fleetfs_raft 0.6.0

// Copyright 2019 TiKV Project Authors. Licensed under Apache-2.0.

// Copyright 2015 The etcd Authors
//
// 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.

/*!

## Creating a Raft node

You can use [`RawNode::new`] to create the Raft node. To create the Raft node, you need to
provide a [`Storage`] component, and a [`Config`] to the [`RawNode::new`] function.

```rust
use fleetfs_raft::{
    Config,
    storage::MemStorage,
    raw_node::RawNode,
};
use slog::{Drain, o};

// Select some defaults, then change what we need.
let config = Config {
    id: 1,
    ..Default::default()
};
// Initialize logger.
let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
// ... Make any configuration changes.
// After, make sure it's valid!
config.validate().unwrap();
// We'll use the built-in `MemStorage`, but you will likely want your own.
// Finally, create our Raft node!
let storage = MemStorage::new_with_conf_state((vec![1], vec![]));
let mut node = RawNode::new(&config, storage, &logger).unwrap();
```

## Ticking the Raft node

Use a timer to tick the Raft node at regular intervals. See the following example using Rust
channel `recv_timeout` to drive the Raft node at least every 100ms, calling
[`tick()`](RawNode::tick) each time.

```rust
# use slog::{Drain, o};
# use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode};
# let config = Config { id: 1, ..Default::default() };
# let store = MemStorage::new_with_conf_state((vec![1], vec![]));
# let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
# let mut node = RawNode::new(&config, store, &logger).unwrap();
# node.raft.become_candidate();
# node.raft.become_leader();
use std::{sync::mpsc::{channel, RecvTimeoutError}, time::{Instant, Duration}};

// We're using a channel, but this could be any stream of events.
let (tx, rx) = channel();
let timeout = Duration::from_millis(100);
let mut remaining_timeout = timeout;

// Send the `tx` somewhere else...

loop {
    let now = Instant::now();

    match rx.recv_timeout(remaining_timeout) {
        Ok(()) => {
            // Let's save this for later.
            unimplemented!()
        },
        Err(RecvTimeoutError::Timeout) => (),
        Err(RecvTimeoutError::Disconnected) => unimplemented!(),
    }

    let elapsed = now.elapsed();
    if elapsed >= remaining_timeout {
        remaining_timeout = timeout;
        // We drive Raft every 100ms.
        node.tick();
    } else {
        remaining_timeout -= elapsed;
    }
#    break;
}
```

## Proposing to, and stepping the Raft node

Using the `propose` function you can drive the Raft node when the client sends a request to the
Raft server. You can call `propose` to add the request to the Raft log explicitly.

In most cases, the client needs to wait for a response for the request. For example, if the
client writes a value to a key and wants to know whether the write succeeds or not, but the
write flow is asynchronous in Raft, so the write log entry must be replicated to other followers,
then committed and at last applied to the state machine, so here we need a way to notify the client
after the write is finished.

One simple way is to use a unique ID for the client request, and save the associated callback
function in a hash map. When the log entry is applied, we can get the ID from the decoded entry,
call the corresponding callback, and notify the client.

You can call the `step` function when you receive the Raft messages from other nodes.

