Golang topoproto.KeyspaceShardString函数代码示例

// findDestinationMasters finds for each destination shard the current master.
func (scw *SplitCloneWorker) findDestinationMasters(ctx context.Context) error {
	scw.setState(WorkerStateFindTargets)

	// Make sure we find a master for each destination shard and log it.
	scw.wr.Logger().Infof("Finding a MASTER tablet for each destination shard...")
	for _, si := range scw.destinationShards {
		waitCtx, waitCancel := context.WithTimeout(ctx, *waitForHealthyTabletsTimeout)
		defer waitCancel()
		if err := scw.tsc.WaitForTablets(waitCtx, scw.cell, si.Keyspace(), si.ShardName(), []topodatapb.TabletType{topodatapb.TabletType_MASTER}); err != nil {
			return fmt.Errorf("cannot find MASTER tablet for destination shard for %v/%v (in cell: %v): %v", si.Keyspace(), si.ShardName(), scw.cell, err)
		}
		masters := scw.tsc.GetHealthyTabletStats(si.Keyspace(), si.ShardName(), topodatapb.TabletType_MASTER)
		if len(masters) == 0 {
			return fmt.Errorf("cannot find MASTER tablet for destination shard for %v/%v (in cell: %v) in HealthCheck: empty TabletStats list", si.Keyspace(), si.ShardName(), scw.cell)
		}
		master := masters[0]

		// Get the MySQL database name of the tablet.
		keyspaceAndShard := topoproto.KeyspaceShardString(si.Keyspace(), si.ShardName())
		scw.destinationDbNames[keyspaceAndShard] = topoproto.TabletDbName(master.Tablet)

		// TODO(mberlin): Verify on the destination master that the
		// _vt.blp_checkpoint table has the latest schema.

		scw.wr.Logger().Infof("Using tablet %v as destination master for %v/%v", topoproto.TabletAliasString(master.Tablet.Alias), si.Keyspace(), si.ShardName())
	}
	scw.wr.Logger().Infof("NOTE: The used master of a destination shard might change over the course of the copy e.g. due to a reparent. The HealthCheck module will track and log master changes and any error message will always refer the actually used master address.")

	return nil
}

// Run is part of the Task interface.
func (t *SplitCloneTask) Run(parameters map[string]string) ([]*automationpb.TaskContainer, string, error) {
	// TODO(mberlin): Add parameters for the following options?
	//                        '--source_reader_count', '1',
	//                        '--destination_writer_count', '1',
	args := []string{"SplitClone"}
	if online := parameters["online"]; online != "" {
		args = append(args, "--online="+online)
	}
	if offline := parameters["offline"]; offline != "" {
		args = append(args, "--offline="+offline)
	}
	if excludeTables := parameters["exclude_tables"]; excludeTables != "" {
		args = append(args, "--exclude_tables="+excludeTables)
	}
	if writeQueryMaxRows := parameters["write_query_max_rows"]; writeQueryMaxRows != "" {
		args = append(args, "--write_query_max_rows="+writeQueryMaxRows)
	}
	if writeQueryMaxSize := parameters["write_query_max_size"]; writeQueryMaxSize != "" {
		args = append(args, "--write_query_max_size="+writeQueryMaxSize)
	}
	if minHealthyRdonlyTablets := parameters["min_healthy_rdonly_tablets"]; minHealthyRdonlyTablets != "" {
		args = append(args, "--min_healthy_rdonly_tablets="+minHealthyRdonlyTablets)
	}
	args = append(args, topoproto.KeyspaceShardString(parameters["keyspace"], parameters["source_shard"]))
	output, err := ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], args)

	// TODO(mberlin): Remove explicit reset when vtworker supports it implicility.
	if err == nil {
		// Ignore output and error of the Reset.
		ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], []string{"Reset"})
	}
	return nil, output, err
}

