Phased Bulk Load Migration from PostgreSQL

A involves in several phases. Data can be sliced per tenant, per service, per region, or per table to suit the needs of the migration. In this approach, you stop application traffic to the source database only for the tables in a particular slice of data. You then migrate that phase of data to the target cluster using during a downtime window. Application traffic is then cut over to those target tables after schema finalization and data verification. This process is repeated for each phase of data.

Data is migrated to the target .
This approach does not utilize .
is manual. If you wish to roll back before the target has received any writes that are not present on the source database, nothing needs to be done. If you wish to roll back after the target has received writes that are not present on the source database, you must manually replicate these new rows on the source.

This approach is comparable to the , but dividing the data into multiple phases allows each downtime window to be shorter, and it allows each phase of the migration to be less complex. Depending on how you divide the data, it also may allow your downtime windows to affect only a subset of users. For example, dividing the data per region could mean that, when migrating the data from Region A, application usage in Region B may remain unaffected. This approach may increase overall migration complexity: its duration is longer, you will need to do the work of partitioning the data, and you will have a longer period when you run both the source and the target database concurrently. This approach is best for databases that are too large to migrate all at once, internal tools, dev/staging environments, and production environments that can handle business disruption. It can only be performed if your system can handle downtime for each migration phase, and if your source database can easily be divided into the phases you need. This page describes an example scenario. While the commands provided can be copy-and-pasted, they may need to be altered or reconsidered to suit the needs of your specific environment.

Example scenario

You have a moderately-sized (500GB) database that provides the data store for a web application. You want to migrate the entirety of this database to a new CockroachDB cluster. You will divide this migration into four geographic regions (A, B, C, and D). You schedule a maintenance window for each region over four subsequent evenings, and you announce them to your users (per region) several weeks in advance. The application runs on a Kubernetes cluster with an NGINX Ingress Controller. Estimated system downtime: 4 hours per region.

Before the migration

Install the tools.
Review the documentation.
and .
Recommended: Perform a dry run of this full set of instructions in a development environment that closely resembles your production environment. This can help you get a realistic sense of the time and complexity it requires.
Announce the maintenance window to your users on a per-region basis.
Understand the prerequisites and limitations of the MOLT tools:

Limitations

MOLT Fetch limitations

Only tables with types of , , or can be sharded with .
GEOMETRY and GEOGRAPHY types are not supported.
OID LOB types in PostgreSQL are not supported, although similar types like BYTEA are supported.

MOLT Verify limitations

MOLT Verify compares 20,000 rows at a time by default, and row values can change between batches, potentially resulting in temporary inconsistencies in data. To configure the row batch size, use the --row_batch_size .
MOLT Verify checks for collation mismatches on columns. This may cause validation to fail when a is used as a primary key and the source and target databases are using different .
MOLT Verify might give an error in case of schema changes on either the source or target database.
cannot yet be compared.

Step 1: Prepare the source database

In this step, you will:

Create a dedicated migration user on your source database.

Create migration user on source database

Create a dedicated migration user (for example, MIGRATION_USER) on the source database. This user is responsible for reading data from source tables during the migration. You will pass this username in the source connection string.

CREATE USER migration_user WITH PASSWORD 'password';

Grant the user privileges to connect, view schema objects, and select the tables you migrate.

GRANT CONNECT ON DATABASE migration_db TO migration_user;
GRANT USAGE ON SCHEMA migration_schema TO migration_user;
GRANT SELECT ON ALL TABLES IN SCHEMA migration_schema TO migration_user;
ALTER DEFAULT PRIVILEGES IN SCHEMA migration_schema GRANT SELECT ON TABLES TO migration_user;

Step 2: Prepare the target database

In this step, you will:

Provision and run a new CockroachDB cluster.
Define the tables on the target cluster to match those on the source.
Create a SQL user on the target cluster with the necessary write permissions.

Provision a CockroachDB cluster

Use one of the following options to create and run a new CockroachDB cluster. This is your migration target.

Option 1: Create a secure cluster locally

If you have the CockroachDB binary installed locally, you can manually deploy a multi-node, self-hosted CockroachDB cluster on your local machine. Learn how to .

Option 2: Create a CockroachDB Self-Hosted cluster on AWS

You can manually deploy a multi-node, self-hosted CockroachDB cluster on Amazon’s AWS EC2 platform, using AWS’s managed load-balancing service to distribute client traffic. Learn how to .