Here is a simple example to use `propose` and `step`:

```rust
# use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode, eraftpb::Message};
# use std::{
#     sync::mpsc::{channel, RecvTimeoutError},
#     time::{Instant, Duration},
#     collections::HashMap
# };
# use slog::{Drain, o};
#
# let config = Config { id: 1, ..Default::default() };
# let store = MemStorage::new_with_conf_state((vec![1], vec![]));
# let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
# let mut node = RawNode::new(&config, store, &logger).unwrap();
# node.raft.become_candidate();
# node.raft.become_leader();
#
# let (tx, rx) = channel();
# let timeout = Duration::from_millis(100);
# let mut remaining_timeout = timeout;
#
enum Msg {
    Propose {
        id: u8,
        callback: Box<dyn Fn() + Send>,
    },
    Raft(Message),
}

// Simulate a message coming down the stream.
tx.send(Msg::Propose { id: 1, callback: Box::new(|| ()) });

let mut cbs = HashMap::new();
loop {
    let now = Instant::now();

    match rx.recv_timeout(remaining_timeout) {
        Ok(Msg::Propose { id, callback }) => {
            cbs.insert(id, callback);
            node.propose(vec![], vec![id]).unwrap();
        }
        Ok(Msg::Raft(m)) => node.step(m).unwrap(),
        Err(RecvTimeoutError::Timeout) => (),
        Err(RecvTimeoutError::Disconnected) => unimplemented!(),
    }

    let elapsed = now.elapsed();
    if elapsed >= remaining_timeout {
        remaining_timeout = timeout;
        // We drive Raft every 100ms.
        node.tick();
    } else {
        remaining_timeout -= elapsed;
    }
    break;
}
```

In the above example, we use a channel to receive the `propose` and `step` messages. We only
propose the request ID to the Raft log. In your own practice, you can embed the ID in your request
and propose the encoded binary request data.

## Processing the `Ready` State

When your Raft node is ticked and running, Raft should enter a `Ready` state. You need to first use
`has_ready` to check whether Raft is ready. If yes, use the `ready` function to get a `Ready`
state:

```rust
# use slog::{Drain, o};
# use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode};
#
# let config = Config { id: 1, ..Default::default() };
# config.validate().unwrap();
# let store = MemStorage::new_with_conf_state((vec![1], vec![]));
# let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
# let mut node = RawNode::new(&config, store, &logger).unwrap();
#
if !node.has_ready() {
    return;
}

// The Raft is ready, we can do something now.
let mut ready = node.ready();
```

The `Ready` state contains quite a bit of information, and you need to check and process them one
by one:

1. Check whether `messages` is empty or not. If not, it means that the node will send messages to
other nodes:

    ```rust
    # use slog::{Drain, o};
    # use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode, StateRole};
    #
    # let config = Config { id: 1, ..Default::default() };
    # config.validate().unwrap();
    # let store = MemStorage::new_with_conf_state((vec![1], vec![]));
    # let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
    # let mut node = RawNode::new(&config, store, &logger).unwrap();
    #
    # if !node.has_ready() {
    #   return;
    # }
    # let mut ready = node.ready();
    #
    if !ready.messages().is_empty() {
        for msg in ready.take_messages() {
            // Send messages to other peers.
        }
    }
    ```

2. Check whether `snapshot` is empty or not. If not empty, it means that the Raft node has received
a Raft snapshot from the leader and we must apply the snapshot:

    ```rust
    # use slog::{Drain, o};
    # use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode};
    #
    # let config = Config { id: 1, ..Default::default() };
    # config.validate().unwrap();
    # let store = MemStorage::new_with_conf_state((vec![1], vec![]));
    # let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
    # let mut node = RawNode::new(&config, store, &logger).unwrap();
    #
    # if !node.has_ready() {
    #   return;
    # }
    # let mut ready = node.ready();
    #
    if !ready.snapshot().is_empty() {
        // This is a snapshot, we need to apply the snapshot at first.
        node.mut_store()
            .wl()
            .apply_snapshot(ready.snapshot().clone())
            .unwrap();
    }

    ```

3. Check whether `committed_entires` is empty or not. If not, it means that there are some newly
committed log entries which you must apply to the state machine. Of course, after applying, you
need to update the applied index and resume `apply` later:

