Distributed transactions
Comparison
https://docs.google.com/spreadsheets/d/1Sw0T4R6-Bb3orF0abwkmiZRBbCioevH1jvyjnw7aqhs/edit?usp=sharing
Two phase commit
Success case flowchart
Failure case flowchart
Cons
The nodes are in blocked status during the execution.
The coordinator could be the single point of failure in the entire process.
If any single failure happens among participants, all participants need to rollback transaction. And this could be high performance cost.
Three phase commit
This introduces another phase before prepare/commit phase to guarantee that the "Cons 3" in two phase commit get resolved.
However, this seldomly gets used in practice because the benefits are limited.
TCC
Definition
Each of three phases corresponds to a local transaction.
Try: Prepare transaction but not commit. Insert data.
Confirm: Corresponds to "commit phase" in two phase commit.
Cancel: Corresponds to "rollback phase" in two phase commit.
Success case flowchart
Failure case flowchart
Typically it won't fail in Try Phase. And it will fail only in Confirm phase.
Fix consistency flow
TCC is an eventual consistency model.
Asynchronously fix inconsistency
Fix inconsistency on discovery
Saga
Flowchart
Compensation
Compensation is more accurate than rollback in saga context because
Rollback applies in uncommitted transaction
Compensation applies in committed transaction
In saga, compensation is a better fit because it reverse committed local transactions.
AT transaction
AT transaction applies when you operate on multiple databases concurrently.
Delayed transactions
Flowchart
When sharding middlware begins transactions on sharded DB, it has two options:
At "Begin", start transactions on all DB shards.
When executing a certain SQL statement, decide which shards to start transactions
Fix consistency
Retry on failure
Human intervention
Asynchronously fix inconsistency
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