Refactor model stuff, including cleaner CRDTs

src/api/s3_copy.rs

@@ -66,25 +66,28 @@ pub async fn handle_copy(
 				.await?;
 			let source_version = source_version.ok_or(Error::NotFound)?;
 
-			let dest_version = Version::new(
+			let mut dest_version = Version::new(
 				new_uuid,
 				dest_bucket.to_string(),
 				dest_key.to_string(),
 				false,
-				source_version.blocks().to_vec(),
 			);
+			for (bk, bv) in source_version.blocks.items().iter() {
+				dest_version.blocks.put(*bk, *bv);
+			}
 			let dest_object = Object::new(
 				dest_bucket.to_string(),
 				dest_key.to_string(),
 				vec![dest_object_version],
 			);
 			let dest_block_refs = dest_version
-				.blocks()
+				.blocks
+				.items()
 				.iter()
 				.map(|b| BlockRef {
-					block: b.hash,
+					block: b.1.hash,
 					version: new_uuid,
-					deleted: false,
+					deleted: false.into(),
 				})
 				.collect::<Vec<_>>();
 			futures::try_join!(

src/api/s3_get.rs

@@ -146,9 +146,10 @@ pub async fn handle_get(
 			let version = version.ok_or(Error::NotFound)?;
 
 			let mut blocks = version
-				.blocks()
+				.blocks
+				.items()
 				.iter()
-				.map(|vb| (vb.hash, None))
+				.map(|(_, vb)| (vb.hash, None))
 				.collect::<Vec<_>>();
 			blocks[0].1 = Some(first_block);
 
@@ -219,11 +220,12 @@ pub async fn handle_get_range(
 			// file (whereas block.offset designates the offset of the block WITHIN THE PART
 			// block.part_number, which is not the same in the case of a multipart upload)
 			let mut blocks = Vec::with_capacity(std::cmp::min(
-				version.blocks().len(),
+				version.blocks.len(),
-				4 + ((end - begin) / std::cmp::max(version.blocks()[0].size as u64, 1024)) as usize,
+				4 + ((end - begin) / std::cmp::max(version.blocks.items()[0].1.size as u64, 1024))
+					as usize,
 			));
 			let mut true_offset = 0;
-			for b in version.blocks().iter() {
+			for (_, b) in version.blocks.items().iter() {
 				if true_offset >= end {
 					break;
 				}

src/api/s3_put.rs

@@ -94,7 +94,7 @@ pub async fn handle_put(
 	garage.object_table.insert(&object).await?;
 
 	// Initialize corresponding entry in version table
-	let version = Version::new(version_uuid, bucket.into(), key.into(), false, vec![]);
+	let version = Version::new(version_uuid, bucket.into(), key.into(), false);
 	let first_block_hash = sha256sum(&first_block[..]);
 
 	// Transfer data and verify checksum
@@ -242,19 +242,18 @@ async fn put_block_meta(
 ) -> Result<(), GarageError> {
 	// TODO: don't clone, restart from empty block list ??
 	let mut version = version.clone();
-	version
+	version.blocks.put(
-		.add_block(VersionBlock {
+		VersionBlockKey {
 			part_number,
 			offset,
-			hash,
+		},
-			size,
+		VersionBlock { hash, size },
-		})
+	);
-		.unwrap();
 
 	let block_ref = BlockRef {
 		block: hash,
 		version: version.uuid,
-		deleted: false,
+		deleted: false.into(),
 	};
 
 	futures::try_join!(
@@ -389,7 +388,7 @@ pub async fn handle_put_part(
 	}
 
 	// Copy block to store
-	let version = Version::new(version_uuid, bucket, key, false, vec![]);
+	let version = Version::new(version_uuid, bucket, key, false);
 	let first_block_hash = sha256sum(&first_block[..]);
 	let (_, md5sum_arr, sha256sum) = read_and_put_blocks(
 		&garage,
@@ -454,7 +453,7 @@ pub async fn handle_complete_multipart_upload(
 	};
 
 	let version = version.ok_or(Error::BadRequest(format!("Version not found")))?;
-	if version.blocks().len() == 0 {
+	if version.blocks.len() == 0 {
 		return Err(Error::BadRequest(format!("No data was uploaded")));
 	}
 
@@ -466,9 +465,10 @@ pub async fn handle_complete_multipart_upload(
 	// Check that the list of parts they gave us corresponds to the parts we have here
 	// TODO: check MD5 sum of all uploaded parts? but that would mean we have to store them somewhere...
 	let mut parts = version
-		.blocks()
+		.blocks
+		.items()
 		.iter()
-		.map(|x| x.part_number)
+		.map(|x| x.0.part_number)
 		.collect::<Vec<_>>();
 	parts.dedup();
 	let same_parts = body_list_of_parts
@@ -485,8 +485,8 @@ pub async fn handle_complete_multipart_upload(
 	// shouldn't impact compatibility as the S3 docs specify that
 	// the ETag is an opaque value in case of a multipart upload.
 	// See also: https://teppen.io/2018/06/23/aws_s3_etags/
-	let num_parts = version.blocks().last().unwrap().part_number
+	let num_parts = version.blocks.items().last().unwrap().0.part_number
-		- version.blocks().first().unwrap().part_number
+		- version.blocks.items().first().unwrap().0.part_number
 		+ 1;
 	let etag = format!(
 		"{}-{}",
@@ -495,17 +495,18 @@ pub async fn handle_complete_multipart_upload(
 	);
 
 	let total_size = version
-		.blocks()
+		.blocks
+		.items()
 		.iter()
-		.map(|x| x.size)
+		.map(|x| x.1.size)
 		.fold(0, |x, y| x + y);
 	object_version.state = ObjectVersionState::Complete(ObjectVersionData::FirstBlock(
 		ObjectVersionMeta {
 			headers,
 			size: total_size,
-			etag: etag,
+			etag,
 		},
-		version.blocks()[0].hash,
+		version.blocks.items()[0].1.hash,
 	));
 
 	let final_object = Object::new(bucket.clone(), key.clone(), vec![object_version]);

src/garage/repair.rs

@@ -97,7 +97,7 @@ impl Repair {
 			pos = item_key.to_vec();
 
 			let version = rmp_serde::decode::from_read_ref::<_, Version>(item_bytes.as_ref())?;
-			if version.deleted {
+			if version.deleted.get() {
 				continue;
 			}
 			let object = self
@@ -127,7 +127,6 @@ impl Repair {
 						version.bucket,
 						version.key,
 						true,
-						vec![],
 					))
 					.await?;
 			}
@@ -146,7 +145,7 @@ impl Repair {
 			pos = item_key.to_vec();
 