    ```rust
    # use slog::{Drain, o};
    # use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode, eraftpb::EntryType};
    #
    # let config = Config { id: 1, ..Default::default() };
    # config.validate().unwrap();
    # let store = MemStorage::new_with_conf_state((vec![1], vec![]));
    # let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
    # let mut node = RawNode::new(&config, store, &logger).unwrap();
    #
    # if !node.has_ready() {
    #   return;
    # }
    # let mut ready = node.ready();
    #
    # fn handle_conf_change(e:  fleetfs_raft::eraftpb::Entry) {
    # }
    #
    # fn handle_conf_change_v2(e:  fleetfs_raft::eraftpb::Entry) {
    # }
    #
    # fn handle_normal(e:  fleetfs_raft::eraftpb::Entry) {
    # }
    #
    let mut _last_apply_index = 0;
    for entry in ready.take_committed_entries() {
        // Mostly, you need to save the last apply index to resume applying
        // after restart. Here we just ignore this because we use a Memory storage.
        _last_apply_index = entry.index;

        if entry.data.is_empty() {
            // Emtpy entry, when the peer becomes Leader it will send an empty entry.
            continue;
        }

        match entry.get_entry_type() {
            EntryType::EntryNormal => handle_normal(entry),
            // It's recommended to always use `EntryType::EntryConfChangeV2.
            EntryType::EntryConfChange => handle_conf_change(entry),
            EntryType::EntryConfChangeV2 => handle_conf_change_v2(entry),
        }
    }
    ```

    Note, although Raft guarentees only persisted committed entries will be applied,
    but it doesn't guarentee commit index is persisted before being applied. For example,
    if application is restarted after applying committed entries before persisting
    commit index, apply index can be larger than commit index and cause panic. To
    solve the problem, persisting commit index with or before applying entries.
    You can also always assign commit index to the `max(commit_index, applied_index)`
    after restarting, *it may work but potential log loss may also be ignored silently*.

4. Check whether `entries` is empty or not. If not empty, it means that there are newly added
entries but have not been committed yet, we must append the entries to the Raft log:

    ```rust
    # use slog::{Drain, o};
    # use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode};
    #
    # let config = Config { id: 1, ..Default::default() };
    # config.validate().unwrap();
    # let store = MemStorage::new_with_conf_state((vec![1], vec![]));
    # let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
    # let mut node = RawNode::new(&config, store, &logger).unwrap();
    #
    # if !node.has_ready() {
    #   return;
    # }
    # let mut ready = node.ready();
    #
    if !ready.entries().is_empty() {
        // Append entries to the Raft log
        node.mut_store().wl().append(ready.entries()).unwrap();
    }

    ```

5. Check whether `hs` is empty or not. If not empty, it means that the `HardState` of the node has
changed. For example, the node may vote for a new leader, or the commit index has been increased.
We must persist the changed `HardState`:

    ```rust
    # use slog::{Drain, o};
    # use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode};
    #
    # let config = Config { id: 1, ..Default::default() };
    # config.validate().unwrap();
    # let store = MemStorage::new_with_conf_state((vec![1], vec![]));
    # let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
    # let mut node = RawNode::new(&config, store, &logger).unwrap();
    #
    # if !node.has_ready() {
    #   return;
    # }
    # let mut ready = node.ready();
    #
    if let Some(hs) = ready.hs() {
        // Raft HardState changed, and we need to persist it.
        node.mut_store().wl().set_hardstate(hs.clone());
    }
    ```

6. Check whether `persisted_messages` is empty or not. If not, it means that the node will send messages to
other nodes after persisting hardstate, entries and snapshot:

    ```rust
    # use slog::{Drain, o};
    # use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode, StateRole};
    #
    # let config = Config { id: 1, ..Default::default() };
    # config.validate().unwrap();
    # let store = MemStorage::new_with_conf_state((vec![1], vec![]));
    # let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
    # let mut node = RawNode::new(&config, store, &logger).unwrap();
    #
    # if !node.has_ready() {
    #   return;
    # }
    # let mut ready = node.ready();
    #
    if !ready.persisted_messages().is_empty() {
        for msg in ready.take_persisted_messages() {
            // Send persisted messages to other peers.
        }
    }
    ```

7. Call `advance` to notify that the previous work is completed. Get the return value `LightReady`
and handle its `messages` and `committed_entries` like step 1 and step 3 does. Then call `advance_apply`
to advance the applied index inside.