Option 3: Create a CockroachDB Cloud cluster

CockroachDB Cloud is a fully-managed service run by Cockroach Labs, which simplifies the deployment and management of CockroachDB. Sign up for a CockroachDB Cloud account and using .

Define the target tables

Convert the source table definitions into CockroachDB-compatible equivalents. CockroachDB supports the PostgreSQL wire protocol and is largely .

The source and target table definitions must match. Review to understand which source types can be mapped to CockroachDB types. For example, a PostgreSQL source table defined as CREATE TABLE migration_schema.tbl (pk INT PRIMARY KEY); must have a corresponding schema and table in CockroachDB:
CREATE SCHEMA migration_schema; CREATE TABLE migration_schema.tbl (pk INT PRIMARY KEY);
- MOLT Fetch can automatically define matching CockroachDB tables using the option.
- If you define the target tables manually, review how MOLT Fetch handles . You can use the MOLT Schema Conversion Tool to create matching table definitions.
Every table must have an explicit primary key.
Avoid using sequential keys. To learn more about the performance issues that can result from their use, refer to the . If a sequential key is necessary in your CockroachDB table, you must create it manually, after using to load and replicate the data.
Review to understand how computed columns and partitioned tables can be mapped to the target, and how target tables can be renamed.
By default on CockroachDB, INT is an alias for INT8, which creates 64-bit signed integers. PostgreSQL and MySQL default to 32-bit integers. Depending on your source database or application requirements, you may need to change the integer size to 4. For more information, refer to .

Schema Conversion Tool

The (SCT) converts source table definitions to CockroachDB-compatible syntax. It requires a free .

Upload a source .sql file to convert the syntax and identify in the table definitions.
Import the converted table definitions to a CockroachDB cluster:
- When migrating to CockroachDB Cloud, the Schema Conversion Tool automatically .
- When migrating to a self-hosted CockroachDB cluster, and pipe the directly into .

Drop constraints and indexes

To optimize data load performance, drop all non-PRIMARY KEY and on the target CockroachDB database before migrating:

(you do not need to drop this constraint when using drop-on-target-and-recreate for table handling)

Do not drop constraints.

You can recreate the constraints and indexes after loading the data.

Create the SQL user

Create a SQL user in the CockroachDB cluster that has the necessary privileges. To create a user crdb_user in the default database (you will pass this username in the target connection string):

CREATE USER crdb_user WITH PASSWORD 'password';

Grant database-level privileges for schema creation within the target database:

GRANT ALL ON DATABASE defaultdb TO crdb_user;

Grant permission to modify cluster settings:

GRANT SYSTEM MODIFYCLUSTERSETTING TO crdb_user;

Grant usage on schemas being migrated:

GRANT USAGE ON SCHEMA migration_schema TO crdb_user;
GRANT USAGE ON SCHEMA public TO crdb_user;

Grant user privileges to create tables in the migration_schema schema and internal MOLT tables like _molt_fetch_exceptions in the public CockroachDB schema:

Ensure that you are connected to the target database.

GRANT CREATE ON SCHEMA migration_schema TO crdb_user;
GRANT CREATE ON SCHEMA public TO crdb_user;

If you manually defined the target tables (which means that drop-on-target-and-recreate will not be used), grant the following privileges on the schema:

GRANT SELECT, INSERT, UPDATE, DELETE ON ALL TABLES IN SCHEMA migration_schema TO crdb_user;
ALTER DEFAULT PRIVILEGES IN SCHEMA migration_schema
GRANT SELECT, INSERT, UPDATE, DELETE ON TABLES TO crdb_user;

Grant the same privileges for internal MOLT tables in the public CockroachDB schema:

GRANT SELECT, INSERT, UPDATE, DELETE ON ALL TABLES IN SCHEMA public TO crdb_user;
ALTER DEFAULT PRIVILEGES IN SCHEMA public
GRANT SELECT, INSERT, UPDATE, DELETE ON TABLES TO crdb_user;

If you will be running Fetch with drop-on-target-and-recreate, and the target schema has pre-existing tables that were created by a different user, you may need to change table ownership to allow the migration user to drop those tables. Make the following alteration for each table:

ALTER TABLE migration_schema.table1 OWNER TO crdb_user;

Depending on the MOLT Fetch you will use, grant the necessary privileges to run either IMPORT INTO or COPY FROM on the target tables:

`IMPORT INTO` privileges

Grant SELECT, INSERT, and DROP (required because the table is taken offline during the IMPORT INTO) privileges on all tables being migrated:

GRANT SELECT, INSERT, DROP ON ALL TABLES IN SCHEMA migration_schema TO crdb_user;

If you plan to use cloud storage with implicit authentication for data load, grant the EXTERNALIOIMPLICITACCESS :

GRANT SYSTEM EXTERNALIOIMPLICITACCESS TO crdb_user;

`COPY FROM` privileges

Grant privileges to the user:

GRANT admin TO crdb_user;

Set session variable

Ensure that the statement_timeout is set to 0s for the user:

ALTER ROLE crdb_user SET statement_timeout = '0s';

Migrate each phase

Steps 3-7 are run for each phase of the data migration. Within the first migration downtime window, you will run through these steps for Region A. You will repeat these steps for the other regions during each subsequent downtime window.

Step 3: Stop application traffic

With both the source and target databases prepared for the data load, it’s time to stop application traffic to the source for a particular region. If the Kubernetes cluster that deploys the application has pre-region deployments (for example, app-us, app-eu, app-apac), you can scale down only the deployment for that region.

kubectl scale deploy/app-eu --replicas=0

Or this can be handled by the NGINX Ingress Controller, by including the following to your NGINX configuration, ensuring that the conditional statement is suitable for your deployment:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: app
  annotations:
    nginx.ingress.kubernetes.io/server-snippet: |
      if ($http_x_region = "eu") {
        return 503;
      }
spec:
  ingressClassName: nginx
  rules:
  - host: api.example.com
    http:
      paths:
      - path: /
        pathType: Prefix
        backend:
          service:
            name: app
            port:
              number: 80

Application downtime begins now, for users in the given region.It is strongly recommended that you perform a dry run of this migration in a test environment. This will allow you to practice using the MOLT tools in real time, and it will give you an accurate sense of how long application downtime might last.

Step 4: Load data into CockroachDB

In this step, you will:

Configure MOLT Fetch with the flags needed for your migration.
Run MOLT Fetch.
Understand how to continue a load after an interruption.

Configure MOLT Fetch

The includes detailed information about how to , and how to . When you run molt fetch, you can configure the following options for data load:

: Specify URL‑encoded source and target connections.
: Specify schema and table names to migrate. Important for a phased migration.
: Export data to cloud storage or a local file server.
: Specifies whether data will only be loaded into/from intermediate storage.
: Divide larger tables into multiple shards during data export.
: Choose between IMPORT INTO and COPY FROM.
: Determine how existing target tables are initialized before load.
: Define any row-level transformations to apply to the data before it reaches the target.
: Configure metrics collection during initial data load.

Read through the documentation to understand how to configure your molt fetch command and its flags. Follow , especially those related to security. At minimum, the molt fetch command should include the source, target, data path, and flags. For a phased migration, you may also choose to include or flags:

molt fetch \
--source $SOURCE \
--target $TARGET \
--table-filter '.*user.*' \
--bucket-path 's3://bucket/path' \
--ignore-replication-check

However, depending on the needs of your migration, you may have many more flags set, and you may need to prepare some accompanying .json files.

Run MOLT Fetch

Perform the bulk load of the source data.

Run the command to move the source data into CockroachDB. This example command passes the source and target connection strings as environment variables, writes intermediate files to S3 storage, and uses the truncate-if-exists table handling mode to truncate the target tables before loading data. It limits the migration to a single schema and filters for three specific tables. The defaults to IMPORT INTO. Include the --ignore-replication-check flag to skip replication checkpoint queries, which eliminates the need to configure the source database for logical replication.
molt fetch \ --source $SOURCE \ --target $TARGET \ --schema-filter 'migration_schema' \ --table-filter 'employees|payments|orders' \ --bucket-path 's3://migration/data/cockroach' \ --table-handling truncate-if-exists \ --ignore-replication-check

Check the output to observe fetch progress. A starting fetch message indicates that the task has started:

{"level":"info","type":"summary","num_tables":3,"cdc_cursor":"0/43A1960","time":"2025-02-10T14:28:11-05:00","message":"starting fetch"}

data extraction messages are written for each table that is exported to the location in --bucket-path:

{"level":"info","table":"migration_schema.employees","time":"2025-02-10T14:28:11-05:00","message":"data extraction phase starting"}

