Migrations

This topic explains how to use LaunchDarkly SDKs to manage migrations or modernizations.

Overview

This topic explains how to use LaunchDarkly SDKs to manage migrations or modernizations. You might use this feature if you are optimizing queries, upgrading to new tech stacks, migrating from one database to another, or other similar technology changes. This feature is available for server-side and edge SDKs only.

Prerequisites

Before you configure your SDK to manage a migration, you must complete the following prerequisites:

• Create a migration feature flag. This is a temporary flag used to migrate data or systems while keeping your application available and disruption free. Migration flags break up the switch from an old to a new implementation into a series of recommended stages where movement from one stage to the next is done in incremental steps.
• Determine how many stages your migration will have. You can select from the following options as part of creating a migration feature flag:
  • Two stages: For migrations where you cannot run the new system and old system at the same time
  • Four stages: For migrations that can run both the new and old systems at the same time
  • Six stages: For migrations where you need to migrate READS and WRITES separatelyTo learn more, read Migration flags.

Use SDKs to manage a migration

Depending on how you created your migration feature flag, your migration will have two, four, or six stages. At each stage, you will be reading data from the old destination, the new destination, or both. You will also be writing data to the old destination, the new destination, or both. At each stage, only one of these destinations is considered the authoritative source. In the LaunchDarkly SDK, you can determine which stage of the migration your application is currently in, execute the appropriate read and write methods, and then compare the results to check correctness and view any errors or changes in latency.

The following table describes the stages and which destination is authoritative. Remember that not all migrations will use all stages.

| Stage

| Read from

| Write to

Authoritative

| ---

| ---

| --- | | off

| old

| old

| old | | dualwrite

| old

| old, new

| old | | shadow

| both

| old, new

| old | | live

| both

| new, old

| new | | rampdown

| new

| new, old

| new | | complete

| new

| new

| new |

To manage your migration:

1. Configure the migration. In your SDK configuration, define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.
2. Call the read and write methods you defined, using the SDK’s migrator. The migrator determines the migration stage of the feature flag controlling the migration, and performs reads and writes to the old and new systems based on the migration stage.

For details of how to perform each step, read Server-side SDKs or Edge SDKs, below.During the migration, you can check the consistency, errors, and latency as you manage your migration. This information is available from the flag’s Targeting tab. To learn more, read Migration flags.

Customize your migration

Customizing your migration is rare. If you have additional metrics that you want to track, or if your migration or modernization involves reading and writing from the new and old systems in a different configuration than the two, four, or six -stage migrations provided, you can also use the SDK to customize your migration.

Here’s how:

1. Configure the migration. In your SDK configuration, define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.
2. Use the migrationVariation method to evaluate your feature flag and determine the migration stage.
3. Use your own logic to perform the appropriate migration operations for the stage. Record any metrics that you are interested in.
4. When the migration operation is complete, call the trackMigration method to record your metrics.

For details of how to perform each step, read Server-side SDKs or Edge SDKs, below.

Server-side SDKs

This feature is available in the following server-side SDKs:

• .NET (server-side)
• Go
• Java
• Node.js (server-side)
• PHP
• Python
• Ruby
• Rust

.NET (server-side)

Expand .NET (server-side) code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call Read or Write. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

.NET SDK v8.0 (C#)

      LDContext context = Context.Builder("example-context-key")
          .Build();

      // this is the migration stage to use if the flag's migration stage
      // is not available from LaunchDarkly
      var defaultStage = MigrationStage.Off;

      var readResult = migration.Read("example-migration-flag-key", context, defaultStage, payload);

      var writeResult = migration.Write("example-migration-flag-key", context, defaultStage, payload);

To learn more, read Read and Write.

You can check for consistency, errors, or latency under “Migration insights” on the Targeting tab of your migration flag in the LaunchDarkly user interface. To learn more, read Migration flags.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the MigrationVariation method to evaluate your feature flag and determine its migration stage. This method returns the migration stage and a tracker that you can use to build the analytics event to send back to LaunchDarkly.

Here’s how:

.NET SDK v8.0 (C#)

        LDContext context = Context.Builder("example-context-key")
            .Build();

        var (stage, tracker) = client.MigrationVariation("example-migration-flag-key", context, MigrationStage.Off);

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

.NET SDK v8.0 (C#)

        // define the combination of reads and writes from the new and old systems
        // that should occur at each migration stage

        switch (stage)
            {
                case MigrationStage.Off:
                case MigrationStage.DualWrite:
                case MigrationStage.Shadow:
                case MigrationStage.Live:
                case MigrationStage.RampDown:
                case MigrationStage.Complete:
                default:
                    // throw an error
            }

Finally, when the migration operation is complete, call the TrackMigration method to record your metrics:

.NET SDK v8.0 (C#)

        client.TrackMigration(tracker);

To learn more, read IMigration, MigrationVariation, and TrackMigration.