// shardsWithTablesSources returns all the shards that have SourceShards set
// to one value, with an array of Tables.
func shardsWithTablesSources(ctx context.Context, wr *wrangler.Wrangler) ([]map[string]string, error) {
	shortCtx, cancel := context.WithTimeout(ctx, *remoteActionsTimeout)
	keyspaces, err := wr.TopoServer().GetKeyspaces(shortCtx)
	cancel()
	if err != nil {
		return nil, fmt.Errorf("failed to get list of keyspaces: %v", err)
	}

	wg := sync.WaitGroup{}
	mu := sync.Mutex{} // protects result
	result := make([]map[string]string, 0, len(keyspaces))
	rec := concurrency.AllErrorRecorder{}
	for _, keyspace := range keyspaces {
		wg.Add(1)
		go func(keyspace string) {
			defer wg.Done()
			shortCtx, cancel := context.WithTimeout(ctx, *remoteActionsTimeout)
			shards, err := wr.TopoServer().GetShardNames(shortCtx, keyspace)
			cancel()
			if err != nil {
				rec.RecordError(fmt.Errorf("failed to get list of shards for keyspace '%v': %v", keyspace, err))
				return
			}
			for _, shard := range shards {
				wg.Add(1)
				go func(keyspace, shard string) {
					defer wg.Done()
					shortCtx, cancel := context.WithTimeout(ctx, *remoteActionsTimeout)
					si, err := wr.TopoServer().GetShard(shortCtx, keyspace, shard)
					cancel()
					if err != nil {
						rec.RecordError(fmt.Errorf("failed to get details for shard '%v': %v", topoproto.KeyspaceShardString(keyspace, shard), err))
						return
					}

					if len(si.SourceShards) == 1 && len(si.SourceShards[0].Tables) > 0 {
						mu.Lock()
						result = append(result, map[string]string{
							"Keyspace": keyspace,
							"Shard":    shard,
						})
						mu.Unlock()
					}
				}(keyspace, shard)
			}
		}(keyspace)
	}
	wg.Wait()

	if rec.HasErrors() {
		return nil, rec.Error()
	}
	if len(result) == 0 {
		return nil, fmt.Errorf("there are no shards with SourceShards")
	}
	return result, nil
}

// Run is part of the Task interface.
func (t *VerticalSplitDiffTask) Run(parameters map[string]string) ([]*automationpb.TaskContainer, string, error) {
	args := []string{"VerticalSplitDiff"}
	args = append(args, topoproto.KeyspaceShardString(parameters["dest_keyspace"], parameters["shard"]))
	output, err := ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], args)

	// TODO(mberlin): Remove explicit reset when vtworker supports it implicility.
	if err == nil {
		// Ignore output and error of the Reset.
		ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], []string{"Reset"})
	}
	return nil, output, err
}

// Run is part of the Task interface.
func (t *MigrateServedTypesTask) Run(parameters map[string]string) ([]*automationpb.TaskContainer, string, error) {
	args := []string{"MigrateServedTypes"}
	if cells := parameters["cells"]; cells != "" {
		args = append(args, "--cells="+cells)
	}
	if reverse := parameters["reverse"]; reverse != "" {
		args = append(args, "--reverse="+reverse)
	}
	args = append(args,
		topoproto.KeyspaceShardString(parameters["keyspace"], parameters["source_shard"]),
		parameters["type"])
	output, err := ExecuteVtctl(context.TODO(), parameters["vtctld_endpoint"], args)
	return nil, output, err
}

// Run is part of the Task interface.
func (t *VerticalSplitDiffTask) Run(parameters map[string]string) ([]*automationpb.TaskContainer, string, error) {
	// Run a "Reset" first to clear the state of a previous finished command.
	// This reset is best effort. We ignore the output and error of it.
	// TODO(mberlin): Remove explicit reset when vtworker supports it implicility.
	ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], []string{"Reset"})

	args := []string{"VerticalSplitDiff"}
	if minHealthyRdonlyTablets := parameters["min_healthy_rdonly_tablets"]; minHealthyRdonlyTablets != "" {
		args = append(args, "--min_healthy_rdonly_tablets="+minHealthyRdonlyTablets)
	}
	args = append(args, topoproto.KeyspaceShardString(parameters["dest_keyspace"], parameters["shard"]))

	output, err := ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], args)
	return nil, output, err
}

func (scw *SplitCloneWorker) createThrottlers() error {
	scw.throttlersMu.Lock()
	defer scw.throttlersMu.Unlock()

	for _, si := range scw.destinationShards {
		// Set up the throttler for each destination shard.
		keyspaceAndShard := topoproto.KeyspaceShardString(si.Keyspace(), si.ShardName())
		t, err := throttler.NewThrottler(keyspaceAndShard, "transactions", scw.destinationWriterCount, scw.maxTPS, scw.maxReplicationLag)
		if err != nil {
			return fmt.Errorf("cannot instantiate throttler: %v", err)
		}
		scw.throttlers[keyspaceAndShard] = t
	}
	return nil
}

