Load and Replicate Separately

On this page

Perform an initial bulk load of the source data using data-load mode, then use replication-only mode to replicate ongoing changes to the target.

Before you begin

• Create a CockroachDB Cloud or self-hosted cluster.
• Install the MOLT (Migrate Off Legacy Technology) tools.
• Review the MOLT Fetch best practices.
• Review Migration Strategy.

SELECT log_mode FROM v$database;
SHUTDOWN IMMEDIATE;
STARTUP MOUNT;
ALTER DATABASE ARCHIVELOG;
ALTER DATABASE OPEN;
SELECT log_mode FROM v$database;

LOG_MODE
--------
ARCHIVELOG

1 row selected.

ALTER DATABASE ADD SUPPLEMENTAL LOG DATA (PRIMARY KEY) COLUMNS;
SELECT supplemental_log_data_min, supplemental_log_data_pk FROM v$database;

SUPPLEMENTAL_LOG_DATA_MIN SUPPLEMENTAL_LOG_DATA_PK
------------------------- ------------------------
IMPLICIT                  YES

1 row selected.

ALTER DATABASE FORCE LOGGING;

Limitations

• Running DDL on the source or target while replication is in progress can cause replication failures.

Prepare the source database

Create migration user on source database

Create a dedicated migration user (e.g., MIGRATION_USER) on the source database. This user is responsible for reading data from source tables during the migration.

CREATE USER MIGRATION_USER WITH PASSWORD 'password';

GRANT CONNECT, RESOURCE ON DATABASE source_database 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;

CREATE USER MIGRATION_USER@'%' IDENTIFIED BY 'password';

GRANT SELECT ON source_database.* TO MIGRATION_USER@'%';
FLUSH PRIVILEGES;

CREATE USER MIGRATION_USER IDENTIFIED BY '{password}';

-- Basic access
GRANT CONNECT TO C##MIGRATION_USER;
GRANT CREATE SESSION TO C##MIGRATION_USER;

-- General metadata access
GRANT EXECUTE_CATALOG_ROLE TO C##MIGRATION_USER;
GRANT SELECT_CATALOG_ROLE TO C##MIGRATION_USER;

-- Access to necessary V$ views
GRANT SELECT ON V_$DATABASE TO C##MIGRATION_USER;

-- Direct grants to specific DBA views
GRANT SELECT ON ALL_USERS TO C##MIGRATION_USER;
GRANT SELECT ON DBA_USERS TO C##MIGRATION_USER;
GRANT SELECT ON DBA_OBJECTS TO C##MIGRATION_USER;
GRANT SELECT ON DBA_SYNONYMS TO C##MIGRATION_USER;
GRANT SELECT ON DBA_TABLES TO C##MIGRATION_USER;

-- Allow C##MIGRATION_USER to connect to the PDB and see active transaction metadata
GRANT CONNECT TO C##MIGRATION_USER;
GRANT CREATE SESSION TO C##MIGRATION_USER;

-- General metadata access
GRANT SELECT_CATALOG_ROLE TO C##MIGRATION_USER;

-- Access to necessary V$ views
GRANT SELECT ON V_$SESSION TO C##MIGRATION_USER;
GRANT SELECT ON V_$TRANSACTION TO C##MIGRATION_USER;

-- Grant these two for every table to migrate in the migration_schema
GRANT SELECT ON migration_schema.tbl TO C##MIGRATION_USER;
GRANT FLASHBACK ON migration_schema.tbl TO C##MIGRATION_USER;

-- Basic access
GRANT CONNECT TO MIGRATION_USER;
GRANT CREATE SESSION TO MIGRATION_USER;

-- General metadata access
GRANT SELECT_CATALOG_ROLE TO MIGRATION_USER;
GRANT EXECUTE_CATALOG_ROLE TO MIGRATION_USER;

-- Access to necessary V$ views
GRANT SELECT ON V_$DATABASE TO MIGRATION_USER;
GRANT SELECT ON V_$SESSION TO MIGRATION_USER;
GRANT SELECT ON V_$TRANSACTION TO MIGRATION_USER;