Go

Expand Go code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call Read or Write. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

Go SDK v7.13.4+, using LDScopedClient

      context := ldcontext.New("example-context-key")

      // this is the migration stage to use if the flag's migration stage
      // is not available from LaunchDarkly
      defaultStage := ldmigration.Off

      // There is not an AllFlagsState method in the LDScopedClient.
      // If you are using scoped clients, pass in the scoped client's current context
      // LDScopedClient is in beta and may change without notice.

      readResult := migrator.Read("example-migration-flag-key", scopedClient.CurrentContext(), defaultStage, nil)

      writeResult := migrator.Write("example-migration-flag-key", scopedClient.CurrentContext(), defaultStage, nil)

To learn more, read Read and Write.

You can check for consistency, errors, or latency under “Migration insights” on the Targeting tab of your migration flag in the LaunchDarkly user interface. To learn more, read Migration flags.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the MigrationVariation method to evaluate your feature flag and determine its migration stage. This method returns the migration stage, a tracker that you can use to build the analytics event to send back to LaunchDarkly, and an error.

Here’s how:

Go SDK v7.13.4+, using LDScopedClient

        context := ldcontext.New("example-context-key")
        scopedClient := ld.NewScopedClient(client, context)
        // LDScopedClient is in beta and may change without notice.

        stage, tracker, err := scopedClient.MigrationVariation("example-migration-flag-key", ldmigration.Off)

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

Go SDK v7+

        // define the combination of reads and writes from the new and old systems
        // that should occur at each migration stage

        switch stage {
          case ldmigration.Off:
          case ldmigration.DualWrite:
          case ldmigration.Shadow:
          case ldmigration.Live:
          case ldmigration.RampDown:
          case ldmigration.Complete:
          default: {
            // throw an error
          }
        }

Finally, when the migration operation is complete, call the TrackMigrationOp method to record your metrics:

Go SDK v7.13.4+, using LDScopedClient

        event, _ := tracker.Build();

        err := scopedClient.TrackMigrationOp(*event);
        // LDScopedClient is in beta and may change without notice.

To learn more, read Migration and TrackMigrationOp.

Java

Expand Java code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call read or write. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

Java SDK v7.0

      LDContext context = LDContext.builder("example-context-key")
        .build();

      // this is the migration stage to use if the flag's migration stage
      // is not available from LaunchDarkly
      MigrationStage defaultStage = MigrationStage.OFF;

      Migration.MigrationResult<String> readResult = migration.read("example-migration-flag-key", context, defaultStage);

      Migration.MigrationWriteResult<String> writeResult = migration.write("example-migration-flag-key", context, defaultStage);

To learn more, read Migration.

You can check for consistency, errors, or latency under “Migration insights” on the Targeting tab of your migration flag in the LaunchDarkly user interface. To learn more, read Migration flags.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the migrationVariation method to evaluate your feature flag and determine its migration stage. This method returns the migration stage, a tracker that you can use to build the analytics event to send back to LaunchDarkly, and an error.

Here’s how:

Java SDK v7.0

        LDContext context = LDContext.builder("example-context-key")
          .build();

        MigrationVariation migrationVariation = client.migrationVariation("example-migration-flag-key", context, MigrationStage.OFF);

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

Java SDK v7.0

        // define the combination of reads and writes from the new and old systems
        // that should occur at each migration stage

        switch (migrationVariation.getStage()) {
          case OFF:
          case DUAL_WRITE:
          case SHADOW:
          case LIVE:
          case RAMP_DOWN:
          case COMPLETE:
          default: {
            // throw an error
          }
        }

Finally, when the migration operation is complete, call the trackMigration method to record your metrics:

Java SDK v7.0

        MigrationOpTracker tracker = migrationVariation.getTracker();

        client.trackMigration(tracker);

To learn more, read migrationVariation and trackMigration.