 			let block_ref = rmp_serde::decode::from_read_ref::<_, BlockRef>(item_bytes.as_ref())?;
-			if block_ref.deleted {
+			if block_ref.deleted.get() {
 				continue;
 			}
 			let version = self
@@ -155,7 +154,7 @@ impl Repair {
 				.get(&block_ref.version, &EmptyKey)
 				.await?;
 			let ref_exists = match version {
-				Some(v) => !v.deleted,
+				Some(v) => !v.deleted.get(),
 				None => {
 					warn!(
 						"Block ref repair: version for block ref {:?} not found, skipping.",
@@ -174,7 +173,7 @@ impl Repair {
 					.insert(&BlockRef {
 						block: block_ref.block,
 						version: block_ref.version,
-						deleted: true,
+						deleted: true.into(),
 					})
 					.await?;
 			}

src/model/block.rs

@@ -420,7 +420,7 @@ impl BlockManager {
 			if Some(&block_ref.block) == last_hash.as_ref() {
 				continue;
 			}
-			if !block_ref.deleted {
+			if !block_ref.deleted.get() {
 				last_hash = Some(block_ref.block);
 				self.put_to_resync(&block_ref.block, 0)?;
 			}

src/model/block_ref_table.rs

@@ -1,9 +1,9 @@
 use serde::{Deserialize, Serialize};
 use std::sync::Arc;
 
-use garage_util::background::*;
 use garage_util::data::*;
 
+use garage_table::crdt::CRDT;
 use garage_table::*;
 
 use crate::block::*;
@@ -17,7 +17,7 @@ pub struct BlockRef {
 	pub version: UUID,
 
 	// Keep track of deleted status
-	pub deleted: bool,
+	pub deleted: crdt::Bool,
 }
 
 impl Entry<Hash, UUID> for BlockRef {
@@ -27,16 +27,15 @@ impl Entry<Hash, UUID> for BlockRef {
 	fn sort_key(&self) -> &UUID {
 		&self.version
 	}
+}
 
+impl CRDT for BlockRef {
 	fn merge(&mut self, other: &Self) {
-		if other.deleted {
+		self.deleted.merge(&other.deleted);
-			self.deleted = true;
-		}
 	}
 }
 
 pub struct BlockRefTable {
-	pub background: Arc<BackgroundRunner>,
 	pub block_manager: Arc<BlockManager>,
 }
 
@@ -48,8 +47,8 @@ impl TableSchema for BlockRefTable {
 
 	fn updated(&self, old: Option<Self::E>, new: Option<Self::E>) {
 		let block = &old.as_ref().or(new.as_ref()).unwrap().block;
-		let was_before = old.as_ref().map(|x| !x.deleted).unwrap_or(false);
+		let was_before = old.as_ref().map(|x| !x.deleted.get()).unwrap_or(false);
-		let is_after = new.as_ref().map(|x| !x.deleted).unwrap_or(false);
+		let is_after = new.as_ref().map(|x| !x.deleted.get()).unwrap_or(false);
 		if is_after && !was_before {
 			if let Err(e) = self.block_manager.block_incref(block) {
 				warn!("block_incref failed for block {:?}: {}", block, e);
@@ -63,6 +62,6 @@ impl TableSchema for BlockRefTable {
 	}
 
 	fn matches_filter(entry: &Self::E, filter: &Self::Filter) -> bool {
-		filter.apply(entry.deleted)
+		filter.apply(entry.deleted.get())
 	}
 }

src/model/bucket_table.rs

@@ -89,7 +89,9 @@ impl Entry<EmptyKey, String> for Bucket {
 	fn sort_key(&self) -> &String {
 		&self.name
 	}
+}
 
+impl CRDT for Bucket {
 	fn merge(&mut self, other: &Self) {
 		self.state.merge(&other.state);
 	}

src/model/garage.rs

@@ -79,7 +79,6 @@ impl Garage {
 		info!("Initialize block_ref_table...");
 		let block_ref_table = Table::new(
 			BlockRefTable {
-				background: background.clone(),
 				block_manager: block_manager.clone(),
 			},
 			data_rep_param.clone(),

src/model/key_table.rs

@@ -1,6 +1,6 @@
 use serde::{Deserialize, Serialize};
 
-use garage_table::crdt::CRDT;
+use garage_table::crdt::*;
 use garage_table::*;
 
 use model010::key_table as prev;
@@ -66,6 +66,10 @@ pub struct PermissionSet {
 	pub allow_write: bool,
 }
 
+impl AutoCRDT for PermissionSet {
+	const WARN_IF_DIFFERENT: bool = true;
+}
+
 impl Entry<EmptyKey, String> for Key {
 	fn partition_key(&self) -> &EmptyKey {
 		&EmptyKey
@@ -73,18 +77,19 @@ impl Entry<EmptyKey, String> for Key {
 	fn sort_key(&self) -> &String {
 		&self.key_id
 	}
+}
 
+impl CRDT for Key {
 	fn merge(&mut self, other: &Self) {
 		self.name.merge(&other.name);
 		self.deleted.merge(&other.deleted);
 
 		if self.deleted.get() {
 			self.authorized_buckets.clear();
-			return;
+		} else {
-		}
-
 			self.authorized_buckets.merge(&other.authorized_buckets);
 		}
+	}
 }
 
 pub struct KeyTable;

src/model/object_table.rs

@@ -5,6 +5,7 @@ use std::sync::Arc;
 use garage_util::background::BackgroundRunner;
 use garage_util::data::*;
 
+use garage_table::crdt::*;
 use garage_table::table_sharded::*;
 use garage_table::*;
 
@@ -70,7 +71,7 @@ pub enum ObjectVersionState {
 	Aborted,
 }
 
-impl ObjectVersionState {
+impl CRDT for ObjectVersionState {
 	fn merge(&mut self, other: &Self) {
 		use ObjectVersionState::*;
 		match other {
@@ -91,37 +92,30 @@ impl ObjectVersionState {
 	}
 }
 
-#[derive(PartialEq, Clone, Debug, Serialize, Deserialize)]
+#[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Debug, Serialize, Deserialize)]
 pub enum ObjectVersionData {
 	DeleteMarker,
 	Inline(ObjectVersionMeta, #[serde(with = "serde_bytes")] Vec<u8>),
 	FirstBlock(ObjectVersionMeta, Hash),
 }
 
