex-types.workspace = true
+reth-exex-types = { workspace = true, features = ["serde"] }
+reth-fs-util.workspace = true
 reth-metrics.workspace = true
 reth-node-api.workspace = true
 reth-node-core.workspace = true
@@ -41,6 +42,8 @@ tokio.workspace = true
 ## misc
 eyre.workspace = true
 metrics.workspace = true
+serde_json.workspace = true
+tracing.workspace = true
 
 [dev-dependencies]
 reth-blockchain-tree.workspace = true
@@ -53,6 +56,7 @@ reth-provider = { workspace = true, features = ["test-utils"] }
 reth-testing-utils.workspace = true
 
 secp256k1.workspace = true
+tempfile.workspace = true
 
 [features]
 default = []

crates/exex/exex/src/lib.rs

@@ -46,6 +46,8 @@ pub use event::*;
 mod manager;
 pub use manager::*;
 
+mod wal;
+
 // Re-export exex types
 #[doc(inline)]
 pub use reth_exex_types::*;

crates/exex/exex/src/wal/cache.rs

@@ -0,0 +1,137 @@
+use std::collections::{BTreeMap, VecDeque};
+
+use reth_exex_types::ExExNotification;
+use reth_primitives::BlockNumHash;
+
+/// The block cache of the WAL. Acts as a mapping of `File ID -> List of Blocks`.
+///
+/// For each notification written to the WAL, there will be an entry per block written to
+/// the cache with the same file ID. I.e. for each notification, there may be multiple blocks in the
+/// cache.
+///
+/// This cache is needed to avoid walking the WAL directory every time we want to find a
+/// notification corresponding to a block.
+#[derive(Debug)]
+pub(super) struct BlockCache(BTreeMap<u64, VecDeque<CachedBlock>>);
+
+impl BlockCache {
+    /// Creates a new instance of [`BlockCache`].
+    pub(super) const fn new() -> Self {
+        Self(BTreeMap::new())
+    }
+
+    /// Returns `true` if the cache is empty.
+    pub(super) fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    /// Returns a front-to-back iterator.
+    pub(super) fn iter(&self) -> impl Iterator<Item = (u64, CachedBlock)> + '_ {
+        self.0.iter().flat_map(|(k, v)| v.iter().map(move |b| (*k, *b)))
+    }
+
+    /// Provides a reference to the first block from the cache, or `None` if the cache is
+    /// empty.
+    pub(super) fn front(&self) -> Option<(u64, CachedBlock)> {
+        self.0.first_key_value().and_then(|(k, v)| v.front().map(|b| (*k, *b)))
+    }
+
+    /// Provides a reference to the last block from the cache, or `None` if the cache is
+    /// empty.
+    pub(super) fn back(&self) -> Option<(u64, CachedBlock)> {
+        self.0.last_key_value().and_then(|(k, v)| v.back().map(|b| (*k, *b)))
+    }
+
+    /// Removes the notification with the given file ID.
+    pub(super) fn remove_notification(&mut self, key: u64) -> Option<VecDeque<CachedBlock>> {
+        self.0.remove(&key)
+    }
+
+    /// Pops the first block from the cache. If it resulted in the whole file entry being empty,
+    /// it will also remove the file entry.
+    pub(super) fn pop_front(&mut self) -> Option<(u64, CachedBlock)> {
+        let first_entry = self.0.first_entry()?;
+        let key = *first_entry.key();
+        let blocks = first_entry.into_mut();
+        let first_block = blocks.pop_front().unwrap();
+        if blocks.is_empty() {
+            self.0.remove(&key);
+        }
+
+        Some((key, first_block))
+    }
+
+    /// Pops the last block from the cache. If it resulted in the whole file entry being empty,
+    /// it will also remove the file entry.