-- Direct grants to specific DBA views
GRANT SELECT ON ALL_USERS TO MIGRATION_USER;
GRANT SELECT ON DBA_USERS TO MIGRATION_USER;
GRANT SELECT ON DBA_OBJECTS TO MIGRATION_USER;
GRANT SELECT ON DBA_SYNONYMS TO MIGRATION_USER;
GRANT SELECT ON DBA_TABLES TO MIGRATION_USER;

-- Grant these two for every table to migrate in the migration_schema
GRANT SELECT ON migration_schema.tbl TO MIGRATION_USER;
GRANT FLASHBACK ON migration_schema.tbl TO MIGRATION_USER;

Configure source database for replication

ALTER SYSTEM SET wal_level = 'logical';

CREATE TABLE migration_schema."_replicator_sentinel" (
  keycol NUMBER PRIMARY KEY,
  lastSCN NUMBER
);

GRANT SELECT, INSERT, UPDATE ON migration_schema."_replicator_sentinel" TO C##MIGRATION_USER;

-- Access to necessary V$ views
GRANT SELECT ON V_$LOG TO C##MIGRATION_USER;
GRANT SELECT ON V_$LOGFILE TO C##MIGRATION_USER;
GRANT SELECT ON V_$LOGMNR_CONTENTS TO C##MIGRATION_USER;
GRANT SELECT ON V_$ARCHIVED_LOG TO C##MIGRATION_USER;
GRANT SELECT ON V_$LOG_HISTORY TO C##MIGRATION_USER;

-- SYS-prefixed views (for full dictionary access)
GRANT SELECT ON SYS.V_$LOGMNR_DICTIONARY TO C##MIGRATION_USER;
GRANT SELECT ON SYS.V_$LOGMNR_LOGS TO C##MIGRATION_USER;
GRANT SELECT ON SYS.V_$LOGMNR_PARAMETERS TO C##MIGRATION_USER;
GRANT SELECT ON SYS.V_$LOGMNR_SESSION TO C##MIGRATION_USER;

-- Access to LogMiner views and controls
GRANT LOGMINING TO C##MIGRATION_USER;
GRANT EXECUTE ON DBMS_LOGMNR TO C##MIGRATION_USER;

SELECT
    l.GROUP#,
    lf.MEMBER,
    l.FIRST_CHANGE# AS START_SCN,
    l.NEXT_CHANGE# AS END_SCN
FROM
    V$LOG l
JOIN
    V$LOGFILE lf
ON
    l.GROUP# = lf.GROUP#;

   GROUP# MEMBER                                       START_SCN                END_SCN 
_________ _________________________________________ ____________ ______________________ 
        3 /opt/oracle/oradata/ORCLCDB/redo03.log         1232896    9295429630892703743 
        2 /opt/oracle/oradata/ORCLCDB/redo02.log         1155042                1232896 
        1 /opt/oracle/oradata/ORCLCDB/redo01.log         1141934                1155042 

3 rows selected.

SELECT CURRENT_SCN FROM V$DATABASE;

CURRENT_SCN
-----------
2358840

1 row selected.

EXEC DBMS_LOGMNR.START_LOGMNR(
  STARTSCN => {current-scn},
  ENDSCN   => 2358840,
  OPTIONS  => DBMS_LOGMNR.DICT_FROM_ONLINE_CATALOG
);

PL/SQL procedure successfully completed.

Prepare the target database

Create the target schema

Convert the source schema into a CockroachDB-compatible schema. CockroachDB supports the PostgreSQL wire protocol and is largely compatible with PostgreSQL syntax.

• The source and target schemas must match. Review Type mapping to understand which source types can be mapped to CockroachDB types.

  For example, a 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 create a matching CockroachDB schema using the drop-on-target-and-recreate option.
  • If you create the target schema manually, review how MOLT Fetch handles type mismatches. You can use the MOLT Schema Conversion Tool to create a matching schema.

  • By default, table and column names are case-insensitive in MOLT Fetch. If using the --case-sensitive flag, schema, table, and column names must match Oracle's default uppercase identifiers. Use quoted names on the target to preserve case. For example, the following CockroachDB SQL statement will error:

    CREATE TABLE co.stores (... store_id ...);
    

    It should be written as:

    CREATE TABLE "CO"."STORES" (... "STORE_ID" ...);
    

    When using --case-sensitive, quote all identifiers and match the case exactly (for example, use "CO"."STORES" and "STORE_ID").