Node.js (server-side)

Expand Node.js (server-side) code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call the read or write methods from the LDMigration interface. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

Node.js (server-side) SDK v9

      import { LDContext, LD MigrationState, createMigration } from '@launchdarkly/node-server-sdk';

      const context: LDContext = {
         kind: 'user',
         key: 'example-user-key',
         name: 'Sandy',
      };

      // this is the migration stage to use if the flag's migration stage
      // is not available from LaunchDarkly
      let defaultStage: LDMigrationStage = LDMigrationStage.Off;

      const migration = createMigration(client, options);

      // when you need to perform a read in your application
      migration.read(
        'example-migration-flag-key',
        context,
        defaultStage
      );

      // when you need to perform a write in your application
      migration.write(
        'example-migration-flag-key',
        context,
        defaultStage
      );

To learn more, read LDMigration.

You can check for consistency, errors, or latency under “Migration insights” on the Targeting tab of your migration flag in the LaunchDarkly user interface. To learn more, read Migration flags.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the migrationVariation method to evaluate your feature flag and determine its migration stage. This method returns a promise that is resolved with the result LDMigrationVariation. This result includes the migration stage and a tracker that you can use to build the analytics event to send back to LaunchDarkly.

Here’s how:

Node.js (server-side) SDK v9

          import { LDContext } from '@launchdarkly/node-server-sdk');

          const context: LDContext = {
             kind: 'user',
             key: 'example-user-key',
             name: 'Sandy',
          };

          const { value, tracker } = await client.migrationVariation(
            'example-migration-flag-key',
            context,
            false
          );

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

Node.js (server-side) SDK v9

        // define the combination of reads and writes from the new and old systems
        // that should occur at each migration stage

        switch (value) {
          case LDMigrationStage.Off: { },
          case LDMigrationStage.DualWrite: { },
          case LDMigrationStage.Shadow: { },
          case LDMigrationStage.Live: { },
          case LDMigrationStage.RampDown: { },
          case LDMigrationStage.Complete: { },
          default: {
            // throw an error
          }
        }

Finally, when the migration operation is complete, call the trackMigration method to record your metrics:

Node.js (server-side) SDK v9

        const event = tracker.createEvent();

        if (event) {
          client.trackMigration(event);
        }

To learn more, read LDMigration. and LDMigrationOpEvent.

PHP

Expand PHP code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call the read or write methods from the Migrator interface. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

PHP SDK v6

      $context = LaunchDarkly\LDContext::builder("example-context-key")->build();

      // this is the migration stage to use if the flag's migration stage
      // is not available from LaunchDarkly
      $defaultStage = Migrations\Stage::OFF;

      $result = $builder->build();
      if (!$result->isSuccessful()) {
          throw new \Exception($result->error);
      }

      $migrator = $result->value;

      // if you need to pass additional information from the call site
      // to your read/write methods, use a mixed type payload
      $payload = ['index' => 'useful information'];

      // when you need to perform a read in your application
      $migrator->read('example-migration-flag-key', $context, $defaultStage, $payload);

      // when you need to perform a write in your application
      $migrator->write('example-migration-flag-key', $context, $defaultStage, $payload);

To learn more, read Migrator.

You can check for consistency, errors, or latency under “Migration insights” on the Targeting tab of your migration flag in the LaunchDarkly user interface. To learn more, read Migration flags.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the migrationVariation method to evaluate your feature flag and determine its migration stage. This method returns the migration stage and a tracker that you can use to build the analytics event to send back to LaunchDarkly.

Here’s how:

PHP SDK v6

        $context = LaunchDarkly\LDContext::builder("example-context-key")->build();

        $result = $client->migrationVariation('example-migration-flag-key', $context, Migrations\Stage::OFF);

        /** @var Migrations\Stage */
        $stage = $result['stage'];
        /** @var Migrations\OpTracker */
        $tracker = $result['tracker'];

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

PHP SDK v6

        // define the combination of reads and writes from the new and old systems
        // that should occur at each migration stage

        switch ($stage) {
          case Migrations\Stage::OFF:
          case Migrations\Stage::DUALWRITE:
          case Migrations\Stage::SHADOW:
          case Migrations\Stage::LIVE:
          case Migrations\Stage::RAMPDOWN:
          case Migrations\Stage::COMPLETE:
          default:
            // throw an error
        }

Finally, when the migration operation is complete, call the trackMigrationOperation method to record your metrics:

PHP SDK v6

        $client->trackMigrationOperation($tracker);

To learn more, read Migrator and trackMigrationOperation.