-#[derive(PartialEq, Clone, Debug, Serialize, Deserialize)]
+impl AutoCRDT for ObjectVersionData {
+	const WARN_IF_DIFFERENT: bool = true;
+}
+
+#[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Debug, Serialize, Deserialize)]
 pub struct ObjectVersionMeta {
 	pub headers: ObjectVersionHeaders,
 	pub size: u64,
 	pub etag: String,
 }
 
-#[derive(PartialEq, Clone, Debug, Serialize, Deserialize)]
+#[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Debug, Serialize, Deserialize)]
 pub struct ObjectVersionHeaders {
 	pub content_type: String,
 	pub other: BTreeMap<String, String>,
 }
 
-impl ObjectVersionData {
-	fn merge(&mut self, b: &Self) {
-		if *self != *b {
-			warn!(
-				"Inconsistent object version data: {:?} (local) vs {:?} (remote)",
-				self, b
-			);
-		}
-	}
-}
-
 impl ObjectVersion {
 	fn cmp_key(&self) -> (u64, UUID) {
 		(self.timestamp, self.uuid)
@@ -154,8 +148,11 @@ impl Entry<String, String> for Object {
 	fn sort_key(&self) -> &String {
 		&self.key
 	}
+}
 
+impl CRDT for Object {
 	fn merge(&mut self, other: &Self) {
+		// Merge versions from other into here
 		for other_v in other.versions.iter() {
 			match self
 				.versions
@@ -169,6 +166,9 @@ impl Entry<String, String> for Object {
 				}
 			}
 		}
+
+		// Remove versions which are obsolete, i.e. those that come
+		// before the last version which .is_complete().
 		let last_complete = self
 			.versions
 			.iter()
@@ -212,13 +212,8 @@ impl TableSchema for ObjectTable {
 						}
 					};
 					if newly_deleted {
-						let deleted_version = Version::new(
+						let deleted_version =
-							v.uuid,
+							Version::new(v.uuid, old_v.bucket.clone(), old_v.key.clone(), true);
-							old_v.bucket.clone(),
-							old_v.key.clone(),
-							true,
-							vec![],
-						);
 						version_table.insert(&deleted_version).await?;
 					}
 				}

src/model/version_table.rs

@@ -4,6 +4,7 @@ use std::sync::Arc;
 use garage_util::background::BackgroundRunner;
 use garage_util::data::*;
 
+use garage_table::crdt::*;
 use garage_table::table_sharded::*;
 use garage_table::*;
 
@@ -15,8 +16,8 @@ pub struct Version {
 	pub uuid: UUID,
 
 	// Actual data: the blocks for this version
-	pub deleted: bool,
+	pub deleted: crdt::Bool,
-	blocks: Vec<VersionBlock>,
+	pub blocks: crdt::Map<VersionBlockKey, VersionBlock>,
 
 	// Back link to bucket+key so that we can figure if
 	// this was deleted later on
@@ -25,56 +26,45 @@ pub struct Version {
 }
 
 impl Version {
-	pub fn new(
+	pub fn new(uuid: UUID, bucket: String, key: String, deleted: bool) -> Self {
-		uuid: UUID,
+		Self {
-		bucket: String,
-		key: String,
-		deleted: bool,
-		blocks: Vec<VersionBlock>,
-	) -> Self {
-		let mut ret = Self {
 			uuid,
-			deleted,
+			deleted: deleted.into(),
-			blocks: vec![],
+			blocks: crdt::Map::new(),
 			bucket,
 			key,
-		};
-		for b in blocks {
-			ret.add_block(b)
-				.expect("Twice the same VersionBlock in Version constructor");
 		}
-		ret
-	}
-	/// Adds a block if it wasn't already present
-	pub fn add_block(&mut self, new: VersionBlock) -> Result<(), ()> {
-		match self
-			.blocks
-			.binary_search_by(|b| b.cmp_key().cmp(&new.cmp_key()))
-		{
-			Err(i) => {
-				self.blocks.insert(i, new);
-				Ok(())
-			}
-			Ok(_) => Err(()),
-		}
-	}
-	pub fn blocks(&self) -> &[VersionBlock] {
-		&self.blocks[..]
 	}
 }
 
-#[derive(PartialEq, Clone, Debug, Serialize, Deserialize)]
+#[derive(PartialEq, Eq, Clone, Copy, Debug, Serialize, Deserialize)]
-pub struct VersionBlock {
+pub struct VersionBlockKey {
 	pub part_number: u64,
 	pub offset: u64,
+}
+
+impl Ord for VersionBlockKey {
+	fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+		self.part_number
+			.cmp(&other.part_number)
+			.then(self.offset.cmp(&other.offset))
+	}
+}
+
+impl PartialOrd for VersionBlockKey {
+	fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+		Some(self.cmp(other))
+	}
+}
+
+#[derive(PartialEq, Eq, Ord, PartialOrd, Clone, Copy, Debug, Serialize, Deserialize)]
+pub struct VersionBlock {
 	pub hash: Hash,
 	pub size: u64,
 }
 
-impl VersionBlock {
+impl AutoCRDT for VersionBlock {
-	fn cmp_key(&self) -> (u64, u64) {
+	const WARN_IF_DIFFERENT: bool = true;
-		(self.part_number, self.offset)
-	}
 }
 
 impl Entry<Hash, EmptyKey> for Version {
@@ -84,23 +74,16 @@ impl Entry<Hash, EmptyKey> for Version {
 	fn sort_key(&self) -> &EmptyKey {
 		&EmptyKey
 	}
+}
 
+impl CRDT for Version {
 	fn merge(&mut self, other: &Self) {
-		if other.deleted {
+		self.deleted.merge(&other.deleted);
-			self.deleted = true;
+
+		if self.deleted.get() {
 			self.blocks.clear();
-		} else if !self.deleted {
+		} else {
-			for bi in other.blocks.iter() {
+			self.blocks.merge(&other.blocks);
-				match self
-					.blocks
-					.binary_search_by(|x| x.cmp_key().cmp(&bi.cmp_key()))
-				{
-					Ok(_) => (),
-					Err(pos) => {
-						self.blocks.insert(pos, bi.clone());
-					}
-				}
-			}
 		}
 	}
 }
@@ -121,14 +104,15 @@ impl TableSchema for VersionTable {
 		self.background.spawn(async move {
 			if let (Some(old_v), Some(new_v)) = (old, new) {
 				// Propagate deletion of version blocks
-				if new_v.deleted && !old_v.deleted {
+				if new_v.deleted.get() && !old_v.deleted.get() {
 					let deleted_block_refs = old_v
 						.blocks
+						.items()
 						.iter()
-						.map(|vb| BlockRef {
+						.map(|(_k, vb)| BlockRef {
 							block: vb.hash,
 							version: old_v.uuid,
-							deleted: true,
+							deleted: true.into(),
 						})
 						.collect::<Vec<_>>();
 					block_ref_table.insert_many(&deleted_block_refs[..]).await?;
@@ -139,6 +123,6 @@ impl TableSchema for VersionTable {
 	}
 