+    pub(super) fn pop_back(&mut self) -> Option<(u64, CachedBlock)> {
+        let last_entry = self.0.last_entry()?;
+        let key = *last_entry.key();
+        let blocks = last_entry.into_mut();
+        let last_block = blocks.pop_back().unwrap();
+        if blocks.is_empty() {
+            self.0.remove(&key);
+        }
+
+        Some((key, last_block))
+    }
+
+    /// Appends a block to the back of the specified file entry.
+    pub(super) fn insert(&mut self, file_id: u64, block: CachedBlock) {
+        self.0.entry(file_id).or_default().push_back(block);
+    }
+
+    /// Inserts the blocks from the notification into the cache with the given file ID.
+    ///
+    /// First, inserts the reverted blocks (if any), then the committed blocks (if any).
+    pub(super) fn insert_notification_blocks_with_file_id(
+        &mut self,
+        file_id: u64,
+        notification: &ExExNotification,
+    ) {
+        let reverted_chain = notification.reverted_chain();
+        let committed_chain = notification.committed_chain();
+
+        if let Some(reverted_chain) = reverted_chain {
+            for block in reverted_chain.blocks().values() {
+                self.insert(
+                    file_id,
+                    CachedBlock {
+                        action: CachedBlockAction::Revert,
+                        block: (block.number, block.hash()).into(),
+                    },
+                );
+            }
+        }
+
+        if let Some(committed_chain) = committed_chain {
+            for block in committed_chain.blocks().values() {
+                self.insert(
+                    file_id,
+                    CachedBlock {
+                        action: CachedBlockAction::Commit,
+                        block: (block.number, block.hash()).into(),
+                    },
+                );
+            }
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(super) struct CachedBlock {
+    pub(super) action: CachedBlockAction,
+    /// The block number and hash of the block.
+    pub(super) block: BlockNumHash,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(super) enum CachedBlockAction {
+    Commit,
+    Revert,
+}
+
+impl CachedBlockAction {
+    pub(super) const fn is_commit(&self) -> bool {
+        matches!(self, Self::Commit)
+    }
+}

crates/exex/exex/src/wal/mod.rs

@@ -0,0 +1,492 @@
+#![allow(dead_code)]
+
+mod cache;
+mod storage;
+
+use std::path::Path;
+
+use cache::BlockCache;
+use reth_exex_types::ExExNotification;
+use reth_primitives::BlockNumHash;
+use reth_tracing::tracing::{debug, instrument};
+use storage::Storage;
+
+/// WAL is a write-ahead log (WAL) that stores the notifications sent to a particular ExEx.
+///
+/// WAL is backed by a directory of binary files represented by [`Storage`] and a block cache
+/// represented by [`BlockCache`]. The role of the block cache is to avoid walking the WAL directory
+/// and decoding notifications every time we want to rollback/finalize the WAL.
+///
+/// The expected mode of operation is as follows:
+/// 1. On every new canonical chain notification, call [`Wal::commit`].
+/// 2. When ExEx is on a wrong fork, rollback the WAL using [`Wal::rollback`]. The caller is
+///    expected to create reverts from the removed notifications and backfill the blocks between the
+///    returned block and the given rollback block. After that, commit new notifications as usual
+///    with [`Wal::commit`].
+/// 3. When the chain is finalized, call [`Wal::finalize`] to prevent the infinite growth of the
+///    WAL.
+#[derive(Debug)]
+pub(crate) struct Wal {
+    /// The underlying WAL storage backed by a file.
+    storage: Storage,
+    /// WAL block cache. See [`cache::BlockCache`] docs for more details.
+    block_cache: BlockCache,
+}
+
+impl Wal {
+    /// Creates a new instance of [`Wal`].
+    pub(crate) fn new(directory: impl AsRef<Path>) -> eyre::Result<Self> {
+        let mut wal = Self { storage: Storage::new(directory)?, block_cache: BlockCache::new() };
+        wal.fill_block_cache()?;
+        Ok(wal)
+    }
+
+    /// Fills the block cache with the notifications from the storage.
