Alexey Scherbakov created IGNITE-21077:
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Summary: Unify transactions for SQL and KV layers
Key: IGNITE-21077
URL: https://issues.apache.org/jira/browse/IGNITE-21077
Project: Ignite
Issue Type: Improvement
Affects Versions: 3.0
Reporter: Alexey Scherbakov
Fix For: 3.0
Currently KV and SQL engines use different approach to handling transactions,
which leads to different execution paths. As a consequence, performance
improvements for KV does not apply to SQL.
The problem here is due to distributed nature of SQL execution, in contrary to
KV API, which can be coordinated from a single txn coordinator.
The idea is to introduce nested transactions, which works in the following way:
# Query initiator starts parent transaction.
# SQL engine should always use KV api to access data for further SQL execution.
# If SQL query is mapped to multiple execution nodes, each node accesses data
using KV API and starts nested transaction having the ID of a parent
transaction.
# Nested transaction can'be directly committed or rolled back.
# If at least one nested transaction is created, we need additional PREPARE
step. On PREPARE step query initiator sends prepare request to each node
containing nested transaction, which is ACKED when a nested txn is safe to
commit.
# On receiving all acks parent transaction sends commit request to commit
partition.
# PREPARE step introduces additional latency, so it's always OK to map query
to a single execution node. For example, SQL transaction containing only
unicast operations, can be always executed from a initiator, so PREPARE step is
not required.
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