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This version will demonstrate locking and the health check but will not
run any commands other than the health check.  The next minor version
will have an ergonomic CLI and will run the service commands.
This commit is contained in:
James Andariese 2024-07-23 19:59:24 -05:00
commit 44677c8a90
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[package]
name = "putex"
version = "0.1.0"
edition = "2021"
license = "Apache-2.0"
repository = "https://git.strudelline.net/james/putex.git"
description = "process mutex"
[dependencies]
clap = {version = "4.0", features = ["derive"]}
clap-verbosity-flag = "2.2.0"
anyhow = "1.0"
env_logger = "0.11.3"
log = "0.4.21"
async-nats = "0.35.1"
tokio = { version = "1.38.1", features = ["full"] }
bytes = "1.6.1"

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Copyright 2024 James Andariese
Licensed under the Apache License, Version 2.0 (the "License");
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# `putex`
Process Mutex
Used to manage a lock and timing components of an at-most-once execution system.
### NATS
This currently uses [NATS](https://nats.io) exclusively for locking but there is
the possibility to extend this to using other locks.
NATS is used via optimistic locking of a kv bucket entry. If it's able to write
the lock with its own value before it's changed by the current owner through
renewal or by another agent becoming the owner, it gets to be the owner. The
agents follow a set of rules and a common execution plan which ensures they will
always elect a single leader and never accidentally overlap execution (as long
as the operator also follows the rules!)
## The Variables
###### `R` - Lock Renewal Interval
How often the active agent will run its health check and assert its activeness.
###### `F` - Failure Threshold
How many `R` must pass without the lock being updated before an agent may
take the lock. This is the maximum timeout of a health check as well because
returning after this time will result in an expected failover anyway.
###### `C` - Confirmation Count
How many `R` must pass between a new agent becomes active and when it
is allowed to fence other nodes and activate its service.
## The Rules
### Operator (YOU)
* healthcheck
* The healthcheck will be given an argument of `active` or `standby` indicating
whether the lock is currently held by this agent. If it is `active`, a more
thorough check should be performed, if possible. If it is `standby`, the
healthcheck should indicate whether we believe that `healthcheck active` would
pass after the confirmation cycles
* The health check should not take more than R in normal circumstances but may
run over R in exceptional circumstances which do _not_ require a failover
such as a switch power cycling somewhere causing a 2 second outage where you
have R=1000 and F=3 (so you have allotted 3 seconds and the health check takes
2 seconds to return a result because of the packet loss -- this is greater than
R but less than R*F so a warning will be produced but failover will not occur.
* The health check MUST NOT take more than R*F to run or else it will have lost
its lock to another agent. If the health check might take more than R*F in
circumstances where failover would _not_ be appropriate, R, F, or both should
be increased, with F increasing the overall failover window and R slowing the
system down overall to allow the health check to keep up.
* activation
* The activation script should first fence the other hosts, if possible and
necessary for the use case. The activation script will have been called after
at least C*R so this should happen immediately followed immediately by the
service being started.
* deactivation
* Deactivation must take place within C*R. If deactivation will need more time
than is allotted, increasing C or R will accomplish this with R affecting all
aspects of the system and slowing it down in general.
### Agent Operation Theory
Uses a specific key in a specific bucket in a NATS cluster to lock a
runner for a specific process.
my-hostname:~$ natslock nats.cluster.local locks spof-service my-hostname
Each token should be unique, generally the hostname. These will also be in the
kv store and can serve to document the current state of the system.
The implementation should ensure that time skew does not affect
synchronization.
R = lock renewal interval
F = lock renewal failures before taking the lock
C = lock renewals to wait before starting after takeover
T = lock timeout = R * F
consider the example of F=1 to ensure the logic makes sense. if R = 1s and
F=1 then a renewal needs to happen within a 1s period from when we check
first to when we want to take the lock. With these parameters, after 1s
with no updates, we would expect to take the lock immediately and so T=R*F.
A client has the lock and should run the service when its token is present in
the key for C full renewal interval(s) or immediately if this is the first
revision or if the current token is empty.