 	fn matches_filter(entry: &Self::E, filter: &Self::Filter) -> bool {
-		filter.apply(entry.deleted)
+		filter.apply(entry.deleted.get())
 	}
 }

src/table/crdt.rs

@@ -1,327 +0,0 @@
-//! This package provides a simple implementation of conflict-free replicated data types (CRDTs)
-//!
-//! CRDTs are a type of data structures that do not require coordination.  In other words, we can
-//! edit them in parallel, we will always find a way to merge it.
-//!
-//! A general example is a counter. Its initial value is 0.  Alice and Bob get a copy of the
-//! counter.  Alice does +1 on her copy, she reads 1.  Bob does +3 on his copy, he reads 3.  Now,
-//! it is easy to merge their counters, order does not count: we always get 4.
-//!
-//! Learn more about CRDT [on Wikipedia](https://en.wikipedia.org/wiki/Conflict-free_replicated_data_type)
-
-use serde::{Deserialize, Serialize};
-
-use garage_util::data::*;
-
-/// Definition of a CRDT - all CRDT Rust types implement this.
-///
-/// A CRDT is defined as a merge operator that respects a certain set of axioms.
-///
-/// In particular, the merge operator must be commutative, associative,
-/// idempotent, and monotonic.
-/// In other words, if `a`, `b` and `c` are CRDTs, and `⊔` denotes the merge operator,
-/// the following axioms must apply:
-///
-/// ```text
-/// a ⊔ b = b ⊔ a                   (commutativity)
-/// (a ⊔ b) ⊔ c = a ⊔ (b ⊔ c)       (associativity)
-/// (a ⊔ b) ⊔ b = a ⊔ b             (idempotence)
-/// ```
-///
-/// Moreover, the relationship `≥` defined by `a ≥ b ⇔ ∃c. a = b ⊔ c` must be a partial order.
-/// This implies a few properties such as: if `a ⊔ b ≠ a`, then there is no `c` such that `(a ⊔ b) ⊔ c = a`,
-/// as this would imply a cycle in the partial order.
-pub trait CRDT {
-	/// Merge the two datastructures according to the CRDT rules.
-	/// `self` is modified to contain the merged CRDT value. `other` is not modified.
-	///
-	/// # Arguments
-	///
-	/// * `other` - the other CRDT we wish to merge with
-	fn merge(&mut self, other: &Self);
-}
-
-/// All types that implement `Ord` (a total order) also implement a trivial CRDT
-/// defined by the merge rule: `a ⊔ b = max(a, b)`.
-impl<T> CRDT for T
-where
-	T: Ord + Clone,
-{
-	fn merge(&mut self, other: &Self) {
-		if other > self {
-			*self = other.clone();
-		}
-	}
-}
-
-// ---- LWW Register ----
-
-/// Last Write Win (LWW)
-///
-/// An LWW CRDT associates a timestamp with a value, in order to implement a
-/// time-based reconciliation rule: the most recent write wins.
-/// For completeness, the LWW reconciliation rule must also be defined for two LWW CRDTs
-/// with the same timestamp but different values.
-///
-/// In our case, we add the constraint that the value that is wrapped inside the LWW CRDT must
-/// itself be a CRDT: in the case when the timestamp does not allow us to decide on which value to
-/// keep, the merge rule of the inner CRDT is applied on the wrapped values.  (Note that all types
-/// that implement the `Ord` trait get a default CRDT implemetnation that keeps the maximum value.
-/// This enables us to use LWW directly with primitive data types such as numbers or strings. It is
-/// generally desirable in this case to never explicitly produce LWW values with the same timestamp
-/// but different inner values, as the rule to keep the maximum value isn't generally the desired
-/// semantics.)
-///
-/// As multiple computers clocks are always desynchronized,
-/// when operations are close enough, it is equivalent to
-/// take one copy and drop the other one.
-///
-/// Given that clocks are not too desynchronized, this assumption
-/// is enough for most cases, as there is few chance that two humans
-/// coordonate themself faster than the time difference between two NTP servers.
-///
-/// As a more concret example, let's suppose you want to upload a file
-/// with the same key (path) in the same bucket at the very same time.
-/// For each request, the file will be timestamped by the receiving server
-/// and may differ from what you observed with your atomic clock!
-///
-/// This scheme is used by AWS S3 or Soundcloud and often without knowing
-/// in entreprise when reconciliating databases with ad-hoc scripts.
-#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
-pub struct LWW<T> {
-	ts: u64,
-	v: T,
-}
-
-impl<T> LWW<T>
-where
-	T: CRDT,
-{
-	/// Creates a new CRDT
-	///
-	/// CRDT's internal timestamp is set with current node's clock.
-	pub fn new(value: T) -> Self {
-		Self {
-			ts: now_msec(),
-			v: value,
-		}
-	}
-
-	/// Build a new CRDT from a previous non-compatible one
-	///
-	/// Compared to new, the CRDT's timestamp is not set to now
-	/// but must be set to the previous, non-compatible, CRDT's timestamp.
-	pub fn migrate_from_raw(ts: u64, value: T) -> Self {
-		Self { ts, v: value }
-	}
-
-	/// Update the LWW CRDT while keeping some causal ordering.
-	///
-	/// The timestamp of the LWW CRDT is updated to be the current node's clock
-	/// at time of update, or the previous timestamp + 1 if that's bigger,
-	/// so that the new timestamp is always strictly larger than the previous one.
-	/// This ensures that merging the update with the old value will result in keeping
-	/// the updated value.
-	pub fn update(&mut self, new_value: T) {
-		self.ts = std::cmp::max(self.ts + 1, now_msec());
-		self.v = new_value;
-	}
-
-	/// Get the CRDT value
-	pub fn get(&self) -> &T {
-		&self.v
-	}
-
-	/// Get a mutable reference to the CRDT's value
-	///
-	/// This is usefull to mutate the inside value without changing the LWW timestamp.
-	/// When such mutation is done, the merge between two LWW values is done using the inner
-	/// CRDT's merge operation. This is usefull in the case where the inner CRDT is a large
-	/// data type, such as a map, and we only want to change a single item in the map.
-	/// To do this, we can produce a "CRDT delta", i.e. a LWW that contains only the modification.
-	/// This delta consists in a LWW with the same timestamp, and the map
-	/// inside only contains the updated value.
-	/// The advantage of such a delta is that it is much smaller than the whole map.
-	///
-	/// Avoid using this if the inner data type is a primitive type such as a number or a string,
