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//! Public Raft interface and data types. use std::collections::HashSet; use std::sync::Arc; use serde::{Deserialize, Serialize}; use tokio::sync::{mpsc, oneshot, watch, Mutex}; use tokio::task::JoinHandle; use crate::config::Config; use crate::core::RaftCore; use crate::error::{ ChangeConfigError, ClientReadError, ClientWriteError, InitializeError, RaftError, RaftResult, }; use crate::metrics::RaftMetrics; use crate::{AppData, AppDataResponse, NodeId, RaftNetwork, RaftStorage}; struct RaftInner<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> { tx_api: mpsc::UnboundedSender<RaftMsg<D, R>>, rx_metrics: watch::Receiver<RaftMetrics>, raft_handle: Mutex<Option<JoinHandle<RaftResult<()>>>>, tx_shutdown: Mutex<Option<oneshot::Sender<()>>>, marker_n: std::marker::PhantomData<N>, marker_s: std::marker::PhantomData<S>, } /// The Raft API. /// /// This type implements the full Raft spec, and is the interface to a running Raft node. /// Applications building on top of Raft will use this to spawn a Raft task and interact with /// the spawned task. /// /// For more information on the Raft protocol, see /// [the specification here](https://raft.github.io/raft.pdf) (**pdf warning**). /// /// For details and discussion on this API, see the /// [Raft API](https://nlv8.github.io/agreed/raft.html) section of the guide. /// /// ### clone /// This type implements `Clone`, and should be cloned liberally. The clone itself is very cheap /// and helps to facilitate use with async workflows. /// /// ### shutting down /// If any of the interfaces returns a `RaftError::ShuttingDown`, this indicates that the Raft node /// is shutting down (potentially for data safety reasons due to a storage error), and the `shutdown` /// method should be called on this type to await the shutdown of the node. If the parent /// application needs to shutdown the Raft node for any reason, calling `shutdown` will do the trick. pub struct Raft<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> { inner: Arc<RaftInner<D, R, N, S>>, } impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Raft<D, R, N, S> { /// Create and spawn a new Raft task. /// /// ### `id` /// The ID which the spawned Raft task will use to identify itself within the cluster. /// Applications must guarantee that the ID provided to this function is stable, and should be /// persisted in a well known location, probably alongside the Raft log and the application's /// state machine. This ensures that restarts of the node will yield the same ID every time. /// /// ### `config` /// Raft's runtime config. See the docs on the `Config` object for more details. /// /// ### `network` /// An implementation of the `RaftNetwork` trait which will be used by Raft for sending RPCs to /// peer nodes within the cluster. See the docs on the `RaftNetwork` trait for more details. /// /// ### `storage` /// An implementation of the `RaftStorage` trait which will be used by Raft for data storage. /// See the docs on the `RaftStorage` trait for more details. pub fn new(id: NodeId, config: Arc<Config>, network: Arc<N>, storage: Arc<S>) -> Self { let (tx_api, rx_api) = mpsc::unbounded_channel(); let (tx_metrics, rx_metrics) = watch::channel(RaftMetrics::new_initial(id)); let (tx_shutdown, rx_shutdown) = oneshot::channel(); let raft_handle = RaftCore::spawn( id, config, network, storage, rx_api, tx_metrics, rx_shutdown, ); let inner = RaftInner { tx_api, rx_metrics, raft_handle: Mutex::new(Some(raft_handle)), tx_shutdown: Mutex::new(Some(tx_shutdown)), marker_n: std::marker::PhantomData, marker_s: std::marker::PhantomData, }; Self { inner: Arc::new(inner), } } /// Submit an AppendEntries RPC to this Raft node. /// /// These RPCs are sent by the cluster leader to replicate log entries (§5.3), and are also /// used as heartbeats (§5.2). #[tracing::instrument(level = "debug", skip(self, rpc))] pub async fn append_entries( &self, rpc: AppendEntriesRequest<D>, ) -> Result<AppendEntriesResponse, RaftError> { let (tx, rx) = oneshot::channel(); self.inner .tx_api .send(RaftMsg::AppendEntries { rpc, tx }) .map_err(|_| RaftError::ShuttingDown)?; Ok(rx .await .map_err(|_| RaftError::ShuttingDown) .and_then(|res| res)?) } /// Submit a VoteRequest (RequestVote