• Every table must have an explicit primary key. For more information, refer to Primary key best practices.

• Review Transformations 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 Considerations for 64-bit signed integers.

Schema Conversion Tool

The MOLT Schema Conversion Tool (SCT) automates target schema creation. It requires a free CockroachDB Cloud account.

1. Upload a source .sql file to convert the syntax and identify unimplemented features and syntax incompatibilities in the schema.

2. Import the converted schema to a CockroachDB cluster:

   • When migrating to CockroachDB Cloud, the Schema Conversion Tool automatically applies the converted schema to a new Cloud database.
   • When migrating to a self-hosted CockroachDB cluster, export a converted schema file and pipe the data definition language (DDL) directly into cockroach sql.

SELECT array_position(ARRAY[4,1,3,2],1);

  array_position
------------------
               2
(1 row)

SELECT * FROM table_a ORDER BY COALESCE(array_position(ARRAY[4,1,3,2],5),999);

Drop constraints and indexes

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

• FOREIGN KEY
• UNIQUE
• Secondary indexes
• CHECK
• DEFAULT
• NOT NULL (you do not need to drop this constraint when using drop-on-target-and-recreate for table handling)

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

Create the SQL user

Create a migration 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 the necessary permissions to run either IMPORT INTO or COPY FROM on the target tables, depending on the MOLT Fetch data load mode) you will use.

IMPORT INTO privileges

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

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 system-level privilege:

GRANT EXTERNALIOIMPLICITACCESS TO crdb_user;

COPY FROM privileges

Make the user a member of the admin role:

ALTER USER crdb_user WITH ADMIN OPTION;

GRANT admin TO crdb_user;

Configure data load

When you run molt fetch, you can configure the following options for data load:

• Connection strings: Specify URL‑encoded source and target connections.
• Intermediate file storage: Export data to cloud storage or a local file server.
• Table handling mode: Determine how existing target tables are initialized before load.
• Schema and table filtering: Specify schema and table names to migrate.
• Data load mode: Choose between IMPORT INTO or COPY FROM.

• Metrics: Configure metrics collection during the load.

• Replication flags: Configure the replicator process.

Connection strings

Define the connection strings for the source and target databases, and keep them secure.

Source connection string

The --source flag specifies the connection string for the source database:

--source 'postgres://{username}:{password}@{host}:{port}/{database}?sslmode=verify-full'

--source 'postgres://postgres:postgres@localhost:5432/molt?sslmode=verify-full'

--source 'mysql://{username}:{password}@{host}:{port}/{database}?sslmode=verify-full&sslcert={path_to_client_crt}&sslkey={path_to_client_key}&sslrootcert={path_to_ca_crt}'

--source 'mysql://user:password@localhost/molt?sslcert=.%2fsource_certs%2fclient.root.crt&sslkey=.%2fsource_certs%2fclient.root.key&sslmode=verify-full&sslrootcert=.%2fsource_certs%2fca.crt'

--source 'oracle://{username}:{password}@{host}:{port}/{service_name}'

--source 'oracle://{username}:{password}@{host}:{port}/{PDB_service_name}'
--source-cdb 'oracle://{username}:{password}@{host}:{port}/{CDB_service_name}'

--source 'oracle://C%23%23MIGRATION_USER:password@host:1521/ORCLPDB1'
--source-cdb 'oracle://C%23%23MIGRATION_USER:password@host:1521/ORCLCDB'

Target connection string

The --target flag specifies the connection string for the target CockroachDB database:

--target 'postgres://{username}:{password}@{host}:{port}/{database}?sslmode=verify-full'

For example:

--target 'postgres://crdb_user@localhost:26257/defaultdb?sslmode=verify-full'

For details, refer to Connect using a URL.

Secure connections

To keep your database credentials out of shell history and logs, follow these best practices when specifying your source and target connection strings:

• Avoid plaintext connection strings.

• URL-encode connection strings for the source database and CockroachDB so special characters in passwords are handled correctly.