* When done after takeover, this gives the former system time to shutdown, if
necessary. C should generally be 1 and the shutdown/fencing mechanism should
be instantaneous (iptables/kill -9).
A client obtains the lock by writing its token to the key.
A client may write to the key when there have been no writes for T seconds or
when the key does not exist.
A client may only write to the key if it does not exist or if it has not been
updated since the last read (in addition to other rules of the protocol).
Does not exist: use create which will fail if the key has been created
Exists: use update with revision to ensure the revision that was read
is also the one being replaced.
Both may be attempted in parallel since only one may possibly succeed.
Any failure to write to the key must result in the underlying application
being killed or otherwise fenced immediately (within R from the last interval).
A client renews its lock after running its health check. The health
check should take less than R but MUST take less than T or else the client
will lose its lock. Since it _must_ lose its lock and _must_ recognize that
this has happened, the health check will always timeout after T and the lock
will be given up immediately.

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/*
Copyright 2024 James Andariese
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 anyhow::{anyhow, Result};
use bytes::Bytes;
use clap::{Parser};
use clap_verbosity_flag::{Verbosity, WarnLevel};
use tokio::process::{Command};
use tokio::time::{sleep, Duration};
use async_nats::{jetstream};
#[derive(Parser)]
struct Cli {
#[command(flatten)]
verbosity: Verbosity<WarnLevel>,
server: String,
bucket: String,
key: String,
token: String,
healthcheck: String,
startup: String,
shutdown: String,
}
struct Invocation {
args: Cli,
js: Option<jetstream::Context>,
latest_revision: u64,
latest_value: Bytes,
r: u64,
f: u64,
c: u64,
starts: u64,
exiting: bool,
active: bool,
}
impl Invocation {
async fn new() -> Result<Self> {
let mut r = Invocation {
args: Cli::parse(),
js: None,
latest_revision: 0,
latest_value: "".into(),
r: 1000,
f: 3,
c: 2,
starts: 0,
exiting: false,
active: false,
};
r.setup().await?;
Ok(r)
}
async fn get_store(&mut self) -> Result<jetstream::kv::Store> {
Ok(self.js.clone().ok_or(anyhow!("Jetstream not connected"))?.get_key_value(&self.args.bucket).await?)
}
async fn update_lock_data(&mut self) -> Result<()> {
let store = self.get_store().await?;
let e = store.entry(&self.args.key).await?;
match e {
Some(k) => {
self.latest_value = k.value.into();
self.latest_revision = k.revision;
}
None => {
self.latest_value = "".into();
self.latest_revision = 0;
}
};
Ok(())
}
async fn clear_lock(&mut self) -> Result<()> {
let store = self.get_store().await?;
store.update(&self.args.key, "".into(), self.latest_revision).await?;
self.update_lock_data().await?;
Ok(())
}
async fn take_lock(&mut self) -> Result<()> {
let store = self.get_store().await?;
store.update(&self.args.key, self.args.token.clone().into(), self.latest_revision).await?;
self.update_lock_data().await?;
Ok(())
}
async fn setup(&mut self) -> Result<()> {
let nc = async_nats::connect(&self.args.server).await?;
println!("info: {:?}", nc.server_info());
self.js = Some(async_nats::jetstream::new(nc));
if self.args.verbosity.is_present() {
env_logger::Builder::new().filter_level(self.args.verbosity.log_level_filter()).init();
} else {
env_logger::Builder::from_env(env_logger::Env::default().default_filter_or("warn")).init();
}
log::error!("error level active");
log::warn!("warn level active");
log::info!("info level active");
log::debug!("debug level active");
log::trace!("trace level active");
Ok(())
}
async fn pump(&mut self) -> Result<()> {
self.update_lock_data().await?;
if self.latest_value == "" || self.latest_value == self.args.token { // lk empty or mine?
