Phased Delta Migration with Failback Replication from Oracle

• Data is migrated to the target .
• This approach utilizes .
• is achieved via failback replication.

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Example scenario

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Before the migration

• Install the tools.
• Review the and 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.
• Understand the prerequisites and limitations of the MOLT tools:

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Prerequisites

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Oracle Instant Client

• On macOS ARM machines, download the Oracle Instant Client. After installation, you should have a new directory at /Users/$USER/Downloads/instantclient_23_3 containing .dylib files. Set the LD_LIBRARY_PATH environment variable to this directory:
  export LD_LIBRARY_PATH=/Users/$USER/Downloads/instantclient_23_3
  
• On Linux machines, install the Oracle Instant Client dependencies and set the LD_LIBRARY_PATH to the client library path:
  sudo apt-get install -yqq --no-install-recommends libaio1t64
  sudo ln -s /usr/lib/x86_64-linux-gnu/libaio.so.1t64 /usr/lib/x86_64-linux-gnu/libaio.so.1
  unzip -d /tmp /tmp/instantclient-basiclite-linux-amd64.zip
  sudo mv /tmp/instantclient_21_13/* /usr/lib
  export LD_LIBRARY_PATH=/usr/lib
  
  You can also download Oracle Instant Client directly from the Oracle site for Linux ARM64 or Linux x86-64.

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Limitations

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MOLT Fetch limitations

• Only tables with types of , , or can be sharded with .
• GEOMETRY and GEOGRAPHY types are not supported.
• Migrations must be performed from a single Oracle schema. You must include so that MOLT Fetch only loads data from the specified schema. Refer to .
  • Specifying is also strongly recommended to ensure that only necessary tables are migrated from the Oracle schema.
• Oracle advises against LONG RAW columns and recommends converting them to BLOB. LONG RAW can only store binary values up to 2GB, and only one LONG RAW column per table is supported.

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MOLT Replicator limitations

• Replication modes require connection to the primary instance (PostgreSQL primary, MySQL primary/master, or Oracle primary). MOLT cannot obtain replication checkpoints or transaction metadata from replicas.
• Running DDL on the source or target while replication is in progress can cause replication failures.
• TRUNCATE operations on the source are not captured. Only INSERT, UPDATE, UPSERT, and DELETE events are replicated.
• Changes to virtual columns are not replicated automatically. To migrate these columns, you must define them explicitly with .
• Replication will not work for tables or column names exceeding 30 characters. This is a limitation of Oracle LogMiner.
• The following data types are not supported for replication:
  • User-defined types (UDTs)
  • Nested tables
  • VARRAY
  • LONGBLOB/CLOB columns (over 4000 characters)
• If your Oracle workload executes UPDATE statements that modify only LOB columns, these UPDATE statements are not supported by Oracle LogMiner and will not be replicated.
• If you are using Oracle 11 and execute UPDATE statements on XMLTYPE or LOB columns, those changes are not supported by Oracle LogMiner and will be excluded from ongoing replication.
• If you are migrating LOB columns from Oracle 12c, use AWS DMS Binary Reader instead of LogMiner. Oracle LogMiner does not support LOB replication in 12c.

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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.

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Step 1: Prepare the source database

• Create a dedicated migration user on your source database.
• Configure the source database for replication.

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Create migration user on source database

CREATE USER MIGRATION_USER IDENTIFIED BY 'password';

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Oracle Multitenant (PDB/CDB) user privileges

-- 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_$LOG TO C##MIGRATION_USER;
GRANT SELECT ON V_$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_$DATABASE TO C##MIGRATION_USER;
GRANT SELECT ON V_$LOG_HISTORY 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, FLASHBACK ON migration_schema.tbl TO C##MIGRATION_USER;

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Single-tenant Oracle user privileges

-- 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, FLASHBACK ON migration_schema.tbl TO MIGRATION_USER;

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Configure source database for replication