  • Given a password a$52&, pass it to the molt escape-password command with single quotes:

    molt escape-password 'a$52&'
    

    Use the encoded password in your --source connection string. For example:

    --source 'postgres://postgres:a%2452%26@localhost:5432/replicationload'
    

• Provide your connection strings as environment variables. For example:

  export SOURCE="postgres://postgres:postgres@localhost:5432/molt?sslmode=verify-full"
  export TARGET="postgres://root@localhost:26257/molt?sslmode=verify-full"
  

  Afterward, reference the environment variables as follows:

  --source $SOURCE
  --target $TARGET
  

• If possible, use an external secrets manager to load the environment variables from stored secrets.

Intermediate file storage

MOLT Fetch can write intermediate files to either a cloud storage bucket or a local file server:

Cloud storage authentication

Ensure that access control is properly configured for Amazon S3, Google Cloud Storage, or Azure Blob Storage.

Table handling mode

MOLT Fetch can initialize target tables on the CockroachDB database in one of three modes using --table-handling:

• Use none when you need to retain existing data and schema.
• Use --table-handling truncate-if-exists to clear existing data while preserving schema definitions.
• Use --table-handling drop-on-target-and-recreate for initial imports or when source and target schemas differ, letting MOLT Fetch generate compatible tables automatically.

To guide schema creation with drop-on-target-and-recreate, you can explicitly map source types to CockroachDB types. Refer to Type mapping.

Schema and table filtering

MOLT Fetch can restrict which schemas (or users) and tables are migrated by using the --schema-filter, --table-filter, and --table-exclusion-filter flags:

--schema-filter 'migration_schema'

import * as api from "replicator@v1";

// List the source tables (matching source names) to include in replication
const allowedTables = ["EMPLOYEES", "PAYMENTS", "ORDERS"];

// Update this to your target CockroachDB database and schema name
api.configureSource("defaultdb.migration_schema", {
  dispatch: (doc: Document, meta: Document): Record<Table, Document[]> | null => {
    // Replicate only if the table matches one of the allowed tables
    if (allowedTables.includes(meta.table)) {
      let ret: Record<Table, Document[]> = {};
      ret[meta.table] = [doc];
      return ret;
    }
    // Ignore all other tables
    return null;
  },
  deletesTo: (doc: Document, meta: Document): Record<Table, Document[]> | null => {
    // Optionally filter deletes the same way
    if (allowedTables.includes(meta.table)) {
      let ret: Record<Table, Document[]> = {};
      ret[meta.table] = [doc];
      return ret;
    }
    return null;
  },
});

--replicator-flags "--userscript table_filter.ts"

import * as api from "replicator@v1";

// List the source tables (matching source names) to include in replication
const allowedTables = ["EMPLOYEES", "PAYMENTS", "ORDERS"];

// Update this to your target CockroachDB database and schema name
api.configureSource("defaultdb.migration_schema", {
  dispatch: (doc: Document, meta: Document): Record<Table, Document[]> | null => {
    // Replicate only if the table matches one of the allowed tables
    if (allowedTables.includes(meta.table)) {
      let ret: Record<Table, Document[]> = {};
      ret[meta.table] = [doc];
      return ret;
    }
    // Ignore all other tables
    return null;
  },
  deletesTo: (doc: Document, meta: Document): Record<Table, Document[]> | null => {
    // Optionally filter deletes the same way
    if (allowedTables.includes(meta.table)) {
      let ret: Record<Table, Document[]> = {};
      ret[meta.table] = [doc];
      return ret;
    }
    return null;
  },
});

--replicator-flags "--userscript table_filter.ts"

Data load mode

MOLT Fetch can use either IMPORT INTO or COPY FROM to load data into CockroachDB:

• Use IMPORT INTO (the default mode) for large datasets, wide rows, or partitioned tables.
• Use --use-copy when tables must remain online during data load.
• Use --direct-copy only when you cannot move data to a public cloud, or want to perform local testing without intermediate storage. In this case, no intermediate file storage is used.

Metrics

By default, MOLT Fetch exports Prometheus metrics at http://127.0.0.1:3030/metrics. You can override the address with --metrics-listen-addr '{host}:{port}', where the endpoint will be http://{host}:{port}/metrics.

Cockroach Labs recommends monitoring the following metrics during data load:

You can also use the sample Grafana dashboard to view the preceding metrics.

Load data into CockroachDB

Perform the initial load of the source data.