in the spec) RPC to this Raft node. /// /// These RPCs are sent by cluster peers which are in candidate state attempting to gather votes (§5.2). #[tracing::instrument(level = "debug", skip(self, rpc))] pub async fn vote(&self, rpc: VoteRequest) -> Result<VoteResponse, RaftError> { let (tx, rx) = oneshot::channel(); self.inner .tx_api .send(RaftMsg::RequestVote { rpc, tx }) .map_err(|_| RaftError::ShuttingDown)?; Ok(rx .await .map_err(|_| RaftError::ShuttingDown) .and_then(|res| res)?) } /// Submit an InstallSnapshot RPC to this Raft node. /// /// These RPCs are sent by the cluster leader in order to bring a new node or a slow node up-to-speed /// with the leader (§7). #[tracing::instrument(level = "debug", skip(self, rpc))] pub async fn install_snapshot( &self, rpc: InstallSnapshotRequest, ) -> Result<InstallSnapshotResponse, RaftError> { let (tx, rx) = oneshot::channel(); self.inner .tx_api .send(RaftMsg::InstallSnapshot { rpc, tx }) .map_err(|_| RaftError::ShuttingDown)?; Ok(rx .await .map_err(|_| RaftError::ShuttingDown) .and_then(|res| res)?) } /// Get the ID of the current leader from this Raft node. /// /// This method is based on the Raft metrics system which does a good job at staying /// up-to-date; however, the `client_read` method must still be used to guard against stale /// reads. This method is perfect for making decisions on where to route client requests. #[tracing::instrument(level = "debug", skip(self))] pub async fn current_leader(&self) -> Option<NodeId> { self.metrics().borrow().current_leader } /// Check to ensure this node is still the cluster leader, in order to guard against stale reads (§8). /// /// The actual read operation itself is up to the application, this method just ensures that /// the read will not be stale. #[tracing::instrument(level = "debug", skip(self))] pub async fn client_read(&self) -> Result<(), ClientReadError> { let (tx, rx) = oneshot::channel(); self.inner .tx_api .send(RaftMsg::ClientReadRequest { tx }) .map_err(|_| ClientReadError::RaftError(RaftError::ShuttingDown))?; Ok(rx .await .map_err(|_| ClientReadError::RaftError(RaftError::ShuttingDown)) .and_then(|res| res)?) } /// Submit a mutating client request to Raft to update the state of the system (§5.1). /// /// It will be appended to the log, committed to the cluster, and then applied to the /// application state machine. The result of applying the request to the state machine will /// be returned as the response from this method. /// /// Our goal for Raft is to implement linearizable semantics. If the leader crashes after committing /// a log entry but before responding to the client, the client may retry the command with a new /// leader, causing it to be executed a second time. As such, clients should assign unique serial /// numbers to every command. Then, the state machine should track the latest serial number /// processed for each client, along with the associated response. If it receives a command whose /// serial number has already been executed, it responds immediately without reexecuting the /// request (§8). The `RaftStorage::apply_entry_to_state_machine` method is the perfect place /// to implement this. /// /// These are application specific requirements, and must be implemented by the application which is /// being built on top of Raft. #[tracing::instrument(level = "debug", skip(self, rpc))] pub async fn client_write( &self, rpc: ClientWriteRequest<D>, ) -> Result<ClientWriteResponse<R>, ClientWriteError<D>> { let (tx, rx) = oneshot::channel(); self.inner .tx_api .send(RaftMsg::ClientWriteRequest { rpc, tx }) .map_err(|_| ClientWriteError::RaftError(RaftError::ShuttingDown))?; Ok(rx .await .map_err(|_| ClientWriteError::RaftError(RaftError::ShuttingDown)) .and_then(|res| res)?) } /// Initialize a pristine Raft node with the given config. /// /// This command should be called on pristine nodes — where the log index is 0 and the node is /// in NonVoter state — as if either of those constraints are false, it indicates that the /// cluster is already formed and in motion. If `InitializeError::NotAllowed` is returned /// from this function, it is safe to ignore, as it simply indicates that the cluster is /// already up and running, which is ultimately the goal of this function. /// /// This