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Enable ARCHIVELOG and FORCE LOGGING

-- Check current log mode
SELECT log_mode FROM v$database;

-- Enable ARCHIVELOG (requires database restart)
SHUTDOWN IMMEDIATE;
STARTUP MOUNT;
ALTER DATABASE ARCHIVELOG;
ALTER DATABASE OPEN;

-- Verify ARCHIVELOG is enabled
SELECT log_mode FROM v$database; -- Expected: ARCHIVELOG

-- Enable minimal supplemental logging for primary keys
ALTER DATABASE ADD SUPPLEMENTAL LOG DATA (PRIMARY KEY) COLUMNS;

-- Verify supplemental logging status
SELECT supplemental_log_data_min, supplemental_log_data_pk FROM v$database;
-- Expected:
--   SUPPLEMENTAL_LOG_DATA_MIN: IMPLICIT (or YES)
--   SUPPLEMENTAL_LOG_DATA_PK: YES

ALTER DATABASE FORCE LOGGING;

-- Verify FORCE LOGGING is enabled
SELECT force_logging FROM v$database; -- Expected: YES

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Create source sentinel table

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;

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Grant LogMiner privileges

-- 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;

• Query redo logs from LogMiner.
• Retrieve active transaction information to determine the starting point for ongoing replication.
• Update the internal REPLICATOR_SENTINEL table created on the Oracle source schema by the DBA.

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Verify LogMiner privileges

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.ADD_LOGFILE(LOGFILENAME => '/opt/oracle/oradata/ORCLCDB/redo01.log', OPTIONS => DBMS_LOGMNR.NEW);
EXEC DBMS_LOGMNR.ADD_LOGFILE(LOGFILENAME => '/opt/oracle/oradata/ORCLCDB/redo02.log', OPTIONS => DBMS_LOGMNR.ADDFILE);
EXEC DBMS_LOGMNR.ADD_LOGFILE(LOGFILENAME => '/opt/oracle/oradata/ORCLCDB/redo03.log', OPTIONS => DBMS_LOGMNR.ADDFILE);

EXEC DBMS_LOGMNR.START_LOGMNR(
  STARTSCN => 2358840,
  OPTIONS  => DBMS_LOGMNR.DICT_FROM_ONLINE_CATALOG
);

PL/SQL procedure successfully completed.

​

Step 2: Prepare the target database

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Define the target tables

• The source and target table definitions must match. Review to understand which source types can be mapped to CockroachDB types. For example, an Oracle 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.
  • By default, table and column names are case-insensitive in MOLT Fetch. If using the 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.
  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 .

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Schema Conversion Tool

1. Upload a source .sql file to convert the syntax and identify in the table definitions.
2. 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 .

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Drop constraints and indexes

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

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Create the SQL user

CREATE USER crdb_user WITH PASSWORD 'password';

GRANT ALL ON DATABASE defaultdb TO crdb_user;

GRANT SYSTEM MODIFYCLUSTERSETTING TO crdb_user;

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

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

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 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;

ALTER TABLE migration_schema.table1 OWNER TO crdb_user;

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IMPORT INTO privileges

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

GRANT SYSTEM EXTERNALIOIMPLICITACCESS TO crdb_user;

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COPY FROM privileges

GRANT admin TO crdb_user;

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Set session variable

ALTER ROLE crdb_user SET statement_timeout = '0s';

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Replication privileges

ALTER USER crdb_user CREATEDB;

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Configure GC TTL

ALTER DATABASE defaultdb CONFIGURE ZONE USING gc.ttlseconds = 86400;

ALTER DATABASE defaultdb CONFIGURE ZONE USING gc.ttlseconds = 300;

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Migrate each phase

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Step 3: Load data into CockroachDB

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

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Configure MOLT Fetch

• : 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.