+    #[instrument(target = "exex::wal", skip(self))]
+    fn fill_block_cache(&mut self) -> eyre::Result<()> {
+        let Some(files_range) = self.storage.files_range()? else { return Ok(()) };
+
+        for entry in self.storage.iter_notifications(files_range) {
+            let (file_id, notification) = entry?;
+
+            let committed_chain = notification.committed_chain();
+            let reverted_chain = notification.reverted_chain();
+
+            debug!(
+                target: "exex::wal",
+                ?file_id,
+                reverted_block_range = ?reverted_chain.as_ref().map(|chain| chain.range()),
+                committed_block_range = ?committed_chain.as_ref().map(|chain| chain.range()),
+                "Inserting block cache entries"
+            );
+
+            self.block_cache.insert_notification_blocks_with_file_id(file_id, &notification);
+        }
+
+        Ok(())
+    }
+
+    /// Commits the notification to WAL.
+    #[instrument(target = "exex::wal", skip_all, fields(
+        reverted_block_range = ?notification.reverted_chain().as_ref().map(|chain| chain.range()),
+        committed_block_range = ?notification.committed_chain().as_ref().map(|chain| chain.range())
+    ))]
+    pub(crate) fn commit(&mut self, notification: &ExExNotification) -> eyre::Result<()> {
+        debug!("Writing notification to WAL");
+        let file_id = self.block_cache.back().map_or(0, |block| block.0 + 1);
+        self.storage.write_notification(file_id, notification)?;
+
+        debug!(?file_id, "Inserting notification blocks into the block cache");
+        self.block_cache.insert_notification_blocks_with_file_id(file_id, notification);
+
+        Ok(())
+    }
+
+    /// Rollbacks the WAL to the given block, inclusive.
+    ///
+    /// 1. Walks the WAL from the end and searches for the first notification where committed chain
+    ///    contains a block with the same number and hash as `to_block`.
+    /// 2. If the notification is found, truncates the WAL. It means that if the found notification
+    ///    contains both given block and blocks before it, the whole notification will be truncated.
+    ///
+    /// # Returns
+    ///
+    /// 1. The block number and hash of the lowest removed block.
+    /// 2. The notifications that were removed.
+    #[instrument(target = "exex::wal", skip(self))]
+    pub(crate) fn rollback(
+        &mut self,
+        to_block: BlockNumHash,
+    ) -> eyre::Result<Option<(BlockNumHash, Vec<ExExNotification>)>> {
+        // First, pop items from the back of the cache until we find the notification with the
+        // specified block. When found, save the file ID of that notification.
+        let mut remove_from_file_id = None;
+        let mut remove_to_file_id = None;
+        let mut lowest_removed_block = None;
+        while let Some((file_id, block)) = self.block_cache.pop_back() {
+            debug!(?file_id, ?block, "Popped back block from the block cache");
+            if block.action.is_commit() && block.block.number == to_block.number {
+                debug!(
+                    ?file_id,
+                    ?block,
+                    ?remove_from_file_id,
+                    ?lowest_removed_block,
+                    "Found the requested block"
+                );
+
+                if block.block.hash != to_block.hash {
+                    eyre::bail!("block hash mismatch in WAL")
+                }
+
+                remove_from_file_id = Some(file_id);
+
+                let notification = self.storage.read_notification(file_id)?;
+                lowest_removed_block = notification
+                    .committed_chain()
+                    .as_ref()
+                    .map(|chain| chain.first())
+                    .map(|block| (block.number, block.hash()).into());
+
+                break
+            }
+
+            remove_from_file_id = Some(file_id);
+            remove_to_file_id.get_or_insert(file_id);
+        }
+
+        // If the specified block is still not found, we can't do anything and just return. The
+        // cache was empty.
+        let Some((remove_from_file_id, remove_to_file_id)) =
+            remove_from_file_id.zip(remove_to_file_id)
+        else {
+            debug!("No blocks were rolled back");
+            return Ok(None)
+        };
+
+        // Remove the rest of the block cache entries for the file ID that we found.
