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use crate::cache::{CacheLookup, CacheLookupState, CacheStore};
use crate::Fetcher;
use std::collections::HashSet;
use std::sync::Arc;
/// Used to batch and cache loads from some datastore. A `Batcher` can be used
/// with any type that implements [`Fetcher`]. `Batcher`s are asynchronous, and
/// designed to be passed and shared between threads or tasks. Cloning a
/// `Batcher` is shallow and can be used to use the same `Fetcher` across
/// multiple threads or tasks.
///
/// A `Batcher` is designed primarily around batching database lookups-- for
/// example, fetching a user from a user ID, where a signle query to retrieve
/// 50 users by ID is significantly faster than 50 separate queries to look up
/// the same set of users.
///
/// A `Batcher` is designed to be ephemeral. In the context of a web service,
/// this means callers should most likely create a new `Batcher` for each
/// request, and **not** a `Batcher` shared across multiple requests.
/// `Batcher`s have no concept of cache invalidation, so old values are stored
/// indefinitely (which means callers may get stale data or may exhaust memory
/// endlessly).
///
/// `Batcher`s introduce a small amount of latency for loads. Each time a
/// `Batcher` receives a key to fetch that hasn't been cached (or a set of
/// keys), it will first wait for more keys to build a batch. The load will only
/// trigger after a timeout is reached or once enough keys have been queued in
/// the batch. See [`BatcherBuilder`](struct.BatcherBuilder.html) for options
/// to tweak latency and batch sizes.
///
/// ## Load semantics
///
/// If the underlying [`Fetcher`] returns an error during the batch request,
/// then all pending [`load`](Batcher::load) and [`load_many`](Batcher::load_many)
/// requests will fail. Subsequent calls to [`load`](Batcher::load) or
/// [`load_many`](Batcher::load_many) with the same keys **will retry**.
///
/// If the underlying [`Fetcher`] succeeds but does not return a value for a
/// given key during a batch request, then the `Batcher` will mark that key as
/// "not found" and an error value of [`NotFound`](LoadError::NotFound) will be
/// returned to all pending [`load`](struct.Batcher.html#method.load) and
/// [`load_many`](struct.Batcher.html#method.load_many) requests. The
/// "not found" status will be preserved, so subsequent calls with the same key
/// will fail and **will not retry**.
pub struct Batcher<F>
where
F: Fetcher,
{
label: String,
cache_store: CacheStore<F::Key, F::Value>,
_fetch_task: Arc<tokio::task::JoinHandle<()>>,
fetch_request_tx: tokio::sync::mpsc::Sender<FetchRequest<F::Key>>,
}
impl<F> Batcher<F>
where
F: Fetcher + Send + Sync + 'static,
{
/// Create a new `Batcher` that uses the given [`Fetcher`] to retrieve data.
/// Returns a [`BatcherBuilder`], which can be used to customize the
/// `Batcher`. Call [`.finish()`](BatcherBuilder::finish) to create the `Batcher`.
///
/// # Examples
///
/// Creating a `Batcher` with default options:
///
/// ```
/// # use async_trait::async_trait;
/// # use ultra_batch::{Batcher, Fetcher, Cache};
/// # struct UserFetcher;
/// # impl UserFetcher {
/// # fn new(db_conn: ()) -> Self { UserFetcher }
/// # }
/// # #[async_trait]
/// # impl Fetcher for UserFetcher {
/// # type Key = ();
/// # type Value = ();
/// # type Error = anyhow::Error;
/// # async fn fetch(&self, keys: &[()], values: &mut Cache<'_, (), ()>) -> anyhow::Result<()> {
/// # unimplemented!();
/// # }
/// # }
/// # #[tokio::main] async fn main() -> anyhow::Result<()> {
/// # let db_conn = ();
/// let user_fetcher = UserFetcher::new(db_conn);
/// let batcher = Batcher::build(user_fetcher).finish();
/// # Ok(())
/// # }
/// ```
///
/// Creating a `Batcher` with custom options:
///
/// ```
/// # use async_trait::async_trait;
/// # use ultra_batch::{Batcher, Fetcher, Cache};
/// # struct UserFetcher;
/// # impl UserFetcher {
/// # fn new(db_conn: ()) -> Self { UserFetcher }
/// # }
/// # #[async_trait]
/// # impl Fetcher for UserFetcher {
/// # type Key = ();
/// # type Value = ();
/// # type Error = anyhow::Error;
/// # async fn fetch(&self, keys: &[()], values: &mut Cache<'_, (), ()>) -> anyhow::Result<()> {
/// # unimplemented!();
/// # }
/// # }
/// # #[tokio::main] async fn main() -> anyhow::Result<()> {
/// # let db_conn = ();
/// let user_fetcher = UserFetcher::new(db_conn);
/// let batcher = Batcher::build(user_fetcher)
/// .eager_batch_size(Some(50))
/// .delay_duration(tokio::time::Duration::from_millis(5))
/// .finish();
/// # Ok(()) }
/// ```
pub fn build(fetcher: F) -> BatcherBuilder<F> {
BatcherBuilder {
fetcher,
delay_duration: tokio::time::Duration::from_millis(10),
eager_batch_size: Some(100),
label: "unlabeled-batcher".to_string(),
}
}
/// Load the value with the associated key, either by calling the `Fetcher`
/// or by loading the cached value. Returns an error if the value could
/// not be loaded or if a value for the given key was not found.