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

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Run MOLT Fetch

1. Issue the command to move the source data to 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 also limits the migration to a single schema and filters three specific tables to migrate. The defaults to IMPORT INTO. The command assumes an Oracle Multitenant (CDB/PDB) source. 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
   
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":"backfillFromSCN=26685444,scn=26685786","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":"backfillFromSCN=26685444,scn=26685786","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":"backfillFromSCN=26685444,scn=26685786","net_duration_ms":6752.847625,"net_duration":"000h 00m 06s","time":"2024-03-18T12:30:37-04:00","message":"fetch complete"}
   
   This message shows the appropriate values for the --backfillFromSCN and --scn flags to use when starting Replicator:
   --backfillFromSCN 26685444
   --scn 26685786
   

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Continue MOLT Fetch after an interruption

{"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)"}

• 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.

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 \
--ignore-replication-check \
--fetch-id f5cb422f-4bb4-4bbd-b2ae-08c4d00d1e7c \
--continuation-file-name shard_01_part_00000001.csv.gz

​

Step 4: Verify the initial data load

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Run MOLT Verify

1. 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'
   
   With Oracle Multitenant deployments, while --source-cdb is required for fetch, it 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 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 5: Finalize the target schema

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Add constraints and indexes

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Step 6: Begin forward replication

• Configure MOLT Replicator with the flags needed for your migration.
• Start MOLT Replicator.
• Understand how to continue replication after an interruption.

​

Configure MOLT Replicator (forward replication)

• Replication connection strings: Specify URL-encoded source and target database connections.
• Replicator flags: Specify required and optional flags to configure replicator behavior.
• Tuning parameters: Optimize replication performance and resource usage.
• Replicator metrics: Monitor replication progress and performance.

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Replication connection strings

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

--sourcePDBConn 'oracle://{username}:{password}@{host}:{port}/{pdb_service_name}'

--targetConn 'postgresql://{username}:{password}@{host}:{port}/{database}'

​

Replicator flags

​

Tuning parameters

​

Replicator metrics

--metricsAddr :30005

​

Start MOLT Replicator (forward replication)

replicator oraclelogminer \
--sourceConn $SOURCE \
--sourcePDBConn $SOURCE_PDB \
--targetConn $TARGET \
--sourceSchema MIGRATION_USER \
--targetSchema defaultdb.migration_schema \
--backfillFromSCN 26685444 \
--scn 26685786 \
--stagingSchema defaultdb._replicator \
--stagingCreateSchema \
--metricsAddr :30005 \
--userscript table_filter.ts \
-v

​

Check that replication is working

DEBUG  [2025-07-03T15:55:12-05:00] registered xid 0f001f0040060000
DEBUG  [2025-07-03T15:55:12-05:00] registered xid 0b001f00bb090000

DEBUG  [2025-07-03T15:55:12-05:00] upserted rows                                 conflicts=0 duration=2.620009ms proposed=13 target="\"molt_movies\".\"USERS\".\"CUSTOMER_CONTACT\"" upserted=13
DEBUG  [2025-07-03T15:55:12-05:00] upserted rows                                 conflicts=0 duration=2.212807ms proposed=16 target="\"molt_movies\".\"USERS\".\"CUSTOMER_DEVICE\"" upserted=16

​

Continue MOLT Replicator after an interruption (forward replication)

replicator oraclelogminer \
--sourceConn $SOURCE \
--sourcePDBConn $SOURCE_PDB \
--sourceSchema MIGRATION_USER \
--targetSchema defaultdb.migration_schema \
--targetConn $TARGET \
--stagingSchema defaultdb._replicator \
--metricsAddr :30005 \
--userscript table_filter.ts \
-v

​

Step 7: Stop application traffic

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

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

​

Step 8: Stop forward replication

1. Wait for replication to drain, which means that all transactions that occurred on the source database have been fully processed and replicated. There are several 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 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]))
     
   • You can also check Prometheus metrics associated with replication lag, including .
2. Cancel replication by entering ctrl-c to issue a SIGTERM signal. This returns an exit code 0.