+        self.block_cache.remove_notification(remove_from_file_id);
+        debug!(?remove_from_file_id, "Block cache was rolled back");
+
+        // Remove notifications from the storage.
+        let removed_notifications =
+            self.storage.take_notifications(remove_from_file_id..=remove_to_file_id)?;
+        debug!(removed_notifications = ?removed_notifications.len(), "Storage was rolled back");
+
+        Ok(Some((lowest_removed_block.expect("qed"), removed_notifications)))
+    }
+
+    /// Finalizes the WAL to the given block, inclusive.
+    ///
+    /// 1. Finds a notification with first unfinalized block (first notification containing a
+    ///    committed block higher than `to_block`).
+    /// 2. Removes the notifications from the beginning of WAL until the found notification. If this
+    ///    notification includes both finalized and non-finalized blocks, it will not be removed.
+    #[instrument(target = "exex::wal", skip(self))]
+    pub(crate) fn finalize(&mut self, to_block: BlockNumHash) -> eyre::Result<()> {
+        // First, walk cache to find the file ID of the notification with the finalized block and
+        // save the file ID with the last unfinalized block. Do not remove any notifications
+        // yet.
+        let mut unfinalized_from_file_id = None;
+        {
+            let mut block_cache = self.block_cache.iter().peekable();
+            while let Some((file_id, block)) = block_cache.next() {
+                debug!(?file_id, ?block, "Iterating over the block cache");
+                if block.action.is_commit() &&
+                    block.block.number == to_block.number &&
+                    block.block.hash == to_block.hash
+                {
+                    let notification = self.storage.read_notification(file_id)?;
+                    if notification.committed_chain().unwrap().blocks().len() == 1 {
+                        unfinalized_from_file_id = block_cache.peek().map(|(file_id, _)| *file_id);
+                    } else {
+                        unfinalized_from_file_id = Some(file_id);
+                    }
+
+                    debug!(
+                        ?file_id,
+                        ?block,
+                        ?unfinalized_from_file_id,
+                        "Found the finalized block in the block cache"
+                    );
+                    break
+                }
+
+                unfinalized_from_file_id = Some(file_id);
+            }
+        }
+
+        // If the finalized block is still not found, we can't do anything and just return.
+        let Some(remove_to_file_id) = unfinalized_from_file_id else {
+            debug!("Could not find the finalized block in WAL");
+            return Ok(())
+        };
+
+        // Remove notifications from the storage from the beginning up to the unfinalized block, not
+        // inclusive.
+        let (mut file_range_start, mut file_range_end) = (None, None);
+        while let Some((file_id, _)) = self.block_cache.front() {
+            if file_id == remove_to_file_id {
+                break
+            }
+            self.block_cache.pop_front();
+
+            file_range_start.get_or_insert(file_id);
+            file_range_end = Some(file_id);
+        }
+        debug!(?remove_to_file_id, "Block cache was finalized");
+
+        // Remove notifications from the storage.