-	/// as you will then rely on the merge function defined on `Ord` types by keeping the maximum
-	/// of both values.
-	pub fn get_mut(&mut self) -> &mut T {
-		&mut self.v
-	}
-}
-
-impl<T> CRDT for LWW<T>
-where
-	T: Clone + CRDT,
-{
-	fn merge(&mut self, other: &Self) {
-		if other.ts > self.ts {
-			self.ts = other.ts;
-			self.v = other.v.clone();
-		} else if other.ts == self.ts {
-			self.v.merge(&other.v);
-		}
-	}
-}
-
-/// Boolean, where `true` is an absorbing state
-#[derive(Clone, Copy, Debug, Serialize, Deserialize, PartialEq)]
-pub struct Bool(bool);
-
-impl Bool {
-	/// Create a new boolean with the specified value
-	pub fn new(b: bool) -> Self {
-		Self(b)
-	}
-	/// Set the boolean to true
-	pub fn set(&mut self) {
-		self.0 = true;
-	}
-	/// Get the boolean value
-	pub fn get(&self) -> bool {
-		self.0
-	}
-}
-
-impl CRDT for Bool {
-	fn merge(&mut self, other: &Self) {
-		self.0 = self.0 || other.0;
-	}
-}
-
-/// Last Write Win Map
-///
-/// This types defines a CRDT for a map from keys to values.
-/// The values have an associated timestamp, such that the last written value
-/// takes precedence over previous ones. As for the simpler `LWW` type, the value
-/// type `V` is also required to implement the CRDT trait.
-/// We do not encourage mutating the values associated with a given key
-/// without updating the timestamp, in fact at the moment we do not provide a `.get_mut()`
-/// method that would allow that.
-///
-/// Internally, the map is stored as a vector of keys and values, sorted by ascending key order.
-/// This is why the key type `K` must implement `Ord` (and also to ensure a unique serialization,
-/// such that two values can be compared for equality based on their hashes). As a consequence,
-/// insertions take `O(n)` time. This means that LWWMap should be used for reasonably small maps.
-/// However, note that even if we were using a more efficient data structure such as a `BTreeMap`,
-/// the serialization cost `O(n)` would still have to be paid at each modification, so we are
-/// actually not losing anything here.
-#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
-pub struct LWWMap<K, V> {
-	vals: Vec<(K, u64, V)>,
-}
-
-impl<K, V> LWWMap<K, V>
-where
-	K: Ord,
-	V: CRDT,
-{
-	/// Create a new empty map CRDT
-	pub fn new() -> Self {
-		Self { vals: vec![] }
-	}
-	/// Used to migrate from a map defined in an incompatible format. This produces
-	/// a map that contains a single item with the specified timestamp (copied from
-	/// the incompatible format). Do this as many times as you have items to migrate,
-	/// and put them all together using the CRDT merge operator.
-	pub fn migrate_from_raw_item(k: K, ts: u64, v: V) -> Self {
-		Self {
-			vals: vec![(k, ts, v)],
-		}
-	}
-	/// Returns a map that contains a single mapping from the specified key to the specified value.
-	/// This map is a mutator, or a delta-CRDT, such that when it is merged with the original map,
-	/// the previous value will be replaced with the one specified here.
-	/// The timestamp in the provided mutator is set to the maximum of the current system's clock
-	/// and 1 + the previous value's timestamp (if there is one), so that the new value will always
-	/// take precedence (LWW rule).
-	///
-	/// Typically, to update the value associated to a key in the map, you would do the following:
-	///
-	/// ```ignore
-	/// let my_update = my_crdt.update_mutator(key_to_modify, new_value);
-	/// my_crdt.merge(&my_update);
-	/// ```
-	///
-	/// However extracting the mutator on its own and only sending that on the network is very
-	/// interesting as it is much smaller than the whole map.
-	pub fn update_mutator(&self, k: K, new_v: V) -> Self {
-		let new_vals = match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(&k)) {
-			Ok(i) => {
-				let (_, old_ts, _) = self.vals[i];
-				let new_ts = std::cmp::max(old_ts + 1, now_msec());
-				vec![(k, new_ts, new_v)]
-			}
-			Err(_) => vec![(k, now_msec(), new_v)],
-		};
-		Self { vals: new_vals }
-	}
-	/// Takes all of the values of the map and returns them. The current map is reset to the
-	/// empty map. This is very usefull to produce in-place a new map that contains only a delta
-	/// that modifies a certain value:
-	///
-	/// ```ignore
-	/// let mut a = get_my_crdt_value();
-	/// let old_a = a.take_and_clear();
-	/// a.merge(&old_a.update_mutator(key_to_modify, new_value));
-	/// put_my_crdt_value(a);
-	/// ```
-	///
-	/// Of course in this simple example we could have written simply
-	/// `pyt_my_crdt_value(a.update_mutator(key_to_modify, new_value))`,
-	/// but in the case where the map is a field in a struct for instance (as is always the case),
-	/// this becomes very handy:
-	///
-	/// ```ignore
-	/// let mut a = get_my_crdt_value();
-	/// let old_a_map = a.map_field.take_and_clear();
-	/// a.map_field.merge(&old_a_map.update_mutator(key_to_modify, new_value));
-	/// put_my_crdt_value(a);
-	/// ```
-	pub fn take_and_clear(&mut self) -> Self {
-		let vals = std::mem::replace(&mut self.vals, vec![]);
-		Self { vals }
-	}
-	/// Removes all values from the map
-	pub fn clear(&mut self) {
-		self.vals.clear();
-	}
-	/// Get a reference to the value assigned to a key
-	pub fn get(&self, k: &K) -> Option<&V> {
-		match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(&k)) {
-			Ok(i) => Some(&self.vals[i].2),
-			Err(_) => None,
-		}
-	}
-	/// Gets a reference to all of the items, as a slice. Usefull to iterate on all map values.
-	/// In most case you will want to ignore the timestamp (second item of the tuple).
-	pub fn items(&self) -> &[(K, u64, V)] {
-		&self.vals[..]
-	}
-}
-
-impl<K, V> CRDT for LWWMap<K, V>
-where
-	K: Clone + Ord,
-	V: Clone + CRDT,
-{
-	fn merge(&mut self, other: &Self) {
-		for (k, ts2, v2) in other.vals.iter() {
-			match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(&k)) {
-				Ok(i) => {
-					let (_, ts1, _v1) = &self.vals[i];
-					if ts2 > ts1 {
-						self.vals[i].1 = *ts2;
-						self.vals[i].2 = v2.clone();
-					} else if ts1 == ts2 {
-						self.vals[i].2.merge(&v2);
-					}
-				}
-				Err(i) => {
-					self.vals.insert(i, (k.clone(), *ts2, v2.clone()));
-				}
-			}
-		}
-	}
-}