    ```rust
    # use slog::{Drain, o};
    # use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode};
    # use fleetfs_raft::eraftpb::{EntryType, Entry, Message};
    #
    # let config = Config { id: 1, ..Default::default() };
    # config.validate().unwrap();
    # let store = MemStorage::new_with_conf_state((vec![1], vec![]));
    # let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
    # let mut node = RawNode::new(&config, store, &logger).unwrap();
    #
    # if !node.has_ready() {
    #   return;
    # }
    # let mut ready = node.ready();
    #
    # fn handle_messages(msgs: Vec<Message>) {
    # }
    #
    # fn handle_committed_entries(committed_entries: Vec<Entry>) {
    # }
    let mut light_rd = node.advance(ready);
    // Like step 1 and 5, you can use functions to make them behave the same.
    handle_messages(light_rd.take_messages());
    handle_committed_entries(light_rd.take_committed_entries());
    node.advance_apply();
    ```

For more information, check out an [example](examples/single_mem_node/main.rs#L113-L179).

Sometimes it's better not to block the raft machine in IO operation, so that latency of
read/write can be more predictable and the fsync frequencey can be controlled. The crate
supports async ready to offload the IO operation to other thread. The usage is the same as
above except:
1. All writes are not required to be persisted immediately, they can be written into memory caches;
2. Persisted messages should be sent after all coresponding writes are persisted;
3. [`advance_append_async`](RawNode::advance_append_async) is used when all writes are finished
    instead of `advance/advance_append`.
4. Only persisted entries can be committed and applied, so to make progress, all writes should
   be persisted at some point.

## Arbitrary Membership Changes

When building a resilient, scalable distributed system there is a strong need to be able to change
the membership of a peer group *dynamically, without downtime.* This Raft crate supports this via
**Joint Consensus**
([Raft paper, section 6](https://web.stanford.edu/~ouster/cgi-bin/papers/raft-atc14)).

It permits resilient arbitrary dynamic membership changes. A membership change can do any or all of
the following:

* Add peer (learner or voter) *n* to the group.
* Remove a learner *n* from the group.
* Promote a learner to a voter.
* Demote a voter back to learner.
* Replace a node *n* with another node *m*.

For example to promote a learner 4 and demote an existing voter 3:
```no_run
# use fleetfs_raft::{Config, storage::MemStorage, raw_node::RawNode, eraftpb::*};
# use protobuf::Message as PbMessage;
# use slog::{Drain, o};
#
# let mut config = Config { id: 1, ..Default::default() };
# let store = MemStorage::new_with_conf_state((vec![1, 2], vec![]));
# let logger = slog::Logger::root(slog_stdlog::StdLog.fuse(), o!());
# let mut node = RawNode::new(&mut config, store, &logger).unwrap();
let steps = vec![
    raft_proto::new_conf_change_single(4, ConfChangeType::AddNode),
    raft_proto::new_conf_change_single(3, ConfChangeType::RemoveNode),
];
let mut cc = ConfChangeV2::default();
cc.set_changes(steps.into());
node.propose_conf_change(vec![], cc).unwrap();
// After the log is committed and applied
// node.apply_conf_change(&cc).unwrap();
```

This process is a two-phase process, during the midst of it the peer group's leader is managing
**two independent, possibly overlapping peer sets**.

> **Note:** In order to maintain resiliency guarantees  (progress while a majority of both peer sets is
active), it is recommended to wait until the entire peer group has exited the transition phase
before taking old, removed peers offline.

*/

#![cfg_attr(not(feature = "cargo-clippy"), allow(unknown_lints))]
#![deny(clippy::all)]
#![deny(missing_docs)]
#![recursion_limit = "128"]
// This is necessary to support prost and rust-protobuf at the same time.
#![allow(clippy::useless_conversion)]
// This lint recommends some bad choices sometimes.
#![allow(clippy::unnecessary_unwrap)]
// We use `default` method a lot to be support prost and rust-protobuf at the
// same time. And reassignment can be optimized by compiler.
#![allow(clippy::field_reassign_with_default)]