// Run is part of the Task interface.
func (t *VerticalSplitCloneTask) Run(parameters map[string]string) ([]*automationpb.TaskContainer, string, error) {
	// TODO(mberlin): Add parameters for the following options?
	//                        '--source_reader_count', '1',
	//                        '--destination_pack_count', '1',
	//                        '--destination_writer_count', '1',
	args := []string{"VerticalSplitClone"}
	args = append(args, "--tables="+parameters["tables"])
	args = append(args, topoproto.KeyspaceShardString(parameters["dest_keyspace"], parameters["shard"]))
	output, err := ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], args)

	// TODO(mberlin): Remove explicit reset when vtworker supports it implicility.
	if err == nil {
		// Ignore output and error of the Reset.
		ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], []string{"Reset"})
	}
	return nil, output, err
}

// Run is part of the Task interface.
func (t *SplitCloneTask) Run(parameters map[string]string) ([]*automationpb.TaskContainer, string, error) {
	// Run a "Reset" first to clear the state of a previous finished command.
	// This reset is best effort. We ignore the output and error of it.
	// TODO(mberlin): Remove explicit reset when vtworker supports it implicility.
	ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], []string{"Reset"})

	// TODO(mberlin): Add parameters for the following options?
	//                        '--source_reader_count', '1',
	//                        '--destination_writer_count', '1',
	args := []string{"SplitClone"}
	if online := parameters["online"]; online != "" {
		args = append(args, "--online="+online)
	}
	if offline := parameters["offline"]; offline != "" {
		args = append(args, "--offline="+offline)
	}
	if excludeTables := parameters["exclude_tables"]; excludeTables != "" {
		args = append(args, "--exclude_tables="+excludeTables)
	}
	if chunkCount := parameters["chunk_count"]; chunkCount != "" {
		args = append(args, "--chunk_count="+chunkCount)
	}
	if minRowsPerChunk := parameters["min_rows_per_chunk"]; minRowsPerChunk != "" {
		args = append(args, "--min_rows_per_chunk="+minRowsPerChunk)
	}
	if writeQueryMaxRows := parameters["write_query_max_rows"]; writeQueryMaxRows != "" {
		args = append(args, "--write_query_max_rows="+writeQueryMaxRows)
	}
	if writeQueryMaxSize := parameters["write_query_max_size"]; writeQueryMaxSize != "" {
		args = append(args, "--write_query_max_size="+writeQueryMaxSize)
	}
	if minHealthyRdonlyTablets := parameters["min_healthy_rdonly_tablets"]; minHealthyRdonlyTablets != "" {
		args = append(args, "--min_healthy_rdonly_tablets="+minHealthyRdonlyTablets)
	}
	if maxTPS := parameters["max_tps"]; maxTPS != "" {
		args = append(args, "--max_tps="+maxTPS)
	}
	if maxReplicationLag := parameters["max_replication_lag"]; maxReplicationLag != "" {
		args = append(args, "--max_replication_lag="+maxReplicationLag)
	}
	args = append(args, topoproto.KeyspaceShardString(parameters["keyspace"], parameters["source_shard"]))

	output, err := ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], args)
	return nil, output, err
}

// Run is part of the Task interface.
func (t *SplitDiffTask) Run(parameters map[string]string) ([]*automationpb.TaskContainer, string, error) {
	args := []string{"SplitDiff"}
	if excludeTables := parameters["exclude_tables"]; excludeTables != "" {
		args = append(args, "--exclude_tables="+excludeTables)
	}
	if minHealthyRdonlyTablets := parameters["min_healthy_rdonly_tablets"]; minHealthyRdonlyTablets != "" {
		args = append(args, "--min_healthy_rdonly_tablets="+minHealthyRdonlyTablets)
	}
	args = append(args, topoproto.KeyspaceShardString(parameters["keyspace"], parameters["dest_shard"]))
	output, err := ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], args)

	// TODO(mberlin): Remove explicit reset when vtworker supports it implicility.
	if err == nil {
		// Ignore output and error of the Reset.
		ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], []string{"Reset"})
	}
	return nil, output, err
}