​

Step 9: Verify the replicated data

​

Step 10: Begin failback replication

• Prepare both databases for failback replication
• Configure MOLT Replicator with the flags needed for your migration.
• Start MOLT Replicator.
• Understand how to continue replication after an interruption.

​

Prepare your source and target databases for failback replication

​

Prepare the CockroachDB cluster

SET CLUSTER SETTING kv.rangefeed.enabled = true;

SET CLUSTER SETTING kv.rangefeed.closed_timestamp_refresh_interval = '250ms';

SET CLUSTER SETTING kv.closed_timestamp.target_duration = '1s';

SET CLUSTER SETTING kv.rangefeed.concurrent_catchup_iterators = 64;

​

Grant target database user permissions

-- Grant INSERT, UPDATE, and FLASHBACK on tables to fail back to
GRANT SELECT, INSERT, UPDATE, FLASHBACK ON migration_schema.employees TO MIGRATION_USER;
GRANT SELECT, INSERT, UPDATE, FLASHBACK ON migration_schema.payments TO MIGRATION_USER;
GRANT SELECT, INSERT, UPDATE, FLASHBACK ON migration_schema.orders TO MIGRATION_USER;

​

Create a CockroachDB changefeed

1. Get the current logical timestamp from CockroachDB, after ensuring that forward replication has fully drained:
   SELECT cluster_logical_timestamp();
   
       cluster_logical_timestamp
   ----------------------------------
     1759246920563173000.0000000000
   
2. Create the CockroachDB changefeed pointing to the MOLT Replicator webhook endpoint. Use cursor to specify the logical timestamp from the preceding step. For details on the webhook sink URI, refer to .
   Explicitly set a default 10s value in the CREATE CHANGEFEED statement. This value ensures that the webhook can report failures in inconsistent networking situations and make crash loops more visible.
   The webhook URL path specifies the schema name on the target Oracle database. Oracle capitalizes identifiers by default. For example, /MIGRATION_SCHEMA routes changes to the MIGRATION_SCHEMA schema.
   CREATE CHANGEFEED FOR TABLE employees, payments, orders \
   INTO 'webhook-https://replicator-host:30004/MIGRATION_SCHEMA?client_cert={base64_encoded_cert}&client_key={base64_encoded_key}&ca_cert={base64_encoded_ca}' \
   WITH updated, resolved = '250ms', min_checkpoint_frequency = '250ms', initial_scan = 'no', cursor = '1759246920563173000.0000000000', webhook_sink_config = '{"Flush":{"Bytes":1048576,"Frequency":"1s"}}', webhook_client_timeout = '10s';
   
   The output shows the job ID:
           job_id
   -----------------------
     1101234051444375553
   
   Ensure that only one changefeed points to MOLT Replicator at a time to avoid mixing streams of incoming data.
3. Monitor the changefeed status, specifying the job ID:
   SHOW CHANGEFEED JOB 1101234051444375553;
   
           job_id        | ... | status  |              running_status               | ...
   ----------------------+-----+---------+-------------------------------------------+----
     1101234051444375553 | ... | running | running: resolved=1759246920563173000,0 | ...
   
   To confirm the changefeed is active and replicating changes to the target database, check that status is running and running_status shows running: resolved={timestamp}.
   running: resolved may be reported even if data isn’t being sent properly. This typically indicates incorrect host/port configuration or network connectivity issues.
4. Verify that Replicator is reporting incoming HTTP requests from the changefeed. To do so, check the MOLT Replicator logs. Since you enabled debug logging with -v, you should see periodic HTTP request successes:
   DEBUG  [2025-08-25T11:52:47-05:00]  httpRequest="&{0x14000b068c0 45 200 3 9.770958ms   false false}"
   DEBUG  [2025-08-25T11:52:48-05:00]  httpRequest="&{0x14000d1a000 45 200 3 13.438125ms   false false}"
   
   These debug messages confirm successful changefeed connections to MOLT Replicator. You can disable verbose logging after verifying the connection.

