Eagerly compute stats as fallback to the cache.

- Refactor all around to avoid spawning indexes more times than necessary

index-scheduler/src/batch.rs

@@ -847,8 +847,10 @@ impl IndexScheduler {
                 // this is a non-critical operation. If it fails, we should not fail
                 // the entire batch.
                 let res = || -> Result<()> {
+                    let index_rtxn = index.read_txn()?;
+                    let stats = crate::index_mapper::IndexStats::new(&index, &index_rtxn)?;
                     let mut wtxn = self.env.write_txn()?;
-                    self.index_mapper.compute_and_store_stats_of(&mut wtxn, &index_uid)?;
+                    self.index_mapper.store_stats_of(&mut wtxn, &index_uid, stats)?;
                     wtxn.commit()?;
                     Ok(())
                 }();
@@ -888,6 +890,10 @@ impl IndexScheduler {
                     )?;
                     index_wtxn.commit()?;
                 }
+
+                // drop rtxn before starting a new wtxn on the same db
+                rtxn.commit()?;
+
                 task.status = Status::Succeeded;
                 task.details = Some(Details::IndexInfo { primary_key });
 
@@ -897,7 +903,9 @@ impl IndexScheduler {
                 // the entire batch.
                 let res = || -> Result<()> {
                     let mut wtxn = self.env.write_txn()?;
-                    self.index_mapper.compute_and_store_stats_of(&mut wtxn, &index_uid)?;
+                    let index_rtxn = index.read_txn()?;
+                    let stats = crate::index_mapper::IndexStats::new(&index, &index_rtxn)?;
+                    self.index_mapper.store_stats_of(&mut wtxn, &index_uid, stats)?;
                     wtxn.commit()?;
                     Ok(())
                 }();

index-scheduler/src/index_mapper/mod.rs

@@ -54,8 +54,11 @@ pub struct IndexMapper {
 
     /// Map an index name with an index uuid currently available on disk.
     pub(crate) index_mapping: Database<Str, UuidCodec>,
-    /// Map an index name with the cached stats associated to the index.
-    pub(crate) index_stats: Database<Str, SerdeJson<IndexStats>>,
+    /// Map an index UUID with the cached stats associated to the index.
+    ///
+    /// Using an UUID forces to use the index_mapping table to recover the index behind a name, ensuring
+    /// consistency wrt index swapping.
+    pub(crate) index_stats: Database<UuidCodec, SerdeJson<IndexStats>>,
 
     /// Path to the folder where the LMDB environments of each index are.
     base_path: PathBuf,
@@ -80,15 +83,39 @@ pub enum IndexStatus {
     Available(Index),
 }
 
+/// The statistics that can be computed from an `Index` object.
 #[derive(Serialize, Deserialize, Debug)]
 pub struct IndexStats {
+    /// Number of documents in the index.
     pub number_of_documents: u64,
+    /// Size of the index' DB, in bytes.
     pub database_size: u64,
+    /// Association of every field name with the number of times it occurs in the documents.
     pub field_distribution: FieldDistribution,
+    /// Creation date of the index.
     pub created_at: OffsetDateTime,
+    /// Date of the last update of the index.
     pub updated_at: OffsetDateTime,
 }
 
+impl IndexStats {
+    /// Compute the stats of an index
+    ///
+    /// # Parameters
+    ///
+    /// - rtxn: a RO transaction for the index, obtained from `Index::read_txn()`.
+    pub fn new(index: &Index, rtxn: &RoTxn) -> Result<Self> {
+        let database_size = index.on_disk_size()?;
+        Ok(IndexStats {
+            number_of_documents: index.number_of_documents(rtxn)?,
+            database_size,
+            field_distribution: index.field_distribution(rtxn)?,
+            created_at: index.created_at(rtxn)?,
+            updated_at: index.updated_at(rtxn)?,
+        })
+    }
+}
+
 impl IndexMapper {
     pub fn new(
         env: &Env,
@@ -149,12 +176,14 @@ impl IndexMapper {
     /// Removes the index from the mapping table and the in-memory index map
     /// but keeps the associated tasks.
     pub fn delete_index(&self, mut wtxn: RwTxn, name: &str) -> Result<()> {
-        self.index_stats.delete(&mut wtxn, name)?;
         let uuid = self
             .index_mapping
             .get(&wtxn, name)?
             .ok_or_else(|| Error::IndexNotFound(name.to_string()))?;
 