// Run is part of the Task interface.
func (t *VerticalSplitCloneTask) Run(parameters map[string]string) ([]*automationpb.TaskContainer, string, error) {
	// TODO(mberlin): Add parameters for the following options?
	//                        '--source_reader_count', '1',
	//                        '--destination_writer_count', '1',
	args := []string{"VerticalSplitClone"}
	args = append(args, "--tables="+parameters["tables"])
	if online := parameters["online"]; online != "" {
		args = append(args, "--online="+online)
	}
	if offline := parameters["offline"]; offline != "" {
		args = append(args, "--offline="+offline)
	}
	if chunkCount := parameters["chunk_count"]; chunkCount != "" {
		args = append(args, "--chunk_count="+chunkCount)
	}
	if minRowsPerChunk := parameters["min_rows_per_chunk"]; minRowsPerChunk != "" {
		args = append(args, "--min_rows_per_chunk="+minRowsPerChunk)
	}
	if writeQueryMaxRows := parameters["write_query_max_rows"]; writeQueryMaxRows != "" {
		args = append(args, "--write_query_max_rows="+writeQueryMaxRows)
	}
	if writeQueryMaxSize := parameters["write_query_max_size"]; writeQueryMaxSize != "" {
		args = append(args, "--write_query_max_size="+writeQueryMaxSize)
	}
	if minHealthyRdonlyTablets := parameters["min_healthy_rdonly_tablets"]; minHealthyRdonlyTablets != "" {
		args = append(args, "--min_healthy_rdonly_tablets="+minHealthyRdonlyTablets)
	}
	if maxTPS := parameters["max_tps"]; maxTPS != "" {
		args = append(args, "--max_tps="+maxTPS)
	}
	if maxReplicationLag := parameters["max_replication_lag"]; maxReplicationLag != "" {
		args = append(args, "--max_replication_lag="+maxReplicationLag)
	}
	args = append(args, topoproto.KeyspaceShardString(parameters["dest_keyspace"], parameters["shard"]))
	output, err := ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], args)

	// TODO(mberlin): Remove explicit reset when vtworker supports it implicility.
	if err == nil {
		// Ignore output and error of the Reset.
		ExecuteVtworker(context.TODO(), parameters["vtworker_endpoint"], []string{"Reset"})
	}
	return nil, output, err
}

// findTargets phase:
// - find one rdonly in the source shard
// - mark it as 'worker' pointing back to us
// - get the aliases of all the targets
func (vscw *VerticalSplitCloneWorker) findTargets(ctx context.Context) error {
	vscw.setState(WorkerStateFindTargets)

	// find an appropriate tablet in the source shard
	var err error
	vscw.sourceAlias, err = FindWorkerTablet(ctx, vscw.wr, vscw.cleaner, nil /* tsc */, vscw.cell, vscw.sourceKeyspace, "0", vscw.minHealthyRdonlyTablets)
	if err != nil {
		return fmt.Errorf("FindWorkerTablet() failed for %v/%v/0: %v", vscw.cell, vscw.sourceKeyspace, err)
	}
	vscw.wr.Logger().Infof("Using tablet %v as the source", topoproto.TabletAliasString(vscw.sourceAlias))

	// get the tablet info for it
	shortCtx, cancel := context.WithTimeout(ctx, *remoteActionsTimeout)
	ti, err := vscw.wr.TopoServer().GetTablet(shortCtx, vscw.sourceAlias)
	cancel()
	if err != nil {
		return fmt.Errorf("cannot read tablet %v: %v", topoproto.TabletAliasString(vscw.sourceAlias), err)
	}
	vscw.sourceTablet = ti.Tablet

	// stop replication on it
	shortCtx, cancel = context.WithTimeout(ctx, *remoteActionsTimeout)
	err = vscw.wr.TabletManagerClient().StopSlave(shortCtx, vscw.sourceTablet)
	cancel()
	if err != nil {
		return fmt.Errorf("cannot stop replication on tablet %v", topoproto.TabletAliasString(vscw.sourceAlias))
	}

	wrangler.RecordStartSlaveAction(vscw.cleaner, vscw.sourceTablet)

