Improve reliability, simplicity and recoverability of BGSAVE (#153)
* Create a new file on writing to flock-ed file This fix is a very important one in two ways. Say we have an user A. They go ahead and launch skyd. skyd creates a data.bin file. Now A just deletes the data.bin file for fun. Funny enough, this never causes flock to error! Why? Well because the descriptor/handle is still valid and was just unlinked from the current directory. But this might seem silly since the user exits with a 'successfully saved notice' only to find that the file never existed and all of their data was lost. That's bad. There's a hidden problem in our current approach too, apart from this. Our writing process begins by truncating the old file and then writing to it by placing the cursor at 0. Nice, but what if this operation just crashes. So we lost the current data AND the old data. Not good. This commit does a better thing: it creates a new temporary file, locks it before writing and then flushes the current data to the temporary file. Once that succeeds, it replaces the old data.bin file with the newly created file. This solves both the problems mentioned here for us: 1. No more of the silly error 2. If BGSAVE crashes in between, we can be sure that at least the last data.bin file is in proper shape and not half truncated or so. This commit further moves the background services into their own module(s) for easy management. * Fix CI scripts Fixes: 1. Our custom runner (drone/.ci.yml) was modified to kill the skyd process once done since this pipeline is not ephemeral. 2. GHA for some reason ignores any error in the test step and proceeds to kill the skyd process without erroring. Since GHA runners are ephemeral, we don't need to do this manually.next
Sayan
3 years ago
committed by
GitHub
parent
b4c44e4684
commit
790558d2c7
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/*
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* Created on Sun May 16 2021
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*
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* This file is a part of Skytable
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* Skytable (formerly known as TerrabaseDB or Skybase) is a free and open-source
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* NoSQL database written by Sayan Nandan ("the Author") with the
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* vision to provide flexibility in data modelling without compromising
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* on performance, queryability or scalability.
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*
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* Copyright (c) 2021, Sayan Nandan <ohsayan@outlook.com>
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU Affero General Public License for more details.
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*
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* You should have received a copy of the GNU Affero General Public License
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* along with this program. If not, see <https://www.gnu.org/licenses/>.
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*
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*/
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use crate::config::BGSave;
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use crate::coredb::CoreDB;
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use crate::dbnet::Terminator;
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use crate::diskstore::{self, flock};
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#[cfg(not(test))]
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use diskstore::PERSIST_FILE;
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use libsky::TResult;
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use std::fs;
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use tokio::time::{self, Duration};
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const SKY_TEMP_FILE: &str = "__skydata_file.bin";
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#[cfg(test)]
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/// The **test location** for the PERSIST_FILE
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pub const BGSAVE_DIRECTORY_TESTING_LOC: &str = "skydata_bgsavetest.bin";
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/// The bgsave_scheduler calls the bgsave task in `CoreDB` after `every` seconds
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///
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/// The time after which the scheduler will wake up the BGSAVE task is determined by
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/// `bgsave_cfg` which is to be passed as an argument. If BGSAVE is disabled, this function
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/// immediately returns
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pub async fn bgsave_scheduler(handle: CoreDB, bgsave_cfg: BGSave, mut terminator: Terminator) {
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match bgsave_cfg {
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BGSave::Enabled(duration) => {
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// If we're here - the user doesn't trust his power supply or just values
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// his data - which is good! So we'll turn this into a `Duration`
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let duration = Duration::from_secs(duration);
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loop {
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tokio::select! {
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// Sleep until `duration` from the current time instant
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_ = time::sleep_until(time::Instant::now() + duration) => {
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let cloned_handle = handle.clone();
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// we spawn this process just to ensure that it doesn't block the runtime's workers
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// dedicated to async tasks (non-blocking)
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tokio::task::spawn_blocking(move || {
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let owned_handle = cloned_handle;
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let _ = bgsave_blocking_section(owned_handle);
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}).await.expect("Something caused the background service to panic");
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}
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// Otherwise wait for a notification
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_ = terminator.receive_signal() => {
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// we got a notification to quit; so break out
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break;
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}
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}
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}
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}
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BGSave::Disabled => {
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// the user doesn't bother about his data; cool, let's not bother about it either
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}
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}
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log::info!("BGSAVE service has exited");
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}
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/// This is a _raw_ version of what Sky's persistence does and is **blocking in nature** since it does
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/// a good amount of disk I/O (which totally depends on the size of the dataset though)
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/// There's nothing dangerous about this really and hence it isn't as _raw_ as it sounds. This method accepts
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/// a handle to a [`coredb::CoreDB`] and uses that to acquire a read lock. This method will create a temporary
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/// file and lock it. It then passes an immutable HTable reference to [`diskstore::flush_data`] which flushes the data to our
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/// temporary locked file. Once the data is successfully flushed, the new temporary file replaces the old data file
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/// by using [`fs::rename`]. This provides us with two gurantees:
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/// 1. No silly logic is seen if the user deletes the data.bin file and yet BGSAVE doesn't complain
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/// 2. If this method crashes midway, we can still be sure that the old file is intact
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pub fn _bgsave_blocking_section(handle: &CoreDB) -> TResult<()> {
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// first lock our temporary file
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let mut file = flock::FileLock::lock(SKY_TEMP_FILE)?;