1. Issue the MOLT Fetch command to move the source data to CockroachDB, specifying --mode data-load to perform a one-time data load. 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 also limits the migration to a single schema and filters three specific tables to migrate. The data load mode defaults to IMPORT INTO.

   Specify a replication slot name with --pglogical-replication-slot-name. This is required for replication in a subsequent step.

   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 \
   --pglogical-replication-slot-name cdc_slot \
   --mode data-load
   

   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 \
   --mode data-load
   

   The command assumes an Oracle Multitenant (CDB/PDB) source. --source-cdb specifies the container database (CDB) connection string.

   molt fetch \
   --source $SOURCE \
   --source-cdb $SOURCE_CDB \
   --target $TARGET \
   --schema-filter 'migration_schema' \
   --table-filter 'employees|payments|orders' \
   --bucket-path 's3://migration/data/cockroach' \
   --table-handling truncate-if-exists \
   --mode data-load
   

2. 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"}
   

   {"level":"info","type":"summary","num_tables":3,"cdc_cursor":"4c658ae6-e8ad-11ef-8449-0242ac140006:1-28","time":"2025-02-10T14:28:11-05:00","message":"starting fetch"}
   

   {"level":"info","type":"summary","num_tables":3,"cdc_cursor":"2358840","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"}
   

   {"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":"4c658ae6-e8ad-11ef-8449-0242ac140006:1-29","time":"2025-02-10T14:28:13-05:00","message":"data import on target for table complete"}
   

   {"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":"2358840","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"],"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"}
   

   {"level":"info","type":"summary","fetch_id":"f5cb422f-4bb4-4bbd-b2ae-08c4d00d1e7c","num_tables":3,"tables":["migration_schema.employees","migration_schema.payments","migration_schema.payments"],"cdc_cursor":"4c658ae6-e8ad-11ef-8449-0242ac140006:1-29","net_duration_ms":6752.847625,"net_duration":"000h 00m 06s","time":"2024-03-18T12:30:37-04:00","message":"fetch complete"}
   

   This message includes a cdc_cursor value. You must set --defaultGTIDSet to this value when starting replication-only mode:

   --defaultGTIDSet 4c658ae6-e8ad-11ef-8449-0242ac140006:1-29
   

   {"level":"info","type":"summary","fetch_id":"f5cb422f-4bb4-4bbd-b2ae-08c4d00d1e7c","num_tables":3,"tables":["migration_schema.employees","migration_schema.payments","migration_schema.payments"],"cdc_cursor":"2358840","net_duration_ms":6752.847625,"net_duration":"000h 00m 06s","time":"2024-03-18T12:30:37-04:00","message":"fetch complete"}
   

   The following message contains the appropriate --backfillFromSCN and --scn values to set when startingreplication-only mode:

   replication-only mode should include the following replicator flags: --backfillFromSCN 26685444 --scn 26685786
   

Verify the data load

Use MOLT Verify to confirm that the source and target data is consistent. This ensures that the data load was successful.

1. Issue the MOLT Verify command, specifying the source and target connection strings and the tables to validate.

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

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

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

   Note:

   With Oracle Multitenant deployments, --source-cdb is not necessary for verify.

2. 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 containing a summary is written after each table is compared. For details on the summary fields, refer to the MOLT Verify 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"}
   

Replicate changes to CockroachDB

With initial load complete, start replication of ongoing changes on the source to CockroachDB.

In the molt fetch command, use --replicator-flags to pass options to the included replicator process that handles continuous replication. For details on all available flags, refer to the MOLT Fetch documentation.

1. Issue the MOLT Fetch command to start replication on CockroachDB, specifying --mode replication-only. For details on --replicator-flags, refer to Replication flags. In this example, the --metricsAddr :30005 replication flag enables a Prometheus endpoint at http://localhost:30005/_/varz where replication metrics will be served. You can use these metrics to verify that replication has drained in a later step.