Python

Expand Python code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call the read or write methods from the Migrator interface. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

Python SDK v9

      from ldclient import Context
      from ldclient.migrations import Stage

      context = Context.builder("example-context-key").build()

      # this is the migration stage to use if the flag's migration stage
      # is not available from LaunchDarkly
      default_stage = Stage.OFF

      migrator = builder.build()

      # when you need to perform a read in your application
      migrator.read(
          'example-migration-flag-key',
          context,
          default_stage
      )

      # when you need to perform a write in your application
      migrator.write(
          'example-migration-flag-key',
          context,
          default_stage
      )

To learn more, read ldclient.migrations.

You can check for consistency, errors, or latency under “Migration insights” on the Targeting tab of your migration flag in the LaunchDarkly user interface. To learn more, read Migration flags.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the migration_variation method to evaluate your feature flag and determine its migration stage. This method returns the migration stage and a tracker that you can use to build the analytics event to send back to LaunchDarkly.

Here’s how:

Python SDK v9

        context = Context.builder("example-context-key").build()

        stage, tracker = ldclient.get().migration_variation('example-migration-flag-key', context, Stage.OFF)

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

Python SDK v9

        # define the combination of reads and writes from the new and old systems
        # that should occur at each migration stage

        if stage == Stage.OFF:
        elif stage == Stage.DUALWRITE:
        elif stage == Stage.SHADOW:
        elif stage == Stage.LIVE:
        elif stage == Stage.RAMPDOWN:
        elif stage == Stage.COMPLETE:
        else:
          # throw an error

Finally, when the migration operation is complete, call the track_migration_op method to record your metrics:

Python SDK v9

        ldclient.get().track_migration_op(tracker)

To learn more, read ldclient.migrations and track_migration_op.

Ruby

Expand Ruby code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call read or write. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

Ruby SDK v8.0

      context = LaunchDarkly::LDContext.create({key: "example-user-key", kind:"user"})

      # this is the migration stage to use if the flag's migration stage
      # is not available from LaunchDarkly
      default_stage = LaunchDarkly::Migrations::STAGE_OFF

      read_result = migrator.read("example-migration-flag-key", context, default_stage, payload)

      write_result = migrator.write("example-migration-flag-key", context, default_stage, payload)

To learn more, read read and write.

You can check for consistency, errors, or latency under “Migration insights” on the Targeting tab of your migration flag in the LaunchDarkly user interface. To learn more, read Migration flags.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the migration_variation method to evaluate your feature flag and determine its migration stage. This method returns the migration stage and a tracker that you can use to build the analytics event to send back to LaunchDarkly.

Here’s how:

Ruby SDK v8.0

        context = LaunchDarkly::LDContext.create({key: "example-user-key", kind:"user"})

        stage, tracker = client.migration_variation(
          "example-migration-flag-key",
          context,
          LaunchDarkly::Migrations::STAGE_OFF
        )

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

Ruby SDK v8.0

        # define the combination of reads and writes from the new and old systems
        # that should occur at each migration stage

        case stage
        when LaunchDarkly::Migrations::STAGE_OFF
        when LaunchDarkly::Migrations::STAGE_DUALWRITE
        when LaunchDarkly::Migrations::STAGE_SHADOW
        when LaunchDarkly::Migrations::STAGE_LIVE
        when LaunchDarkly::Migrations::STAGE_RAMPDOWN
        when LaunchDarkly::Migrations::STAGE_COMPLETE
        else
          # throw an error
        end

Finally, when the migration operation is complete, call the track_migration_op method to record your metrics:

Ruby SDK v8.0

        client.track_migration_op(tracker);

To learn more, read Migrator and track_migration_op.

Rust

Expand Rust code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call read or write. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

Rust SDK v2.2

      let context = ContextBuilder::new("example-user-key")
          .kind("user")
          .build()
          .expect("Context failed to build");

      // this is the migration stage to use if the flag's migration stage
      // is not available from LaunchDarkly
      let default_stage = Stage::Off;

      let read_result = migrator
          .read(
              &context,
              "example-migration-flag-key".into(),
              default_stage,
              "example-payload".into(),
          )
          .await;

      let write_result = migrator
          .write(
              &context,
              "example-migration-flag-key".into(),
              default_stage,
              "example-payload".into(),
          )
          .await;

To learn more, read read and write.

You can check for consistency, errors, or latency under “Migration insights” on the Targeting tab of your migration flag in the LaunchDarkly user interface. To learn more, read Migration flags.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the migration_variation method to evaluate your feature flag and determine its migration stage. This method returns the migration stage and a tracker that you can use to build the analytics event to send back to LaunchDarkly.

