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feat(server): Introduce transaction clock.

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Partially implements #6.
Before, each shard lazily updated its clock used for the expiry evaluation.
Now, the clock value is set during the transaction scheduling phase and is assigned
to each transaction. From now on DbSlice methods use this value when checking whether
the entry is expired via passed DbContext argument.

Also, implemented transactionally consistent TIME command and
verify that time is the same during the transaction. See
https://ably.com/blog/redis-keys-do-not-expire-atomically for motivation.

Still have not implemented any lamport style updates for background processes
(not sure if it's the right way to proceed).
This commit is contained in:
Roman Gershman 2022-09-25 16:18:59 +03:00 • committed by Roman Gershman
parent 347a619b44
commit 0925829afb
28 changed files with 448 additions and 387 deletions
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15
src/server/transaction.cc
View file

@ -24,7 +24,7 @@ thread_local Transaction::TLTmpSpace Transaction::tmp_space;
namespace {
std::atomic_uint64_t op_seq{1};
atomic_uint64_t op_seq{1};
[[maybe_unused]] constexpr size_t kTransSize = sizeof(Transaction);
@ -449,10 +449,12 @@ void Transaction::ScheduleInternal() {
}
while (true) {
txid_ = op_seq.fetch_add(1, std::memory_order_relaxed);
txid_ = op_seq.fetch_add(1, memory_order_relaxed);
std::atomic_uint32_t lock_granted_cnt{0};
std::atomic_uint32_t success{0};
atomic_uint32_t lock_granted_cnt{0};
atomic_uint32_t success{0};
time_now_ms_ = GetCurrentTimeMs();
auto cb = [&](EngineShard* shard) {
pair<bool, bool> res = ScheduleInShard(shard);
@ -551,6 +553,7 @@ OpStatus Transaction::ScheduleSingleHop(RunnableType cb) {
// above.
// IsArmedInShard() first checks run_count_ before accessing shard_data.
run_count_.fetch_add(1, memory_order_release);
time_now_ms_ = GetCurrentTimeMs();
// Please note that schedule_cb can not update any data on ScheduleSingleHop stack
// since the latter can exit before ScheduleUniqueShard returns.
@ -805,7 +808,7 @@ bool Transaction::ScheduleUniqueShard(EngineShard* shard) {
}
// we can do it because only a single thread writes into txid_ and sd.
txid_ = op_seq.fetch_add(1, std::memory_order_relaxed);
txid_ = op_seq.fetch_add(1, memory_order_relaxed);
sd.pq_pos = shard->txq()->Insert(this);
DCHECK_EQ(0, sd.local_mask & KEYLOCK_ACQUIRED);
@ -1111,7 +1114,7 @@ inline uint32_t Transaction::DecreaseRunCnt() {
::boost::intrusive_ptr guard(this);
// We use release so that no stores will be reordered after.
uint32_t res = run_count_.fetch_sub(1, std::memory_order_release);
uint32_t res = run_count_.fetch_sub(1, memory_order_release);
if (res == 1) {
run_ec_.notify();
}