Make searchable snapshot cache size effectively zero on non-frozen nodes

docs/reference/searchable-snapshots/index.asciidoc

@@ -172,9 +172,9 @@ xpack.searchable.snapshot.shared_cache.size: 4TB
 ----
 
 IMPORTANT: Currently, you can configure
-`xpack.searchable.snapshot.shared_cache.size` on any node. In a future release,
-you will only be able to configure this setting on nodes with the
-<<data-frozen-node,`data_frozen`>> role.
+`xpack.searchable.snapshot.shared_cache.size` on any node. However, if the cache size is set on any
+node that does not have the <<data-frozen-node,`data_frozen`>> role, it will be treated as though it
+is set to `0b`.
 
 You can set `xpack.searchable.snapshot.shared_cache.size` to any size between a
 couple of gigabytes up to 90% of available disk space. We only recommend larger

x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/allocation/decider/HasFrozenCacheAllocationDecider.java

@@ -9,6 +9,7 @@
 
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.cluster.node.DiscoveryNodeRole;
 import org.elasticsearch.cluster.routing.RoutingNode;
 import org.elasticsearch.cluster.routing.ShardRouting;
 import org.elasticsearch.cluster.routing.allocation.RoutingAllocation;
@@ -42,7 +43,9 @@ public class HasFrozenCacheAllocationDecider extends AllocationDecider {
         NAME,
         "node setting ["
             + SNAPSHOT_CACHE_SIZE_SETTING.getKey()
-            + "] is set to zero, so frozen searchable snapshot shards cannot be allocated to this node"
+            + "] is set to zero, or the node is not a ["
+            + DiscoveryNodeRole.DATA_FROZEN_NODE_ROLE.roleName()
+            + "] node, so frozen searchable snapshot shards cannot be allocated to this node"
     );
 
     private static final Decision UNKNOWN_FROZEN_CACHE = Decision.single(

x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/cache/shared/FrozenCacheService.java

@@ -30,12 +30,12 @@
 import org.elasticsearch.common.util.concurrent.KeyedLock;
 import org.elasticsearch.env.NodeEnvironment;
 import org.elasticsearch.index.shard.ShardId;
-import org.elasticsearch.xpack.searchablesnapshots.cache.common.CacheKey;
-import org.elasticsearch.xpack.searchablesnapshots.cache.common.SparseFileTracker;
 import org.elasticsearch.node.NodeRoleSettings;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.xpack.core.DataTier;
 import org.elasticsearch.xpack.searchablesnapshots.cache.common.ByteRange;
+import org.elasticsearch.xpack.searchablesnapshots.cache.common.CacheKey;
+import org.elasticsearch.xpack.searchablesnapshots.cache.common.SparseFileTracker;
 
 import java.io.IOException;
 import java.io.UncheckedIOException;
@@ -46,6 +46,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentLinkedQueue;
 import java.util.concurrent.Executor;
@@ -92,7 +93,7 @@ private static Setting.Validator<ByteSizeValue> getPageSizeAlignedByteSizeValueV
         };
     }
 
-    public static final Setting<ByteSizeValue> SNAPSHOT_CACHE_SIZE_SETTING = new Setting<>(
+    public static final Setting<ByteSizeValue> SNAPSHOT_CACHE_SIZE_SETTING = new Setting<ByteSizeValue>(
         SHARED_CACHE_SETTINGS_PREFIX + "size",
         ByteSizeValue.ZERO.getStringRep(),
         s -> ByteSizeValue.parseBytesSizeValue(s, SHARED_CACHE_SETTINGS_PREFIX + "size"),
@@ -132,7 +133,19 @@ public Iterator<Setting<?>> settings() {
 
         },
         Setting.Property.NodeScope
-    );
+    ) {
+        @Override
+        public ByteSizeValue get(final Settings settings) {
+            final ByteSizeValue value = super.get(settings);
+            final List<DiscoveryNodeRole> roles = NodeRoleSettings.NODE_ROLES_SETTING.get(settings);
+            final Set<DiscoveryNodeRole> roleSet = new HashSet<>(roles);
+            if (DataTier.isFrozenNode(roleSet)) {
+                return value;
+            } else {
+                return ByteSizeValue.ZERO;
+            }
+        }
+    };
 
     public static final Setting<ByteSizeValue> FROZEN_CACHE_RECOVERY_RANGE_SIZE_SETTING = Setting.byteSizeSetting(
         SHARED_CACHE_SETTINGS_PREFIX + "recovery_range_size",

x-pack/plugin/searchable-snapshots/src/test/java/org/elasticsearch/xpack/searchablesnapshots/cache/shared/FrozenCacheServiceTests.java

@@ -24,6 +24,7 @@
 import java.io.IOException;
 
 import static org.elasticsearch.node.Node.NODE_NAME_SETTING;
+import static org.hamcrest.Matchers.equalTo;
 
 public class FrozenCacheServiceTests extends ESTestCase {
 
@@ -210,6 +211,23 @@ public void testCacheSizeDeprecatedOnNonFrozenNodes() {
         );
     }
 
+    public void testCacheSizeReturnsZeroOnNonFrozenNodes() {
+        final Settings settings = Settings.builder()
+            .put(FrozenCacheService.SNAPSHOT_CACHE_SIZE_SETTING.getKey(), new ByteSizeValue(size(500)).getStringRep())
+            .put(FrozenCacheService.SNAPSHOT_CACHE_REGION_SIZE_SETTING.getKey(), new ByteSizeValue(size(100)).getStringRep())
+            .putList(NodeRoleSettings.NODE_ROLES_SETTING.getKey(), DiscoveryNodeRole.DATA_HOT_NODE_ROLE.roleName())
+            .build();
+        final ByteSizeValue value = FrozenCacheService.SNAPSHOT_CACHE_SIZE_SETTING.get(settings);
+        assertThat(value, equalTo(ByteSizeValue.ZERO));
+        assertWarnings(
+            "setting ["
+                + FrozenCacheService.SNAPSHOT_CACHE_SIZE_SETTING.getKey()
+                + "] to be positive ["
+                + new ByteSizeValue(size(500)).getStringRep()
+                + "] on node without the data_frozen role is deprecated, roles are [data_hot]"
+        );
+    }
+
     private static CacheKey generateCacheKey() {
         return new CacheKey(
             randomAlphaOfLength(10),