+        if let Some((file_range_start, file_range_end)) = file_range_start.zip(file_range_end) {
+            let removed_notifications =
+                self.storage.remove_notifications(file_range_start..=file_range_end)?;
+            debug!(?removed_notifications, "Storage was finalized");
+        } else {
+            debug!("No notifications were finalized from the storage");
+        }
+
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+
+    use eyre::OptionExt;
+    use reth_exex_types::ExExNotification;
+    use reth_provider::Chain;
+    use reth_testing_utils::generators::{
+        self, random_block, random_block_range, BlockParams, BlockRangeParams,
+    };
+
+    use crate::wal::{
+        cache::{CachedBlock, CachedBlockAction},
+        Wal,
+    };
+
+    fn read_notifications(wal: &Wal) -> eyre::Result<Vec<ExExNotification>> {
+        let Some(files_range) = wal.storage.files_range()? else { return Ok(Vec::new()) };
+
+        wal.storage
+            .iter_notifications(files_range)
+            .map(|entry| Ok(entry?.1))
+            .collect::<eyre::Result<_>>()
+    }
+
+    #[test]
+    fn test_wal() -> eyre::Result<()> {
+        reth_tracing::init_test_tracing();
+
+        let mut rng = generators::rng();
+
+        // Create an instance of the WAL in a temporary directory
+        let temp_dir = tempfile::tempdir()?;
+        let mut wal = Wal::new(&temp_dir)?;
+        assert!(wal.block_cache.is_empty());
+
+        // Create 4 canonical blocks and one reorged block with number 2
+        let blocks = random_block_range(&mut rng, 0..=3, BlockRangeParams::default())
+            .into_iter()
+            .map(|block| block.seal_with_senders().ok_or_eyre("failed to recover senders"))
+            .collect::<eyre::Result<Vec<_>>>()?;
+        let block_1_reorged = random_block(
+            &mut rng,
+            1,
+            BlockParams { parent: Some(blocks[0].hash()), ..Default::default() },
+        )
+        .seal_with_senders()
+        .ok_or_eyre("failed to recover senders")?;
+        let block_2_reorged = random_block(
+            &mut rng,
+            2,
+            BlockParams { parent: Some(blocks[1].hash()), ..Default::default() },
+        )
+        .seal_with_senders()
+        .ok_or_eyre("failed to recover senders")?;
+
+        // Create notifications for the above blocks.
+        // 1. Committed notification for blocks with number 0 and 1
+        // 2. Reverted notification for block with number 1
+        // 3. Committed notification for block with number 1 and 2
+        // 4. Reorged notification for block with number 2 that was reverted, and blocks with number
+        //    2 and 3 that were committed
+        let committed_notification_1 = ExExNotification::ChainCommitted {
+            new: Arc::new(Chain::new(
+                vec![blocks[0].clone(), blocks[1].clone()],
+                Default::default(),
+                None,
+            )),
+        };
+        let reverted_notification = ExExNotification::ChainReverted {
+            old: Arc::new(Chain::new(vec![blocks[1].clone()], Default::default(), None)),
+        };
+        let committed_notification_2 = ExExNotification::ChainCommitted {
+            new: Arc::new(Chain::new(
+                vec![block_1_reorged.clone(), blocks[2].clone()],
+                Default::default(),
+                None,
+            )),
+        };
+        let reorged_notification = ExExNotification::ChainReorged {
+            old: Arc::new(Chain::new(vec![blocks[2].clone()], Default::default(), None)),
+            new: Arc::new(Chain::new(
+                vec![block_2_reorged.clone(), blocks[3].clone()],
+                Default::default(),
+                None,
+            )),
+        };
+
+        // Commit notifications, verify that the block cache is updated and the notifications are
+        // written to WAL.
+
+        // First notification (commit block 0, 1)
+        let file_id = 0;
+        let committed_notification_1_cache = vec![
+            (
+                file_id,
+                CachedBlock {
+                    action: CachedBlockAction::Commit,
+                    block: (blocks[0].number, blocks[0].hash()).into(),
+                },
+            ),
+            (
+                file_id,
+                CachedBlock {
+                    action: CachedBlockAction::Commit,
+                    block: (blocks[1].number, blocks[1].hash()).into(),
+                },
+            ),
+        ];
+        wal.commit(&committed_notification_1)?;
+        assert_eq!(wal.block_cache.iter().collect::<Vec<_>>(), committed_notification_1_cache);
+        assert_eq!(read_notifications(&wal)?, vec![committed_notification_1.clone()]);
+
+        // Second notification (revert block 1)
+        wal.commit(&reverted_notification)?;
+        let file_id = 1;
+        let reverted_notification_cache = vec![(
+            file_id,
+            CachedBlock {
+                action: CachedBlockAction::Revert,
+                block: (blocks[1].number, blocks[1].hash()).into(),
+            },
+        )];
+        assert_eq!(
+            wal.block_cache.iter().collect::<Vec<_>>(),
+            [committed_notification_1_cache.clone(), reverted_notification_cache.clone()].concat()
+        );
+        assert_eq!(
+            read_notifications(&wal)?,
+            vec![committed_notification_1.clone(), reverted_notification.clone()]
+        );
+
+        // Now, rollback to block 1 and verify that both the block cache and the storage are
+        // empty. We expect the rollback to delete the first notification (commit block 0, 1),
+        // because we can't delete blocks partly from the notification, and also the second
+        // notification (revert block 1). Additionally, check that the block that the rolled
+        // back to is the block with number 0.