	// Initialize healthcheck and add destination shards to it.
	vscw.healthCheck = discovery.NewHealthCheck(*remoteActionsTimeout, *healthcheckRetryDelay, *healthCheckTimeout)
	vscw.tsc = discovery.NewTabletStatsCache(vscw.healthCheck, vscw.cell)
	watcher := discovery.NewShardReplicationWatcher(vscw.wr.TopoServer(), vscw.healthCheck,
		vscw.cell, vscw.destinationKeyspace, vscw.destinationShard,
		*healthCheckTopologyRefresh, discovery.DefaultTopoReadConcurrency)
	vscw.destinationShardWatchers = append(vscw.destinationShardWatchers, watcher)

	// Make sure we find a master for each destination shard and log it.
	vscw.wr.Logger().Infof("Finding a MASTER tablet for each destination shard...")
	waitCtx, waitCancel := context.WithTimeout(ctx, *waitForHealthyTabletsTimeout)
	defer waitCancel()
	if err := vscw.tsc.WaitForTablets(waitCtx, vscw.cell, vscw.destinationKeyspace, vscw.destinationShard, []topodatapb.TabletType{topodatapb.TabletType_MASTER}); err != nil {
		return fmt.Errorf("cannot find MASTER tablet for destination shard for %v/%v (in cell: %v): %v", vscw.destinationKeyspace, vscw.destinationShard, vscw.cell, err)
	}
	masters := vscw.tsc.GetHealthyTabletStats(vscw.destinationKeyspace, vscw.destinationShard, topodatapb.TabletType_MASTER)
	if len(masters) == 0 {
		return fmt.Errorf("cannot find MASTER tablet for destination shard for %v/%v (in cell: %v) in HealthCheck: empty TabletStats list", vscw.destinationKeyspace, vscw.destinationShard, vscw.cell)
	}
	master := masters[0]

	// Get the MySQL database name of the tablet.
	keyspaceAndShard := topoproto.KeyspaceShardString(vscw.destinationKeyspace, vscw.destinationShard)
	vscw.destinationDbNames[keyspaceAndShard] = topoproto.TabletDbName(master.Tablet)

	// TODO(mberlin): Verify on the destination master that the
	// _vt.blp_checkpoint table has the latest schema.

	vscw.wr.Logger().Infof("Using tablet %v as destination master for %v/%v", topoproto.TabletAliasString(master.Tablet.Alias), vscw.destinationKeyspace, vscw.destinationShard)
	vscw.wr.Logger().Infof("NOTE: The used master of a destination shard might change over the course of the copy e.g. due to a reparent. The HealthCheck module will track and log master changes and any error message will always refer the actually used master address.")

	return nil
}

func (p *shardTabletProvider) description() string {
	return topoproto.KeyspaceShardString(p.keyspace, p.shard)
}

// copy phase:
//	- copy the data from source tablets to destination masters (with replication on)
// Assumes that the schema has already been created on each destination tablet
// (probably from vtctl's CopySchemaShard)
func (scw *LegacySplitCloneWorker) copy(ctx context.Context) error {
	scw.setState(WorkerStateCloneOffline)
	start := time.Now()
	defer func() {
		statsStateDurationsNs.Set(string(WorkerStateCloneOffline), time.Now().Sub(start).Nanoseconds())
	}()

	// get source schema from the first shard
	// TODO(alainjobart): for now, we assume the schema is compatible
	// on all source shards. Furthermore, we estimate the number of rows
	// in each source shard for each table to be about the same
	// (rowCount is used to estimate an ETA)
	shortCtx, cancel := context.WithTimeout(ctx, *remoteActionsTimeout)
	sourceSchemaDefinition, err := scw.wr.GetSchema(shortCtx, scw.sourceAliases[0], nil, scw.excludeTables, false /* includeViews */)
	cancel()
	if err != nil {
		return fmt.Errorf("cannot get schema from source %v: %v", topoproto.TabletAliasString(scw.sourceAliases[0]), err)
	}
	if len(sourceSchemaDefinition.TableDefinitions) == 0 {
		return fmt.Errorf("no tables matching the table filter in tablet %v", topoproto.TabletAliasString(scw.sourceAliases[0]))
	}
	for _, td := range sourceSchemaDefinition.TableDefinitions {
		if len(td.Columns) == 0 {
			return fmt.Errorf("schema for table %v has no columns", td.Name)
		}
	}
	scw.wr.Logger().Infof("Source tablet 0 has %v tables to copy", len(sourceSchemaDefinition.TableDefinitions))
	scw.tableStatusList.initialize(sourceSchemaDefinition)