src/table/crdt/bool.rs

@@ -0,0 +1,34 @@
+use serde::{Deserialize, Serialize};
+
+use crate::crdt::crdt::*;
+
+/// Boolean, where `true` is an absorbing state
+#[derive(Clone, Copy, Debug, Serialize, Deserialize, PartialEq)]
+pub struct Bool(bool);
+
+impl Bool {
+	/// Create a new boolean with the specified value
+	pub fn new(b: bool) -> Self {
+		Self(b)
+	}
+	/// Set the boolean to true
+	pub fn set(&mut self) {
+		self.0 = true;
+	}
+	/// Get the boolean value
+	pub fn get(&self) -> bool {
+		self.0
+	}
+}
+
+impl From<bool> for Bool {
+	fn from(b: bool) -> Bool {
+		Bool::new(b)
+	}
+}
+
+impl CRDT for Bool {
+	fn merge(&mut self, other: &Self) {
+		self.0 = self.0 || other.0;
+	}
+}

src/table/crdt/crdt.rs

@@ -0,0 +1,73 @@
+use garage_util::data::*;
+
+/// Definition of a CRDT - all CRDT Rust types implement this.
+///
+/// A CRDT is defined as a merge operator that respects a certain set of axioms.
+///
+/// In particular, the merge operator must be commutative, associative,
+/// idempotent, and monotonic.
+/// In other words, if `a`, `b` and `c` are CRDTs, and `⊔` denotes the merge operator,
+/// the following axioms must apply:
+///
+/// ```text
+/// a ⊔ b = b ⊔ a                   (commutativity)
+/// (a ⊔ b) ⊔ c = a ⊔ (b ⊔ c)       (associativity)
+/// (a ⊔ b) ⊔ b = a ⊔ b             (idempotence)
+/// ```
+///
+/// Moreover, the relationship `≥` defined by `a ≥ b ⇔ ∃c. a = b ⊔ c` must be a partial order.
+/// This implies a few properties such as: if `a ⊔ b ≠ a`, then there is no `c` such that `(a ⊔ b) ⊔ c = a`,
+/// as this would imply a cycle in the partial order.
+pub trait CRDT {
+	/// Merge the two datastructures according to the CRDT rules.
+	/// `self` is modified to contain the merged CRDT value. `other` is not modified.
+	///
+	/// # Arguments
+	///
+	/// * `other` - the other CRDT we wish to merge with
+	fn merge(&mut self, other: &Self);
+}
+
+/// All types that implement `Ord` (a total order) can also implement a trivial CRDT
+/// defined by the merge rule: `a ⊔ b = max(a, b)`. Implement this trait for your type
+/// to enable this behavior.
+pub trait AutoCRDT: Ord + Clone + std::fmt::Debug {
+	/// WARN_IF_DIFFERENT: emit a warning when values differ. Set this to true if
+	/// different values in your application should never happen. Set this to false
+	/// if you are actually relying on the semantics of `a ⊔ b = max(a, b)`.
+	const WARN_IF_DIFFERENT: bool;
+}
+
+impl<T> CRDT for T
+where
+	T: AutoCRDT,
+{
+	fn merge(&mut self, other: &Self) {
+		if Self::WARN_IF_DIFFERENT && self != other {
+			warn!(
+				"Different CRDT values should be the same (logic error!): {:?} vs {:?}",
+				self, other
+			);
+			if other > self {
+				*self = other.clone();
+			}
+			warn!("Making an arbitrary choice: {:?}", self);
+		} else {
+			if other > self {
+				*self = other.clone();
+			}
+		}
+	}
+}
+
+impl AutoCRDT for String {
+	const WARN_IF_DIFFERENT: bool = true;
+}
+
+impl AutoCRDT for bool {
+	const WARN_IF_DIFFERENT: bool = true;
+}
+
+impl AutoCRDT for FixedBytes32 {
+	const WARN_IF_DIFFERENT: bool = true;
+}

