> ## Documentation Index > Fetch the complete documentation index at: https://cockroachlabs.mintlify.site/llms.txt > Use this file to discover all available pages before exploring further. # Delta Migration from PostgreSQL export const InternalLink = ({version, path = "", children, ...props}) => { let detectedVersion = version || "stable"; if (typeof window !== 'undefined' && !version) { const match = window.location.pathname.match(/\/docs\/([^/]+)/); if (match) { detectedVersion = match[1]; } } const normalizedPath = path.startsWith("/") ? path.slice(1) : path; return {children} ; }; A *Delta Migration* uses an initial data load, followed by continuous replication, to migrate data to CockroachDB. In this approach, you migrate most application data to the target using MOLT Fetch **before** stopping application traffic to the source database. You then use MOLT Replicator to keep the target database in sync with any changes in the source database (the migration *delta*), before finally halting traffic to the source and cutting over to the target after schema finalization and data verification. * All source data is migrated to the target at once. * This approach utilizes continuous replication. * Failback replication is supported, though this example will not use it. See Phased Delta Migration with Failback Replication for an example of a migration that uses failback replication. This approach is best for production environments that need to minimize system downtime. 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. Delta migration flow

## Example scenario You have a small (300 GB) database that provides the data store for a web application. You want to migrate the entirety of this database to a new CockroachDB cluster. Business cannot accommodate a full maintenance window, but it can accommodate a brief (\<60 second) halt in traffic. The application runs on a Kubernetes cluster. **Estimated system downtime:** 3-5 minutes. ## Before the migration * Install the MOLT (Migrate Off Legacy Technology) tools. * Review the MOLT Fetch and MOLT Replicator documentation. * Develop a migration plan and prepare for the migration. * **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: ### Limitations #### MOLT Fetch limitations * Only tables with primary key types of `INT`, `FLOAT`, or `UUID` can be sharded with `--export-concurrency`. * `GEOMETRY` and `GEOGRAPHY` types are not supported. * `OID LOB` types in PostgreSQL are not supported, although similar types like `BYTEA` are supported. #### 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 transformation rules. #### 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` flag. * MOLT Verify checks for collation mismatches on primary key columns. This may cause validation to fail when a `STRING` is used as a primary key and the source and target databases are using different collations. * MOLT Verify might give an error in case of schema changes on either the source or target database. * Geospatial types 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). * [Configure the source database for replication](#configure-source-database-for-replication). ### 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](#source-connection-string). ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} CREATE USER migration_user WITH PASSWORD 'password'; ``` Grant the user privileges to connect, view schema objects, and select the tables you migrate. ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} 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; ``` Grant the `SUPERUSER` role to the user (recommended for replication configuration): ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} ALTER USER migration_user WITH SUPERUSER; ``` Alternatively, grant the following permissions to create replication slots, access replication data, create publications, and add tables to publications: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} ALTER USER migration_user WITH LOGIN REPLICATION; GRANT CREATE ON DATABASE migration_db TO migration_user; ALTER TABLE migration_schema.table_name OWNER TO migration_user; ``` Run the `ALTER TABLE` command for each table to replicate. ### Configure source database for replication Connect to the primary instance (PostgreSQL primary, MySQL primary/master, or Oracle primary), **not** a replica. Replicas cannot provide the necessary replication checkpoints and transaction metadata required for ongoing replication. Verify that you are connected to the primary server: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} SELECT pg_is_in_recovery(); ``` You should get a false result: ``` pg_is_in_recovery ------------------- f ``` Enable logical replication by setting `wal_level` to `logical` in `postgresql.conf` or in the SQL shell. For example: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} ALTER SYSTEM SET wal_level = 'logical'; ``` ## Step 2: Prepare the target database ### Define the target tables Convert the source table definitions into CockroachDB-compatible equivalents. CockroachDB supports the PostgreSQL wire protocol and is largely compatible with PostgreSQL syntax. * The source and target table definitions must **match**. Review Type mapping 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: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} CREATE SCHEMA migration_schema; CREATE TABLE migration_schema.tbl (pk INT PRIMARY KEY); ``` * MOLT Fetch can automatically define matching CockroachDB tables using the `drop-on-target-and-recreate` option. * If you define the target tables manually, review how MOLT Fetch handles type mismatches. You can use the [MOLT Schema Conversion Tool](#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 guidance on indexing with sequential keys. If a sequential key is necessary in your CockroachDB table, you must create it manually, after using MOLT Fetch to load and replicate the data. * 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) converts source table definitions to CockroachDB-compatible syntax. 