Here’s how:

Rust SDK v2.2

        let context = ContextBuilder::new("example-user-key")
            .kind("user")
            .build()
            .expect("Context failed to build");

        let (stage, tracker) =
            client.migration_variation(&context, "example-migration-flag-key", Stage::Off);

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

Rust SDK v2.2

        // define the combination of reads and writes from the new and old systems
        // that should occur at each migration stage

        match stage {
            Stage::Off => todo!(),
            Stage::DualWrite => todo!(),
            Stage::Live => todo!(),
            Stage::Shadow => todo!(),
            Stage::Rampdown => todo!(),
            Stage::Complete => todo!(),
            _ => todo!(),
        };

Finally, when the migration operation is complete, call the track_migration_op method to record your metrics:

Rust SDK v2.2

        client.track_migration_op(tracker);

To learn more, read Migrator and track_migration_op.

Edge SDKs

This feature is available in the following edge SDKs:

• Akamai
• Cloudflare
• Vercel

Akamai

Expand Akamai code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call the read or write methods from the LDMigration interface. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

Akamai SDK v1.0.9+

      import {
        createMigration,
        LDContext,
        LDMigrationStage,
      } from '@launchdarkly/akamai-server-edgekv-sdk';

      const context: LDContext = {
         kind: 'user',
         key: 'example-user-key',
         name: 'Sandy',
      };

      // this is the migration stage to use if the flag's migration stage
      // is not available from LaunchDarkly
      let defaultStage: LDMigrationStage = LDMigrationStage.Off;

      const migration = createMigration(client, options);

      // when you need to perform a read in your application
      migration.read(
        'example-migration-flag-key',
        context,
        defaultStage
      );

      // when you need to perform a write in your application
      migration.write(
        'example-migration-flag-key',
        context,
        defaultStage
      );

To learn more, read LDMigration.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the migrationVariation method to evaluate your feature flag and determine its migration stage. This method returns a promise that is resolved with the result LDMigrationVariation. This result includes the migration stage. It also returns a tracker, which you can ignore. (The tracker is normally used to build an analytics event to send back to LaunchDarkly. However, the Akamai SDK does not support sending events, so there is no need to build one.)

Here’s how:

Akamai SDK v1.0.9+

          import {
            LDContext,
          } from '@launchdarkly/akamai-server-edgekv-sdk';

          const context: LDContext = {
             kind: 'user',
             key: 'example-user-key',
             name: 'Sandy',
          };

          const { value, tracker } = await client.migrationVariation(
            'example-migration-flag-key',
            context,
            false
          );

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

Akamai SDK v1.0.9+

        import { LDMigrationStage } from '@launchdarkly/akamai-server-edgekv-sdk';

        // define the combination of reads and writes from the new and old systems
        // that should occur at each migration stage

        switch (value) {
          case LDMigrationStage.Off: { },
          case LDMigrationStage.DualWrite: { },
          case LDMigrationStage.Shadow: { },
          case LDMigrationStage.Live: { },
          case LDMigrationStage.RampDown: { },
          case LDMigrationStage.Complete: { },
          default: {
            // throw an error
          }
        }

To learn more, read LDMigration.

Cloudflare

Expand Cloudflare code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call the read or write methods from the LDMigration interface. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

Cloudflare SDK v2.2.2+

      import {
        createMigration,
        LDContext,
        LDMigrationStage,
      } from '@launchdarkly/cloudflare-server-sdk';

      const context: LDContext = {
         kind: 'user',
         key: 'example-user-key',
         name: 'Sandy',
      };

      // this is the migration stage to use if the flag's migration stage
      // is not available from LaunchDarkly
      let defaultStage: LDMigrationStage = LDMigrationStage.Off;

      const migration = createMigration(client, options);

      // when you need to perform a read in your application
      migration.read(
        'example-migration-flag-key',
        context,
        defaultStage
      );

      // when you need to perform a write in your application
      migration.write(
        'example-migration-flag-key',
        context,
        defaultStage
      );

To learn more, read LDMigration.

You can check for consistency, errors, or latency under “Migration insights” on the Targeting tab of your migration flag in the LaunchDarkly user interface. To learn more, read Migration flags.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the migrationVariation method to evaluate your feature flag and determine its migration stage. This method returns a promise that is resolved with the result LDMigrationVariation. This result includes the migration stage and a tracker that you can use to build the analytics event to send back to LaunchDarkly.

