incorporating review comments

Signed-off-by: Sarthak Aggarwal <sarthagg@amazon.com>

plugins/custom-codecs/src/main/java/org/opensearch/index/codec/customcodecs/Lucene95CustomCodec.java

@@ -15,6 +15,9 @@
 import org.opensearch.index.codec.PerFieldMappingPostingFormatCodec;
 import org.opensearch.index.mapper.MapperService;
 
+import java.util.Collections;
+import java.util.Set;
+
 /**
  *
  * Extends {@link FilterCodec} to reuse the functionality of Lucene Codec.
@@ -32,20 +35,24 @@ public enum Mode {
         /**
          * ZStandard mode with dictionary
          */
-        ZSTD("ZSTD"),
+        ZSTD("ZSTD", Set.of("zstd")),
         /**
          * ZStandard mode without dictionary
          */
-        ZSTD_NO_DICT("ZSTDNODICT"),
+        ZSTD_NO_DICT("ZSTDNODICT", Set.of("zstd_no_dict")),
         /**
-         * Default ZStandard mode
+         * Deprecated ZStandard mode, added for backward compatibility to support indices created in 2.9.0 where
+         * both ZSTD and ZSTD_NO_DICT used Lucene95CustomCodec underneath. This should not be used to
+         * create new indices.
          */
-        ZSTD_DEPRECATED("Lucene95CustomCodec");
+        ZSTD_DEPRECATED("Lucene95CustomCodec", Collections.emptySet());
 
         private final String codec;
+        private final Set<String> aliases;
 
-        Mode(String codec) {
+        Mode(String codec, Set<String> aliases) {
             this.codec = codec;
+            this.aliases = aliases;
         }
 
         /**
@@ -54,6 +61,13 @@ public enum Mode {
         public String getCodec() {
             return codec;
         }
+
+        /**
+         * Returns the aliases of the Codec
+         */
+        public Set<String> getAliases() {
+            return aliases;
+        }
     }
 
     private final StoredFieldsFormat storedFieldsFormat;

plugins/custom-codecs/src/main/java/org/opensearch/index/codec/customcodecs/ZstdCodec.java

@@ -62,6 +62,6 @@ public boolean supports(Setting<?> setting) {
 
     @Override
     public Set<String> aliases() {
-        return Set.of("zstd");
+        return Mode.ZSTD.getAliases();
     }
 }

plugins/custom-codecs/src/main/java/org/opensearch/index/codec/customcodecs/ZstdDeprecatedCodec.java

@@ -10,18 +10,16 @@
 
 import org.apache.logging.log4j.Logger;
 import org.opensearch.common.settings.Setting;
-import org.opensearch.index.codec.CodecAliases;
 import org.opensearch.index.codec.CodecSettings;
 import org.opensearch.index.engine.EngineConfig;
 import org.opensearch.index.mapper.MapperService;
 
-import java.util.Set;
-
 /**
- * ZstdDefaultCodec provides ZSTD compressor using the <a href="https://github.com/luben/zstd-jni">zstd-jni</a> library.
+ * ZstdDeprecatedCodec provides ZSTD compressor using the <a href="https://github.com/luben/zstd-jni">zstd-jni</a> library.
+ * Added to support backward compatibility for indices created with Lucene95CustomCodec as codec name.
  */
 @Deprecated(since = "2.10")
-public class ZstdDeprecatedCodec extends Lucene95CustomCodec implements CodecSettings, CodecAliases {
+public class ZstdDeprecatedCodec extends Lucene95CustomCodec implements CodecSettings {
 
     /**
      * Creates a new ZstdDefaultCodec instance with the default compression level.
@@ -60,9 +58,4 @@ public String toString() {
     public boolean supports(Setting<?> setting) {
         return setting.equals(EngineConfig.INDEX_CODEC_COMPRESSION_LEVEL_SETTING);
     }
-
-    @Override
-    public Set<String> aliases() {
-        return Set.of();
-    }
 }

plugins/custom-codecs/src/main/java/org/opensearch/index/codec/customcodecs/ZstdNoDictCodec.java

@@ -62,6 +62,6 @@ public boolean supports(Setting<?> setting) {
 
     @Override
     public Set<String> aliases() {
-        return Set.of("zstd_no_dict");
+        return Mode.ZSTD_NO_DICT.getAliases();
     }
 }

plugins/custom-codecs/src/test/java/org/opensearch/index/codec/customcodecs/CustomCodecTests.java

@@ -93,6 +93,14 @@ public void testZstdNoDict() throws Exception {
         assertEquals(Lucene95CustomCodec.DEFAULT_COMPRESSION_LEVEL, storedFieldsFormat.getCompressionLevel());
     }
 
+    public void testZstdDeprecatedCodec() {
+        final IllegalArgumentException e = expectThrows(
+            IllegalArgumentException.class,
+            () -> createCodecService(false).codec("ZSTD_DEPRECATED")
+        );
+        assertTrue(e.getMessage().startsWith("failed to find codec"));
+    }
+
     public void testZstdWithCompressionLevel() throws Exception {
         int randomCompressionLevel = randomIntBetween(1, 6);
         Codec codec = createCodecService(randomCompressionLevel, "zstd").codec("zstd");

server/src/main/java/org/opensearch/index/codec/CodecAliases.java

@@ -24,5 +24,7 @@ public interface CodecAliases {
      *
      * @return A non-null set of alias strings. If no aliases are available, an empty set should be returned.
      */
-    Set<String> aliases();
+    default Set<String> aliases() {
+        return Set.of();
+    }
 }