{"level":"info","table":"migration_schema.employees","type":"summary","num_rows":200000,"export_duration_ms":1000,"export_duration":"000h 00m 01s","time":"2025-02-10T14:28:12-05:00","message":"data extraction from source complete"}

data import messages are written for each table that is loaded into CockroachDB:

{"level":"info","table":"migration_schema.employees","time":"2025-02-10T14:28:12-05:00","message":"starting data import on target"}

{"level":"info","table":"migration_schema.employees","type":"summary","net_duration_ms":1899.748333,"net_duration":"000h 00m 01s","import_duration_ms":1160.523875,"import_duration":"000h 00m 01s","export_duration_ms":1000,"export_duration":"000h 00m 01s","num_rows":200000,"cdc_cursor":"0/43A1960","time":"2025-02-10T14:28:13-05:00","message":"data import on target for table complete"}

A fetch complete message is written when the fetch task succeeds:

{"level":"info","type":"summary","fetch_id":"f5cb422f-4bb4-4bbd-b2ae-08c4d00d1e7c","num_tables":3,"tables":["migration_schema.employees","migration_schema.payments","migration_schema.payments"],"skipped_unmigratable_tables":[],"cdc_cursor":"0/3F41E40","net_duration_ms":6752.847625,"net_duration":"000h 00m 06s","time":"2024-03-18T12:30:37-04:00","message":"fetch complete"}

Continue MOLT Fetch after an interruption

If MOLT Fetch fails while loading data into CockroachDB from intermediate files, it exits with an error message, fetch ID, and continuation token for each table that failed to load on the target database.

{"level":"info","table":"public.employees","file_name":"shard_01_part_00000001.csv.gz","message":"creating or updating token for duplicate key value violates unique constraint \"employees_pkey\"; Key (id)=(1) already exists."}
{"level":"info","table":"public.employees","continuation_token":"a1b2c3d4-e5f6-7890-abcd-ef1234567890","message":"created continuation token"}
{"level":"info","fetch_id":"f5cb422f-4bb4-4bbd-b2ae-08c4d00d1e7c","message":"continue from this fetch ID"}
{"level":"error","message":"Error: error from fetching table for public.employees: error importing data: duplicate key value violates unique
  constraint \"employees_pkey\" (SQLSTATE 23505)"}

You can use this information to where it was interrupted. Continuation is only possible under the following conditions:

All data has been exported from the source database into intermediate files on or .
The initial load of source data into the target CockroachDB database is incomplete.
The load uses IMPORT INTO rather than COPY FROM.

Only one fetch ID and set of continuation tokens, each token corresponding to a table, are active at any time. See .

The following command reattempts the data load starting from a specific continuation file, but you can also use individual continuation tokens to .

molt fetch \
--source $SOURCE \
--target $TARGET \
--schema-filter 'migration_schema' \
--table-filter 'employees|payments|orders' \
--bucket-path 's3://migration/data/cockroach' \
--table-handling truncate-if-exists \
--ignore-replication-check \
--fetch-id f5cb422f-4bb4-4bbd-b2ae-08c4d00d1e7c \
--continuation-file-name shard_01_part_00000001.csv.gz

Step 5: Verify the data

In this step, you will use to confirm that the source and target data is consistent. This ensures that the data load was successful. Use MOLT Verify’s or to select only the tables that are relevant for the given phase.

Run MOLT Verify

Run the command, specifying the source and target connection strings and the tables to validate.

molt verify \
--source $SOURCE \
--target $TARGET \
--table-filter 'employees|payments|orders'

Check the output to observe verify progress. A verification in progress indicates that the task has started:

{"level":"info","time":"2025-02-10T15:35:04-05:00","message":"verification in progress"}

starting verify messages are written for each specified table:

{"level":"info","time":"2025-02-10T15:35:04-05:00","message":"starting verify on public.employees, shard 1/1"}

A finished row verification message is written after each table is compared. If num_success equals num_truth_rows and the error counters (num_missing, num_mismatch, num_extraneous, and num_column_mismatch) are all 0, the table verified successfully. Any non-zero values in the error counters indicate data discrepancies that need investigation. For details on each field, refer to the page.

