suuporting dynamic reconfiguration of max-load

Author: Shlomi Noach

build.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 #
 #
-RELEASE_VERSION="0.8.7"
+RELEASE_VERSION="0.8.8"
 
 buildpath=/tmp/gh-ost
 target=gh-ost

doc/interactive-commands.md

@@ -19,6 +19,8 @@ Both interfaces may serve at the same time. Both respond to simple text command,
 - `throttle`: force migration suspend
 - `no-throttle`: cancel forced suspension (though other throttling reasons may still apply)
 - `chunk-size=<newsize>`: modify the `chunk-size`; applies on next running copy-iteration
+- `max-load=<max-load-thresholds>`: modify the `max-load` config; applies on next running copy-iteration
+  The `max-load` format must be: `some_status=<numeric-threshold>[,some_status=<numeric-threshold>...]`. For example: `Threads_running=50,threads_connected=1000`, and you would then write/echo `max-load=Threads_running=50,threads_connected=1000` to the socket.
 
 ### Examples
 

doc/perks.md

@@ -12,13 +12,22 @@ You started with a `chunk-size=5000` but you find out it's too much. You want to
 $ echo "chunk-size=250" | nc -U /tmp/gh-ost.test.sample_data_0.sock
 ```
 
+Likewise, you can change the `max-load` configuration:
+
+```shell
+$ echo "max-load=Threads_running=50,threads_connected=1000" | nc -U /tmp/gh-ost.test.sample_data_0.sock
+```
+
+The `max-load` format must be: `some_status=<numeric-threshold>[,some_status=<numeric-threshold>...]`.
+In case of parsing error the command is ignored.
+
 Read more about [interactive commands](interactive-commands.md)
 
 ### What's the status?
 
 You do not have to have access to the `screen` where the migration is issued. You have two ways to get current status:
 
-0. Use [interactive commands](interactive-commands.md). Via unix socket file or via `TCP` you can get current status:
+1. Use [interactive commands](interactive-commands.md). Via unix socket file or via `TCP` you can get current status:
 
 ```shell
 $ echo status | nc -U /tmp/gh-ost.test.sample_data_0.sock
@@ -31,7 +40,7 @@ $ echo status | nc -U /tmp/gh-ost.test.sample_data_0.sock
 Copy: 0/2915 0.0%; Applied: 0; Backlog: 0/100; Elapsed: 40s(copy), 41s(total); streamer: mysql-bin.000550:49942; ETA: throttled, flag-file
 ```
 
-0. `gh-ost` creates and uses a changelog table for internal bookkeeping. This table has the `_osc` suffix (the tool creates and announces this table upon startup) If you like, you can SQL your status:
+2. `gh-ost` creates and uses a changelog table for internal bookkeeping. This table has the `_osc` suffix (the tool creates and announces this table upon startup) If you like, you can SQL your status:
 
 ```
 > select * from _sample_data_0_osc order by id desc limit 1 \G

doc/why-triggerless.md

@@ -45,6 +45,10 @@ Thus, triggers must keep operating. On busy servers, we have seen that even as t
 
 Read more about [`gh-ost` throttling](throttle.md)
 
+### Triggers, multiple migrations
+
+We are interested in being able to run multiple concurrent migrations (not on the same table, of course). Given all the above, we do not have trust that running multiple trigger-based migrations is a safe operation. In our current, past and shared experiences we have never done so; we are unaware of anyone who is doing so.
+
 ### Trigger based migration, no reliable production test
 
 We sometimes wish to experiment with a migration, or know in advance how much time it would take. A trigger-based solution allows us to run a migration on a replica, provided it uses Statement Based Replication.

go/base/context.go

@@ -65,7 +65,8 @@ type MigrationContext struct {
 	ThrottleFlagFile                    string
 	ThrottleAdditionalFlagFile          string
 	ThrottleCommandedByUser             int64
-	MaxLoad                             map[string]int64
+	maxLoad                             map[string]int64
+	maxLoadMutex                        *sync.Mutex
 	PostponeCutOverFlagFile             string
 	SwapTablesTimeoutSeconds            int64
 
@@ -141,7 +142,8 @@ func newMigrationContext() *MigrationContext {
 		ApplierConnectionConfig:             mysql.NewConnectionConfig(),
 		MaxLagMillisecondsThrottleThreshold: 1000,
 		SwapTablesTimeoutSeconds:            3,
-		MaxLoad:                             make(map[string]int64),
+		maxLoad:                             make(map[string]int64),
+		maxLoadMutex:                        &sync.Mutex{},
 		throttleMutex:                       &sync.Mutex{},
 		ThrottleControlReplicaKeys:          mysql.NewInstanceKeyMap(),
 		configMutex:                         &sync.Mutex{},
@@ -269,12 +271,29 @@ func (this *MigrationContext) IsThrottled() (bool, string) {
 	return this.isThrottled, this.throttleReason
 }
 
