Rocksdb事务隔离性指的是多线程并发事务使用时候，事务与事务之间的隔离性，通过加锁机制来实现，本文重点剖析Read Commited隔离级别下，Rocksdb的加锁机制。
- Rocksdb事务相关类族
Rocksdb的事务相关的类图如下图所示。主要有两个类族，Transaction和DB，默认采用PessimisticTransaction，而PessimisticTransaction内部的加锁机制通过TransactionLockMgr来实现的。
TransactionLockMgr内部维护了LockMap。TransactionLockMgr根据每个记录的Key计算hash值，再对num_stripes取模，在LockMap中的向量Std::vector
LockMap的数据成员如下
Size_t num_stripes LockMapStripe个数，默认16个
Std::vector
LockMapStripe的数据成员如下
std::shared_ptr
std::shared_ptr
std::unordered_map
LockInfo的数据成员如下
bool exclusive : 排它锁，还是共享锁
uint64_t expiration_time : 锁的过期时间
autovector
2. Rocksdb事务流程分析
上述流程，是应用创建TransactionDB，然后Put一条记录，再Commit的协作流程图，在Put阶段调用TransactionLockMgr的TryLock方法，Commit阶段调用TransactionLockMgr的UnLock方法。
TransactionLockMgr：：TryLock内部的主要逻辑在AcquireLocked函数中，TransactionLockMgr：：UnLock内部的主要逻辑在UnlockKey函数中，下面具体分析这两个函数。绿色部分字体为个人注解。
AcquireLocked
Status TransactionLockMgr::AcquireLocked(LockMap* lock_map,
LockMapStripe* stripe,
const std::string& key, //记录的Key值
Env* env,
LockInfo&& txn_lock_info, //当前事务锁信息
uint64_t* expire_time, //锁的过期时间
autovector
{
Status result;
auto stripe_iter = stripe->keys.find(key); // 检查这条记录的Key是否已经被加锁了。
if (stripe_iter != stripe->keys.end()) { // 这条记录的Key已经被之前事务加过锁
LockInfo& lock_info = stripe_iter->second;
if (lock_info.exclusive || txn_lock_info.exclusive) { //之前事务或者当前事务加的是排他锁，
if (lock_info.txn_ids.size() == 1 &&
lock_info.txn_ids[0] == txn_lock_info.txn_ids[0]) { //之前加锁的事务就是当前事务
lock_info.exclusive = txn_lock_info.exclusive;
lock_info.expiration_time = txn_lock_info.expiration_time;
} else { //之前加锁的事务不是当前事务
if (IsLockExpired(txn_lock_info.txn_ids[0], lock_info, env,
expire_time)) { // 之前事务加的锁已经过期，可以清除
lock_info.txn_ids = txn_lock_info.txn_ids;
lock_info.exclusive = txn_lock_info.exclusive;
lock_info.expiration_time = txn_lock_info.expiration_time;
} else {
result = Status::TimedOut(Status::SubCode::kLockTimeout);
*txn_ids = lock_info.txn_ids; // 返回之前事务列表
}
}
} else { //当前事务加的是共享锁
lock_info.txn_ids.push_back(txn_lock_info.txn_ids[0]);
lock_info.expiration_time =
std::max(lock_info.expiration_time, txn_lock_info.expiration_time);
}
} else { // 这条记录的Key没有被之前事务加过锁
if (max_num_locks_ > 0 &&
lock_map->lock_cnt.load(std::memory_order_acquire) >= max_num_locks_) {
result = Status::Busy(Status::SubCode::kLockLimit);
} else {
// 当前事务执行加锁操作
stripe->keys.emplace(key, std::move(txn_lock_info));
if (max_num_locks_) {
lock_map->lock_cnt++;
}
}
}
return result;
}
UnlockKey逻辑相对简单一些，主要是删除加锁的记录，并且唤醒被阻塞的事务。
void TransactionLockMgr::UnLockKey(const PessimisticTransaction* txn,
const std::string& key,
LockMapStripe* stripe, LockMap* lock_map,
Env* env) {
TransactionID txn_id = txn->GetID();
auto stripe_iter = stripe->keys.find(key);
if (stripe_iter != stripe->keys.end()) {
auto& txns = stripe_iter->second.txn_ids;
auto txn_it = std::find(txns.begin(), txns.end(), txn_id);
// Found the key we locked. unlock it.
if (txn_it != txns.end()) {
if (txns.size() == 1) {
stripe->keys.erase(stripe_iter);
} else {
auto last_it = txns.end() - 1;
if (txn_it != last_it) {
*txn_it = *last_it;
}
txns.pop_back();
}
if (max_num_locks_ > 0) {
// Maintain lock count if there is a limit on the number of locks.
assert(lock_map->lock_cnt.load(std::memory_order_relaxed) > 0);
lock_map->lock_cnt--;
}
}
} else {
// This key is either not locked or locked by someone else. This should
// only happen if the unlocking transaction has expired.
assert(txn->GetExpirationTime() > 0 &&
txn->GetExpirationTime() < env->NowMicros());
}
}