///
/// See the type-level docs for [`Batcher`](#load-semantics) for more
/// detailed loading semantics.
#[tracing::instrument(skip_all, fields(batcher = %self.label))]
pub async fn load(&self, key: F::Key) -> Result<F::Value, LoadError> {
let mut values = self.load_keys(&[key]).await?;
Ok(values.remove(0))
}
/// Load all the values for the given keys, either by calling the `Fetcher`
/// or by loading cached values. Values are returned in the same order as
/// the input keys. Returns an error if _any_ load fails.
///
/// See the type-level docs for [`Batcher`](#load-semantics) for more
/// detailed loading semantics.
#[tracing::instrument(skip_all, fields(batcher = %self.label, num_keys = keys.len()))]
pub async fn load_many(&self, keys: &[F::Key]) -> Result<Vec<F::Value>, LoadError> {
let values = self.load_keys(keys).await?;
Ok(values)
}
async fn load_keys(&self, keys: &[F::Key]) -> Result<Vec<F::Value>, LoadError> {
let mut cache_lookup = CacheLookup::new(keys.to_vec());
match cache_lookup.lookup(&self.cache_store) {
CacheLookupState::Done(result) => {
tracing::debug!(batcher = %self.label, "all keys have already been looked up");
return result;
}
CacheLookupState::Pending => {}
}
let pending_keys = cache_lookup.pending_keys();
let fetch_request_tx = self.fetch_request_tx.clone();
let (result_tx, result_rx) = tokio::sync::oneshot::channel();
tracing::debug!(
num_pending_keys = pending_keys.len(),
batcher = %self.label,
"sending a batch of keys to fetch",
);
let fetch_request = FetchRequest {
keys: pending_keys,
result_tx,
};
fetch_request_tx
.send(fetch_request)
.await
.map_err(|_| LoadError::SendError)?;
match result_rx.await {
Ok(Ok(())) => {
tracing::debug!(batcher = %self.label, "fetch response returned successfully");
}
Ok(Err(fetch_error)) => {
tracing::info!("error returned while fetching keys: {}", fetch_error);
return Err(LoadError::FetchError(fetch_error));
}
Err(recv_error) => {
panic!(
"Batch result channel for batcher {batcher} hung up with error: {error}",
batcher = self.label,
error = recv_error,
);
}
}
match cache_lookup.lookup(&self.cache_store) {
CacheLookupState::Done(result) => {
tracing::debug!("all keys have now been looked up");
result
}
CacheLookupState::Pending => {
panic!(
"Batch result for batcher {batcher} is still pending after result channel was sent",
batcher = self.label,
);
}
}
}
}
impl<F> Clone for Batcher<F>
where
F: Fetcher,
{
fn clone(&self) -> Self {
Batcher {
cache_store: self.cache_store.clone(),
_fetch_task: self._fetch_task.clone(),
fetch_request_tx: self.fetch_request_tx.clone(),
label: self.label.clone(),
}
}
}
/// Used to configure a new [`Batcher`]. A `BatcherBuilder` is returned from
/// [`Batcher::build`].
pub struct BatcherBuilder<F>
where
F: Fetcher + Send + Sync + 'static,
{
fetcher: F,
delay_duration: tokio::time::Duration,
eager_batch_size: Option<usize>,
label: String,
}
impl<F> BatcherBuilder<F>
where
F: Fetcher + Send + Sync + 'static,
{
/// The maximum amount of time the [`Batcher`] will wait to queue up more
/// keys before calling the [`Fetcher`].
pub fn delay_duration(mut self, delay: tokio::time::Duration) -> Self {
self.delay_duration = delay;
self
}
/// The maximum number of keys to wait for before eagerly calling the
/// [`Fetcher`]. A value of `Some(n)` will load the batch once `n` or more
/// keys have been queued (or once the timeout set by
/// [`delay_duration`](BatcherBuilder::delay_duration) is reached, whichever
/// comes first). A value of `None` will never eagerly dispatch the queue,
/// and the [`Batcher`] will always wait for the timeout set by
/// [`delay_duration`](BatcherBuilder::delay_duration).
