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// This file is part of lfchring-rs. // // Copyright 2021 Christos Katsakioris // // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // http://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. use std::borrow::Borrow; use std::fmt::{Display, Formatter}; use std::sync::atomic::Ordering; use std::sync::Arc; use crossbeam_epoch::{self as epoch, Atomic, Guard, Owned}; use log::trace; use crate::{ iter::Iter, state::HashRingState, types::{Adjacency, DefaultStdHasher, HashRingError, Hasher, Node, Result, Update, Vnid}, vnode::VirtualNode, }; /// The consistent hashing ring data structure. /// /// Users will probably interact with this crate mostly through this type, as it is central to its /// API. /// /// In multi-threaded contexts, it needs to be wrapped in [`Arc`]. /// /// To find out more general information regarding its use, refer to the crate-level documentation. #[derive(Debug)] pub struct HashRing<N, H> where N: Node + ?Sized, H: Hasher, { inner: Atomic<HashRingState<N, H>>, } impl<N, H> Clone for HashRing<N, H> where N: Node + ?Sized, H: Hasher, { fn clone(&self) -> Self { // Pin the current thread. let guard = epoch::pin(); // Atomically load the pointer. let inner = self.inner.load(Ordering::Acquire, &guard); // Dereference it. // SAFETY: `self.inner` is not null because after its initialization, it is always // `HashRing::update` ever setting it, and it is never set to null. Furthermore, we always // use Acquire/Release ordering. let inner = unsafe { inner.as_ref().expect("inner HashRingState is null!") }; // Clone the copy of the inner state and wrap it in a new `Atomic` and a new `HashRing`. Self { inner: Atomic::new(inner.clone()), } } } impl<N> HashRing<N, DefaultStdHasher> where N: Node + ?Sized, { /// Create a new [`HashRing<N, H>`] configured with the given parameters (i.e., the number of /// *virtual nodes* per ring node and the *replication factor*) and initialize it with the /// provided `Node`s (the ring will be populated by their [`VirtualNode`]s automatically). /// /// The new [`HashRing<N, H>`] will employ the built-in [`Hasher`] that is based on standard /// library's [`DefaultHasher`][DefaultHasher]. /// /// # Errors /// /// - Returns [`HashRingError::InvalidConfiguration`] if either the number of virtual nodes per /// distinct ring node or the replication factor is `0`. /// /// - Returns [`HashRingError::VirtualNodeAlreadyExists`] in the case that a hash collision /// occurs while attempting to insert the given [`Node`]s in the new consistent hashing ring. /// The reason for this can be one of the following: /// - the output of [`Node::hashring_node_id`] for two (or more) of the [`Node`]s provided /// for insertion is equal; /// - the provided [`Hasher`] produces equal hash digests for different outputs of /// [`Node::hashring_node_id`] for two (or more) [`Node`]s among those provided for /// insertion. /// /// /// [DefaultHasher]: https://doc.rust-lang.org/std/collections/hash_map/struct.DefaultHasher.html #[inline] pub fn with_nodes( vnodes_per_node: Vnid, replication_factor: u8, nodes: &[Arc<N>], ) -> Result<Self> { Self::with_hasher_and_nodes( DefaultStdHasher::default(), vnodes_per_node, replication_factor, nodes, ) } /// Create a new [`HashRing<N, H>`] configured with the given parameters (i.e., the number of /// *virtual nodes* per ring node and the *replication factor*), which is initially empty of /// `Node`s (and, of course, empty of [`VirtualNode`]s too). /// /// The new [`HashRing<N, H>`] will employ the built-in [`Hasher`] that is based on standard /// library's [`DefaultHasher`][DefaultHasher]. /// /// # Errors /// /// Returns [`HashRingError::InvalidConfiguration`] if either the number of virtual nodes per /// distinct ring node or the replication factor is `0`. /// /// /// [DefaultHasher]: https://doc.rust-lang.org/std/collections/hash_map/struct.DefaultHasher.html #[inline] pub fn new(vnodes_per_node: Vnid, replication_factor: u8) -> Result<Self> { Self::with_hasher_and_nodes( DefaultStdHasher::default(), vnodes_per_node, replication_factor, &[], ) } } impl<N, H> HashRing<N, H> where N: Node + ?Sized, H: Hasher, { /// Create a new [`HashRing<N, H>`] configured with the given parameters (i.e., the number of /// *virtual nodes* per ring node and the *replication factor*) and initialize it with the /// provided `Node`s (the ring will be populated by their [`VirtualNode`]s automatically). /// /// The new [`HashRing<N, H>`] will