command will work for single-node or multi-node cluster formation. This command /// should be called with all discovered nodes which need to be part of cluster, and as such /// it is recommended that applications be configured with an initial cluster formation delay /// which will allow time for the initial members of the cluster to be discovered (by the /// parent application) for this call. /// /// If successful, this routine will set the given config as the active config, only in memory, /// and will start an election. /// /// It is recommended that applications call this function based on an initial call to /// `RaftStorage.get_initial_state`. If the initial state indicates that the hard state's /// current term is `0` and the `last_log_index` is `0`, then this routine should be called /// in order to initialize the cluster. /// /// Once a node becomes leader and detects that its index is 0, it will commit a new config /// entry (instead of the normal blank entry created by new leaders). /// /// Every member of the cluster should perform these actions. This routine is race-condition /// free, and Raft guarantees that the first node to become the cluster leader will propagate /// only its own config. #[tracing::instrument(level = "debug", skip(self))] pub async fn initialize(&self, members: HashSet<NodeId>) -> Result<(), InitializeError> { let (tx, rx) = oneshot::channel(); self.inner .tx_api .send(RaftMsg::Initialize { members, tx }) .map_err(|_| RaftError::ShuttingDown)?; Ok(rx .await .map_err(|_| InitializeError::RaftError(RaftError::ShuttingDown)) .and_then(|res| res)?) } /// Synchronize a new Raft node, bringing it up-to-speed (§6). /// /// Applications built on top of Raft will typically have some peer discovery mechanism for /// detecting when new nodes come online and need to be added to the cluster. This API /// facilitates the ability to request that a new node be synchronized with the leader, so /// that it is up-to-date and ready to be added to the cluster. /// /// Calling this API will add the target node as a non-voter, starting the syncing process. /// Once the node is up-to-speed, this function will return. It is the responsibility of the /// application to then call `change_membership` once all of the new nodes are synced. /// /// If this Raft node is not the cluster leader, then this call will fail. #[tracing::instrument(level = "debug", skip(self))] pub async fn add_non_voter(&self, id: NodeId) -> Result<(), ChangeConfigError> { let (tx, rx) = oneshot::channel(); self.inner .tx_api .send(RaftMsg::AddNonVoter { id, tx }) .map_err(|_| RaftError::ShuttingDown)?; Ok(rx .await .map_err(|_| ChangeConfigError::RaftError(RaftError::ShuttingDown)) .and_then(|res| res)?) } /// Cancel replication to the specified Non-Voter node. /// /// Although the Non-Voter role and the machinery around it mainly serves the purpose of /// syncing and bringing up a node to speed prior to adding it as a Voter member (to prevent /// disrupting the cluster), it can very well be used as a read-only replication/change data capture /// mechanism. To support the latter use-case, this API can be used to remove Non-Voter nodes. /// /// If the node is being synced as part of a configuration change (causing the leader to be in /// `CatchingUp` consensus state) then it cannot be removed. You have to wait for the /// configuration change to take place. /// /// If this Raft node is not the cluster leader, then this call will fail. #[tracing::instrument(level = "debug", skip(self))] pub async fn remove_non_voter(&self, old_non_voter: NodeId) -> Result<(), ChangeConfigError> { let (tx, rx) = oneshot::channel(); self.inner .tx_api .send(RaftMsg::RemoveNonVoter { id: old_non_voter, tx, }) .map_err(|_| RaftError::ShuttingDown)?; Ok(rx .await .map_err(|_| ChangeConfigError::RaftError(RaftError::ShuttingDown)) .and_then(|res| res)?) } /// Request the addition of the specified node as a Voter member of the cluster. Will return /// once the configuration change has finished. /// /// This will cause the leader to begin a cluster membership configuration change. If the new node /// is not already registered as a Non-Voter — from an earlier call to `add_non_voter` — then the /// new node will first be synced as a