+        let rollback_result = wal.rollback((blocks[1].number, blocks[1].hash()).into())?;
+        assert_eq!(wal.block_cache.iter().collect::<Vec<_>>(), vec![]);
+        assert_eq!(read_notifications(&wal)?, vec![]);
+        assert_eq!(
+            rollback_result,
+            Some((
+                (blocks[0].number, blocks[0].hash()).into(),
+                vec![committed_notification_1.clone(), reverted_notification.clone()]
+            ))
+        );
+
+        // Commit notifications 1 and 2 again
+        wal.commit(&committed_notification_1)?;
+        assert_eq!(
+            wal.block_cache.iter().collect::<Vec<_>>(),
+            [committed_notification_1_cache.clone()].concat()
+        );
+        assert_eq!(read_notifications(&wal)?, vec![committed_notification_1.clone()]);
+        wal.commit(&reverted_notification)?;
+        assert_eq!(
+            wal.block_cache.iter().collect::<Vec<_>>(),
+            [committed_notification_1_cache.clone(), reverted_notification_cache.clone()].concat()
+        );
+        assert_eq!(
+            read_notifications(&wal)?,
+            vec![committed_notification_1.clone(), reverted_notification.clone()]
+        );
+
+        // Third notification (commit block 1, 2)
+        wal.commit(&committed_notification_2)?;
+        let file_id = 2;
+        let committed_notification_2_cache = vec![
+            (
+                file_id,
+                CachedBlock {
+                    action: CachedBlockAction::Commit,
+                    block: (block_1_reorged.number, block_1_reorged.hash()).into(),
+                },
+            ),
+            (
+                file_id,
+                CachedBlock {
+                    action: CachedBlockAction::Commit,
+                    block: (blocks[2].number, blocks[2].hash()).into(),
+                },
+            ),
+        ];
+        assert_eq!(
+            wal.block_cache.iter().collect::<Vec<_>>(),
+            [
+                committed_notification_1_cache.clone(),
+                reverted_notification_cache.clone(),
+                committed_notification_2_cache.clone()
+            ]
+            .concat()
+        );
+        assert_eq!(
+            read_notifications(&wal)?,
+            vec![
+                committed_notification_1.clone(),
+                reverted_notification.clone(),
+                committed_notification_2.clone()
+            ]
+        );
+
+        // Fourth notification (revert block 2, commit block 2, 3)
+        wal.commit(&reorged_notification)?;
+        let file_id = 3;
+        let reorged_notification_cache = vec![
+            (
+                file_id,
+                CachedBlock {
+                    action: CachedBlockAction::Revert,
+                    block: (blocks[2].number, blocks[2].hash()).into(),
+                },
+            ),
+            (
+                file_id,
+                CachedBlock {
+                    action: CachedBlockAction::Commit,
+                    block: (block_2_reorged.number, block_2_reorged.hash()).into(),
+                },
+            ),
+            (
+                file_id,
+                CachedBlock {
+                    action: CachedBlockAction::Commit,
+                    block: (blocks[3].number, blocks[3].hash()).into(),
+                },
+            ),
+        ];
+        assert_eq!(
+            wal.block_cache.iter().collect::<Vec<_>>(),
+            [
+                committed_notification_1_cache,
+                reverted_notification_cache,
+                committed_notification_2_cache.clone(),
+                reorged_notification_cache.clone()
+            ]
+            .concat()
+        );
+        assert_eq!(
+            read_notifications(&wal)?,
+            vec![
+                committed_notification_1.clone(),
+                reverted_notification.clone(),
+                committed_notification_2.clone(),
+                reorged_notification.clone()
+            ]
+        );
+
+        // Now, finalize the WAL up to the block 1. Block 1 was in the third notification that also
+        // had block 2 committed. In this case, we can't split the notification into two parts, so
+        // we preserve the whole notification in both the block cache and the storage, and delete
+        // the notifications before it.