src/table/crdt/lww.rs

@@ -0,0 +1,114 @@
+use serde::{Deserialize, Serialize};
+
+use garage_util::data::now_msec;
+
+use crate::crdt::crdt::*;
+
+/// Last Write Win (LWW)
+///
+/// An LWW CRDT associates a timestamp with a value, in order to implement a
+/// time-based reconciliation rule: the most recent write wins.
+/// For completeness, the LWW reconciliation rule must also be defined for two LWW CRDTs
+/// with the same timestamp but different values.
+///
+/// In our case, we add the constraint that the value that is wrapped inside the LWW CRDT must
+/// itself be a CRDT: in the case when the timestamp does not allow us to decide on which value to
+/// keep, the merge rule of the inner CRDT is applied on the wrapped values.  (Note that all types
+/// that implement the `Ord` trait get a default CRDT implemetnation that keeps the maximum value.
+/// This enables us to use LWW directly with primitive data types such as numbers or strings. It is
+/// generally desirable in this case to never explicitly produce LWW values with the same timestamp
+/// but different inner values, as the rule to keep the maximum value isn't generally the desired
+/// semantics.)
+///
+/// As multiple computers clocks are always desynchronized,
+/// when operations are close enough, it is equivalent to
+/// take one copy and drop the other one.
+///
+/// Given that clocks are not too desynchronized, this assumption
+/// is enough for most cases, as there is few chance that two humans
+/// coordonate themself faster than the time difference between two NTP servers.
+///
+/// As a more concret example, let's suppose you want to upload a file
+/// with the same key (path) in the same bucket at the very same time.
+/// For each request, the file will be timestamped by the receiving server
+/// and may differ from what you observed with your atomic clock!
+///
+/// This scheme is used by AWS S3 or Soundcloud and often without knowing
+/// in entreprise when reconciliating databases with ad-hoc scripts.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct LWW<T> {
+	ts: u64,
+	v: T,
+}
+
+impl<T> LWW<T>
+where
+	T: CRDT,
+{
+	/// Creates a new CRDT
+	///
+	/// CRDT's internal timestamp is set with current node's clock.
+	pub fn new(value: T) -> Self {
+		Self {
+			ts: now_msec(),
+			v: value,
+		}
+	}
+
+	/// Build a new CRDT from a previous non-compatible one
+	///
+	/// Compared to new, the CRDT's timestamp is not set to now
+	/// but must be set to the previous, non-compatible, CRDT's timestamp.
+	pub fn migrate_from_raw(ts: u64, value: T) -> Self {
+		Self { ts, v: value }
+	}
+
+	/// Update the LWW CRDT while keeping some causal ordering.
+	///
+	/// The timestamp of the LWW CRDT is updated to be the current node's clock
+	/// at time of update, or the previous timestamp + 1 if that's bigger,
+	/// so that the new timestamp is always strictly larger than the previous one.
+	/// This ensures that merging the update with the old value will result in keeping
+	/// the updated value.
+	pub fn update(&mut self, new_value: T) {
+		self.ts = std::cmp::max(self.ts + 1, now_msec());
+		self.v = new_value;
+	}
+
+	/// Get the CRDT value
+	pub fn get(&self) -> &T {
+		&self.v
+	}
+
+	/// Get a mutable reference to the CRDT's value
+	///
+	/// This is usefull to mutate the inside value without changing the LWW timestamp.
+	/// When such mutation is done, the merge between two LWW values is done using the inner
+	/// CRDT's merge operation. This is usefull in the case where the inner CRDT is a large
+	/// data type, such as a map, and we only want to change a single item in the map.
+	/// To do this, we can produce a "CRDT delta", i.e. a LWW that contains only the modification.
+	/// This delta consists in a LWW with the same timestamp, and the map
+	/// inside only contains the updated value.
+	/// The advantage of such a delta is that it is much smaller than the whole map.
+	///
+	/// Avoid using this if the inner data type is a primitive type such as a number or a string,
+	/// as you will then rely on the merge function defined on `Ord` types by keeping the maximum
+	/// of both values.
+	pub fn get_mut(&mut self) -> &mut T {
+		&mut self.v
+	}
+}
+
+impl<T> CRDT for LWW<T>
+where
+	T: Clone + CRDT,
+{
+	fn merge(&mut self, other: &Self) {
+		if other.ts > self.ts {
+			self.ts = other.ts;
+			self.v = other.v.clone();
+		} else if other.ts == self.ts {
+			self.v.merge(&other.v);
+		}
+	}
+}

src/table/crdt/lww_map.rs

@@ -0,0 +1,145 @@
+use serde::{Deserialize, Serialize};
+
+use garage_util::data::now_msec;
+
+use crate::crdt::crdt::*;
+
+/// Last Write Win Map
+///
+/// This types defines a CRDT for a map from keys to values.
+/// The values have an associated timestamp, such that the last written value
+/// takes precedence over previous ones. As for the simpler `LWW` type, the value
+/// type `V` is also required to implement the CRDT trait.
+/// We do not encourage mutating the values associated with a given key
+/// without updating the timestamp, in fact at the moment we do not provide a `.get_mut()`
+/// method that would allow that.
+///
+/// Internally, the map is stored as a vector of keys and values, sorted by ascending key order.
+/// This is why the key type `K` must implement `Ord` (and also to ensure a unique serialization,
+/// such that two values can be compared for equality based on their hashes). As a consequence,
+/// insertions take `O(n)` time. This means that LWWMap should be used for reasonably small maps.
+/// However, note that even if we were using a more efficient data structure such as a `BTreeMap`,
+/// the serialization cost `O(n)` would still have to be paid at each modification, so we are
+/// actually not losing anything here.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct LWWMap<K, V> {
+	vals: Vec<(K, u64, V)>,
+}
+
+impl<K, V> LWWMap<K, V>
+where
+	K: Ord,
+	V: CRDT,
+{
+	/// Create a new empty map CRDT
+	pub fn new() -> Self {
+		Self { vals: vec![] }
+	}
+	/// Used to migrate from a map defined in an incompatible format. This produces
+	/// a map that contains a single item with the specified timestamp (copied from
+	/// the incompatible format). Do this as many times as you have items to migrate,
+	/// and put them all together using the CRDT merge operator.
+	pub fn migrate_from_raw_item(k: K, ts: u64, v: V) -> Self {
+		Self {
+			vals: vec![(k, ts, v)],
+		}
+	}
+	/// Returns a map that contains a single mapping from the specified key to the specified value.
+	/// This map is a mutator, or a delta-CRDT, such that when it is merged with the original map,
+	/// the previous value will be replaced with the one specified here.
+	/// The timestamp in the provided mutator is set to the maximum of the current system's clock
+	/// and 1 + the previous value's timestamp (if there is one), so that the new value will always
+	/// take precedence (LWW rule).
+	///
+	/// Typically, to update the value associated to a key in the map, you would do the following:
+	///
+	/// ```ignore
+	/// let my_update = my_crdt.update_mutator(key_to_modify, new_value);
+	/// my_crdt.merge(&my_update);
+	/// ```
+	///
+	/// However extracting the mutator on its own and only sending that on the network is very
+	/// interesting as it is much smaller than the whole map.
+	pub fn update_mutator(&self, k: K, new_v: V) -> Self {
+		let new_vals = match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(&k)) {
+			Ok(i) => {
+				let (_, old_ts, _) = self.vals[i];
+				let new_ts = std::cmp::max(old_ts + 1, now_msec());
+				vec![(k, new_ts, new_v)]
+			}
+			Err(_) => vec![(k, now_msec(), new_v)],
+		};
+		Self { vals: new_vals }
+	}
+	/// Takes all of the values of the map and returns them. The current map is reset to the
+	/// empty map. This is very usefull to produce in-place a new map that contains only a delta
+	/// that modifies a certain value:
+	///
+	/// ```ignore
+	/// let mut a = get_my_crdt_value();
+	/// let old_a = a.take_and_clear();
+	/// a.merge(&old_a.update_mutator(key_to_modify, new_value));
+	/// put_my_crdt_value(a);
+	/// ```
+	///
+	/// Of course in this simple example we could have written simply
+	/// `pyt_my_crdt_value(a.update_mutator(key_to_modify, new_value))`,
+	/// but in the case where the map is a field in a struct for instance (as is always the case),
+	/// this becomes very handy:
+	///
+	/// ```ignore
+	/// let mut a = get_my_crdt_value();
+	/// let old_a_map = a.map_field.take_and_clear();
+	/// a.map_field.merge(&old_a_map.update_mutator(key_to_modify, new_value));
+	/// put_my_crdt_value(a);
+	/// ```
+	pub fn take_and_clear(&mut self) -> Self {
+		let vals = std::mem::replace(&mut self.vals, vec![]);
+		Self { vals }
+	}
+	/// Removes all values from the map
+	pub fn clear(&mut self) {
+		self.vals.clear();
+	}
+	/// Get a reference to the value assigned to a key
+	pub fn get(&self, k: &K) -> Option<&V> {
+		match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(&k)) {
+			Ok(i) => Some(&self.vals[i].2),
+			Err(_) => None,
+		}
+	}
+	/// Gets a reference to all of the items, as a slice. Usefull to iterate on all map values.
+	/// In most case you will want to ignore the timestamp (second item of the tuple).
+	pub fn items(&self) -> &[(K, u64, V)] {
+		&self.vals[..]
+	}
+	/// Returns the number of items in the map
+	pub fn len(&self) -> usize {
+		self.vals.len()
+	}
+}
+
+impl<K, V> CRDT for LWWMap<K, V>
+where
+	K: Clone + Ord,
+	V: Clone + CRDT,
+{
+	fn merge(&mut self, other: &Self) {
+		for (k, ts2, v2) in other.vals.iter() {
+			match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(&k)) {
+				Ok(i) => {
+					let (_, ts1, _v1) = &self.vals[i];
+					if ts2 > ts1 {
+						self.vals[i].1 = *ts2;
+						self.vals[i].2 = v2.clone();
+					} else if ts1 == ts2 {
+						self.vals[i].2.merge(&v2);
+					}
+				}
+				Err(i) => {
+					self.vals.insert(i, (k.clone(), *ts2, v2.clone()));
+				}
+			}
+		}
+	}
+}