+func (this *MigrationContext) GetMaxLoad() map[string]int64 {
+	this.maxLoadMutex.Lock()
+	defer this.maxLoadMutex.Unlock()
+
+	tmpMaxLoadMap := make(map[string]int64)
+	for k, v := range this.maxLoad {
+		tmpMaxLoadMap[k] = v
+	}
+	return tmpMaxLoadMap
+}
+
 // ReadMaxLoad parses the `--max-load` flag, which is in multiple key-value format,
 // such as: 'Threads_running=100,Threads_connected=500'
+// It only applies changes in case there's no parsing error.
 func (this *MigrationContext) ReadMaxLoad(maxLoadList string) error {
 	if maxLoadList == "" {
 		return nil
 	}
+	this.maxLoadMutex.Lock()
+	defer this.maxLoadMutex.Unlock()
+
+	tmpMaxLoadMap := make(map[string]int64)
+
 	maxLoadConditions := strings.Split(maxLoadList, ",")
 	for _, maxLoadCondition := range maxLoadConditions {
 		maxLoadTokens := strings.Split(maxLoadCondition, "=")
@@ -287,9 +306,11 @@ func (this *MigrationContext) ReadMaxLoad(maxLoadList string) error {
 		if n, err := strconv.ParseInt(maxLoadTokens[1], 10, 0); err != nil {
 			return fmt.Errorf("Error parsing numeric value in max-load condition: %s", maxLoadCondition)
 		} else {
-			this.MaxLoad[maxLoadTokens[0]] = n
+			tmpMaxLoadMap[maxLoadTokens[0]] = n
 		}
 	}
+
+	this.maxLoad = tmpMaxLoadMap
 	return nil
 }
 

go/logic/migrator.go

@@ -130,7 +130,8 @@ func (this *Migrator) shouldThrottle() (result bool, reason string) {
 		}
 	}
 
-	for variableName, threshold := range this.migrationContext.MaxLoad {
+	maxLoad := this.migrationContext.GetMaxLoad()
+	for variableName, threshold := range maxLoad {
 		value, err := this.applier.ShowStatusVariable(variableName)
 		if err != nil {
 			return true, fmt.Sprintf("%s %s", variableName, err)
@@ -530,7 +531,9 @@ func (this *Migrator) stopWritesAndCompleteMigrationOnReplica() (err error) {
 }
 
 func (this *Migrator) onServerCommand(command string, writer *bufio.Writer) (err error) {
-	tokens := strings.Split(command, "=")
+	defer writer.Flush()
+
+	tokens := strings.SplitN(command, "=", 2)
 	command = strings.TrimSpace(tokens[0])
 	arg := ""
 	if len(tokens) > 1 {
@@ -553,12 +556,21 @@ func (this *Migrator) onServerCommand(command string, writer *bufio.Writer) (err
 	case "chunk-size":
 		{
 			if chunkSize, err := strconv.Atoi(arg); err != nil {
+				fmt.Fprintf(writer, "%s\n", err.Error())
 				return log.Errore(err)
 			} else {
 				this.migrationContext.SetChunkSize(int64(chunkSize))
 				this.printMigrationStatusHint(writer)
 			}
 		}
+	case "max-load":
+		{
+			if err := this.migrationContext.ReadMaxLoad(arg); err != nil {
+				fmt.Fprintf(writer, "%s\n", err.Error())
+				return log.Errore(err)
+			}
+			this.printMigrationStatusHint(writer)
+		}
 	case "throttle", "pause", "suspend":
 		{
 			atomic.StoreInt64(&this.migrationContext.ThrottleCommandedByUser, 1)
@@ -568,9 +580,10 @@ func (this *Migrator) onServerCommand(command string, writer *bufio.Writer) (err
 			atomic.StoreInt64(&this.migrationContext.ThrottleCommandedByUser, 0)
 		}
 	default:
-		return fmt.Errorf("Unknown command: %s", command)
+		err = fmt.Errorf("Unknown command: %s", command)
+		fmt.Fprintf(writer, "%s\n", err.Error())
+		return err
 	}
-	writer.Flush()
 	return nil
 }
 
@@ -644,10 +657,11 @@ func (this *Migrator) printMigrationStatusHint(writers ...io.Writer) {
 	fmt.Fprintln(w, fmt.Sprintf("# Migration started at %+v",
 		this.migrationContext.StartTime.Format(time.RubyDate),
 	))
+	maxLoad := this.migrationContext.GetMaxLoad()
 	fmt.Fprintln(w, fmt.Sprintf("# chunk-size: %+v; max lag: %+vms; max-load: %+v",
 		atomic.LoadInt64(&this.migrationContext.ChunkSize),
 		atomic.LoadInt64(&this.migrationContext.MaxLagMillisecondsThrottleThreshold),
-		this.migrationContext.MaxLoad,
+		maxLoad,
 	))
 	if this.migrationContext.ThrottleFlagFile != "" {
 		fmt.Fprintln(w, fmt.Sprintf("# Throttle flag file: %+v",