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 table definitions. 2. Import the converted table definitions to a CockroachDB cluster: * When migrating to CockroachDB Cloud, the Schema Conversion Tool automatically applies the converted table definitions to a new Cloud database. * When migrating to a self-hosted CockroachDB cluster, export a converted DDL file and pipe the data definition language (DDL) directly into `cockroach sql`. #### 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](#table-handling-mode)) Do **not** drop `PRIMARY KEY` constraints. You can [recreate the constraints and indexes after loading the data](#add-constraints-and-indexes). ### 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](#target-connection-string)): ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} CREATE USER crdb_user WITH PASSWORD 'password'; ``` Grant database-level privileges for schema creation within the target database: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} GRANT ALL ON DATABASE defaultdb TO crdb_user; ``` Grant permission to modify cluster settings: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} GRANT SYSTEM MODIFYCLUSTERSETTING TO crdb_user; ``` Grant usage on schemas being migrated: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} 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. ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} 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`](#table-handling-mode) will not be used), grant the following privileges on the schema: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} 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: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} 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`](#table-handling-mode), 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: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} ALTER TABLE migration_schema.table1 OWNER TO crdb_user; ``` Depending on the MOLT Fetch data load mode you will use, grant the necessary privileges to run either [`IMPORT INTO`](#import-into-privileges) or [`COPY FROM`](#copy-from-privileges) 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: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} GRANT SELECT, INSERT, DROP ON ALL TABLES IN SCHEMA migration_schema TO crdb_user; ``` If you plan to use [cloud storage with implicit authentication](#cloud-storage-authentication) for data load, grant the `EXTERNALIOIMPLICITACCESS` system-level privilege: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} GRANT SYSTEM EXTERNALIOIMPLICITACCESS TO crdb_user; ``` #### `COPY FROM` privileges Grant `admin` privileges to the user: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} GRANT admin TO crdb_user; ``` #### Set session variable Ensure that the `statement_timeout` session variable is set to `0s` for the user: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} ALTER ROLE crdb_user SET statement_timeout = '0s'; ``` #### Replication privileges Grant permissions to create the staging schema for replication: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} ALTER USER crdb_user CREATEDB; ``` ### Configure GC TTL Before starting the [initial data load](#run-molt-fetch), configure the garbage collection (GC) TTL on the source CockroachDB cluster to ensure that historical data remains available when replication begins. The GC TTL must be long enough to cover the full duration of the data load. Increase the GC TTL before starting the data load. For example, to set the GC TTL to 24 hours: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} ALTER DATABASE defaultdb CONFIGURE ZONE USING gc.ttlseconds = 86400; ``` The GC TTL duration must be higher than your expected time for the initial data load. Once replication has started successfully (which automatically protects its own data range), you can restore the GC TTL to its original value. For example, to restore to 5 minutes: ```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} ALTER DATABASE defaultdb CONFIGURE ZONE USING gc.ttlseconds = 300; ``` For details, refer to Protect Changefeed Data from Garbage Collection. ## Step 3: Load data into CockroachDB In this step, you will: * [Configure MOLT Fetch with the flags needed for your migration](#configure-molt-fetch). * [Run MOLT Fetch](#run-molt-fetch). * [Understand how to continue a load after an interruption](#continue-molt-fetch-after-an-interruption). ### Configure MOLT Fetch The MOLT Fetch documentation includes detailed information about how to configure MOLT Fetch, and how to monitor MOLT Fetch metrics. When you run `molt fetch`, you can configure the following options for data load: * Specify source and target databases: Specify URL‑encoded source and target connections. * Select data to migrate: Specify schema and table names to migrate. * Define intermediate file storage: Export data to cloud storage or a local file server. * Define fetch mode: Specifies whether data will only be loaded into/from intermediate storage. * Shard tables: Divide larger tables into multiple shards during data export. * Data load mode: Choose between `IMPORT INTO` and `COPY FROM`. * Table handling mode: Determine how existing target tables are initialized before load. * Define data transformations: Define any row-level transformations to apply to the data before it reaches the target. * Monitor fetch metrics: Configure metrics collection during initial data load. Read through the documentation to understand how to configure your `molt fetch` command and its flags. Follow best practices, especially those related to security. At minimum, the `molt fetch` command should include the source, target, data path, and `--ignore-replication-check` flags: ```shell theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} molt fetch \ --source $SOURCE \ --target $TARGET \ --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 initial load of the source data. 1. Issue the MOLT Fetch command to move the source data to CockroachDB. This example command passes the source and target connection strings [as environment variables](#secure-connections), writes [intermediate files](#intermediate-file-storage) to S3 storage, and uses the `truncate-if-exists` [table handling mode](#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`. You **must** include `--pglogical-replication-slot-name` and `--pglogical-publication-and-slot-drop-and-recreate` to automatically create the publication and replication slot during the data load. ```shell theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} 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 molt_slot \ --pglogical-publication-and-slot-drop-and-recreate ``` 2. Check the output to observe `fetch` progress. If you included the `--pglogical-replication-slot-name` and `--pglogical-publication-and-slot-drop-and-recreate` flags, a publication named `molt_fetch` is automatically created: ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"level":"info","time":"2025-02-10T14:28:11-05:00","message":"dropping and recreating publication molt_fetch"} ``` A `starting fetch` message indicates that the task has started: ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"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`: ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"level":"info","table":"migration_schema.employees","time":"2025-02-10T14:28:11-05:00","message":"data extraction phase starting"} ``` ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"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: ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"level":"info","table":"migration_schema.employees","time":"2025-02-10T14:28:12-05:00","message":"starting data import on target"} ``` ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"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: ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"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. ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"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 continue the task from the *continuation point* 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 cloud or local storage. * The *initial load* of source data into the target CockroachDB database is incomplete. * The load uses [`IMPORT INTO` rather than `COPY FROM`](#data-load-mode). Only one fetch ID and set of continuation tokens, each token corresponding to a table, are active at any time. See List active continuation tokens. The following command reattempts the data load starting from a specific continuation file, but you can also use individual continuation tokens to reattempt the data load for individual tables. ```shell theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} 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 4: Verify the initial data load Use MOLT Verify to confirm that the source and target data is consistent. This ensures that the data load was successful. ### Run MOLT Verify 1. Run the MOLT Verify command, specifying the source and target [connection strings](#connection-strings) and the tables to validate. ```shell theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} molt verify \ --source $SOURCE \ --target $TARGET \ --table-filter 'employees|payments|orders' ``` 2. Check the output to observe `verify` progress. A `verification in progress` indicates that the task has started: ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"level":"info","time":"2025-02-10T15:35:04-05:00","message":"verification in progress"} ``` `starting verify` messages are written for each specified table: ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"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 MOLT Verify page. ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"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: ```json theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} {"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 ### Add constraints and indexes Add any constraints or indexes that you previously [removed from the CockroachDB schema](#drop-constraints-and-indexes) to facilitate data load. If you used the `--table-handling drop-on-target-and-recreate` option for data load, only `PRIMARY KEY` and `NOT NULL` constraints are preserved. You **must** manually recreate all other constraints and indexes. For the appropriate SQL syntax, refer to `ALTER TABLE ... ADD CONSTRAINT` and `CREATE INDEX`. Review the best practices for creating secondary indexes on CockroachDB. ## Step 6: Begin forward replication In this step, you will: * [Configure MOLT Replicator with the flags needed for your migration](#configure-molt-replicator). * [Start MOLT Replicator](#start-molt-replicator). * [Understand how to continue replication after an interruption](#continue-molt-replicator-after-an-interruption). ### Configure MOLT Replicator When you run `replicator`, you can configure the following