log::trace!("lk empty or mine? - yes - Found an empty lock or my own token present");
if self.healthcheck().await? {
log::trace!("health check succeeded");
let max_i = self.f;
for i in 1..=max_i {
if self.take_lock().await.is_ok() {
self.unfence_and_enable_process().await?;
log::trace!("successfully unfenced and enabled process. ending pump cycle.");
return Ok(());
} else {
log::trace!("failed to take lock. attempt {i}/{max_i}");
}
}
} else {
log::info!("sunny side health check failed");
}
// lock failed to write or health check failed at this point
log::trace!("attempting to clear lock and kill process");
match self.kill_process().await {
Ok(()) => {
log::info!("killed process");
}
e => {
log::info!("failed to kill process: {e:?}");
}
}
match self.clear_lock().await {
Ok(()) => {
log::info!("cleared lock");
}
e => {
log::info!("failed to clear lock (will not retry but will wait 1R extra): {e:?}");
self.wait_r(1).await;
}
}
log::trace!("waiting F*R");
self.wait_r(self.f).await;
return Ok(());
} else {
log::trace!("lk empty or mine? - no - Found a different token in the lock");
self.kill_process().await?;
log::trace!("kill_process asserted");
if !self.healthcheck().await? {
log::trace!("initial takeover health check failed. not eligible for takeover. ending pump cycle.");
return Ok(());
}
log::trace!("initial takeover health check succeeded. waiting F for takeover confirmation.");
self.wait_r(self.f).await;
if !self.healthcheck().await? {
log::trace!("confirmation takeover health check failed. no longer eligible for takeover. ending pump cycle.");
return Ok(());
}
log::trace!("confirmation takeover health check succeeded.");
if self.take_lock().await.is_ok() {
log::info!("lock taken. waiting C for other services to die.");
self.wait_r(self.c).await;
} else {
// this happens on every cycle whether the lock _needs_ to be taken or not so
// logging it every time is not helpful.
log::trace!("failed to take lock. ending cycle");
return Ok(());
}
}
log::trace!("ending pump cycle normally.");
Ok(())
}
async fn run(&mut self) -> Result<()> {
let mut round = 0;
self.starts = self.starts + 1;
let this_start = self.starts;
// we must wait C*R before participating in case we had just
// taken the lock and the program crashed and then restarted
// within .00001s because this is the year 2050 and computers
// are really fast now. This is because we must guarantee
// that the previous owner agent has C*R to disable itself.
// we can guarantee at all other times that we've waited the
// requisite time except in circumstances where we have left
// the control of the round_timer loop.
//
// For the same reason, this is also done whenever pump has
// failed. See below.
self.wait_r(self.c).await;
loop {
round = round + 1;
if self.exiting {
log::info!("exiting before round {round}");
break;
}
// BEWARE vvv
// this must be awaited before the loop ends.
let round_timer = sleep(Duration::from_millis(self.r));
// BEWARE ^^^
match self.pump().await {
Ok(()) => {
log::info!("pump {this_start}.{round}: success");
}
e => {
// See longer comment above about C*R
self.wait_r(self.c).await;
log::error!("pump {this_start}.{round}: error: {e:?}");
}
}
// BEWARL PART DEUX
round_timer.await;
}
Ok(())
}
async fn kill_process(&mut self) -> Result<()> {
log::trace!("attempting to kill process");
if self.active {
log::warn!("killing process");
}
self.active = false;
Ok(())
}
async fn unfence_and_enable_process(&mut self) -> Result<()> {
log::trace!("attempting to unfence and enable process");
if !self.active {
log::warn!("starting process");
}
self.active = true;
Ok(())
}
async fn healthcheck(&mut self) -> Result<bool> {
let mut child = Command::new("/bin/sh")
.arg("-c")
.arg(self.args.healthcheck.clone())
.arg("healthcheck")
.arg(if self.active {"active"} else {"standby"})
.spawn()?;
tokio::select! {
status = child.wait() => {
Ok(status?.success())
}
_ = sleep(Duration::from_millis(self.r * self.f)) => {
log::warn!("health check timed out.");
Ok(false)
}
}
}
async fn wait_r(&mut self, count: u64) {
sleep(Duration::from_millis(self.r * count)).await;
}
}
#[tokio::main]
async fn main() -> Result<()> {
log::info!("here we come");
let mut inv = Invocation::new().await?;
log::info!("here we are");
// inv.check().await?;
let rr = inv.run().await;
log::info!("{rr:?}");
Ok(())
}