![]() ![]() With timestamp ordering, on the other hand, if the timestamp of In 2PL, if T2 arrives after T1 is finished, Using 2PL? Yes! consider this simple example. Uses for accepting/rejecting transactions, but are there any advantages in 2PL is much more complicated than the simple rules that DynamoDB Figure 6 shows the difference between DynamoDB and Spanner at a very high level. Thus, Unlike DynamoDB, Spanner does NOT need timestamps to guarantee serializability for its read-write transactions, as that is guaranteed by 2PL. Thus, to avoid duplicate mutation, nodes have toĭe-duplicate messages from the TC (see this post for more info on exactly-once The coordinator makes sure that mutations of an accepted transaction are delivered to the state of all shards exactly once. Handles the failure of a TC and continues pending transactions recorded on the The state of the transaction on theĬoordinator is stored on a durable ledger. Stored (see this post): 1) the vote to accept/reject by the nodes, 2) There are two points of no returns in 2PC that must be durably Transaction, the TC asks them to complete the transactions by making the Each node sees if it can execute the transaction or not and A TC sends the transaction request to the Although Terry says DynamoDB avoids 2PC, it seemsĢPC is exactly what the TCs do. There is a fleet of stateless transaction coordinators that manage the execution of transactions. This design tries to satisfy the "No impact on non-transactional operation" requirement mentionedĪbove. Non-transaction operations go through separate routes. We will see how, with this simple approach, DynamoDB provides ACID transactions while avoiding the complexities of more sophisticated protocols.įigure 2 shows the overall architecture of DynamoDB. Thus, the job of the database is to abort any transaction that violates this illusion. The timestamp ordering approach used by DynamoDB is very simple: assign timestamps to transactions and let these timestamps define the serialization order, i.e., a transaction with a smaller timestamp must appear to be executed before the one with a larger timestamp. It does not need MVCC and avoids any locking scheme (e.g. ![]() DynamoDB does not require tight clock synchronization, and unlike Spanner does not assume any uncertainty window. In this post, we cover DynamoDB which uses timestamp ordering that perhaps is the simplest approach compared to those we have covered so far. We saw Spanner uses 2PC+2PL over Paxos leaders and uses TrueTime for external consistency, Calvin uses a deterministic approach, and FoundationDB uses OCC. We have covered several protocols for distributed transactions in this blog so far. ![]()
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