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// get a read lock on the coretable
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let lock = handle.acquire_read();
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diskstore::flush_data(&mut file, lock.get_ref())?;
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// now rename the file
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#[cfg(not(test))]
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fs::rename(SKY_TEMP_FILE, &*PERSIST_FILE)?;
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#[cfg(test)]
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fs::rename(SKY_TEMP_FILE, BGSAVE_DIRECTORY_TESTING_LOC)?;
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// now unlock the file
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file.unlock()?;
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// close the file
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drop(file);
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// drop the lock since we're done writing the file
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drop(lock);
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Ok(())
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}
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/// This just wraps around [`_bgsave_blocking_section`] and prints nice log messages depending on the outcome
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fn bgsave_blocking_section(handle: CoreDB) -> bool {
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match _bgsave_blocking_section(&handle) {
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Ok(_) => {
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log::info!("BGSAVE completed successfully");
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drop(handle.unpoison());
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true
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}
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Err(e) => {
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log::info!("BGSAVE failed with error: {}", e);
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drop(handle.poison());
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false
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}
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}
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}
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@ -0,0 +1,28 @@
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/*
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* Created on Sun May 16 2021
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*
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* This file is a part of Skytable
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* Skytable (formerly known as TerrabaseDB or Skybase) is a free and open-source
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* NoSQL database written by Sayan Nandan ("the Author") with the
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* vision to provide flexibility in data modelling without compromising
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* on performance, queryability or scalability.
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*
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* Copyright (c) 2021, Sayan Nandan <ohsayan@outlook.com>
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU Affero General Public License for more details.
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*
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* You should have received a copy of the GNU Affero General Public License
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* along with this program. If not, see <https://www.gnu.org/licenses/>.
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*
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*/
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pub mod bgsave;
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pub mod snapshot;
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@ -0,0 +1,73 @@
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/*
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* Created on Sun May 16 2021
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*
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* This file is a part of Skytable
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* Skytable (formerly known as TerrabaseDB or Skybase) is a free and open-source
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* NoSQL database written by Sayan Nandan ("the Author") with the
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* vision to provide flexibility in data modelling without compromising
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* on performance, queryability or scalability.
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*
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* Copyright (c) 2021, Sayan Nandan <ohsayan@outlook.com>
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU Affero General Public License for more details.
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*
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* You should have received a copy of the GNU Affero General Public License
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* along with this program. If not, see <https://www.gnu.org/licenses/>.
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*
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*/
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use crate::config::SnapshotConfig;
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use crate::dbnet::Terminator;
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use crate::diskstore::snapshot::SnapshotEngine;
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use crate::CoreDB;
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use tokio::time::{self, Duration};
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/// The snapshot service
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///
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/// This service calls `SnapEngine::mksnap()` periodically to create snapshots. Whenever
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/// the interval for snapshotting expires or elapses, we create a snapshot. The snapshot service
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/// keeps creating snapshots, as long as the database keeps running. Once [`dbnet::run`] broadcasts
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/// a termination signal, we're ready to quit
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pub async fn snapshot_service(
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handle: CoreDB,
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ss_config: SnapshotConfig,
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mut termination_signal: Terminator,
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) {
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match ss_config {
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SnapshotConfig::Disabled => {
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// since snapshotting is disabled, we'll imediately return
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return;
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}
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SnapshotConfig::Enabled(configuration) => {
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let (duration, atmost) = configuration.decompose();
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let duration = Duration::from_secs(duration);
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let mut sengine = match SnapshotEngine::new(atmost, &handle, None) {
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Ok(ss) => ss,
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Err(e) => {
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log::error!("Failed to initialize snapshot service with error: '{}'", e);
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return;
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}
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};
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loop {
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tokio::select! {
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_ = time::sleep_until(time::Instant::now() + duration) => {
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let _ = sengine.mksnap().await;
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},
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_ = termination_signal.receive_signal() => {
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// time to terminate; goodbye!
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break;
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}
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}
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}
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}
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}
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log::info!("Snapshot service has exited");
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}
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