Non-Voter before actually performing the membership change. /// While the synchronization takes place, the Leader enters `CatchingUp` consensus state, preventing /// other configuration changes. As this process may take some time, it is recommended that /// `add_non_voter` be called first for the new node, and then once the new node has been synchronized, /// call this method to start reconfiguration. /// /// If this Raft node is not the cluster leader or there is already another configuration change /// in progress, then the change will be rejected. pub async fn add_voter(&self, new_voter: NodeId) -> Result<(), ChangeConfigError> { let (tx, rx) = oneshot::channel(); self.inner .tx_api .send(RaftMsg::AddVoter { id: new_voter, tx }) .map_err(|_| RaftError::ShuttingDown)?; Ok(rx .await .map_err(|_| ChangeConfigError::RaftError(RaftError::ShuttingDown)) .and_then(|res| res)?) } /// Request the removal of the specified Voter node from the cluster. Will return once the /// configuration change has finished. /// /// Please note, that it is not guaranteed to be safe to immediately shut down the removed node /// once this call returns. The node is only safe to be shut down once it replicated the config /// change. Afterwards, it will not receive entries anymore. Shutting down the node early will /// cause the cluster leader to constantly attempt to replicate the config change to the removed /// node. /// /// If this Raft node is not the cluster leader or there is already another configuration change /// in progress, then the change will be rejected. pub async fn remove_voter(&self, old_voter: NodeId) -> Result<(), ChangeConfigError> { let (tx, rx) = oneshot::channel(); self.inner .tx_api .send(RaftMsg::RemoveVoter { id: old_voter, tx }) .map_err(|_| RaftError::ShuttingDown)?; Ok(rx .await .map_err(|_| ChangeConfigError::RaftError(RaftError::ShuttingDown)) .and_then(|res| res)?) } /// Get a handle to the metrics channel. pub fn metrics(&self) -> watch::Receiver<RaftMetrics> { self.inner.rx_metrics.clone() } /// Shutdown this Raft node. pub async fn shutdown(&self) -> anyhow::Result<()> { if let Some(tx) = self.inner.tx_shutdown.lock().await.take() { let _ = tx.send(()); } if let Some(handle) = self.inner.raft_handle.lock().await.take() { let _ = handle.await?; } Ok(()) } } impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Clone for Raft<D, R, N, S> { fn clone(&self) -> Self { Self { inner: self.inner.clone(), } } } pub(crate) type ClientWriteResponseTx<D, R> = oneshot::Sender<Result<ClientWriteResponse<R>, ClientWriteError<D>>>; pub(crate) type ClientReadResponseTx = oneshot::Sender<Result<(), ClientReadError>>; pub(crate) type ChangeMembershipTx = oneshot::Sender<Result<(), ChangeConfigError>>; /// A message coming from the Raft API. pub(crate) enum RaftMsg<D: AppData, R: AppDataResponse> { AppendEntries { rpc: AppendEntriesRequest<D>, tx: oneshot::Sender<Result<AppendEntriesResponse, RaftError>>, }, RequestVote { rpc: VoteRequest, tx: oneshot::Sender<Result<VoteResponse, RaftError>>, }, InstallSnapshot { rpc: InstallSnapshotRequest, tx: oneshot::Sender<Result<InstallSnapshotResponse, RaftError>>, }, ClientWriteRequest { rpc: ClientWriteRequest<D>, tx: ClientWriteResponseTx<D, R>, }, ClientReadRequest { tx: ClientReadResponseTx, }, Initialize { members: HashSet<NodeId>, tx: oneshot::Sender<Result<(), InitializeError>>, }, AddNonVoter { id: NodeId, tx: ChangeMembershipTx, }, RemoveNonVoter { id: NodeId, tx: ChangeMembershipTx, }, AddVoter { id: NodeId, tx: ChangeMembershipTx, }, RemoveVoter { id: NodeId, tx: ChangeMembershipTx, }, } ////////////////////////////////////////////////////////////////////////////////////////////////// ////////////////////////////////////////////////////////////////////////////////////////////////// /// An RPC sent by a cluster leader to replicate log entries (§5.3), and as a heartbeat (§5.2). #[derive(Debug, Serialize, Deserialize)] pub struct AppendEntriesRequest<D: AppData> { /// The leader's current term. pub term: u64, /// The leader's ID. Useful in redirecting clients. pub leader_id: u64, /// The index of the log entry immediately preceding the new entries. pub prev_log_index: u64, /// The term of the `prev_log_index` entry. pub prev_log_term: u64, /// The new log entries