Atomic commit write batches in the command EXEC

[git-hulk]

Search before asking

• I had searched in the issues and found no similar issues.

Motivation

As mentioned in #487, the current transaction is NOT an atomic operation, some operations may be committed to DB but some are not if they crashed in the middle way of executing the command EXEC.

To fix this issue, we need to gather all write operations into one RocksDB's WriteBatch, then commit it at once.

Solution

We can add a new function to create a shared WriteBatch for the Multi-Exec command like the below:

Status Storage::Begin() {
    is_txn_mode = true;
    txn_write_batch = make_shared<*rocksdb::WriteBatch>();
}

rocksdb::WriteBatch *Storage::GetWriteBatch() {
   if (is_txn_mode) {
      return txn_write_batch.get();
   }
   return make_shared<*rocksdb::WriteBatch>();
}

Status Storage::Commit() {
    is_txn_mode = false;
    // write txn WriteBatch to RocksDB
    txn_write_batch.reset();
}

For the Command Exec, we need to explicitly call the Begin() and Commit() to enter and leave the transaction mode. So that Kvrocks will create a new WriteBatch for each writes operation if it's NOT in transaction mode, and use the shared WriteBatch to collect all write operations if it's in the transaction mode.

Are you willing to submit a PR?

• I'm willing to submit a PR!

[aleksraiden]

Very important to me, waiting for this in upstream. Lot of thanks!