macro_rules! fatal {
    ($logger:expr, $msg:expr) => {{
        let owned_kv = ($logger).list();
        let s = crate::util::format_kv_list(&owned_kv);
        if s.is_empty() {
            panic!("{}", $msg)
        } else {
            panic!("{}, {}", $msg, s)
        }
    }};
    ($logger:expr, $fmt:expr, $($arg:tt)+) => {{
        fatal!($logger, format_args!($fmt, $($arg)+))
    }};
}

mod confchange;
mod config;
mod errors;
mod log_unstable;
mod quorum;
#[cfg(test)]
pub mod raft;
#[cfg(not(test))]
mod raft;
mod raft_log;
pub mod raw_node;
mod read_only;
mod status;
pub mod storage;
mod tracker;
pub mod util;

pub use crate::raft::{
    vote_resp_msg_type, Raft, SoftState, StateRole, CAMPAIGN_ELECTION, CAMPAIGN_PRE_ELECTION,
    CAMPAIGN_TRANSFER, INVALID_ID, INVALID_INDEX,
};
pub use confchange::{Changer, MapChange};
pub use config::Config;
pub use errors::{Error, Result, StorageError};
pub use log_unstable::Unstable;
pub use quorum::joint::Configuration as JointConfig;
pub use quorum::majority::Configuration as MajorityConfig;
pub use raft_log::{RaftLog, NO_LIMIT};
pub use raft_proto::eraftpb;
#[allow(deprecated)]
pub use raw_node::is_empty_snap;
pub use raw_node::{LightReady, Peer, RawNode, Ready, SnapshotStatus};
pub use read_only::{ReadOnlyOption, ReadState};
pub use status::Status;
pub use storage::{RaftState, Storage};
pub use tracker::{Inflights, Progress, ProgressState, ProgressTracker};
pub use util::majority;

pub mod prelude {
    //! A "prelude" for crates using the `raft` crate.
    //!
    //! This prelude is similar to the standard library's prelude in that you'll
    //! almost always want to import its entire contents, but unlike the standard
    //! library's prelude you'll have to do so manually:
    //!
    //! ```
    //! use fleetfs_raft::prelude::*;
    //! ```
    //!
    //! The prelude may grow over time as additional items see ubiquitous use.

    pub use raft_proto::prelude::*;

    pub use crate::config::Config;
    pub use crate::raft::Raft;

    pub use crate::storage::{RaftState, Storage};

    pub use crate::raw_node::{Peer, RawNode, Ready, SnapshotStatus};

    pub use crate::Progress;

    pub use crate::status::Status;

    pub use crate::read_only::{ReadOnlyOption, ReadState};
}

/// The default logger we fall back to when passed `None` in external facing constructors.
///
/// Currently, this is a `log` adaptor behind a `Once` to ensure there is no clobbering.
#[cfg(any(test, feature = "default-logger"))]
pub fn default_logger() -> slog::Logger {
    use slog::{o, Drain};
    use std::sync::{Mutex, Once};

    static LOGGER_INITIALIZED: Once = Once::new();
    static mut LOGGER: Option<slog::Logger> = None;

    let logger = unsafe {
        LOGGER_INITIALIZED.call_once(|| {
            let decorator = slog_term::TermDecorator::new().build();
            let drain = slog_term::CompactFormat::new(decorator).build();
            let drain = slog_envlogger::new(drain);
            LOGGER = Some(slog::Logger::root(Mutex::new(drain).fuse(), o!()));
        });
        LOGGER.as_ref().unwrap()
    };
    if let Some(case) = std::thread::current()
        .name()
        .and_then(|v| v.split(':').last())
    {
        logger.new(o!("case" => case.to_string()))
    } else {
        logger.new(o!())
    }
}

type DefaultHashBuilder = std::hash::BuildHasherDefault<fxhash::FxHasher>;
type HashMap<K, V> = std::collections::HashMap<K, V, DefaultHashBuilder>;
type HashSet<K> = std::collections::HashSet<K, DefaultHashBuilder>;