Session

The session package (and related packages such as sessionctx and sessionctx/variable) are responsible for maintaining the state of both sessions and transactions.

New session origins

New connections are first established in the server package. After some initial protocol negotiation, the server package calls session.CreateSession(). This function then calls session.createSessionWithOpt() (via CreateSessionWithOpt()) which creates the session.

Sessions used for internal server operations are usually created in a different manner, with the sessionctx being retrieved from a pool of sessions maintained by domain. For example:

dom := domain.GetDomain(e.ctx)
sysSessionPool := dom.SysSessionPool()
ctx, err := sysSessionPool.Get()
if err != nil {
    return nil, err
}
restrictedCtx := ctx.(sessionctx.Context)
restrictedCtx.GetSessionVars().InRestrictedSQL = true

Internal sessions will not show up in the output of SHOW PROCESSLIST, and skip all privilege checks because they do not have a privilege manager handle attached.

System variable state

System variables follow similar semantics to MySQL:

• If a variable includes SESSION scope, the value is copied to the session state when the session is created.

• Any changes to the GLOBAL value will not apply to any existing sessions.

The state of the variables is stored in sessionVars. The raw string values are stored in a map named systems. This string value is used for persistence in the mysql.global_variables table.

For many variables, as well as a string value there is a typed field in sessionVars. For example:

SessionVars.systems["tidb_skip_utf8_check"] (string) maps to SessionVars.SkipUTF8Check (bool).

The typed value is set when the SetSession attached to the system variable definition is called. For tidb_skip_utf8_check this is as follows:

{Scope: ScopeGlobal | ScopeSession, Name: TiDBSkipUTF8Check, Value: BoolToOnOff(DefSkipUTF8Check), Type: TypeBool, SetSession: func(s *SessionVars, val string) error {
	s.SkipUTF8Check = TiDBOptOn(val)
	return nil
}},

The SetSession function can also be considered an Init function, since it is called when the session is created and the values are copied from global scope. To disable SetSession from being called on creation, skipInit can be set to true. For example with CharsetDatabase:

{Scope: ScopeGlobal | ScopeSession, Name: CharsetDatabase, Value: mysql.DefaultCharset, skipInit: true, Validation: func(vars *SessionVars, normalizedValue string, originalValue string, scope ScopeFlag) (string, error) {
    return checkCharacterSet(normalizedValue, CharsetDatabase)
}, SetSession: func(s *SessionVars, val string) error {
	if cs, err := charset.GetCharsetInfo(val); err == nil {
		s.systems[CollationDatabase] = cs.DefaultCollation
	}
	return nil
}},

In the above example, skipping the SetSession function is useful because it prevents the CollationDatabase from being overwritten when the session is initialized. This is only expected if the user issues a statement to change the CharsetDatabase value.

Differences from MySQL

In TiDB, changes to GLOBAL scoped system variables are propagated to other TiDB servers in the cluster and persist across restarts. The notification event to other servers is sent via an etcd channel in the call domain.GetDomain(s).NotifyUpdateSysVarCache():

// replaceGlobalVariablesTableValue executes restricted sql updates the variable value
// It will then notify the etcd channel that the value has changed.
func (s *session) replaceGlobalVariablesTableValue(ctx context.Context, varName, val string) error {
	stmt, err := s.ParseWithParams(ctx, `REPLACE INTO %n.%n (variable_name, variable_value) VALUES (%?, %?)`, mysql.SystemDB, mysql.GlobalVariablesTable, varName, val)
	if err != nil {
		return err
	}
	_, _, err = s.ExecRestrictedStmt(ctx, stmt)
	domain.GetDomain(s).NotifyUpdateSysVarCache() // <-- the notification happens here
	return err
}

Because GLOBAL scoped variables are propagated to other servers, TiDB also has a special concept of "instance-scoped variables". An instance scoped variable is actually a SESSION scoped variable that has a GetSession method which returns data that is specific to an instance. For example, tidb_general_log:

{Scope: ScopeSession, Name: TiDBGeneralLog, Value: BoolToOnOff(DefTiDBGeneralLog), Type: TypeBool, skipInit: true, SetSession: func(s *SessionVars, val string) error {
	ProcessGeneralLog.Store(TiDBOptOn(val))
	return nil
}, GetSession: func(s *SessionVars) (string, error) {
	return BoolToOnOff(ProcessGeneralLog.Load()), nil
}},

The decision to make an option such as tidb_general_log instance scoped is because it references a file on the local filesystem. This may create issues when global, as the path may not be writable on each tidb-server in the cluster.

As you can see by the Scope: Session, instance-scoped variables are not natively handled by the sysvar framework, but are instead denoted by the GetSession() function reading from a global location. The documentation for tidb_general_log also notes it as "instance" scoped by convention.

Transaction state

The session struct (s.txn) is responsible for keeping modified key-value pairs in a LazyTxn until the transaction commits. A commit statement only sets the session variable state that it is no longer in an active transaction:

func (e *SimpleExec) executeCommit(s *ast.CommitStmt) {
	e.ctx.GetSessionVars().SetInTxn(false)
}

The function autoCommitAfterStmt() which is called as part of finishStmt() is responsible for committing the transaction:

if !sessVars.InTxn() {
	if err := se.CommitTxn(ctx); err != nil {
		if _, ok := sql.(*executor.ExecStmt).StmtNode.(*ast.CommitStmt); ok {
			err = errors.Annotatef(err, "previous statement: %s", se.GetSessionVars().PrevStmt)
		}
		return err
	}
	return nil
}

The session.CommitTxn() function will handle the commit, including retry (if permitted). There is also special handling for both pessimistic and optimistic transactions, as well as removing the key-value pairs which apply to temporary tables from the transaction buffer.

See also

• The lifecycle of a statement
• Privilege management
• Transaction