Add link to MAX_RETRY allocation explain docs (elastic#113657)

Author: matthewabbott

docs/reference/cluster/allocation-explain.asciidoc

@@ -159,6 +159,7 @@ node.
 <5> The decider which led to the `no` decision for the node.
 <6> An explanation as to why the decider returned a `no` decision, with a helpful hint pointing to the setting that led to the decision. In this example, a newly created index has <<indices-get-settings,an index setting>> that requires that it only be allocated to a node named `nonexistent_node`, which does not exist, so the index is unable to allocate.
 
+[[maximum-number-of-retries-exceeded]]
 ====== Maximum number of retries exceeded
 
 The following response contains an allocation explanation for an unassigned
@@ -195,17 +196,19 @@ primary shard that has reached the maximum number of allocation retry attempts.
         {
           "decider": "max_retry",
           "decision" : "NO",
-          "explanation": "shard has exceeded the maximum number of retries [5] on failed allocation attempts - manually call [/_cluster/reroute?retry_failed=true] to retry, [unassigned_info[[reason=ALLOCATION_FAILED], at[2024-07-30T21:04:12.166Z], failed_attempts[5], failed_nodes[[mEKjwwzLT1yJVb8UxT6anw]], delayed=false, details[failed shard on node [mEKjwwzLT1yJVb8UxT6anw]: failed recovery, failure RecoveryFailedException], allocation_status[deciders_no]]]"
+          "explanation": "shard has exceeded the maximum number of retries [5] on failed allocation attempts - manually call [POST /_cluster/reroute?retry_failed] to retry, [unassigned_info[[reason=ALLOCATION_FAILED], at[2024-07-30T21:04:12.166Z], failed_attempts[5], failed_nodes[[mEKjwwzLT1yJVb8UxT6anw]], delayed=false, details[failed shard on node [mEKjwwzLT1yJVb8UxT6anw]: failed recovery, failure RecoveryFailedException], allocation_status[deciders_no]]]"
         }
       ]
     }
   ]
 }
 ----
 // NOTCONSOLE
-
-If decider message indicates a transient allocation issue, use
-the <<cluster-reroute,cluster reroute>> API to retry allocation.
+When Elasticsearch is unable to allocate a shard, it will attempt to retry allocation up to
+the maximum number of retries allowed. After this, Elasticsearch will stop attempting to
+allocate the shard in order to prevent infinite retries which may impact cluster
+performance. Run the <<cluster-reroute,cluster reroute>> API to retry allocation, which
+will allocate the shard if the issue preventing allocation has been resolved.
 
 [[no-valid-shard-copy]]
 ====== No valid shard copy

server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/MaxRetryAllocationDecider.java

@@ -14,6 +14,7 @@
 import org.elasticsearch.cluster.routing.ShardRouting;
 import org.elasticsearch.cluster.routing.UnassignedInfo;
 import org.elasticsearch.cluster.routing.allocation.RoutingAllocation;
+import org.elasticsearch.common.ReferenceDocs;
 import org.elasticsearch.common.settings.Setting;
 
 /**
@@ -72,9 +73,11 @@ private static Decision debugDecision(Decision decision, UnassignedInfo info, in
             return Decision.single(
                 Decision.Type.NO,
                 NAME,
-                "shard has exceeded the maximum number of retries [%d] on failed allocation attempts - manually call [%s] to retry, [%s]",
+                "shard has exceeded the maximum number of retries [%d] on failed allocation attempts - "
+                    + "manually call [%s] to retry, and for more information, see [%s] [%s]",
                 maxRetries,
                 RETRY_FAILED_API,
+                ReferenceDocs.ALLOCATION_EXPLAIN_MAX_RETRY,
                 info.toString()
             );
         } else {

server/src/main/java/org/elasticsearch/common/ReferenceDocs.java

@@ -82,6 +82,7 @@ public enum ReferenceDocs {
     FORMING_SINGLE_NODE_CLUSTERS,
     CIRCUIT_BREAKER_ERRORS,
     ALLOCATION_EXPLAIN_NO_COPIES,
+    ALLOCATION_EXPLAIN_MAX_RETRY,
     // this comment keeps the ';' on the next line so every entry above has a trailing ',' which makes the diff for adding new links cleaner
     ;
 

server/src/main/resources/org/elasticsearch/common/reference-docs-links.txt

@@ -44,3 +44,4 @@ X_OPAQUE_ID                                                     api-conventions.
 FORMING_SINGLE_NODE_CLUSTERS                                    modules-discovery-bootstrap-cluster.html#modules-discovery-bootstrap-cluster-joining
 CIRCUIT_BREAKER_ERRORS                                          circuit-breaker-errors.html
 ALLOCATION_EXPLAIN_NO_COPIES                                    cluster-allocation-explain.html#no-valid-shard-copy
+ALLOCATION_EXPLAIN_MAX_RETRY                                    cluster-allocation-explain.html#maximum-number-of-retries-exceeded