emkornfield commented on a change in pull request #8023:
URL: https://github.com/apache/arrow/pull/8023#discussion_r485038390



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File path: cpp/src/parquet/key_toolkit.h
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@@ -0,0 +1,123 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#pragma once
+
+#include <cstdint>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "parquet/key_encryption_key.h"
+#include "parquet/kms_client.h"
+#include "parquet/kms_client_factory.h"
+#include "parquet/platform.h"
+#include "parquet/two_level_cache_with_expiration.h"
+
+namespace parquet {
+namespace encryption {
+
+class KeyWithMasterID {
+ public:
+  KeyWithMasterID(const std::string& key_bytes, const std::string& master_id)
+      : key_bytes_(key_bytes), master_id_(master_id) {}
+
+  const std::string& data_key() const { return key_bytes_; }
+  const std::string& master_id() const { return master_id_; }
+
+ private:
+  std::string key_bytes_;
+  std::string master_id_;
+};
+
+class PARQUET_EXPORT KeyToolkit {
+ public:
+  class KmsClientCache {
+   public:
+    static KmsClientCache& GetInstance() {
+      static KmsClientCache instance;

Review comment:
       I'm generally skeptical for forcing singletons into the API.  For 
instance in Java, I would assume that applications would maintain the an 
instance of KeyToolkit as singleton (not enforced at the language level).  
   
   Its not clear if this would be an issue, but if this would the use of 
singleton here work well if the class was being used in a multi-tenant system?




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