options for replication: * [Replication connection strings](#replication-connection-strings): Specify URL-encoded source and target database connections. * [Replicator flags](#replicator-flags): Specify required and optional flags to configure replicator behavior. * [Tuning parameters](#tuning-parameters): Optimize replication performance and resource usage. * [Replicator metrics](#replicator-metrics): Monitor replication progress and performance. #### Replication connection strings MOLT Replicator uses `--sourceConn` and `--targetConn` to specify the source and target database connections. `--sourceConn` specifies the connection string of the source database: ``` --sourceConn 'postgresql://{username}:{password}@{host}:{port}/{database}' ``` `--targetConn` specifies the target CockroachDB connection string: ``` --targetConn 'postgresql://{username}:{password}@{host}:{port}/{database}' ``` Follow best practices for securing connection strings. Refer to [Secure connections](#secure-connections). #### Replicator flags Configure the following MOLT Replicator flags for continuous replication. For details on all available flags, refer to Replicator Flags. | Flag | Description | | ------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | `--slotName` | **Required.** PostgreSQL replication slot name. Must match the slot name specified with `--pglogical-replication-slot-name` in the [MOLT Fetch command](#start-fetch). | | `--publicationName` | **Required.** PostgreSQL publication name. Must match the publication name created either manually or automatically with `--pglogical-publication-and-slot-drop-and-recreate` in the [MOLT Fetch command](#start-fetch). | | `--targetSchema` | **Required.** Target schema name on CockroachDB where tables will be replicated. Schema name must be fully qualified in the format `database.schema`. | | `--stagingSchema` | **Required.** Staging schema name on CockroachDB for replication metadata and checkpoints. Schema name must be fully qualified in the format `database.schema`. | | `--stagingCreateSchema` | Automatically create the staging schema if it does not exist. Include this flag when starting replication for the first time. | | `--metricsAddr` | Enable Prometheus metrics at a specified `{host}:{port}`. Metrics are served at `http://{host}:{port}/_/varz`. | | `--userscript` | Path to a userscript that enables data filtering, routing, or transformations. For examples, refer to Userscript Cookbook. | #### Tuning parameters Configure the following `replicator` flags to optimize replication throughput and resource usage. Test different combinations in a pre-production environment to find the optimal balance of stability and performance for your workload. The following parameters apply to PostgreSQL, Oracle, and CockroachDB (failback) sources. | Flag | Description | | --------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `--parallelism` | Control the maximum number of concurrent target transactions. Higher values increase throughput but require more target connections. Start with a conservative value and increase based on target database capacity. | | `--flushSize` | Balance throughput and latency. Controls how many mutations are batched into each query to the target. Increase for higher throughput at the cost of higher latency. | | `--targetApplyQueueSize` | Control memory usage during operation. Increase to allow higher throughput at the expense of memory; decrease to apply backpressure and limit memory consumption. | | `--targetMaxPoolSize` | Set larger than `--parallelism` by a safety factor to avoid exhausting target pool connections. Replicator enforces setting parallelism to 80% of this value. | | `--collapseMutations` | Reduce the number of queries to the target by combining multiple mutations on the same primary key within each batch. Disable only if exact mutation order matters more than end state. | | `--enableParallelApplies` | Improve apply throughput for independent tables and table groups that share foreign key dependencies. Increases memory and target connection usage, so ensure you increase `--targetMaxPoolSize` or reduce `--parallelism`. | | `--flushPeriod` | Set to the maximum allowable time between flushes (for example, `10s` if data must be applied within 10 seconds). Works with `--flushSize` to control when buffered mutations are committed to the target. | | `--quiescentPeriod` | Lower this value if constraint violations resolve quickly on your workload to make retries more frequent and reduce latency. Do not lower if constraint violations take time to resolve. | | `--scanSize` | Applies to failback (`replicator start`) scenarios **only**. Balance memory usage and throughput. Increase to read more rows at once from the CockroachDB staging cluster for higher throughput, at the cost of memory pressure. Decrease to reduce memory pressure and increase stability. | #### Replicator metrics MOLT Replicator metrics are not enabled by default. Enable Replicator metrics by specifying the `--metricsAddr` flag with a port (or `host:port`) when you start Replicator. This exposes Replicator metrics at `http://{host}:{port}/_/varz`. For example, the following flag exposes metrics on port `30005`: ``` --metricsAddr :30005 ``` For guidelines on using and interpreting replication metrics, refer to Replicator Metrics. ### Start MOLT Replicator With initial load complete, start replication of ongoing changes on the source to CockroachDB using MOLT Replicator. MOLT Fetch captures a consistent point-in-time checkpoint at the start of the data load (shown as `cdc_cursor` in the fetch output). Starting replication from this checkpoint ensures that all changes made during and after the data load are replicated to CockroachDB, preventing data loss or duplication. The following steps use the checkpoint values from the fetch output to start replication at the correct position. 