Addressed Sagar's comments

Signed-off-by: Peter Alfonsi <petealft@amazon.com>

plugins/cache-ehcache/src/main/java/org/opensearch/cache/store/disk/EhcacheDiskCache.java

@@ -94,7 +94,7 @@ public class EhcacheDiskCache<K, V> implements ICache<K, V> {
     // A Cache manager can create many caches.
     private final PersistentCacheManager cacheManager;
 
-    // Disk cache. Because of a bug in ehcache, we have to store ByteArrayWrapper rather than byte[].
+    // Disk cache. Using ByteArrayWrapper to compare two byte[] by values rather than the default reference checks
     private Cache<K, ByteArrayWrapper> cache;
     private final long maxWeightInBytes;
     private final String storagePath;
@@ -188,6 +188,10 @@ public Duration getExpiryForUpdate(K key, Supplier<? extends ByteArrayWrapper> o
                     )
                     .withKeySerializer(new KeySerializerWrapper<K>(keySerializer))
                     .withValueSerializer(new ByteArrayWrapperSerializer())
+                // We pass ByteArrayWrapperSerializer as ehcache's value serializer. If V is an interface, and we pass its
+                // serializer directly to ehcache, ehcache requires the classes match exactly before/after serialization.
+                // This is not always feasible or necessary, like for BytesReference. So, we handle the value serialization
+                // before V hits ehcache.
             );
         } catch (IllegalArgumentException ex) {
             logger.error("Ehcache disk cache initialization failed due to illegal argument: {}", ex.getMessage());

server/src/main/java/org/opensearch/indices/IRCKeyWriteableSerializer.java

@@ -26,12 +26,15 @@ public IRCKeyWriteableSerializer() {}
 
     @Override
     public byte[] serialize(IndicesRequestCache.Key object) {
+        if (object == null) {
+            return null;
+        }
         try {
             BytesStreamOutput os = new BytesStreamOutput();
             object.writeTo(os);
             return BytesReference.toBytes(os.bytes());
         } catch (IOException e) {
-            throw new OpenSearchException(e);
+            throw new OpenSearchException("Unable to serialize IndicesRequestCache.Key", e);
         }
     }
 
@@ -44,7 +47,7 @@ public IndicesRequestCache.Key deserialize(byte[] bytes) {
             BytesStreamInput is = new BytesStreamInput(bytes, 0, bytes.length);
             return new IndicesRequestCache.Key(is);
         } catch (IOException e) {
-            throw new OpenSearchException(e);
+            throw new OpenSearchException("Unable to deserialize byte[] to IndicesRequestCache.Key", e);
         }
     }