Add the simple strategy to cluster settings (#49414)

This is related to #49067. This commit adds the simple connection
strategy settings and strategy mode setting to the cluster settings
registry. With these changes, the simple connection mode can be used.
Additionally, it adds validation to ensure that settings cannot be
misconfigured.

qa/multi-cluster-search/src/test/resources/rest-api-spec/test/multi_cluster/100_connection_mode_configuration.yml

@@ -0,0 +1,212 @@
+---
+"Add transient remote cluster in simple mode with invalid sniff settings":
+  - do:
+      cluster.get_settings:
+        include_defaults: true
+
+  - set: { defaults.cluster.remote.my_remote_cluster.seeds.0: remote_ip }
+
+  - do:
+      catch: bad_request
+      cluster.put_settings:
+        flat_settings: true
+        body:
+          transient:
+            cluster.remote.test_remote_cluster.mode: "simple"
+            cluster.remote.test_remote_cluster.sniff.node_connections: "5"
+            cluster.remote.test_remote_cluster.simple.addresses: $remote_ip
+
+  - match: { status: 400 }
+  - match: { error.root_cause.0.type: "illegal_argument_exception" }
+  - match: { error.root_cause.0.reason: "Setting \"cluster.remote.test_remote_cluster.sniff.node_connections\" cannot be
+  used with the configured \"cluster.remote.test_remote_cluster.mode\" [required=SNIFF, configured=SIMPLE]" }
+
+  - do:
+      catch: bad_request
+      cluster.put_settings:
+        flat_settings: true
+        body:
+          transient:
+            cluster.remote.test_remote_cluster.mode: "simple"
+            cluster.remote.test_remote_cluster.sniff.seeds: $remote_ip
+            cluster.remote.test_remote_cluster.simple.addresses: $remote_ip
+
+  - match: { status: 400 }
+  - match: { error.root_cause.0.type: "illegal_argument_exception" }
+  - match: { error.root_cause.0.reason: "Setting \"cluster.remote.test_remote_cluster.sniff.seeds\" cannot be
+   used with the configured \"cluster.remote.test_remote_cluster.mode\" [required=SNIFF, configured=SIMPLE]" }
+
+---
+"Add transient remote cluster in sniff mode with invalid simple settings":
+  - do:
+      cluster.get_settings:
+        include_defaults: true
+
+  - set: { defaults.cluster.remote.my_remote_cluster.seeds.0: remote_ip }
+
+  - do:
+      catch: bad_request
+      cluster.put_settings:
+        flat_settings: true
+        body:
+          transient:
+            cluster.remote.test_remote_cluster.simple.socket_connections: "20"
+            cluster.remote.test_remote_cluster.sniff.seeds: $remote_ip
+
+  - match: { status: 400 }
+  - match: { error.root_cause.0.type: "illegal_argument_exception" }
+  - match: { error.root_cause.0.reason: "Setting \"cluster.remote.test_remote_cluster.simple.socket_connections\" cannot be
+  used with the configured \"cluster.remote.test_remote_cluster.mode\" [required=SIMPLE, configured=SNIFF]" }
+
+  - do:
+      catch: bad_request
+      cluster.put_settings:
+        flat_settings: true
+        body:
+          transient:
+            cluster.remote.test_remote_cluster.simple.addresses: $remote_ip
+            cluster.remote.test_remote_cluster.sniff.seeds: $remote_ip
+
+  - match: { status: 400 }
+  - match: { error.root_cause.0.type: "illegal_argument_exception" }
+  - match: { error.root_cause.0.reason: "Setting \"cluster.remote.test_remote_cluster.simple.addresses\" cannot be
+   used with the configured \"cluster.remote.test_remote_cluster.mode\" [required=SIMPLE, configured=SNIFF]" }
+
+---
+"Add transient remote cluster using simple connection mode using valid settings":
+  - do:
+      cluster.get_settings:
+        include_defaults: true
+
+  - set: { defaults.cluster.remote.my_remote_cluster.seeds.0: remote_ip }
+
+  - do:
+      cluster.put_settings:
+        flat_settings: true
+        body:
+          transient:
+            cluster.remote.test_remote_cluster.mode: "simple"
+            cluster.remote.test_remote_cluster.simple.socket_connections: "3"
+            cluster.remote.test_remote_cluster.simple.addresses: $remote_ip
+
+  - match: {transient.cluster\.remote\.test_remote_cluster\.mode: "simple"}
+  - match: {transient.cluster\.remote\.test_remote_cluster\.simple\.socket_connections: "3"}
+  - match: {transient.cluster\.remote\.test_remote_cluster\.simple\.addresses: $remote_ip}
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        index: test_remote_cluster:test_index
+
+  - is_false: num_reduce_phases
+  - match: {_clusters.total: 1}
+  - match: {_clusters.successful: 1}
+  - match: {_clusters.skipped: 0}
+  - match: { _shards.total: 3 }
+  - match: { hits.total: 6 }
+  - match: { hits.hits.0._index: "test_remote_cluster:test_index" }
+
+---
+"Add transient remote cluster using sniff connection mode using valid settings":
+  - do:
+      cluster.get_settings:
+        include_defaults: true
+
+  - set: { defaults.cluster.remote.my_remote_cluster.seeds.0: remote_ip }
+
+  - do:
+      cluster.put_settings:
+        flat_settings: true
+        body:
+          transient:
+            cluster.remote.test_remote_cluster.mode: "sniff"
+            cluster.remote.test_remote_cluster.sniff.node_connections: "3"
+            cluster.remote.test_remote_cluster.sniff.seeds: $remote_ip
+
+  - match: {transient.cluster\.remote\.test_remote_cluster\.mode: "sniff"}
+  - match: {transient.cluster\.remote\.test_remote_cluster\.sniff\.node_connections: "3"}
+  - match: {transient.cluster\.remote\.test_remote_cluster\.sniff\.seeds: $remote_ip}
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        index: test_remote_cluster:test_index
+
+  - is_false: num_reduce_phases
+  - match: {_clusters.total: 1}
+  - match: {_clusters.successful: 1}
+  - match: {_clusters.skipped: 0}
+  - match: { _shards.total: 3 }
+  - match: { hits.total: 6 }
+  - match: { hits.hits.0._index: "test_remote_cluster:test_index" }
+
+---
+"Switch connection mode for configured cluster":
+  - do:
+      cluster.get_settings:
+        include_defaults: true
+
+  - set: { defaults.cluster.remote.my_remote_cluster.seeds.0: remote_ip }
+
+  - do:
+      cluster.put_settings:
+        flat_settings: true
+        body:
+          transient:
+            cluster.remote.test_remote_cluster.mode: "sniff"
+            cluster.remote.test_remote_cluster.sniff.seeds: $remote_ip
+
+  - match: {transient.cluster\.remote\.test_remote_cluster\.mode: "sniff"}
+  - match: {transient.cluster\.remote\.test_remote_cluster\.sniff\.seeds: $remote_ip}
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        index: test_remote_cluster:test_index
+
+  - is_false: num_reduce_phases
+  - match: {_clusters.total: 1}
+  - match: {_clusters.successful: 1}
+  - match: {_clusters.skipped: 0}
+  - match: { _shards.total: 3 }
+  - match: { hits.total: 6 }
+  - match: { hits.hits.0._index: "test_remote_cluster:test_index" }
+
+  - do:
+      catch: bad_request
+      cluster.put_settings:
+        flat_settings: true
+        body:
+          transient:
+            cluster.remote.test_remote_cluster.mode: "simple"
+            cluster.remote.test_remote_cluster.simple.addresses: $remote_ip
+
+  - match: { status: 400 }
+  - match: { error.root_cause.0.type: "illegal_argument_exception" }
+  - match: { error.root_cause.0.reason: "Setting \"cluster.remote.test_remote_cluster.sniff.seeds\" cannot be
+  used with the configured \"cluster.remote.test_remote_cluster.mode\" [required=SNIFF, configured=SIMPLE]" }
+
+  - do:
+      cluster.put_settings:
+        flat_settings: true
+        body:
+          transient:
+            cluster.remote.test_remote_cluster.mode: "simple"
+            cluster.remote.test_remote_cluster.sniff.seeds: null
+            cluster.remote.test_remote_cluster.simple.addresses: $remote_ip
+
+  - match: {transient.cluster\.remote\.test_remote_cluster\.mode: "simple"}
+  - match: {transient.cluster\.remote\.test_remote_cluster\.simple\.addresses: $remote_ip}
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        index: test_remote_cluster:test_index
+
+  - is_false: num_reduce_phases
+  - match: {_clusters.total: 1}
+  - match: {_clusters.successful: 1}
+  - match: {_clusters.skipped: 0}
+  - match: { _shards.total: 3 }
+  - match: { hits.total: 6 }
+  - match: { hits.hits.0._index: "test_remote_cluster:test_index" }