   Be sure to specify the same --pglogical-replication-slot-name value that you provided in Load data into CockroachDB.

   molt fetch \
   --source $SOURCE \ 
   --target $TARGET \
   --table-filter 'employees|payments|orders' \
   --pglogical-replication-slot-name cdc_slot \
   --replicator-flags '--metricsAddr :30005' \
   --mode replication-only
   

   molt fetch \
   --source $SOURCE \ 
   --target $TARGET \
   --table-filter 'employees|payments|orders' \
   --non-interactive \
   --replicator-flags '--defaultGTIDSet 4c658ae6-e8ad-11ef-8449-0242ac140006:1-29 --metricsAddr :30005 --userscript table_filter.ts' \
   --mode replication-only
   

   molt fetch \
   --source $SOURCE \
   --source-cdb $SOURCE_CDB \
   --target $TARGET \
   --schema-filter 'migration_schema' \
   --table-filter 'employees|payments|orders' \
   --replicator-flags '--backfillFromSCN 26685444 --scn 26685786 --metricsAddr :30005 --userscript table_filter.ts' \
   --mode 'replication-only'
   

2. Check the output to observe replicator progress.

   A starting replicator message indicates that the task has started:

   {"level":"info","time":"2025-02-10T14:28:13-05:00","message":"starting replicator"}
   

   The staging database name message contains the name of the staging schema. The schema name contains a replication marker for streaming changes, which is used for resuming replication, or performing failback to the source database.

   {"level":"info","time":"2025-02-10T14:28:13-05:00","message":"staging database name: _replicator_1739215693817700000"}
   

   upserted rows log messages indicate that changes were replicated to CockroachDB:

   DEBUG  [Jan 22 13:52:40] upserted rows                                 conflicts=0 duration=7.620208ms proposed=1 target="\"molt\".\"migration_schema\".\"employees\"" upserted=1
   

   Tip:

   If replication is interrupted, you can resume replication.

Stop replication and verify data

1. Stop application traffic to your source database. This begins downtime.

2. Wait for replication to drain, which means that all transactions that occurred on the source database have been fully processed and replicated to CockroachDB. There are two ways to determine that replication has fully drained:

   • When replication is caught up, you will not see new upserted rows logs.

   • If you set up the replication metrics endpoint with --metricsAddr in the preceding steps, metrics are available at:

     http://{host}:{port}/_/varz
     

     Use the following Prometheus alert expression to observe when the combined rate of upserts and deletes is 0 for each schema:

     sum by (schema) (rate(apply_upserts_total[$__rate_interval]) + rate(apply_deletes_total[$__rate_interval]))
     

3. Cancel replication to CockroachDB by entering ctrl-c to issue a SIGTERM signal. This returns an exit code 0.

4. Repeat Verify the data load to verify the updated data.

Modify the CockroachDB schema

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

For the appropriate SQL syntax, refer to ALTER TABLE ... ADD CONSTRAINT and CREATE INDEX. Review the best practices for creating secondary indexes on CockroachDB.

Cutover

Perform a cutover by resuming application traffic, now to CockroachDB.

Troubleshooting

Fetch exits early due to mismatches

molt fetch exits early in the following cases, and will output a log with a corresponding mismatch_tag and failable_mismatch set to true:

• A source table is missing a primary key.
• A source and table primary key have mismatching types.
• A STRING primary key has a different collation on the source and target.
• A source and target column have mismatching types that are not allowable mappings.
• A target table is missing a column that is in the corresponding source table.
• A source column is nullable, but the corresponding target column is not nullable (i.e., the constraint is more strict on the target).

molt fetch can continue in the following cases, and will output a log with a corresponding mismatch_tag and failable_mismatch set to false:

• A target table has a column that is not in the corresponding source table.
• A source column has a NOT NULL constraint, and the corresponding target column is nullable (i.e., the constraint is less strict on the target).
• A DEFAULT, CHECK, FOREIGN KEY, or UNIQUE constraint is specified on a target column and not on the source column.

-- change UNLIMITED to a suitable limit for the table owner
ALTER USER migration_schema QUOTA UNLIMITED ON USERS;

GRANT SELECT, FLASHBACK ON migration_schema.employees TO C##MIGRATION_USER;
GRANT SELECT, FLASHBACK ON migration_schema.payments TO C##MIGRATION_USER;
GRANT SELECT, FLASHBACK ON migration_schema.orders TO C##MIGRATION_USER;

See also

• Migration Overview
• Migration Strategy
• MOLT Schema Conversion Tool
• MOLT Fetch
• MOLT Verify
• Migration Failback