employ the provided [`Hasher`] for placing the /// [`VirtualNode`]s on the consistent hashing ring. /// /// # Errors /// /// - Returns [`HashRingError::InvalidConfiguration`] if either the number of virtual nodes per /// distinct ring node or the replication factor is `0`. /// /// - Returns [`HashRingError::VirtualNodeAlreadyExists`] in the case that a hash collision /// occurs while attempting to insert the given [`Node`]s in the new consistent hashing ring. /// The reason for this can be one of the following: /// - the output of [`Node::hashring_node_id`] for two (or more) of the [`Node`]s provided /// for insertion is equal; /// - the provided [`Hasher`] produces equal hash digests for different outputs of /// [`Node::hashring_node_id`] for two (or more) [`Node`]s among those provided for /// insertion. pub fn with_hasher_and_nodes( hasher: H, vnodes_per_node: Vnid, replication_factor: u8, nodes: &[Arc<N>], ) -> Result<Self> { if replication_factor == 0 || vnodes_per_node == 0 { return Err(HashRingError::InvalidConfiguration( replication_factor, vnodes_per_node, )); } let mut inner = HashRingState::with_capacity(nodes.len(), hasher, vnodes_per_node, replication_factor); inner.insert(nodes)?; Ok(Self { inner: Atomic::new(inner), }) } /// Create a new [`HashRing<N, H>`] configured with the given parameters (i.e., the number of /// *virtual nodes* per ring node and the *replication factor*), which is initially empty of /// `Node`s (and, of course, empty of [`VirtualNode`]s too). /// /// The new [`HashRing<N, H>`] will employ the provided [`Hasher`] for placing the /// [`VirtualNode`]s on the consistent hashing ring. /// /// # Errors /// /// Returns [`HashRingError::InvalidConfiguration`] if either the number of virtual nodes per /// distinct ring node or the replication factor is `0`. #[inline] pub fn with_hasher(hasher: H, vnodes_per_node: Vnid, replication_factor: u8) -> Result<Self> { Self::with_hasher_and_nodes(hasher, vnodes_per_node, replication_factor, &[]) } /// Returns the number of distinct ring nodes that currently populate the consistent hashing /// ring. pub fn len_nodes(&self) -> usize { let guard = epoch::pin(); let inner = self.inner.load(Ordering::Acquire, &guard); // SAFETY: `self.inner` is not null because after its initialization, it is always // `HashRing::update` ever setting it, and it is never set to null. Furthermore, we always // use Acquire/Release ordering. unsafe { inner.as_ref().expect("inner HashRingState is null!") }.len_nodes() //unsafe { inner.deref() }.len_nodes() } /// Returns the number of *virtual nodes* that currently populate the consistent hashing ring. /// /// This should always be equal to the result of [`HashRing::len_nodes`] multiplied by the /// `vnodes_per_node` for a particular [`HashRing<N, H>`]. pub fn len_virtual_nodes(&self) -> usize { let guard = epoch::pin(); let inner = self.inner.load(Ordering::Acquire, &guard); // SAFETY: `self.inner` is not null because after its initialization, it is always // `HashRing::update` ever setting it, and it is never set to null. Furthermore, we always // use Acquire/Release ordering. unsafe { inner.as_ref().expect("inner HashRingState is null!") }.len_virtual_nodes() //unsafe { inner.deref() }.len_virtual_nodes() } fn update(&self, op: Update, nodes: &[Arc<N>]) -> Result<()> { // Pin current thread. let guard = epoch::pin(); // Atomically load the pointer and then dereference it to retrieve the pointee, in order to // be able to clone it and then update it. // This is the READ part of the RCU technique. // Using `Ordering::Acquire` we make sure that no reads or writes in the current thread can // be reordered before this load. All writes in other threads that release the same atomic // variable are visible in the current thread. let curr_inner_ptr = self.inner.load(Ordering::Acquire, &guard); // SAFETY: // `self.inner` is not null because after its initialization, it is always us setting it, // and we never set it to null. Furthermore, we always use Acquire/Release ordering. let curr_inner = unsafe { curr_inner_ptr.as_ref() }.expect("old inner HashRingState was null!"); // Clone the current inner HashRingState. This is the COPY part of the RCU technique. let mut new_inner = curr_inner.clone(); // Modify the local copy of the inner state as deemed necessary (i.e., insert the new Nodes // to the local copy of the inner state, or remove the provided old ones from it). match op { Update::Insert => new_inner.insert(nodes)?, Update::Remove => new_inner.remove(nodes)?, } let new_inner_ptr = Owned::new(new_inner); // Atomically overwrite the pointer to the inner state with a pointer to the new, updated // one. // This is the UPDATE part of the RCU technique. // Using `Ordering::AcqRel` we make sure that no memory reads or writes in the current // thread can be reordered before or after this store. All writes in other threads that // release the same atomic variable are visible before the modification and the // modification is visible in other threads that acquire the same atomic variable. // We use `compare_exchange()` rather than `swap()` to detect any concurrent modification // (i.e., any modification made by another thread since we last loaded the current inner // state of the HashRing), to give the caller a chance to evaluate possible new options. let old_inner = match self.inner.compare_exchange( curr_inner_ptr, new_inner_ptr, Ordering::AcqRel, Ordering::Acquire, &guard, ) { Ok(_) => { // On success, I think `compare_exchange()` returns `new_inner_ptr` as `Shared`; // therefore, the pointer to the "old" inner state is probably `curr_inner_ptr`. curr_inner_ptr } Err(cas_err) => { trace!( "CAS failed; current: {:?}; new: {:?}", cas_err.current, cas_err.new ); return Err(HashRingError::ConcurrentModification); } }; // Defer the destruction of the old inner state until there are no active (i.e., "pinned") // threads in the current global epoch. // XXX: How is "destruction" defined? A simple deallocation will not do; we must make sure // that `Drop::drop()` is run, since `HashRingState` contains `VirtualNode`s which contain // `Arc<Node>` that must be reference-counted correctly. // - According to The Rust Book, `Drop::drop()` _is_ a destructor. // - It looks like `Guard::(self: &Self, ptr: Shared<'_, T>)` gets the ownership of `ptr` // and does nothing more, hence `drop()`ping it in the end. // Therefore, this should probably be fine...(?) // SAFETY: // `self.inner` is not null because after its initialization, it is always us ever setting // it, and we never set it to null. Furthermore, we always use Acquire/Release ordering. unsafe { guard.defer_destroy(old_inner); } // Flush to accelerate the deferred execution of the destructor. FIXME? guard.flush(); Ok(()) } /// Insert the given [`Node`]s to the consistent hashing ring, thereby expanding it. /// /// # Errors /// /// [`HashRingError::VirtualNodeAlreadyExists`] is returned in the case of a hash collision /// while attempting to insert the given [`Node`]s in the consistent hashing ring. /// This can happen if: /// - the output of [`Node::hashring_node_id`] for one (or more) of the new [`Node`]s is equal /// to that of one of the `Node`s that already exist in the ring; /// - the output of [`Node::hashring_node_id`] for two (or more) of the new [`Node`]s is equal /// to each other; /// - the provided [`Hasher`] produces equal hash digests for different outputs of /// [`Node::hashring_node_id`] for two (or more) [`Node`]s among the new or the already /// existing ones. #[inline] pub fn insert(&self, nodes: &[Arc<N>]) -> Result<()> { self.update(Update::Insert, nodes) } /// Remove the given [`Node`]s from the consistent hashing ring, thereby shrinking it. /// /// # Errors /// /// [`HashRingError::VirtualNodeDoesNotExist`] is returned in the case that one of the /// [`Node`]s provided for removal does not currently exist in the consistent hashing ring. #[inline] pub fn remove(&self, nodes: &[Arc<N>]) -> Result<()> { self.update(Update::Remove, nodes) } /// Returns `true` if the provided `key` corresponds to an existing [`VirtualNode`] of some /// [`Node`] in the consistent hashing ring, or `false` otherwise. pub fn has_virtual_node<K>(&self, key: &K) -> bool where K: Borrow<[u8]>, { let guard = epoch::pin(); let inner = self.inner.load(Ordering::Acquire, &guard); // SAFETY: `self.inner` is not null because after its initialization, it is always // `HashRing::update` ever setting it, and it is never set to null. Furthermore, we always // use Acquire/Release ordering. let inner = unsafe { inner.as_ref().expect("inner HashRingState is null!") }; inner.has_virtual_node(key) } /// Look up in the consistent hashing ring and return a **clone** of the [`VirtualNode`] that /// the given `key` should be assigned on. /// /// # Errors /// /// Returns [`HashRingError::EmptyRing`] if the consistent hashing ring is currently empty of /// [`Node`]s and therefore the given `key` cannot be assigned to any [`VirtualNode`] (as none /// exists). pub