目的

• 提供read-modify-write语义操作，保证原子性。
• Merge-Operator
• 头文件： rocksdb/merge_operator.h

使用场景

• 需求先读取旧的值才能确定新值

• 数据的递增操作

• 比如：

• list 的append操作

• string的拼接操作

• 也可以将多个merge操作合并成一个merge操作（ 叫做Partial merge或 Associative merge)

如何使用

• 如果client要使用rocksdb的mege操作，首先要实现AssociativeMergeOperator或MergeOperator接口实现满足业务逻辑的mege操作，然后在参数Options.merge_operator(shared_ptr类型)设置Merge实例对象。否则打开的Db默认不支持merge操作，调用时会抱Status::NotSupport.

class DB {

// rocksdb提供的mege操作
virtual Status Merge(const WriteOptions& options, const Slice& key, const Slice& value) = 0;
};

Struct Options {
....
// 用户自定义的meger操作类，需要传进来
const std::shared_ptr<MergeOperator> merge_operator;
...
};

• AssociativeMergeOperator 接口

 class AssociativeMergeOperator : public MergeOperator {
 // Gives the client a way to express the read -> modify -> write semantics
      // key:           (IN) The key that's associated with this merge operation.，可以根据key来识别key的业务类型，做不同的merge操作。
      // existing_value:(IN) null indicates the key does not exist before this op
      // value:         (IN) the value to update/merge the existing_value with 对已有值进行update得值，比如对已有值累计1，那么该value应该是1。RocksDb的mege方法中的value值会被传到这里
      // new_value:    (OUT) Client is responsible for filling the merge result here
      // logger:        (IN) Client could use this to log errors during merge.
      //
      // Return true on success. Return false failure / error / corruption.
      virtual bool Merge(const Slice& key,
                         const Slice* existing_value,
                         const Slice& value,
                         std::string* new_value,
                         Logger* logger) const = 0;

 // The name of the MergeOperator. Used to check for MergeOperator
      // mismatches (i.e., a DB created with one MergeOperator is
      // accessed using a different MergeOperator)
      virtual const char* Name() const = 0;
   ...
    };

• MergeOperator接口，AssociativeMergeOperator 是MergeOperator的子类

class MergeOperator {
// Gives the client a way to express the read -> modify -> write semantics
      // key:         (IN) The key that's associated with this merge operation.
      // existing:    (IN) null indicates that the key does not exist before this op
      // operand_list:(IN) the sequence of merge operations to apply, front() first.
      // new_value:  (OUT) Client is responsible for filling the merge result here
      // logger:      (IN) Client could use this to log errors during merge.
      //
      // Return true on success. Return false failure / error / corruption.
// 对key上的积攒的一批merge操作（operand_list）进行merge
      virtual bool FullMerge(const Slice& key,
                             const Slice* existing_value,
                             const std::deque<std::string>& operand_list,
                             std::string* new_value,
                             Logger* logger) const = 0;
};
// This function performs merge(left_op, right_op)
      // when both the operands are themselves merge operation types.
      // Save the result in *new_value and return true. If it is impossible
      // or infeasible to combine the two operations, return false instead.
// 如果两个merge操作可以合并成一个，则调用该方法。
      virtual bool PartialMerge(const Slice& key,
                                const Slice& left_operand,
                                const Slice& right_operand,
                                std::string* new_value,
                                Logger* logger) const = 0;

MergeOperator和AssociativeMergeOperator的选用

• when to use merge

• 使用AssociativeMergeOperator的场景

  • merge操作数的值的格式和Put的值的格式相同
  • 把多个操作数合并进一个也是ok的

• MergeOperator的场景

  • 如果AssociativeMergeOperator的使用条件不满足，则使用MergeOperator。当然MergeOperator可以实现AssociativeMergeOperator的功能。
  • 如果有时候可以把多个merge操作合并成一个(允许出现不可以合并的情况)可以使用MergeOperator，并实现其PartialMerge()方法，返回true表示两个操作被合并成了一个。
  • 案例：如果value是json，要对json中元素同merge进行修改，这时该操作不满足AssociativeMergeOperator的使用条件。因为其初始值可后续的merge操作值不能同等对待。具体看案例。

案例

• 实现累加器，方法void Add(const string& key, unit64_t value)，对key中的已有值累加value，如果key不存在则默认其初始值为0。

1. 先实现AssociativeMergeOperator接口类

// A 'model' merge operator with uint64 addition semantics
    class UInt64AddOperator : public AssociativeMergeOperator {
     public:
      virtual bool Merge(
        const Slice& key,
        const Slice* existing_value,
        const Slice& value,
        std::string* new_value,
        Logger* logger) const override {

        // assuming 0 if no existing value
        uint64_t existing = 0;
        if (existing_value) {
          if (!Deserialize(*existing_value, &existing)) {
            // if existing_value is corrupted, treat it as 0
            Log(logger, "existing value corruption");
            existing = 0;
          }
        }

        uint64_t oper;
        if (!Deserialize(value, &oper)) {
          // if operand is corrupted, treat it as 0
          Log(logger, "operand value corruption");
          oper = 0;
        }

        auto new = existing + oper;
        *new_value = Serialize(new);
        return true;        // always return true for this, since we treat all errors as "zero".
      }

      virtual const char* Name() const override {
        return "UInt64AddOperator";
       }
    };

• 实现Add方法

    // Implement 'add' directly with the new Merge operation
    class MergeBasedCounters : public RocksCounters {
     public:
      MergeBasedCounters(std::shared_ptr<DB> db);

      // mapped to a leveldb Merge operation
      virtual void Add(const string& key, uint64_t value) override {
        string serialized = Serialize(value);
        db_->Merge(merge_option_, key, serialized);
      }
    };

• 打开DB设置merge_operator，调用Add

    DB* dbp;
    Options options;
    options.merge_operator.reset(new UInt64AddOperator);
    DB::Open(options, "/tmp/db", &dbp);
    std::shared_ptr<DB> db(dbp);
    MergeBasedCounters counters(db);
    counters.Add("a", 1);
    ...
    uint64_t v;
    counters.Get("a", &v);

注意

• rocksdb在执行的get，iteration，compaction等操作时，与merge operator的操作行为是有关联的。
• 如果在没有设置merge operator的Db上调用merge操作会返回Status::NotSupport
• 同一个Db要打开时要使用相同的merge operator。
• 用户通过实现Merge operator接口来定义满足业务要求的merge语义，所以不同的实现往往有不同的业务含义。比如，累加器的mege是对原来的值加1，而json的mege操作是修改原有json中字段的值，list是mege操作是追加内容等等。
• rocksDB 的put和merge操作在实际中不会被立即执行，可能在下一次Get时候或者compaction时候才执行。How do these methods work?

链接

• • Merge+Compaction Implementation Details: For RocksDB engineers who want to know how MergeOperator affects their code.