src/table/crdt/map.rs

@@ -0,0 +1,83 @@
+use serde::{Deserialize, Serialize};
+
+use crate::crdt::crdt::*;
+
+/// Simple CRDT Map
+///
+/// This types defines a CRDT for a map from keys to values. Values are CRDT types which
+/// can have their own updating logic.
+///
+/// Internally, the map is stored as a vector of keys and values, sorted by ascending key order.
+/// This is why the key type `K` must implement `Ord` (and also to ensure a unique serialization,
+/// such that two values can be compared for equality based on their hashes). As a consequence,
+/// insertions take `O(n)` time. This means that Map should be used for reasonably small maps.
+/// However, note that even if we were using a more efficient data structure such as a `BTreeMap`,
+/// the serialization cost `O(n)` would still have to be paid at each modification, so we are
+/// actually not losing anything here.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct Map<K, V> {
+	vals: Vec<(K, V)>,
+}
+
+impl<K, V> Map<K, V>
+where
+	K: Clone + Ord,
+	V: Clone + CRDT,
+{
+	/// Create a new empty map CRDT
+	pub fn new() -> Self {
+		Self { vals: vec![] }
+	}
+
+	/// Returns a map that contains a single mapping from the specified key to the specified value.
+	/// This can be used to build a delta-mutator:
+	/// when merged with another map, the value will be added or CRDT-merged if a previous
+	/// value already exists.
+	pub fn put_mutator(k: K, v: V) -> Self {
+		Self { vals: vec![(k, v)] }
+	}
+
+	pub fn put(&mut self, k: K, v: V) {
+		self.merge(&Self::put_mutator(k, v));
+	}
+
+	/// Removes all values from the map
+	pub fn clear(&mut self) {
+		self.vals.clear();
+	}
+
+	/// Get a reference to the value assigned to a key
+	pub fn get(&self, k: &K) -> Option<&V> {
+		match self.vals.binary_search_by(|(k2, _)| k2.cmp(&k)) {
+			Ok(i) => Some(&self.vals[i].1),
+			Err(_) => None,
+		}
+	}
+	/// Gets a reference to all of the items, as a slice. Usefull to iterate on all map values.
+	pub fn items(&self) -> &[(K, V)] {
+		&self.vals[..]
+	}
+	/// Returns the number of items in the map
+	pub fn len(&self) -> usize {
+		self.vals.len()
+	}
+}
+
+impl<K, V> CRDT for Map<K, V>
+where
+	K: Clone + Ord,
+	V: Clone + CRDT,
+{
+	fn merge(&mut self, other: &Self) {
+		for (k, v2) in other.vals.iter() {
+			match self.vals.binary_search_by(|(k2, _)| k2.cmp(&k)) {
+				Ok(i) => {
+					self.vals[i].1.merge(&v2);
+				}
+				Err(i) => {
+					self.vals.insert(i, (k.clone(), v2.clone()));
+				}
+			}
+		}
+	}
+}

src/table/crdt/mod.rs

@@ -0,0 +1,22 @@
+//! This package provides a simple implementation of conflict-free replicated data types (CRDTs)
+//!
+//! CRDTs are a type of data structures that do not require coordination.  In other words, we can
+//! edit them in parallel, we will always find a way to merge it.
+//!
+//! A general example is a counter. Its initial value is 0.  Alice and Bob get a copy of the
+//! counter.  Alice does +1 on her copy, she reads 1.  Bob does +3 on his copy, he reads 3.  Now,
+//! it is easy to merge their counters, order does not count: we always get 4.
+//!
+//! Learn more about CRDT [on Wikipedia](https://en.wikipedia.org/wiki/Conflict-free_replicated_data_type)
+
+mod bool;
+mod crdt;
+mod lww;
+mod lww_map;
+mod map;
+
+pub use self::bool::*;
+pub use crdt::*;
+pub use lww::*;
+pub use lww_map::*;
+pub use map::*;

src/table/schema.rs

@@ -2,6 +2,8 @@ use serde::{Deserialize, Serialize};
 
 use garage_util::data::*;
 
+use crate::crdt::CRDT;
+
 pub trait PartitionKey {
 	fn hash(&self) -> Hash;
 }
@@ -35,12 +37,10 @@ impl SortKey for Hash {
 }
 
 pub trait Entry<P: PartitionKey, S: SortKey>:
-	PartialEq + Clone + Serialize + for<'de> Deserialize<'de> + Send + Sync
+	CRDT + PartialEq + Clone + Serialize + for<'de> Deserialize<'de> + Send + Sync
 {
 	fn partition_key(&self) -> &P;
 	fn sort_key(&self) -> &S;
-
-	fn merge(&mut self, other: &Self);
 }
 
 pub trait TableSchema: Send + Sync {

src/table/table.rs

@@ -17,6 +17,7 @@ use garage_rpc::ring::Ring;
 use garage_rpc::rpc_client::*;
 use garage_rpc::rpc_server::*;
 
+use crate::crdt::CRDT;
 use crate::schema::*;
 use crate::table_sync::*;