an atomic cut-over implementation, as per issue #82

build.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 #
 #
-RELEASE_VERSION="0.9.7"
+RELEASE_VERSION="0.9.8"
 
 buildpath=/tmp/gh-ost
 target=gh-ost

go/base/context.go

@@ -30,7 +30,8 @@ const (
 type CutOver int
 
 const (
-	CutOverSafe    CutOver = iota
+	CutOverAtomic  CutOver = iota
+	CutOverSafe            = iota
 	CutOverTwoStep         = iota
 )
 

go/cmd/gh-ost/main.go

@@ -66,7 +66,7 @@ func main() {
 	flag.BoolVar(&migrationContext.OkToDropTable, "ok-to-drop-table", false, "Shall the tool drop the old table at end of operation. DROPping tables can be a long locking operation, which is why I'm not doing it by default. I'm an online tool, yes?")
 	flag.BoolVar(&migrationContext.InitiallyDropOldTable, "initially-drop-old-table", false, "Drop a possibly existing OLD table (remains from a previous run?) before beginning operation. Default is to panic and abort if such table exists")
 	flag.BoolVar(&migrationContext.InitiallyDropGhostTable, "initially-drop-ghost-table", false, "Drop a possibly existing Ghost table (remains from a previous run?) before beginning operation. Default is to panic and abort if such table exists")
-	cutOver := flag.String("cut-over", "", "(mandatory) choose cut-over type (two-step, voluntary-lock)")
+	cutOver := flag.String("cut-over", "atomic", "choose cut-over type (atomic, two-step, voluntary-lock)")
 
 	flag.BoolVar(&migrationContext.SwitchToRowBinlogFormat, "switch-to-rbr", false, "let this tool automatically switch binary log format to 'ROW' on the replica, if needed. The format will NOT be switched back. I'm too scared to do that, and wish to protect you if you happen to execute another migration while this one is running")
 	chunkSize := flag.Int64("chunk-size", 1000, "amount of rows to handle in each iteration (allowed range: 100-100,000)")
@@ -144,7 +144,9 @@ func main() {
 	}
 
 	switch *cutOver {
-	case "safe", "default", "":
+	case "atomic", "default", "":
+		migrationContext.CutOverType = base.CutOverAtomic
+	case "safe":
 		migrationContext.CutOverType = base.CutOverSafe
 	case "two-step":
 		migrationContext.CutOverType = base.CutOverTwoStep

go/logic/applier.go

@@ -20,6 +20,10 @@ import (
 	"github.com/outbrain/golib/sqlutils"
 )
 
+const (
+	atomicCutOverMagicHint = "ghost-cut-over-sentry"
+)
+
 // Applier connects and writes the the applier-server, which is the server where migration
 // happens. This is typically the master, but could be a replica when `--test-on-replica` or
 // `--execute-on-replica` are given.
@@ -77,15 +81,21 @@ func (this *Applier) validateConnection(db *gosql.DB) error {
 	return nil
 }
 
-// tableExists checks if a given table exists in database
-func (this *Applier) tableExists(tableName string) (tableFound bool) {
+// showTableStatus returns the output of `show table status like '...'` command
+func (this *Applier) showTableStatus(tableName string) (rowMap sqlutils.RowMap) {
+	rowMap = nil
 	query := fmt.Sprintf(`show /* gh-ost */ table status from %s like '%s'`, sql.EscapeName(this.migrationContext.DatabaseName), tableName)
-
 	sqlutils.QueryRowsMap(this.db, query, func(m sqlutils.RowMap) error {
-		tableFound = true
+		rowMap = m
 		return nil
 	})
-	return tableFound
+	return rowMap
+}
+
+// tableExists checks if a given table exists in database
+func (this *Applier) tableExists(tableName string) (tableFound bool) {
+	m := this.showTableStatus(tableName)
+	return (m != nil)
 }
 
 // ValidateOrDropExistingTables verifies ghost and changelog tables do not exist,
@@ -775,6 +785,195 @@ func (this *Applier) ExpectProcess(sessionId int64, stateHint, infoHint string)
 	return nil
 }
 