fn virtual_node_for_key<K>(&self, key: &K) -> Result<VirtualNode<N>> where K: Borrow<[u8]>, { let guard = epoch::pin(); let inner = self.inner.load(Ordering::Acquire, &guard); // SAFETY: `self.inner` is not null because after its initialization, it is always // `HashRing::update` ever setting it, and it is never set to null. Furthermore, we always // use Acquire/Release ordering. let inner = unsafe { inner.as_ref().expect("inner HashRingState is null!") }; let vn = inner.virtual_node_for_key(key)?; let mut ret = vn.clone(); ret.replica_owners = Some( vn.replica_owners .as_ref() .expect("Inconsistent access to VirtualNode detected! Please file a bug report.") .clone(), ); Ok(ret) } /// Look up in the consistent hashing ring and return a [`Vec`] of [`Node`]s, each wrapped in /// an [`Arc`], on which a replica of the given `key` should be assigned on. /// /// The [`Node`]s are returned according to their order in the consistent hashing ring. /// Therefore, the first [`Node`] is always the one that the particular [`VirtualNode`] /// originally belongs to. /// /// # Errors /// /// Returns [`HashRingError::EmptyRing`] if the consistent hashing ring is currently empty of /// [`Node`]s and therefore the given `key` cannot be assigned to any [`VirtualNode`] (as none /// exists). pub fn nodes_for_key<K>(&self, key: &K) -> Result<Vec<Arc<N>>> where K: Borrow<[u8]>, { let guard = epoch::pin(); let inner = self.inner.load(Ordering::Acquire, &guard); // SAFETY: `self.inner` is not null because after its initialization, it is always // `HashRing::update` ever setting it, and it is never set to null. Furthermore, we always // use Acquire/Release ordering. let inner = unsafe { inner.as_ref().expect("inner HashRingState is null!") }; let vn = inner.virtual_node_for_key(key)?; Ok(vn .replica_owners .as_ref() .expect("Inconsistent access to VirtualNode detected! Please file a bug report.") .clone()) } fn adjacent<K>(&self, adjacency: Adjacency, key: &K) -> Result<VirtualNode<N>> where K: Borrow<[u8]>, { let guard = epoch::pin(); let inner = self.inner.load(Ordering::Acquire, &guard); // SAFETY: `self.inner` is not null because after its initialization, it is always // `HashRing::update` ever setting it, and it is never set to null. Furthermore, we always // use Acquire/Release ordering. let inner = unsafe { inner.as_ref().expect("inner HashRingState is null!") }; let vn = inner.adjacent(adjacency, key)?; let mut ret = vn.clone(); ret.replica_owners = Some( vn.replica_owners .as_ref() .expect("Inconsistent access to VirtualNode detected! Please file a bug report.") .clone(), ); Ok(ret) } /// Look up in the consistent hashing ring and return the [`VirtualNode`] which is the /// predecessor of the one that the given `key` should be assigned on. /// /// # Errors /// /// Returns [`HashRingError::EmptyRing`] if the consistent hashing ring is currently empty of /// [`Node`]s and therefore the given `key` cannot be assigned to any [`VirtualNode`] (as none /// exists). #[inline] pub fn predecessor<K>(&self, key: &K) -> Result<VirtualNode<N>> where K: Borrow<[u8]>, { self.adjacent(Adjacency::Predecessor, key) } /// Look up in the consistent hashing ring and return the [`VirtualNode`] which is the /// successor of the one that the given `key` should be assigned on. /// /// # Errors /// /// Returns [`HashRingError::EmptyRing`] if the consistent hashing ring is currently empty of /// [`Node`]s and therefore the given `key` cannot be assigned to any [`VirtualNode`] (as none /// exists). #[inline] pub fn successor<K>(&self, key: &K) -> Result<VirtualNode<N>> where K: Borrow<[u8]>, { self.adjacent(Adjacency::Successor, key) } fn adjacent_node<K>(&self, adjacency: Adjacency, key: &K) -> Result<VirtualNode<N>> where K: Borrow<[u8]>, { let guard = epoch::pin(); let inner = self.inner.load(Ordering::Acquire, &guard); // SAFETY: `self.inner` is not null because after its initialization, it is always // `HashRing::update` ever setting it, and it is never set to null. Furthermore, we always // use Acquire/Release ordering. let inner = unsafe { inner.as_ref().expect("inner HashRingState is null!") }; let vn = inner.adjacent_node(adjacency, key)?; let mut ret = vn.clone(); ret.replica_owners = Some( vn.replica_owners .as_ref() .expect("Inconsistent access to VirtualNode detected! Please file a bug report.") .clone(), ); Ok(ret) } /// Look up in the consistent hashing ring and return the first predecessor [`VirtualNode`] to /// the one that the given `key` should be