to store. /// /// This may be empty when the leader is sending heartbeats. Entries /// are batched for efficiency. #[serde(bound = "D: AppData")] pub entries: Vec<Entry<D>>, /// The leader's commit index. pub leader_commit: u64, } /// The response to an `AppendEntriesRequest`. #[derive(Debug, Serialize, Deserialize)] pub struct AppendEntriesResponse { /// The responding node's current term, for leader to update itself. pub term: u64, /// Will be true if follower contained entry matching `prev_log_index` and `prev_log_term`. pub success: bool, /// A value used to implement the _conflicting term_ optimization outlined in §5.3. /// /// This value will only be present, and should only be considered, when `success` is `false`. pub conflict_opt: Option<ConflictOpt>, } /// A struct used to implement the _conflicting term_ optimization outlined in §5.3 for log replication. /// /// This value will only be present, and should only be considered, when an `AppendEntriesResponse` /// object has a `success` value of `false`. /// /// This implementation of Raft uses this value to more quickly synchronize a leader with its /// followers which may be some distance behind in replication, may have conflicting entries, or /// which may be new to the cluster. #[derive(Debug, Serialize, Deserialize)] pub struct ConflictOpt { /// The term of the most recent entry which does not conflict with the received request. pub term: u64, /// The index of the most recent entry which does not conflict with the received request. pub index: u64, } /// A Raft log entry. #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)] pub struct Entry<D: AppData> { /// This entry's term. pub term: u64, /// This entry's index. pub index: u64, /// This entry's payload. #[serde(bound = "D: AppData")] pub payload: EntryPayload<D>, } impl<D: AppData> Entry<D> { /// Create a new snapshot pointer from the given data. /// /// ### index & term /// The index and term of the entry being replaced by this snapshot pointer entry. /// /// ### id /// The ID of the associated snapshot. /// /// ### membership /// The cluster membership config which is contained in the snapshot, which will always be the /// latest membership covered by the snapshot. pub fn new_snapshot_pointer( index: u64, term: u64, id: String, membership: MembershipConfig, ) -> Self { Entry { term, index, payload: EntryPayload::SnapshotPointer(EntrySnapshotPointer { id, membership }), } } } /// Log entry payload variants. #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)] pub enum EntryPayload<D: AppData> { /// An empty payload committed by a new cluster leader. Blank, /// A normal log entry. #[serde(bound = "D: AppData")] Normal(EntryNormal<D>), /// A config change log entry. ConfigChange(EntryConfigChange), /// An entry which points to a snapshot. SnapshotPointer(EntrySnapshotPointer), } /// A normal log entry. #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)] pub struct EntryNormal<D: AppData> { /// The contents of this entry. #[serde(bound = "D: AppData")] pub data: D, } /// A log entry holding a config change. #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)] pub struct EntryConfigChange { /// Details on the cluster's membership configuration. pub membership: MembershipConfig, } /// A log entry pointing to a snapshot. /// /// This will only be present when read from storage. An entry of this type will never be /// transmitted from a leader during replication, an `InstallSnapshotRequest` /// RPC will be sent instead. #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)] pub struct EntrySnapshotPointer { /// The ID of the snapshot, which is application specific, and probably only meaningful to the storage layer. pub id: String, /// The cluster's membership config covered by this snapshot. pub membership: MembershipConfig, } ////////////////////////////////////////////////////////////////////////////////////////////////// ////////////////////////////////////////////////////////////////////////////////////////////////// /// A model of the membership configuration of the cluster. #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)] pub struct MembershipConfig { /// All members of the Raft cluster. pub members: HashSet<NodeId>, } impl MembershipConfig { /// Get an