+// DropAtomicCutOverSentryTableIfExists checks if the "old" table name
+// happens to be a cut-over magic table; if so, it drops it.
+func (this *Applier) DropAtomicCutOverSentryTableIfExists() error {
+	log.Infof("Looking for magic cut-over table")
+	tableName := this.migrationContext.GetOldTableName()
+	rowMap := this.showTableStatus(tableName)
+	if rowMap == nil {
+		// Table does not exist
+		return nil
+	}
+	if rowMap["Comment"].String != atomicCutOverMagicHint {
+		return fmt.Errorf("Expected magic comment on %s, did not find it", tableName)
+	}
+	log.Infof("Dropping magic cut-over table")
+	return this.dropTable(tableName)
+}
+
+// DropAtomicCutOverSentryTableIfExists checks if the "old" table name
+// happens to be a cut-over magic table; if so, it drops it.
+func (this *Applier) CreateAtomicCutOverSentryTable() error {
+	if err := this.DropAtomicCutOverSentryTableIfExists(); err != nil {
+		return err
+	}
+	tableName := this.migrationContext.GetOldTableName()
+
+	query := fmt.Sprintf(`create /* gh-ost */ table %s.%s (
+			id int auto_increment primary key
+		) comment='%s'
+		`,
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(tableName),
+		atomicCutOverMagicHint,
+	)
+	log.Infof("Creating magic cut-over table %s.%s",
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(tableName),
+	)
+	if _, err := sqlutils.ExecNoPrepare(this.db, query); err != nil {
+		return err
+	}
+	log.Infof("Magic cut-over table created")
+
+	return nil
+}
+
+// AtomicCutOverMagicLock
+func (this *Applier) AtomicCutOverMagicLock(sessionIdChan chan int64, tableLocked chan<- error, okToUnlockTable <-chan bool, tableUnlocked chan<- error) error {
+	tx, err := this.db.Begin()
+	if err != nil {
+		tableLocked <- err
+		return err
+	}
+	defer func() {
+		sessionIdChan <- -1
+		tableLocked <- fmt.Errorf("Unexpected error in AtomicCutOverMagicLock(), injected to release blocking channel reads")
+		tableUnlocked <- fmt.Errorf("Unexpected error in AtomicCutOverMagicLock(), injected to release blocking channel reads")
+		tx.Rollback()
+	}()
+
+	var sessionId int64
+	if err := tx.QueryRow(`select connection_id()`).Scan(&sessionId); err != nil {
+		tableLocked <- err
+		return err
+	}
+	sessionIdChan <- sessionId
+
+	lockResult := 0
+	query := `select get_lock(?, 0)`
+	lockName := this.GetSessionLockName(sessionId)
+	log.Infof("Grabbing voluntary lock: %s", lockName)
+	if err := tx.QueryRow(query, lockName).Scan(&lockResult); err != nil || lockResult != 1 {
+		err := fmt.Errorf("Unable to acquire lock %s", lockName)
+		tableLocked <- err
+		return err
+	}
+
+	tableLockTimeoutSeconds := this.migrationContext.SwapTablesTimeoutSeconds * 2
+	log.Infof("Setting LOCK timeout as %d seconds", tableLockTimeoutSeconds)
+	query = fmt.Sprintf(`set session lock_wait_timeout:=%d`, tableLockTimeoutSeconds)
+	if _, err := tx.Exec(query); err != nil {
+		tableLocked <- err
+		return err
+	}
+
+	if err := this.CreateAtomicCutOverSentryTable(); err != nil {
+		tableLocked <- err
+		return err
+	}
+
+	query = fmt.Sprintf(`lock /* gh-ost */ tables %s.%s write, %s.%s write`,
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.OriginalTableName),
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.GetOldTableName()),
+	)
+	log.Infof("Locking %s.%s, %s.%s",
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.OriginalTableName),
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.GetOldTableName()),
+	)
+	this.migrationContext.LockTablesStartTime = time.Now()
+	if _, err := tx.Exec(query); err != nil {
+		tableLocked <- err
+		return err
+	}
+	log.Infof("Tables locked")
+	tableLocked <- nil // No error.
+
+	// From this point on, we are committed to UNLOCK TABLES. No matter what happens,
+	// the UNLOCK must execute (or, alternatively, this connection dies, which gets the same impact)
+
+	// The cut-over phase will proceed to apply remaining backlog onto ghost table,
+	// and issue RENAME. We wait here until told to proceed.
+	<-okToUnlockTable
+	log.Infof("Will now proceed to drop magic table and unlock tables")
+
+	// The magic table is here because we locked it. And we are the only ones allowed to drop it.
+	// And in fact, we will:
+	log.Infof("Dropping magic cut-over table")
+	query = fmt.Sprintf(`drop /* gh-ost */ table if exists %s.%s`,
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.GetOldTableName()),
+	)
+	if _, err := tx.Exec(query); err != nil {
+		log.Errore(err)
+		// We DO NOT return here because we must `UNLOCK TABLES`!
+	}
+
+	// Tables still locked
+	log.Infof("Releasing lock from %s.%s, %s.%s",
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.OriginalTableName),
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.GetOldTableName()),
+	)
+	query = `unlock tables`
+	if _, err := tx.Exec(query); err != nil {
+		tableUnlocked <- err
+		return log.Errore(err)
+	}
+	log.Infof("Tables unlocked")
+	tableUnlocked <- nil
+	return nil
+}
+
+// RenameOriginalTable will attempt renaming the original table into _old
+func (this *Applier) AtomicCutoverRename(sessionIdChan chan int64, tablesRenamed chan<- error) error {
+	tx, err := this.db.Begin()
+	if err != nil {
+		return err
+	}
+	defer func() {
+		tx.Rollback()
+		sessionIdChan <- -1
+		tablesRenamed <- fmt.Errorf("Unexpected error in AtomicCutoverRename(), injected to release blocking channel reads")
+	}()
+	var sessionId int64
+	if err := tx.QueryRow(`select connection_id()`).Scan(&sessionId); err != nil {
+		return err
+	}
+	sessionIdChan <- sessionId
+
+	log.Infof("Setting RENAME timeout as %d seconds", this.migrationContext.SwapTablesTimeoutSeconds)
+	query := fmt.Sprintf(`set session lock_wait_timeout:=%d`, this.migrationContext.SwapTablesTimeoutSeconds)
+	if _, err := tx.Exec(query); err != nil {
+		return err
+	}
+
+	query = fmt.Sprintf(`rename /* gh-ost */ table %s.%s to %s.%s, %s.%s to %s.%s`,
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.OriginalTableName),
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.GetOldTableName()),
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.GetGhostTableName()),
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.OriginalTableName),
+	)
+	log.Infof("Issuing and expecting this to block: %s", query)
+	if _, err := tx.Exec(query); err != nil {
+		tablesRenamed <- err
+		return log.Errore(err)
+	}
+	tablesRenamed <- nil
+	log.Infof("Tables renamed")
+	return nil
+}
+
 func (this *Applier) ShowStatusVariable(variableName string) (result int64, err error) {
 	query := fmt.Sprintf(`show global status like '%s'`, variableName)
 	if err := this.db.QueryRow(query).Scan(&variableName, &result); err != nil {