server/src/main/java/org/elasticsearch/common/settings/ClusterSettings.java

@@ -102,6 +102,8 @@
 import org.elasticsearch.search.fetch.subphase.highlight.FastVectorHighlighter;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.transport.RemoteClusterService;
+import org.elasticsearch.transport.RemoteConnectionStrategy;
+import org.elasticsearch.transport.SimpleConnectionStrategy;
 import org.elasticsearch.transport.SniffConnectionStrategy;
 import org.elasticsearch.transport.TransportSettings;
 import org.elasticsearch.watcher.ResourceWatcherService;
@@ -281,12 +283,15 @@ public void apply(Settings value, Settings current, Settings previous) {
             SearchService.DEFAULT_ALLOW_PARTIAL_SEARCH_RESULTS,
             TransportSearchAction.SHARD_COUNT_LIMIT_SETTING,
             RemoteClusterService.REMOTE_CLUSTER_SKIP_UNAVAILABLE,
-            RemoteClusterService.REMOTE_CONNECTIONS_PER_CLUSTER,
+            SniffConnectionStrategy.REMOTE_CONNECTIONS_PER_CLUSTER,
             RemoteClusterService.REMOTE_INITIAL_CONNECTION_TIMEOUT_SETTING,
             RemoteClusterService.REMOTE_NODE_ATTRIBUTE,
             RemoteClusterService.ENABLE_REMOTE_CLUSTERS,
             RemoteClusterService.REMOTE_CLUSTER_PING_SCHEDULE,
             RemoteClusterService.REMOTE_CLUSTER_COMPRESS,
+            RemoteConnectionStrategy.REMOTE_CONNECTION_MODE,
+            SimpleConnectionStrategy.REMOTE_CLUSTER_ADDRESSES,
+            SimpleConnectionStrategy.REMOTE_SOCKET_CONNECTIONS,
             SniffConnectionStrategy.REMOTE_CLUSTER_SEEDS_OLD,
             SniffConnectionStrategy.REMOTE_CLUSTERS_PROXY,
             SniffConnectionStrategy.REMOTE_CLUSTER_SEEDS,