Add an index->step cache to the PolicyStepsRegistry (#82316)

docs/changelog/82316.yaml

@@ -0,0 +1,5 @@
+pr: 82316
+summary: Add an index->step cache to the `PolicyStepsRegistry`
+area: ILM+SLM
+type: enhancement
+issues: []

x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/ilm/IndexLifecycleService.java

@@ -301,15 +301,25 @@ public class IndexLifecycleService
         if (prevIsMaster != event.localNodeMaster()) {
             this.isMaster = event.localNodeMaster();
             if (this.isMaster) {
+                // we weren't the master, and now we are
                 onMaster(event.state());
             } else {
+                // we were the master, and now we aren't
                 cancelJob();
+                policyRegistry.clear();
             }
         }
 
-        final IndexLifecycleMetadata lifecycleMetadata = event.state().metadata().custom(IndexLifecycleMetadata.TYPE);
-        if (this.isMaster && lifecycleMetadata != null) {
-            triggerPolicies(event.state(), true);
+        // if we're the master, then process deleted indices and trigger policies
+        if (this.isMaster) {
+            for (Index index : event.indicesDeleted()) {
+                policyRegistry.delete(index);
+            }
+
+            final IndexLifecycleMetadata lifecycleMetadata = event.state().metadata().custom(IndexLifecycleMetadata.TYPE);
+            if (lifecycleMetadata != null) {
+                triggerPolicies(event.state(), true);
+            }
         }
     }
 

x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/ilm/PolicyStepsRegistry.java

@@ -18,6 +18,7 @@ import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.TimeValue;
+import org.elasticsearch.core.Tuple;
 import org.elasticsearch.index.Index;
 import org.elasticsearch.license.XPackLicenseState;
 import org.elasticsearch.xcontent.DeprecationHandler;
@@ -48,20 +49,27 @@ import java.util.Optional;
 import java.util.Set;
 import java.util.SortedMap;
 import java.util.TreeMap;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.stream.Collectors;
 
 public class PolicyStepsRegistry {
     private static final Logger logger = LogManager.getLogger(PolicyStepsRegistry.class);
 
+    private final NamedXContentRegistry xContentRegistry;
     private final Client client;
     private final XPackLicenseState licenseState;
+
     // keeps track of existing policies in the cluster state
     private final SortedMap<String, LifecyclePolicyMetadata> lifecyclePolicyMap;
     // keeps track of what the first step in a policy is, the key is policy name
     private final Map<String, Step> firstStepMap;
     // keeps track of a mapping from policy/step-name to respective Step, the key is policy name
     private final Map<String, Map<Step.StepKey, Step>> stepMap;
-    private final NamedXContentRegistry xContentRegistry;
+
+    // tracks an index->step cache, where the indexmetadata is also tracked for cache invalidation/eviction purposes.
+    // for a given index, the step can be cached as long as the indexmetadata (and the policy!) hasn't changed. since
+    // policies change infrequently, the entire cache is cleared on policy change.
+    private final Map<Index, Tuple<IndexMetadata, Step>> cachedSteps = new ConcurrentHashMap<>();
 
     public PolicyStepsRegistry(NamedXContentRegistry xContentRegistry, Client client, XPackLicenseState licenseState) {
         this(new TreeMap<>(), new HashMap<>(), new HashMap<>(), xContentRegistry, client, licenseState);
@@ -98,6 +106,9 @@ public class PolicyStepsRegistry {
     public void update(IndexLifecycleMetadata meta) {
         assert meta != null : "IndexLifecycleMetadata cannot be null when updating the policy steps registry";
 
+        // since the policies (may have) changed, the whole steps cache needs to be thrown out
+        cachedSteps.clear();
+
         DiffableUtils.MapDiff<String, LifecyclePolicyMetadata, Map<String, LifecyclePolicyMetadata>> mapDiff = DiffableUtils.diff(
             lifecyclePolicyMap,
             meta.getPolicyMetadatas(),
@@ -154,6 +165,36 @@ public class PolicyStepsRegistry {
         }
     }
 
+    /**
+     * Remove the entry for an index from the index->step cache.
+     *
+     * We clear the map entirely when the master of the cluster changes, and when any
+     * policy changes, but in a long-lived cluster that doesn't happen to experience
+     * either of those events (and where indices are removed regularly) we still want
+     * the cache to trim deleted indices.
+     *
+     * n.b. even with this, there's still a pretty small chance that a given index
+     * could leak, if we're right in the middle of populating the cache for that
+     * index (in getStep) when we process the delete here, then we'll end up with an
+     * entry that doesn't get deleted until the master changes or a policy changes
+     * -- it's harmless enough
+     */
+    public void delete(Index deleted) {
+        cachedSteps.remove(deleted);
+    }
+
+    /**
+     * Clear internal maps that were populated by update (and others).
+     */
+    public void clear() {
+        // this is potentially large, so it's important to clear it
+        cachedSteps.clear();
+        // these are relatively small, but there's no harm in clearing them
+        lifecyclePolicyMap.clear();
+        firstStepMap.clear();
+        stepMap.clear();
+    }
+
     /**
      * Return all ordered steps for the current policy for the index. Does not
      * resolve steps using the phase caching, but only for the currently existing policy.
@@ -266,6 +307,14 @@ public class PolicyStepsRegistry {
 
     @Nullable
     public Step getStep(final IndexMetadata indexMetadata, final Step.StepKey stepKey) {
+        final Tuple<IndexMetadata, Step> cachedStep = cachedSteps.get(indexMetadata.getIndex());
+        // n.b. we're using instance equality here for the IndexMetadata rather than object equality because it's fast,
+        // this means that we're erring on the side of cache misses (if the IndexMetadata changed in any way, it'll be
+        // a new instance, so we'll miss-and-repopulate the cache for the index in question)
+        if (cachedStep != null && cachedStep.v1() == indexMetadata && cachedStep.v2().getKey().equals(stepKey)) {
+            return cachedStep.v2();
+        }
+
         if (ErrorStep.NAME.equals(stepKey.getName())) {
             return new ErrorStep(new Step.StepKey(stepKey.getPhase(), stepKey.getAction(), ErrorStep.NAME));
         }
@@ -304,7 +353,9 @@ public class PolicyStepsRegistry {
                 + phaseSteps;
 
         // Return the step that matches the given stepKey or else null if we couldn't find it
-        return phaseSteps.stream().filter(step -> step.getKey().equals(stepKey)).findFirst().orElse(null);
+        final Step s = phaseSteps.stream().filter(step -> step.getKey().equals(stepKey)).findFirst().orElse(null);
+        cachedSteps.put(indexMetadata.getIndex(), Tuple.tuple(indexMetadata, s));
+        return s;
     }
 
     /**