assigned on, but which also belongs to a different /// distinct [`Node`] than the latter. /// /// # Errors /// /// - Returns [`HashRingError::EmptyRing`] if the consistent hashing ring is currently empty of /// [`Node`]s and therefore the given `key` cannot be assigned to any [`VirtualNode`] (as none /// exists). /// - Returns [`HashRingError::SingleDistinctNodeRing`] if the consistent hashing ring /// currently consists of a single distinct node and therefore all [`VirtualNode`]s in the ring /// actually belong to the same [`Node`]. pub fn predecessor_node<K>(&self, key: &K) -> Result<VirtualNode<N>> where K: Borrow<[u8]>, { self.adjacent_node(Adjacency::Predecessor, key) } /// Look up in the consistent hashing ring and return the first successor [`VirtualNode`] to /// the one that the given `key` should be assigned on, but which also belongs to a different /// distinct [`Node`] than the latter. /// /// # Errors /// /// - Returns [`HashRingError::EmptyRing`] if the consistent hashing ring is currently empty of /// [`Node`]s and therefore the given `key` cannot be assigned to any [`VirtualNode`] (as none /// exists). /// - Returns [`HashRingError::SingleDistinctNodeRing`] if the consistent hashing ring /// currently consists of a single distinct node and therefore all [`VirtualNode`]s in the ring /// actually belong to the same [`Node`]. pub fn successor_node<K>(&self, key: &K) -> Result<VirtualNode<N>> where K: Borrow<[u8]>, { self.adjacent_node(Adjacency::Successor, key) } /// Returns an [`Iter`], i.e., an iterator to loop through all [`VirtualNode`]s that populate /// the consistent hashing ring. /// /// See the documentation of [`Iter`] for more information regarding its use. // // TODO: Include an example for calling [`HashRing::iter`], both here and in the documentation // of [`Iter`]. // #[inline] pub fn iter<'guard>(&self, guard: &'guard Guard) -> Iter<'guard, N, H> { let inner_ptr = self.inner.load(Ordering::Acquire, guard); // SAFETY: `self.inner` is not null because after its initialization, it is always // `HashRing::update` ever setting it, and it is never set to null. Furthermore, we always // use Acquire/Release ordering. let inner = unsafe { inner_ptr.as_ref() }.expect("Iter's inner HashRingState is null!"); Iter::new(inner_ptr, inner.len_virtual_nodes()) } } impl<N, H> Extend<Arc<N>> for HashRing<N, H> where N: Node + ?Sized, H: Hasher, { /// Extend the [`HashRing<N, H>`] by the [`Node`]s provided through the given [`IntoIterator`] /// over `Arc<N>>`. /// /// Note that, due to the restriction of [`Extend::extend`]'s signature, a `&mut HashRing` is /// required to use this method. /// The preferred way to extend the ring is via [`HashRing::insert`] anyway; read the section /// below for further details. /// /// # Panics /// /// Although the [`Extend`] trait is implemented for [`HashRing<N, H>`], it is not the /// preferred way of extending it. /// The signature of [`Extend::extend`] does not allow to return a `Result::Err` if the /// extension attempt fails. /// Therefore, in case of hash collision (e.g., when inserting an already existing [`Node`] in /// the [`HashRing<N, H>`]) this method fails by panicking (although the ring remains in a /// consistent state, since updating the ring is considered an atomic operation). /// /// The preferred way to add [`Node`]s to the [`HashRing<N, H>`] is via [`HashRing::insert`] /// instead, which returns a `Result::Err` that can be handled in case of a failure. /// Only use this method if you know for sure that hash collisions are extremely unlikely and /// practically impossible (e.g., when employing a cryptographically secure hash algorithm and /// no attempts to re-insert existing [`Node`]s occur). fn extend<I: IntoIterator<Item = Arc<N>>>(&mut self, iter: I) { let mut nodes = vec![]; for node in iter { nodes.push(Arc::clone(&node)); } if let Err(err) = self.update(Update::Insert, &nodes) { panic!("Error inserting new nodes to the ring: {}", err); } } } impl<N, H> Display for HashRing<N, H> where N: Node + ?Sized, H: Hasher, { fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result { let guard = epoch::pin(); let inner = self.inner.load(Ordering::Acquire, &guard); // SAFETY: `self.inner` is not null because after its initialization, it is always // `HashRing::update` ever setting it, and it is never set to null. Furthermore, we always // use Acquire/Release ordering. let inner = unsafe { inner.as_ref().expect("inner HashRingState is null!") }; write!(f, "{}", inner) } }