	// In parallel, setup the channels to send SQL data chunks to for each destination tablet:
	//
	// mu protects the context for cancelation, and firstError
	mu := sync.Mutex{}
	var firstError error

	ctx, cancelCopy := context.WithCancel(ctx)
	processError := func(format string, args ...interface{}) {
		scw.wr.Logger().Errorf(format, args...)
		mu.Lock()
		if firstError == nil {
			firstError = fmt.Errorf(format, args...)
			cancelCopy()
		}
		mu.Unlock()
	}

	insertChannels := make([]chan string, len(scw.destinationShards))
	destinationWaitGroup := sync.WaitGroup{}
	for shardIndex, si := range scw.destinationShards {
		// we create one channel per destination tablet.  It
		// is sized to have a buffer of a maximum of
		// destinationWriterCount * 2 items, to hopefully
		// always have data. We then have
		// destinationWriterCount go routines reading from it.
		insertChannels[shardIndex] = make(chan string, scw.destinationWriterCount*2)

		go func(keyspace, shard string, insertChannel chan string) {
			for j := 0; j < scw.destinationWriterCount; j++ {
				destinationWaitGroup.Add(1)
				go func(threadID int) {
					defer destinationWaitGroup.Done()

					keyspaceAndShard := topoproto.KeyspaceShardString(keyspace, shard)
					throttler := scw.destinationThrottlers[keyspaceAndShard]
					defer throttler.ThreadFinished(threadID)

					executor := newExecutor(scw.wr, scw.tsc, throttler, keyspace, shard, threadID)
					if err := executor.fetchLoop(ctx, insertChannel); err != nil {
						processError("executer.FetchLoop failed: %v", err)
					}
				}(j)
			}
		}(si.Keyspace(), si.ShardName(), insertChannels[shardIndex])
	}

	// read the vschema if needed
	var keyspaceSchema *vindexes.KeyspaceSchema
	if *useV3ReshardingMode {
		kschema, err := scw.wr.TopoServer().GetVSchema(ctx, scw.keyspace)
		if err != nil {
			return fmt.Errorf("cannot load VSchema for keyspace %v: %v", scw.keyspace, err)
		}
		if kschema == nil {
			return fmt.Errorf("no VSchema for keyspace %v", scw.keyspace)
		}

		keyspaceSchema, err = vindexes.BuildKeyspaceSchema(kschema, scw.keyspace)
		if err != nil {
			return fmt.Errorf("cannot build vschema for keyspace %v: %v", scw.keyspace, err)
		}
	}

	// Now for each table, read data chunks and send them to all
	// insertChannels
	sourceWaitGroup := sync.WaitGroup{}
	for shardIndex := range scw.sourceShards {
//.........这里部分代码省略.........

// findTargets phase:
// - find one rdonly in the source shard
// - mark it as 'worker' pointing back to us
// - get the aliases of all the targets
func (scw *LegacySplitCloneWorker) findTargets(ctx context.Context) error {
	scw.setState(WorkerStateFindTargets)
	var err error

	// find an appropriate tablet in the source shards
	scw.sourceAliases = make([]*topodatapb.TabletAlias, len(scw.sourceShards))
	for i, si := range scw.sourceShards {
		scw.sourceAliases[i], err = FindWorkerTablet(ctx, scw.wr, scw.cleaner, scw.tsc, scw.cell, si.Keyspace(), si.ShardName(), scw.minHealthyRdonlyTablets)
		if err != nil {
			return fmt.Errorf("FindWorkerTablet() failed for %v/%v/%v: %v", scw.cell, si.Keyspace(), si.ShardName(), err)
		}
		scw.wr.Logger().Infof("Using tablet %v as source for %v/%v", topoproto.TabletAliasString(scw.sourceAliases[i]), si.Keyspace(), si.ShardName())
	}