+        wal.finalize((block_1_reorged.number, block_1_reorged.hash()).into())?;
+        assert_eq!(
+            wal.block_cache.iter().collect::<Vec<_>>(),
+            [committed_notification_2_cache, reorged_notification_cache].concat()
+        );
+        assert_eq!(read_notifications(&wal)?, vec![committed_notification_2, reorged_notification]);
+
+        Ok(())
+    }
+}

crates/exex/exex/src/wal/storage.rs

@@ -0,0 +1,186 @@
+use std::{
+    fs::File,
+    io::{Read, Write},
+    ops::RangeInclusive,
+    path::{Path, PathBuf},
+};
+
+use eyre::OptionExt;
+use reth_exex_types::ExExNotification;
+use reth_tracing::tracing::debug;
+use tracing::instrument;
+
+/// The underlying WAL storage backed by a directory of files.
+///
+/// Each notification is represented by a single file that contains a MessagePack-encoded
+/// notification.
+#[derive(Debug)]
+pub(super) struct Storage {
+    /// The path to the WAL file.
+    path: PathBuf,
+}
+
+impl Storage {
+    /// Creates a new instance of [`Storage`] backed by the file at the given path and creates
+    /// it doesn't exist.
+    pub(super) fn new(path: impl AsRef<Path>) -> eyre::Result<Self> {
+        reth_fs_util::create_dir_all(&path)?;
+
+        Ok(Self { path: path.as_ref().to_path_buf() })
+    }
+
+    fn file_path(&self, id: u64) -> PathBuf {
+        self.path.join(format!("{id}.wal"))
+    }
+
+    fn parse_filename(filename: &str) -> eyre::Result<u64> {
+        filename
+            .strip_suffix(".wal")
+            .and_then(|s| s.parse().ok())
+            .ok_or_eyre(format!("failed to parse file name: {filename}"))
+    }
+
+    /// Removes notification for the given file ID from the storage.
+    #[instrument(target = "exex::wal::storage", skip(self))]
+    fn remove_notification(&self, file_id: u64) {
+        match reth_fs_util::remove_file(self.file_path(file_id)) {
+            Ok(()) => debug!("Notification was removed from the storage"),
+            Err(err) => debug!(?err, "Failed to remove notification from the storage"),
+        }
+    }
+
+    /// Returns the range of file IDs in the storage.
+    ///
+    /// If there are no files in the storage, returns `None`.
+    pub(super) fn files_range(&self) -> eyre::Result<Option<RangeInclusive<u64>>> {
+        let mut min_id = None;
+        let mut max_id = None;
+
+        for entry in reth_fs_util::read_dir(&self.path)? {
+            let entry = entry?;
+            let file_name = entry.file_name();
+            let file_id = Self::parse_filename(&file_name.to_string_lossy())?;
+
+            min_id = min_id.map_or(Some(file_id), |min_id: u64| Some(min_id.min(file_id)));
+            max_id = max_id.map_or(Some(file_id), |max_id: u64| Some(max_id.max(file_id)));
+        }
+
+        Ok(min_id.zip(max_id).map(|(min_id, max_id)| min_id..=max_id))
+    }
+
+    /// Removes notifications from the storage according to the given range.
