Set Statements

Use Case #

There are multiple applications/frameworks that tweak session system variables at the start of the connection to configure the system in specific ways. Some do it for all connections it uses, and some do it only for specific workloads, such as import, installation, etc.

To support these use cases, Vitess needs to allow SET statements to be set on a per-connection basis.

The recommendation is to change the setting at a global level during Vitess tablet setup. When this is done, sessions can use the normal connection pools, which is the most efficient way to operate. However, this design allows for sessions with VTgate changing these system variables and Vitess will make sure that they are set for all connections with MySQL for that session.

System variable types #

Vitess internally classifies system variables into three types - Ignore, Reserve, and Vitess-aware.

Ignore modification #

These are the system variables for which Vitess validates the current setting and notify users via warning that settings are either ignored or modification was not allowed. E.g. default_storage_engine, debug, etc.

Vitess Aware #

These are the system variables for which Vitess needs to change behavior based on the settings. E.g. autocommit, client_found_rows, etc.

Reserve connection #

These are the system variables that Vitess will send down to the storage engine and modify the settings at the session connection level. E.g. sql_mode, sql_safe_updates, etc.

This document contains the design for this type of system variables.

Design #

VTTablet Reserve Pool #

VTTablet will have a new pool named reserved pool. Connections in the reserved pool are tied to a specific session and are closed when the session is closed.

QueryService interface changes:

  • ReserveExecute: Reserve a connection, execute the set statements, and the query.
  • ReserveBeginExecute: Reserve a connection, execute the set statements, move the reserve connection to active transaction pool and execute begin and the query.
  • ReserveRelease: This closes the reserved connection and also rollback the transaction if it is in an open state.
  • BeginExecute: ReserveId will be added as a parameter to the request.
  • Execute: ReserveId will be added as a parameter to the request.

Connections that have updated session system variables will at the end of a transaction (Commit/Rollback) be moved to the reserve pool - they can’t be reused by other sessions.

VTGate #

When a session issues a SET statement to change a system variable, VTGate will compare the provided value with the already configured value. If it differs, it is stored in the session.

A connection with the vttablet is reserved when a query is sent from the client connection. The stored settings are applied first to the reserved connection and then the query is executed. Storing the changed setting in the session enables VTGate to apply settings to any new connection that is opened with different shards.

StateReservedTransactionRecorded SETs
clearFFF
remember-setsFFT
reservedTFT
inTxFTF
reserved-inTxTTT
From StateQueryVtgate ActionAPI on tabletTo State
clearSET system varRecord statementNo callremember-sets
remember-setsSET system varRecord statementNo callremember-sets
reservedSET system varRecord statementExecutereserved
inTxSET system varRecord statementReserveExecutereserved-inTx
reserved-inTxSET system varRecord statementExecutereserved-inTx
remember-setsQuery ReserveExecutereserved
reservedQuery Executereserved
reserved-inTxQuery Executereserved-inTx
remember-setsBegin + Query ReserveBeginExecutereserved-inTx
reservedBegin + Query BeginExecutereserved-inTx
remember-setsCommit/RollbackNo-opNo callremember-sets
reservedCommit/RollbackNo-opNo callreserved
reserved-inTxCommit/Rollback Commit/Rollbackreserved
remember-setsClose ConnectionClose SessionNo callclear
reservedClose ConnectionClose SessionReserveReleaseclear
reserved-inTxClose ConnectionClose SessionReserveReleaseclear

Release Plan #

The release plan would be to update vttablet < vtctl < vtgate in this particular order.

Assumption #

The order of the SET statements is not maintained. Different Vitess shards might see settings being applied in any order.