Limit number of merge operands in Cassandra merge operator

Summary:
Now that RocksDB supports conditional merging during point lookups (introduced in #2923), Cassandra value merge operator can be updated to pass in a limit. The limit needs to be passed in from the Cassandra code.
Closes https://github.com/facebook/rocksdb/pull/2947

Differential Revision: D5938454

Pulled By: sagar0

fbshipit-source-id: d64a72d53170d8cf202b53bd648475c3952f7d7f

java/rocksjni/cassandra_value_operator.cc

@@ -23,13 +23,14 @@
 /*
  * Class:     org_rocksdb_CassandraValueMergeOperator
  * Method:    newSharedCassandraValueMergeOperator
- * Signature: (I)J
+ * Signature: (II)J
  */
 jlong Java_org_rocksdb_CassandraValueMergeOperator_newSharedCassandraValueMergeOperator(
-    JNIEnv* env, jclass jclazz, jint gcGracePeriodInSeconds) {
+    JNIEnv* env, jclass jclazz, jint gcGracePeriodInSeconds,
+    jint operands_limit) {
   auto* op = new std::shared_ptr<rocksdb::MergeOperator>(
       new rocksdb::cassandra::CassandraValueMergeOperator(
-          gcGracePeriodInSeconds));
+          gcGracePeriodInSeconds, operands_limit));
   return reinterpret_cast<jlong>(op);
 }
 

java/src/main/java/org/rocksdb/CassandraValueMergeOperator.java

@@ -11,10 +11,15 @@ package org.rocksdb;
  */
 public class CassandraValueMergeOperator extends MergeOperator {
   public CassandraValueMergeOperator(int gcGracePeriodInSeconds) {
-    super(newSharedCassandraValueMergeOperator(gcGracePeriodInSeconds));
+    super(newSharedCassandraValueMergeOperator(gcGracePeriodInSeconds, 0));
     }
 
-    private native static long newSharedCassandraValueMergeOperator(int gcGracePeriodInSeconds);
+    public CassandraValueMergeOperator(int gcGracePeriodInSeconds, int operandsLimit) {
+      super(newSharedCassandraValueMergeOperator(gcGracePeriodInSeconds, operandsLimit));
+    }
+
+    private native static long newSharedCassandraValueMergeOperator(
+        int gcGracePeriodInSeconds, int limit);
 
     @Override protected final native void disposeInternal(final long handle);
 }

utilities/cassandra/merge_operator.h

@@ -15,8 +15,10 @@ namespace cassandra {
  */
 class CassandraValueMergeOperator : public MergeOperator {
 public:
- explicit CassandraValueMergeOperator(int32_t gc_grace_period_in_seconds)
-     : gc_grace_period_in_seconds_(gc_grace_period_in_seconds) {}
+ explicit CassandraValueMergeOperator(int32_t gc_grace_period_in_seconds,
+                                      size_t operands_limit = 0)
+     : gc_grace_period_in_seconds_(gc_grace_period_in_seconds),
+       operands_limit_(operands_limit) {}
 
  virtual bool FullMergeV2(const MergeOperationInput& merge_in,
                           MergeOperationOutput* merge_out) const override;
@@ -30,8 +32,13 @@ public:
 
  virtual bool AllowSingleOperand() const override { return true; }
 
+ virtual bool ShouldMerge(const std::vector<Slice>& operands) const override {
+   return operands_limit_ > 0 && operands.size() >= operands_limit_;
+ }
+
 private:
  int32_t gc_grace_period_in_seconds_;
+ size_t operands_limit_;
 };
 } // namespace cassandra
 } // namespace rocksdb