iterator over all nodes in the current config. pub fn all_nodes(&self) -> HashSet<u64> { self.members.clone() } /// Check if the given NodeId exists in this membership config. pub fn contains(&self, x: &NodeId) -> bool { self.members.contains(x) } /// Create a new initial config containing only the given node ID. pub fn new_initial(id: NodeId) -> Self { let mut members = HashSet::new(); members.insert(id); Self { members } } } ////////////////////////////////////////////////////////////////////////////////////////////////// ////////////////////////////////////////////////////////////////////////////////////////////////// /// An RPC sent by candidates to gather votes (§5.2). #[derive(Debug, Serialize, Deserialize)] pub struct VoteRequest { /// The candidate's current term. pub term: u64, /// The candidate's ID. pub candidate_id: u64, /// The index of the candidate’s last log entry (§5.4). pub last_log_index: u64, /// The term of the candidate’s last log entry (§5.4). pub last_log_term: u64, } impl VoteRequest { /// Create a new instance. pub fn new(term: u64, candidate_id: u64, last_log_index: u64, last_log_term: u64) -> Self { Self { term, candidate_id, last_log_index, last_log_term, } } } /// The response to a `VoteRequest`. #[derive(Debug, Serialize, Deserialize)] pub struct VoteResponse { /// The current term of the responding node, for the candidate to update itself. pub term: u64, /// Will be true if the candidate received a vote from the responder. pub vote_granted: bool, } ////////////////////////////////////////////////////////////////////////////////////////////////// ////////////////////////////////////////////////////////////////////////////////////////////////// /// An RPC sent by the Raft leader to send chunks of a snapshot to a follower (§7). #[derive(Clone, Debug, Serialize, Deserialize)] pub struct InstallSnapshotRequest { /// The leader's current term. pub term: u64, /// The leader's ID. Useful in redirecting clients. pub leader_id: u64, /// The snapshot replaces all log entries up through and including this index. pub last_included_index: u64, /// The term of the `last_included_index`. pub last_included_term: u64, /// The byte offset where this chunk of data is positioned in the snapshot file. pub offset: u64, /// The raw bytes of the snapshot chunk, starting at `offset`. pub data: Vec<u8>, /// Will be `true` if this is the last chunk in the snapshot. pub done: bool, } /// The response to an `InstallSnapshotRequest`. #[derive(Debug, Serialize, Deserialize)] pub struct InstallSnapshotResponse { /// The receiving node's current term, for leader to update itself. pub term: u64, } ////////////////////////////////////////////////////////////////////////////////////////////////// ////////////////////////////////////////////////////////////////////////////////////////////////// /// An application specific client request to update the state of the system (§5.1). /// /// The entry of this payload will be appended to the Raft log and then applied to the Raft state /// machine according to the Raft protocol. #[derive(Debug, Serialize, Deserialize)] pub struct ClientWriteRequest<D: AppData> { /// The application specific contents of this client request. #[serde(bound = "D: AppData")] pub(crate) entry: EntryPayload<D>, } impl<D: AppData> ClientWriteRequest<D> { /// Create a new client payload instance with a normal entry type. pub fn new(entry: D) -> Self { Self::new_base(EntryPayload::Normal(EntryNormal { data: entry })) } /// Create a new instance. pub(crate) fn new_base(entry: EntryPayload<D>) -> Self { Self { entry } } /// Generate a new payload holding a config change. pub(crate) fn new_config(membership: MembershipConfig) -> Self { Self::new_base(EntryPayload::ConfigChange(EntryConfigChange { membership })) } /// Generate a new blank payload. /// /// This is used by new leaders when first coming to power. pub(crate) fn new_blank_payload() -> Self { Self::new_base(EntryPayload::Blank) } } /// The response to a `ClientRequest`. #[derive(Debug, Serialize, Deserialize)] pub struct ClientWriteResponse<R: AppDataResponse> { /// The log index of the successfully processed client request. pub index: u64, /// Application specific response data. #[serde(bound = "R: AppDataResponse")] pub data: R, }