///
/// Note that `eager_batch_size` **does not** set an upper limit on the
/// batch! For example, if [`Batcher::load_many`] is called with more than
/// `eager_batch_size` items, then the batch will be sent immediately with
/// _all_ of the provided keys.
pub fn eager_batch_size(mut self, eager_batch_size: Option<usize>) -> Self {
self.eager_batch_size = eager_batch_size;
self
}
/// Set a label for the [`Batcher`]. This is only used to improve diagnostic
/// messages, such as logs.
pub fn label(mut self, label: impl Into<String>) -> Self {
self.label = label.into();
self
}
/// Create and return a [`Batcher`] with the given options.
pub fn finish(self) -> Batcher<F> {
let cache_store = CacheStore::new();
let (fetch_request_tx, mut fetch_request_rx) =
tokio::sync::mpsc::channel::<FetchRequest<F::Key>>(1);
let label = self.label.clone();
let fetch_task = tokio::spawn({
let cache_store = cache_store.clone();
async move {
'task: loop {
// Wait for some keys to come in
let mut pending_keys = HashSet::new();
let mut result_txs = vec![];
tracing::trace!(batcher = %self.label, "waiting for keys to fetch...");
match fetch_request_rx.recv().await {
Some(fetch_request) => {
tracing::trace!(batcher = %self.label, num_fetch_request_keys = fetch_request.keys.len(), "received initial fetch request");
for key in fetch_request.keys {
pending_keys.insert(key);
}
result_txs.push(fetch_request.result_tx);
}
None => {
// Fetch queue closed, so we're done
break 'task;
}
};
// Wait for more keys
'wait_for_more_keys: loop {
let should_run_batch_now = match self.eager_batch_size {
Some(eager_batch_size) => pending_keys.len() >= eager_batch_size,
None => false,
};
if should_run_batch_now {
// We have enough keys already, so don't wait for more
tracing::trace!(
batcher = %self.label,
num_pending_keys = pending_keys.len(),
eager_batch_size = ?self.eager_batch_size,
"batch filled up, ready to fetch keys now",
);
break 'wait_for_more_keys;
}
let delay = tokio::time::sleep(self.delay_duration);
tokio::pin!(delay);
tokio::select! {
fetch_request = fetch_request_rx.recv() => {
match fetch_request {
Some(fetch_request) => {
tracing::trace!(batcher = %self.label, num_fetch_request_keys = fetch_request.keys.len(), "retrieved additional fetch request");
for key in fetch_request.keys {
pending_keys.insert(key);
}
result_txs.push(fetch_request.result_tx);
}
None => {
// Fetch queue closed, so we're done waiting for keys
tracing::debug!(batcher = %self.label, num_pending_keys = pending_keys.len(), "fetch channel closed");
break 'wait_for_more_keys;
}
}
}
_ = &mut delay => {
// Reached delay, so we're done waiting for keys
tracing::trace!(
batcher = %self.label,
num_pending_keys = pending_keys.len(),
"delay reached while waiting for more keys to fetch"
);
break 'wait_for_more_keys;
}
};
}
let result = {
let mut cache = cache_store.as_cache();
tracing::trace!(batcher = %self.label, num_pending_keys = pending_keys.len(), num_pending_channels = result_txs.len(), "fetching keys");
let pending_keys: Vec<_> = pending_keys.into_iter().collect();
let result = self
.fetcher
.fetch(&pending_keys, &mut cache)
.await
.map_err(|error| error.to_string());
if result.is_ok() {
cache.mark_keys_not_found(pending_keys);
}
result
};
for result_tx in result_txs {
// Ignore error if receiver was already closed
let _ = result_tx.send(result.clone());
}
}
}
});
Batcher {
label,
cache_store,
_fetch_task: Arc::new(fetch_task),
fetch_request_tx,
}
}
}
struct FetchRequest<K> {
keys: Vec<K>,
result_tx: tokio::sync::oneshot::Sender<Result<(), String>>,
}
/// Error indicating that loading one or more values from a [`Batcher`]
/// failed.
#[derive(Debug, thiserror::Error)]
pub enum LoadError {
/// The [`Fetcher`] returned an error while loading the batch. The message
/// contains the error message specified by [`Fetcher::Error`].
#[error("error while fetching from batch: {}", _0)]
FetchError(String),
/// The request could not be sent to the [`Batcher`].
#[error("error sending fetch request")]
SendError,
/// The [`Fetcher`] did not return a value for one or more keys in the batch.
#[error("value not found")]
NotFound,
}