	// get the tablet info for them, and stop their replication
	scw.sourceTablets = make([]*topodatapb.Tablet, len(scw.sourceAliases))
	for i, alias := range scw.sourceAliases {
		shortCtx, cancel := context.WithTimeout(ctx, *remoteActionsTimeout)
		ti, err := scw.wr.TopoServer().GetTablet(shortCtx, alias)
		cancel()
		if err != nil {
			return fmt.Errorf("cannot read tablet %v: %v", topoproto.TabletAliasString(alias), err)
		}
		scw.sourceTablets[i] = ti.Tablet

		shortCtx, cancel = context.WithTimeout(ctx, *remoteActionsTimeout)
		err = scw.wr.TabletManagerClient().StopSlave(shortCtx, scw.sourceTablets[i])
		cancel()
		if err != nil {
			return fmt.Errorf("cannot stop replication on tablet %v", topoproto.TabletAliasString(alias))
		}

		wrangler.RecordStartSlaveAction(scw.cleaner, scw.sourceTablets[i])
	}

	// Initialize healthcheck and add destination shards to it.
	scw.healthCheck = discovery.NewHealthCheck(*remoteActionsTimeout, *healthcheckRetryDelay, *healthCheckTimeout)
	scw.tsc = discovery.NewTabletStatsCache(scw.healthCheck, scw.cell)
	for _, si := range scw.destinationShards {
		watcher := discovery.NewShardReplicationWatcher(scw.wr.TopoServer(), scw.healthCheck,
			scw.cell, si.Keyspace(), si.ShardName(),
			*healthCheckTopologyRefresh, discovery.DefaultTopoReadConcurrency)
		scw.destinationShardWatchers = append(scw.destinationShardWatchers, watcher)
	}

	// Make sure we find a master for each destination shard and log it.
	scw.wr.Logger().Infof("Finding a MASTER tablet for each destination shard...")
	for _, si := range scw.destinationShards {
		waitCtx, waitCancel := context.WithTimeout(ctx, 10*time.Second)
		defer waitCancel()
		if err := scw.tsc.WaitForTablets(waitCtx, scw.cell, si.Keyspace(), si.ShardName(), []topodatapb.TabletType{topodatapb.TabletType_MASTER}); err != nil {
			return fmt.Errorf("cannot find MASTER tablet for destination shard for %v/%v: %v", si.Keyspace(), si.ShardName(), err)
		}
		masters := scw.tsc.GetHealthyTabletStats(si.Keyspace(), si.ShardName(), topodatapb.TabletType_MASTER)
		if len(masters) == 0 {
			return fmt.Errorf("cannot find MASTER tablet for destination shard for %v/%v in HealthCheck: empty TabletStats list", si.Keyspace(), si.ShardName())
		}
		master := masters[0]

		// Get the MySQL database name of the tablet.
		shortCtx, cancel := context.WithTimeout(ctx, *remoteActionsTimeout)
		ti, err := scw.wr.TopoServer().GetTablet(shortCtx, master.Tablet.Alias)
		cancel()
		if err != nil {
			return fmt.Errorf("cannot get the TabletInfo for destination master (%v) to find out its db name: %v", topoproto.TabletAliasString(master.Tablet.Alias), err)
		}
		keyspaceAndShard := topoproto.KeyspaceShardString(si.Keyspace(), si.ShardName())
		scw.destinationDbNames[keyspaceAndShard] = ti.DbName()

		// TODO(mberlin): Verify on the destination master that the
		// _vt.blp_checkpoint table has the latest schema.

		scw.wr.Logger().Infof("Using tablet %v as destination master for %v/%v", topoproto.TabletAliasString(master.Tablet.Alias), si.Keyspace(), si.ShardName())
	}
	scw.wr.Logger().Infof("NOTE: The used master of a destination shard might change over the course of the copy e.g. due to a reparent. The HealthCheck module will track and log master changes and any error message will always refer the actually used master address.")

	// Set up the throttler for each destination shard.
	for _, si := range scw.destinationShards {
		keyspaceAndShard := topoproto.KeyspaceShardString(si.Keyspace(), si.ShardName())
		t, err := throttler.NewThrottler(
			keyspaceAndShard, "transactions", scw.destinationWriterCount, scw.maxTPS, throttler.ReplicationLagModuleDisabled)
		if err != nil {
			return fmt.Errorf("cannot instantiate throttler: %v", err)
		}
		scw.destinationThrottlers[keyspaceAndShard] = t
	}

	return nil
}