{"level":"info","type":"summary","table_schema":"public","table_name":"employees","num_truth_rows":200004,"num_success":200004,"num_conditional_success":0,"num_missing":0,"num_mismatch":0,"num_extraneous":0,"num_live_retry":0,"num_column_mismatch":0,"time":"2025-02-10T15:35:05-05:00","message":"finished row verification on public.employees (shard 1/1)"}

A verification complete message is written when the verify task succeeds:

{"level":"info","net_duration_ms":699.804875,"net_duration":"000h 00m 00s","time":"2025-02-10T15:35:05-05:00","message":"verification complete"}

Step 6: Finalize the target schema

Add constraints and indexes

Add any constraints or indexes that you previously removed from the CockroachDB schema to facilitate data load.

If you used the --table-handling drop-on-target-and-recreate option for data load, only and constraints are preserved. You must manually recreate all other constraints and indexes.

For the appropriate SQL syntax, refer to and . Review the on CockroachDB.

Step 7: Cut over application traffic

With the target cluster verified and finalized, it’s time to resume application traffic for the current migration phase.

Modify application code

In the application back end, update the application to route traffic for this migration phase to the CockroachDB cluster. A simple example:

env:
  - name: DATABASE_URL_US_EAST
    value: postgres://root@cockroachdb.us-east:26257/defaultdb?sslmode=verify-full
  - name: DATABASE_URL_US_WEST
    value: postgres://legacy-db.us-west:5432/defaultdb  # Still on source

In your application code, route database connections based on the user’s region:

def get_db_connection(user_region):
    if user_region == "us-east":
        return os.getenv("DATABASE_URL_US_EAST")  # CockroachDB
    else:
        return os.getenv("DATABASE_URL_US_WEST")  # Source database

Resume application traffic

If you halted traffic by scaling down a regional Kubernetes deployment, scale it back up.

kubectl scale deploy/app-eu --replicas=3

Or if this was handled by the NGINX Controller, remove the 503 block that was written in step 3:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: app
#   annotations:
#     nginx.ingress.kubernetes.io/server-snippet: |
#       if ($http_x_region = "eu") {
#         return 503;
#       }
spec:
  ingressClassName: nginx
  rules:
  - host: api.example.com
    http:
      paths:
      - path: /
        pathType: Prefix
        backend:
          service:
            name: app
            port:
              number: 80

This ends downtime for the current migration phase.

Repeat for each phase

During the next scheduled, regional downtime window, return to step 3 to migrate the next phase of data. Repeat steps 3-7 for each phase of data, until every region’s data has been migrated and all application traffic has been cut over to the target.

​Example scenario

​Before the migration

​Limitations

​MOLT Fetch limitations

​MOLT Verify limitations

​Step 1: Prepare the source database

​Create migration user on source database

​Step 2: Prepare the target database

​Provision a CockroachDB cluster

​Option 1: Create a secure cluster locally

​Option 2: Create a CockroachDB Self-Hosted cluster on AWS

​Option 3: Create a CockroachDB Cloud cluster

​Define the target tables

​Schema Conversion Tool

​Drop constraints and indexes

​Create the SQL user

​IMPORT INTO privileges

​COPY FROM privileges

​Set session variable

​Migrate each phase

​Step 3: Stop application traffic

​Step 4: Load data into CockroachDB

​Configure MOLT Fetch

​Run MOLT Fetch

​Continue MOLT Fetch after an interruption

​Step 5: Verify the data

​Run MOLT Verify

​Step 6: Finalize the target schema

​Add constraints and indexes

​Step 7: Cut over application traffic

​Modify application code

​Resume application traffic

​Repeat for each phase

​See also

Example scenario

Before the migration

Limitations

MOLT Fetch limitations

MOLT Verify limitations

Step 1: Prepare the source database

Create migration user on source database

Step 2: Prepare the target database

Provision a CockroachDB cluster

Option 1: Create a secure cluster locally

Option 2: Create a CockroachDB Self-Hosted cluster on AWS

Option 3: Create a CockroachDB Cloud cluster

Define the target tables

Schema Conversion Tool

Drop constraints and indexes

Create the SQL user

`IMPORT INTO` privileges

`COPY FROM` privileges

Set session variable

Migrate each phase

Step 3: Stop application traffic

Step 4: Load data into CockroachDB

Configure MOLT Fetch

Run MOLT Fetch

Continue MOLT Fetch after an interruption

Step 5: Verify the data

Run MOLT Verify

Step 6: Finalize the target schema

Add constraints and indexes

Step 7: Cut over application traffic

Modify application code

Resume application traffic

Repeat for each phase

See also