Improve search job placer logging (#5514)

quickwit/quickwit-search/src/search_job_placer.rs

@@ -28,7 +28,7 @@ use async_trait::async_trait;
 use quickwit_common::pubsub::EventSubscriber;
 use quickwit_common::rendezvous_hasher::{node_affinity, sort_by_rendez_vous_hash};
 use quickwit_proto::search::{ReportSplit, ReportSplitsRequest};
-use tracing::warn;
+use tracing::{info, warn};
 
 use crate::{SearchJob, SearchServiceClient, SearcherPool, SEARCH_METRICS};
 
@@ -80,7 +80,7 @@ impl EventSubscriber<ReportSplitsRequest> for SearchJobPlacer {
                 .max_by_key(|node_addr| node_affinity(*node_addr, &report_split.split_id))
                 // This actually never happens thanks to the if-condition at the
                 // top of this function.
-                .expect("`nodes` should not be empty.");
+                .expect("`nodes` should not be empty");
             splits_per_node
                 .entry(*node_addr)
                 .or_default()
@@ -150,39 +150,51 @@ impl SearchJobPlacer {
         mut jobs: Vec<J>,
         excluded_addrs: &HashSet<SocketAddr>,
     ) -> anyhow::Result<impl Iterator<Item = (SearchServiceClient, Vec<J>)>> {
-        let num_nodes = self.searcher_pool.len();
+        let mut all_nodes = self.searcher_pool.pairs();
 
-        let mut candidate_nodes: Vec<CandidateNodes> = self
-            .searcher_pool
-            .pairs()
+        if all_nodes.is_empty() {
+            bail!(
+                "failed to assign search jobs: there are no available searcher nodes in the \
+                 cluster"
+            );
+        }
+        if !excluded_addrs.is_empty() && excluded_addrs.len() < all_nodes.len() {
+            all_nodes.retain(|(grpc_addr, _)| !excluded_addrs.contains(grpc_addr));
+
+            // This should never happen, but... belt and suspenders policy.
+            if all_nodes.is_empty() {
+                bail!(
+                    "failed to assign search jobs: there are no searcher nodes candidates for \
+                     these jobs"
+                );
+            }
+            info!(
+                "excluded {} nodes from search job placement, {} remaining",
+                excluded_addrs.len(),
+                all_nodes.len()
+            );
+        }
+        let mut candidate_nodes: Vec<_> = all_nodes
             .into_iter()
-            .filter(|(grpc_addr, _)| {
-                excluded_addrs.is_empty()
-                    || excluded_addrs.len() == num_nodes
-                    || !excluded_addrs.contains(grpc_addr)
-            })
-            .map(|(grpc_addr, client)| CandidateNodes {
+            .map(|(grpc_addr, client)| CandidateNode {
                 grpc_addr,
                 client,
                 load: 0,
             })
             .collect();
 
-        if candidate_nodes.is_empty() {
-            bail!(
-                "failed to assign search jobs. there are no available searcher nodes in the pool"
-            );
-        }
         jobs.sort_unstable_by(Job::compare_cost);
 
+        let num_nodes = candidate_nodes.len();
+
         let mut job_assignments: HashMap<SocketAddr, (SearchServiceClient, Vec<J>)> =
             HashMap::with_capacity(num_nodes);
 
         let total_load: usize = jobs.iter().map(|job| job.cost()).sum();
 
         // allow around 5% disparity. Round up so we never end up in a case where
         // target_load * num_nodes < total_load
-        // some of our tests needs 2 splits to be put on 2 different searchers. It makes sens for
+        // some of our tests needs 2 splits to be put on 2 different searchers. It makes sense for
         // these tests to keep doing so (testing root merge). Either we can make the allowed
         // difference stricter, find the right split names ("split6" instead of "split2" works).
         // or modify mock_split_meta() so that not all splits have the same job cost
@@ -239,25 +251,25 @@ impl SearchJobPlacer {
 }
 
 #[derive(Debug, Clone)]
-struct CandidateNodes {
+struct CandidateNode {
     pub grpc_addr: SocketAddr,
     pub client: SearchServiceClient,
     pub load: usize,
 }
 
-impl Hash for CandidateNodes {
+impl Hash for CandidateNode {
     fn hash<H: Hasher>(&self, state: &mut H) {
         self.grpc_addr.hash(state);
     }
 }
 
-impl PartialEq for CandidateNodes {
+impl PartialEq for CandidateNode {
     fn eq(&self, other: &Self) -> bool {
         self.grpc_addr == other.grpc_addr
     }
 }
 
-impl Eq for CandidateNodes {}
+impl Eq for CandidateNode {}
 
 /// Groups jobs by index id and returns a list of `SearchJob` per index
 pub fn group_jobs_by_index_id(