+        // Not an error if the index had no stats in cache.
+        self.index_stats.delete(&mut wtxn, &uuid)?;
+
         // Once we retrieved the UUID of the index we remove it from the mapping table.
         assert!(self.index_mapping.delete(&mut wtxn, name)?);
 
@@ -375,26 +404,42 @@ impl IndexMapper {
         Ok(())
     }
 
-    /// Return the stored stats of an index.
+    /// The stats of an index.
+    ///
+    /// If available in the cache, they are directly returned.
+    /// Otherwise, the `Index` is opened to compute the stats on the fly (the result is not cached).
+    /// The stats for an index are cached after each `Index` update.
     pub fn stats_of(&self, rtxn: &RoTxn, index_uid: &str) -> Result<IndexStats> {
-        self.index_stats
+        let uuid = self
+            .index_mapping
             .get(rtxn, index_uid)?
-            .ok_or_else(|| Error::IndexNotFound(index_uid.to_string()))
+            .ok_or_else(|| Error::IndexNotFound(index_uid.to_string()))?;
+
+        match self.index_stats.get(rtxn, &uuid)? {
+            Some(stats) => Ok(stats),
+            None => {
+                let index = self.index(rtxn, index_uid)?;
+                let index_rtxn = index.read_txn()?;
+                IndexStats::new(&index, &index_rtxn)
+            }
+        }
     }
 
-    /// Return the stats of an index and write it in the index-mapper database.
-    pub fn compute_and_store_stats_of(&self, wtxn: &mut RwTxn, index_uid: &str) -> Result<()> {
-        let index = self.index(wtxn, index_uid)?;
-        let database_size = index.on_disk_size()?;
-        let rtxn = index.read_txn()?;
-        let stats = IndexStats {
-            number_of_documents: index.number_of_documents(&rtxn)?,
-            database_size,
-            field_distribution: index.field_distribution(&rtxn)?,
-            created_at: index.created_at(&rtxn)?,
-            updated_at: index.updated_at(&rtxn)?,
-        };
-        self.index_stats.put(wtxn, index_uid, &stats)?;
+    /// Stores the new stats for an index.
+    ///
+    /// Expected usage is to compute the stats the index using `IndexStats::new`, the pass it to this function.
+    pub fn store_stats_of(
+        &self,
+        wtxn: &mut RwTxn,
+        index_uid: &str,
+        stats: IndexStats,
+    ) -> Result<()> {
+        let uuid = self
+            .index_mapping
+            .get(wtxn, index_uid)?
+            .ok_or_else(|| Error::IndexNotFound(index_uid.to_string()))?;
+
+        self.index_stats.put(wtxn, &uuid, &stats)?;
         Ok(())
     }
 

index-scheduler/src/lib.rs

@@ -1245,9 +1245,14 @@ struct IndexBudget {
     task_db_size: usize,
 }
 
+/// The statistics that can be computed from an `Index` object and the scheduler.
+///
+/// Compared with `index_mapper::IndexStats`, it adds the scheduling status.
 #[derive(Debug)]
 pub struct IndexStats {
+    /// Whether this index is currently performing indexation, according to the scheduler.
     pub is_indexing: bool,
+    /// Internal stats computed from the index.
     pub inner_stats: index_mapper::IndexStats,
 }
 

meilisearch/src/routes/indexes/mod.rs

@@ -220,11 +220,15 @@ pub async fn delete_index(
     Ok(HttpResponse::Accepted().json(task))
 }
 
+/// Stats of an `Index`, as known to the `stats` route.
 #[derive(Serialize, Debug)]
 #[serde(rename_all = "camelCase")]
 pub struct IndexStats {
+    /// Number of documents in the index
     pub number_of_documents: u64,
+    /// Whether the index is currently performing indexation, according to the scheduler.
     pub is_indexing: bool,
+    /// Association of every field name with the number of times it occurs in the documents.
     pub field_distribution: FieldDistribution,
 }