ding to their visibility. This filtering process is known

If I have seen further, it is by standing on ye shoulders of Giants

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Execute a transaction's validation and write phase together as a critical section: while ti being in the val-write phase, no other tk can enter its val-write phase

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• If there is unresolved write intent (uncommitted version in the chain head), it initiates the intent resolution 
• Checks a local txn state map for committed or aborted state - allow read if the state is committed and commitTs <= readTs.
• If not possible, send a TxStateReq  to the txn coordinator - this initiates the coordinator path. Coordinator address is fetched from the txn state map.
• If a coordinator path was not able to resolve the intent, one of the following has happened - the coordinator is dead or txn state is not available in the cache. Calculate a commit partition and send the TxStateReq to its primary replica - this initiates the commit partition path.
• Retry commit partition path until a success or timeout.

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• If the local txn is finished, return the response with the outcome: commit or abort. The txn state is stored in a local cache, mentioned in the RW section.
  
  If the local txn is finishing (waiting for finish state replication), wait for outcome and return response with the outcomeresult: commit or abort
  
  If the outcome result is to commit, additional timestamp check is required: a commit timestamp must be <= readTs. If the condition is not held, the outcome result is changed to abort.
  
  If local txn is active, adjust the txn coordinator node HLC according to readTs to make sure the txn commit timestamp is above the read timestamp. The read timestamp must be installed before txn is started to commit, so commit timestamp is assigned after the read timestamp. This must be achieved by some sort of concurrency control, preferably non-blocking. In this case we must ignore the write intent, so the outcome is to abort.
  
  If txn state is not found in a local cache and txn is not active, return NULL.

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Because read locks are not replicated, it's possible that another transaction is mapped to a new primary replica and invalidates a read for the current not yet committed transaction.

Assume a transactions

T1: a= r1(x), w1(y), c1

T2: r2(y), w2(x), c2

Due to PR1 failure and a lost read lock, a following history is possible.

H = r1(x) w2r2(x) c2 w1(y) c1

Assume the RO transaction T3 and the history

H2 = r1(x) w2(x) c2 r3(x) r3(y) w1(y) c1

The RO transaction will return the inconsistent result to a user, not corresponding to any serial execution of T1, T2, T3

This is illustrated by the diagram:

Image Removed

Note that H2 is not allowed by SS2PL, instead it allows

H3 = r1(x) w1(y) c1 w2(x) c2This history is not serializable.

We prevent this history by not committing TX1 if its commit timestamp lies outside lease ranges.

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In the following diagram the lease refresh discovers that the lease is no longer valid, and the transaction is aborted.

Leases can must be refreshed periodically by TxnCoordinator, depending on the lease duration. Successful refreshing of a lease extends transaction deadline up to a lease upper bound.

The more frequent refreshing reduces the probability of txn abort due to reaching the deadline.The reasonable refresh rate can be calculated as a fraction of the lease duration D, for example  D/3is D/2, where D is a lease duration interval.

Commit reordering for dependent transactions

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This picture shows the possible commit reordering between T1 and T2 due to delay in applying the commit for T1, despite the fact T2 has a larger commit timestamp. This leads to a situation when T2 becomes visible before T1. But it seems not to cause any consistency issues, because up-to-date data is correct and the possible issues can only be with RO transactions at a timestamp T2. Likely Luckily, we already have the intent written for T1 and the write intent resolution procedure will eventually wait for committed intent. So, for now I’m considering this reordering harmless.

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https://github.com/ascherbakoff/ai3-txn-mvp MVP for RW based CC, with data and index versioning support.

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