​

Configure MOLT Replicator (failback replication)

• Connection strings: Specify URL‑encoded source and target connections.
• TLS certificate and key: Configure secure TLS connections.
• Replicator flags: Specify required and optional flags to configure replicator behavior.
• Tuning parameters: Optimize failback performance and resource usage.
• Replicator metrics: Monitor failback replication performance.

​

Replication connection strings

--sourceConn 'postgresql://{username}:{password}@{host}:{port}/{database}'

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

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.
  • Provide your connection strings as environment variables. For example:
    export SOURCE="postgres://migration_user:a%2452%26@localhost:5432/migration_db?sslmode=verify-full"
    export TARGET="postgres://root@localhost:26257/defaultdb?sslmode=verify-full"
    
    Afterward, reference the environment variables in MOLT commands:
    --source $SOURCE
    --target $TARGET
    
  • If possible, use an external secrets manager to load the environment variables from stored secrets.
• Use TLS-enabled connection strings to encrypt data in transit from MOLT to the database. When using TLS certificates, ensure certificate files are accessible to the MOLT binary on the same machine. For example, a PostgreSQL connection string with TLS certificates:
  postgresql://migration_user@db.example.com:5432/appdb?sslmode=verify-full&sslrootcert=/etc/migration_db/certs/ca.pem&sslcert=/etc/migration_db/certs/client.crt&sslkey=/etc/migration_db/certs/client.key
  
• URL-encode connection strings for the source database and 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 --password 'a$52&'
    
    Use the encoded password in your connection string. For example:
    postgres://migration_user:a%2452%26@localhost:5432/migration_db
    
• Remove sslmode=disable from production connection strings.

​

TLS certificate and key

replicator start \
... \
--tlsCertificate ./certs/server.crt \
--tlsPrivateKey ./certs/server.key

base64 -i ./client.crt | jq -R -r '@uri'
base64 -i ./client.key | jq -R -r '@uri'
base64 -i ./ca.crt | jq -R -r '@uri'

CREATE CHANGEFEED FOR TABLE table1, table2
INTO 'webhook-https://host:port/database/schema?client_cert={base64_and_url_encoded_cert}&client_key={base64_and_url_encoded_key}&ca_cert={base64_and_url_encoded_ca}'
WITH ...;

​

Replicator flags

• The staging schema is first created during with .
• When configuring a secure changefeed for failback, you must include and , which specify the paths to the server certificate and private key for the webhook sink connection.

​

Tuning parameters

​

Replicator metrics

--metricsAddr :30005

​

Start MOLT Replicator (failback replication)

replicator start \
--targetConn $TARGET \
--stagingConn $STAGING \
--stagingSchema defaultdb._replicator \
--metricsAddr :30005 \
--bindAddr :30004 \
--tlsCertificate ./certs/server.crt \
--tlsPrivateKey ./certs/server.key \
-v

​

Continue MOLT Replicator after an interruption (failback replication)

replicator oraclelogminer \
--sourceConn $SOURCE \
--sourcePDBConn $SOURCE_PDB \
--sourceSchema MIGRATION_USER \
--targetSchema defaultdb.migration_schema \
--targetConn $TARGET \
--stagingSchema defaultdb._replicator \
--metricsAddr :30005 \
--userscript table_filter.ts \
-v

​

Step 11: Cut over application traffic

​

Modify application code

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

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

kubectl scale deploy/app-eu --replicas=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

​

Step 12: Stop failback replication

1. Wait for replication to drain, which means that all transactions that occurred on the source database have been fully processed and replicated. There are several 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 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]))
     
   • You can also check Prometheus metrics associated with replication lag, including .
2. Cancel replication by entering ctrl-c to issue a SIGTERM signal. This returns an exit code 0.

​

Repeat for each phase

​

See also