Here’s how:

Cloudflare SDK v2.2.2+

          import {
            LDContext,
          } from '@launchdarkly/cloudflare-server-sdk'

          const context: LDContext = {
             kind: 'user',
             key: 'example-user-key',
             name: 'Sandy',
          };

          const { value, tracker } = await client.migrationVariation(
            'example-migration-flag-key',
            context,
            false
          );

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

Cloudflare SDK v2.2.2+

        import { LDMigrationStage } from '@launchdarkly/cloudflare-server-sdk';

        // define the combination of reads and writes from the new and old systems
        // that should occur at each migration stage

        switch (value) {
          case LDMigrationStage.Off: { },
          case LDMigrationStage.DualWrite: { },
          case LDMigrationStage.Shadow: { },
          case LDMigrationStage.Live: { },
          case LDMigrationStage.RampDown: { },
          case LDMigrationStage.Complete: { },
          default: {
            // throw an error
          }
        }

Finally, when the migration operation is complete, call the trackMigration method to record your metrics:

Cloudflare SDK v2.2.2+

        import {
          LDClient,
          LDMigrationTracker,
        } from '@launchdarkly/cloudflare-server-sdk';

        const event = tracker.createEvent();

        if (event) {
          client.trackMigration(event);
        }

To learn more, read LDMigration and LDMigrationOpEvent.

Vercel

Expand Vercel code sample

To manage your migration, first you need to define how to read from and write to the old and new systems, how to check whether two reads are a match, and whether to track errors and latency metrics. To learn more, read Migration configuration.

Then, whenever you need to perform a read or write in the systems that you are modernizing or migrating, call the read or write methods from the LDMigration interface. The SDK evaluates the flag, determines which migration stage the flag is in, and performs the reads or writes in the appropriate system.

Here’s how:

Vercel SDK v1.1.6+

      import {
        createMigration,
        LDContext,
        LDMigrationStage,
      } from '@launchdarkly/vercel-server-sdk';

      const context: LDContext = {
         kind: 'user',
         key: 'example-user-key',
         name: 'Sandy',
      };

      // this is the migration stage to use if the flag's migration stage
      // is not available from LaunchDarkly
      let defaultStage: LDMigrationStage = LDMigrationStage.Off;

      const migration = createMigration(client, options);

      // when you need to perform a read in your application
      migration.read(
        'example-migration-flag-key',
        context,
        defaultStage
      );

      // when you need to perform a write in your application
      migration.write(
        'example-migration-flag-key',
        context,
        defaultStage
      );

To learn more, read LDMigration.

You can check for consistency, errors, or latency under “Migration insights” on the Targeting tab of your migration flag in the LaunchDarkly user interface. To learn more, read Migration flags.

Expand Customizing your migration

Customizing your migration

Customizing your migration is rare. If you want to customize your migration, configure your migration information as before.

Then, use the migrationVariation method to evaluate your feature flag and determine its migration stage. This method returns a promise that is resolved with the result LDMigrationVariation. This result includes the migration stage and a tracker that you can use to build the analytics event to send back to LaunchDarkly.

Here’s how:

Vercel SDK v1.1.6+

          import {
            LDContext,
          } from '@launchdarkly/vercel-server-sdk';

          const context: LDContext = {
             kind: 'user',
             key: 'example-user-key',
             name: 'Sandy',
          };

          const { value, tracker } = await client.migrationVariation(
            'example-migration-flag-key',
            context,
            false
          );

Next, perform the migration for the appropriate stage. At each stage, the migration may involve reading or writing from one or both systems. You must define the behavior for each stage. To learn more about how LaunchDarkly defines the stages, read Use SDKs to manage a migration, above.

The structure looks like this:

Vercel SDK v1.1.6+

        import { LDMigrationStage } from '@launchdarkly/vercel-server-sdk';

        // define the combination of reads and writes from the new and old systems
        // that should occur at each migration stage

        switch (value) {
          case LDMigrationStage.Off: { },
          case LDMigrationStage.DualWrite: { },
          case LDMigrationStage.Shadow: { },
          case LDMigrationStage.Live: { },
          case LDMigrationStage.RampDown: { },
          case LDMigrationStage.Complete: { },
          default: {
            // throw an error
          }
        }

Finally, when the migration operation is complete, call the trackMigration method to record your metrics:

Vercel SDK v1.1.6+

        import {
          LDMigrationTracker,
          LDClient
        } from '@launchdarkly/vercel-server-sdk';

        const event = tracker.createEvent();

        if (event) {
          client.trackMigration(event);
        }

To learn more, read LDMigration and LDMigrationOpEvent.