Merge #5044

5044: Adds new metrics to prometheus r=irevoire a=PedroTurik

not 100% confident in this solution, especially because i couldn't make the "Search Queue searches waiting" metric give me any value other than 0 with my local testing 😆. But i believe it solves the Issue.

# Pull Request

## Related issue
Fixes #4998 

## What does this PR do?
### Adds new metrics to prometheus;
- SearchQueue size, 
- SearchQueue searches running, 
- and Search Queue searches waiting.

## PR checklist
Please check if your PR fulfills the following requirements:
- [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
- [x] Have you read the contributing guidelines?
- [x] Have you made sure that the title is accurate and descriptive of the changes?

Co-authored-by: Pedro Turik Firmino <pedroturik@gmail.com>

crates/meilisearch/src/metrics.rs

@@ -49,4 +49,18 @@ lazy_static! {
     pub static ref MEILISEARCH_IS_INDEXING: IntGauge =
         register_int_gauge!(opts!("meilisearch_is_indexing", "Meilisearch Is Indexing"))
             .expect("Can't create a metric");
+    pub static ref MEILISEARCH_SEARCH_QUEUE_SIZE: IntGauge = register_int_gauge!(opts!(
+        "meilisearch_search_queue_size",
+        "Meilisearch Search Queue Size"
+    ))
+    .expect("Can't create a metric");
+    pub static ref MEILISEARCH_SEARCHES_RUNNING: IntGauge =
+        register_int_gauge!(opts!("meilisearch_searches_running", "Meilisearch Searches Running"))
+            .expect("Can't create a metric");
+    pub static ref MEILISEARCH_SEARCHES_WAITING_TO_BE_PROCESSED: IntGauge =
+        register_int_gauge!(opts!(
+            "meilisearch_searches_waiting_to_be_processed",
+            "Meilisearch Searches Being Processed"
+        ))
+        .expect("Can't create a metric");
 }

crates/meilisearch/src/routes/metrics.rs

@@ -10,6 +10,7 @@ use prometheus::{Encoder, TextEncoder};
 use crate::extractors::authentication::policies::ActionPolicy;
 use crate::extractors::authentication::{AuthenticationError, GuardedData};
 use crate::routes::create_all_stats;
+use crate::search_queue::SearchQueue;
 
 pub fn configure(config: &mut web::ServiceConfig) {
     config.service(web::resource("").route(web::get().to(get_metrics)));
@@ -18,6 +19,7 @@ pub fn configure(config: &mut web::ServiceConfig) {
 pub async fn get_metrics(
     index_scheduler: GuardedData<ActionPolicy<{ actions::METRICS_GET }>, Data<IndexScheduler>>,
     auth_controller: Data<AuthController>,
+    search_queue: web::Data<SearchQueue>,
 ) -> Result<HttpResponse, ResponseError> {
     index_scheduler.features().check_metrics()?;
     let auth_filters = index_scheduler.filters();
@@ -35,6 +37,11 @@ pub async fn get_metrics(
     crate::metrics::MEILISEARCH_USED_DB_SIZE_BYTES.set(response.used_database_size as i64);
     crate::metrics::MEILISEARCH_INDEX_COUNT.set(response.indexes.len() as i64);
 
+    crate::metrics::MEILISEARCH_SEARCH_QUEUE_SIZE.set(search_queue.capacity() as i64);
+    crate::metrics::MEILISEARCH_SEARCHES_RUNNING.set(search_queue.searches_running() as i64);
+    crate::metrics::MEILISEARCH_SEARCHES_WAITING_TO_BE_PROCESSED
+        .set(search_queue.searches_waiting() as i64);
+
     for (index, value) in response.indexes.iter() {
         crate::metrics::MEILISEARCH_INDEX_DOCS_COUNT
             .with_label_values(&[index])

crates/meilisearch/src/search_queue.rs

@@ -18,6 +18,8 @@
 //!                         And should drop the Permit only once you have freed all the RAM consumed by the method.
 
 use std::num::NonZeroUsize;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::Arc;
 use std::time::Duration;
 
 use rand::rngs::StdRng;
@@ -33,6 +35,8 @@ pub struct SearchQueue {
     /// If we have waited longer than this to get a permit, we should abort the search request entirely.
     /// The client probably already closed the connection, but we have no way to find out.
     time_to_abort: Duration,
+    searches_running: Arc<AtomicUsize>,
+    searches_waiting_to_be_processed: Arc<AtomicUsize>,
 }
 
 /// You should only run search requests while holding this permit.
@@ -68,14 +72,41 @@ impl SearchQueue {
         // so let's not allocate any RAM and keep a capacity of 1.
         let (sender, receiver) = mpsc::channel(1);
 
-        tokio::task::spawn(Self::run(capacity, paralellism, receiver));
-        Self { sender, capacity, time_to_abort: Duration::from_secs(60) }
+        let instance = Self {
+            sender,
+            capacity,
+            time_to_abort: Duration::from_secs(60),
+            searches_running: Default::default(),
+            searches_waiting_to_be_processed: Default::default(),
+        };
+
+        tokio::task::spawn(Self::run(
+            capacity,
+            paralellism,
+            receiver,
+            Arc::clone(&instance.searches_running),
+            Arc::clone(&instance.searches_waiting_to_be_processed),
+        ));
+
+        instance
     }
 
     pub fn with_time_to_abort(self, time_to_abort: Duration) -> Self {
         Self { time_to_abort, ..self }
     }
 
+    pub fn capacity(&self) -> usize {
+        self.capacity
+    }
+
+    pub fn searches_running(&self) -> usize {
+        self.searches_running.load(Ordering::Relaxed)
+    }
+
+    pub fn searches_waiting(&self) -> usize {
+        self.searches_waiting_to_be_processed.load(Ordering::Relaxed)
+    }
+
     /// This function is the main loop, it's in charge on scheduling which search request should execute first and
     /// how many should executes at the same time.
     ///
@@ -84,6 +115,8 @@ impl SearchQueue {
         capacity: usize,
         parallelism: NonZeroUsize,
         mut receive_new_searches: mpsc::Receiver<oneshot::Sender<Permit>>,
+        metric_searches_running: Arc<AtomicUsize>,
+        metric_searches_waiting: Arc<AtomicUsize>,
     ) {
         let mut queue: Vec<oneshot::Sender<Permit>> = Default::default();
         let mut rng: StdRng = StdRng::from_entropy();
@@ -133,6 +166,9 @@ impl SearchQueue {
                     queue.push(search_request);
                 },
             }
+
+            metric_searches_running.store(searches_running, Ordering::Relaxed);
+            metric_searches_waiting.store(queue.len(), Ordering::Relaxed);
         }
     }