+    ///
+    /// # Returns
+    ///
+    /// Number of removed notifications.
+    pub(super) fn remove_notifications(&self, range: RangeInclusive<u64>) -> eyre::Result<usize> {
+        for id in range.clone() {
+            self.remove_notification(id);
+        }
+
+        Ok(range.count())
+    }
+
+    /// Removes notifications from the storage according to the given range.
+    ///
+    /// # Returns
+    ///
+    /// Notifications that were removed.
+    pub(super) fn take_notifications(
+        &self,
+        range: RangeInclusive<u64>,
+    ) -> eyre::Result<Vec<ExExNotification>> {
+        let notifications = self.iter_notifications(range).collect::<eyre::Result<Vec<_>>>()?;
+
+        for (id, _) in &notifications {
+            self.remove_notification(*id);
+        }
+
+        Ok(notifications.into_iter().map(|(_, notification)| notification).collect())
+    }
+
+    pub(super) fn iter_notifications(
+        &self,
+        range: RangeInclusive<u64>,
+    ) -> impl Iterator<Item = eyre::Result<(u64, ExExNotification)>> + '_ {
+        range.map(move |id| self.read_notification(id).map(|notification| (id, notification)))
+    }
+
+    /// Reads the notification from the file with the given id.
+    pub(super) fn read_notification(&self, file_id: u64) -> eyre::Result<ExExNotification> {
+        debug!(?file_id, "Reading notification from WAL");
+
+        let file_path = self.file_path(file_id);
+        let mut file = File::open(&file_path)?;
+        read_notification(&mut file)
+    }
+
+    /// Writes the notification to the file with the given id.
+    pub(super) fn write_notification(
+        &self,
+        file_id: u64,
+        notification: &ExExNotification,
+    ) -> eyre::Result<()> {
+        debug!(?file_id, "Writing notification to WAL");
+
+        let file_path = self.file_path(file_id);
+        let mut file = File::create_new(&file_path)?;
+        write_notification(&mut file, notification)?;
+
+        Ok(())
+    }
+}
+
+// TODO(alexey): use rmp-serde when Alloy and Reth serde issues are resolved
+
+fn write_notification(mut w: &mut impl Write, notification: &ExExNotification) -> eyre::Result<()> {
+    // rmp_serde::encode::write(w, notification)?;
+    serde_json::to_writer(&mut w, notification)?;
+    w.flush()?;
+    Ok(())
+}
+
+fn read_notification(r: &mut impl Read) -> eyre::Result<ExExNotification> {
+    // Ok(rmp_serde::from_read(r)?)
+    Ok(serde_json::from_reader(r)?)
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+
+    use eyre::OptionExt;
+    use reth_exex_types::ExExNotification;
+    use reth_provider::Chain;
+    use reth_testing_utils::generators::{self, random_block};
+
+    use super::Storage;
+
+    #[test]
+    fn test_roundtrip() -> eyre::Result<()> {
+        let mut rng = generators::rng();
+
+        let temp_dir = tempfile::tempdir()?;
+        let storage = Storage::new(&temp_dir)?;
+
+        let old_block = random_block(&mut rng, 0, Default::default())
+            .seal_with_senders()
+            .ok_or_eyre("failed to recover senders")?;
+        let new_block = random_block(&mut rng, 0, Default::default())
+            .seal_with_senders()
+            .ok_or_eyre("failed to recover senders")?;
+
+        let notification = ExExNotification::ChainReorged {
+            new: Arc::new(Chain::new(vec![new_block], Default::default(), None)),
+            old: Arc::new(Chain::new(vec![old_block], Default::default(), None)),
+        };
+
+        // Do a round trip serialization and deserialization
+        let file_id = 0;
+        storage.write_notification(file_id, &notification)?;
+        let deserialized_notification = storage.read_notification(file_id)?;
+        assert_eq!(deserialized_notification, notification);
+
+        Ok(())
+    }
+}