1. Run the `replicator` command, using the same slot name that you specified with `--pglogical-replication-slot-name` and the publication name created by `--pglogical-publication-and-slot-drop-and-recreate` in the [Fetch command](#run-molt-fetch). Use `--stagingSchema` to specify a unique name for the staging database, and include `--stagingCreateSchema` to have MOLT Replicator automatically create the staging database: ```shell theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} replicator pglogical \ --sourceConn $SOURCE \ --targetConn $TARGET \ --targetSchema defaultdb.migration_schema \ --slotName molt_slot \ --publicationName molt_fetch \ --stagingSchema defaultdb._replicator \ --stagingCreateSchema \ --metricsAddr :30005 \ -v ``` #### Check that replication is working 1. Verify that Replicator is processing changes successfully. To do so, check the MOLT Replicator logs. Since you enabled debug logging with `-v`, you should see connection and row processing messages: You should see periodic primary keepalive messages: ``` DEBUG [2025-08-25T14:38:10-05:00] primary keepalive received ReplyRequested=false ServerTime="2025-08-25 14:38:09.556773 -0500 CDT" ServerWALEnd=0/49913A58 DEBUG [2025-08-25T14:38:15-05:00] primary keepalive received ReplyRequested=false ServerTime="2025-08-25 14:38:14.556836 -0500 CDT" ServerWALEnd=0/49913E60 ``` When rows are successfully replicated, you should see debug output like the following: ``` DEBUG [2025-08-25T14:40:02-05:00] upserted rows conflicts=0 duration=7.855333ms proposed=1 target="\"molt\".\"public\".\"tbl1\"" upserted=1 DEBUG [2025-08-25T14:40:02-05:00] progressed to LSN: 0/49915DD0 ``` These messages confirm successful replication. You can disable verbose logging after verifying the connection. ### Continue MOLT Replicator after an interruption Run the MOLT Replicator `pglogical` command using the same `--stagingSchema` value from your [initial replication command](#start-molt-replicator). Be sure to specify the same `--slotName` value that you used during your [initial replication command](#start-molt-replicator). The replication slot on the source PostgreSQL database automatically tracks the LSN (Log Sequence Number) checkpoint, so replication will resume from where it left off. ```shell theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} replicator pglogical \ --sourceConn $SOURCE \ --targetConn $TARGET \ --targetSchema defaultdb.migration_schema \ --slotName molt_slot \ --stagingSchema defaultdb._replicator \ --metricsAddr :30005 \ -v ``` Replication resumes from the last checkpoint without performing a fresh load. Monitor the metrics endpoint at `http://localhost:30005/_/varz` to track replication progress. ## Step 7: Stop application traffic Once the inital data load has been verified and the target schema has been finalized, it's time to begin the cutover process. First, stop application traffic to the source. Scale down the Kubernetes cluster to zero pods. ```shell theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} kubectl scale deployment app --replicas=0 ``` Application downtime begins now. It is strongly recommended that you perform a dry run of this migration in a test environment. This will allow you to practice using the MOLT tools in real time, and it will give you an accurate sense of how long application downtime might last. ## Step 8: Stop forward replication Before you can cut over traffic to the target, the changes to the source database need to finish being written to the target. Once the source is no longer receiving write traffic, MOLT Replicator will take some seconds to finish replicating the final changes. This is known as *drainage*. 1. Wait for replication to drain, which means that all transactions that occurred on the source database have been fully processed and replicated. There are several ways to determine that replication has fully drained: * When replication is caught up, you will not see new `upserted rows` logs. * If you set up the replication metrics endpoint with `--metricsAddr` in the preceding steps, metrics are available at: ``` http://{host}:{port}/_/varz ``` Use the following Prometheus alert expression to observe when the combined rate of upserts and deletes is `0` for each schema: ``` sum by (schema) (rate(apply_upserts_total[$__rate_interval]) + rate(apply_deletes_total[$__rate_interval])) ``` * You can also check Prometheus metrics associated with replication lag, including `target_apply_transaction_lag_seconds`, `core_source_lag_seconds`, and `source_commit_to_apply_lag_seconds`. 2. Cancel replication by entering `ctrl-c` to issue a `SIGTERM` signal. This returns an exit code `0`. ## Step 9: Verify the replicated data Repeat [Step 4](#step-4-verify-the-initial-data-load) to verify the updated data. ## Step 10: Cut over application traffic With the target cluster verified and finalized, it's time to resume application traffic. ### Modify application code In the application back end, make sure that the application now directs traffic to the CockroachDB cluster. For example: ```yml theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} env: - name: DATABASE_URL value: postgres://root@localhost:26257/defaultdb?sslmode=verify-full ``` ### Resume application traffic Scale up the Kubernetes deployment to the original number of replicas: ```shell theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}} kubectl scale deployment app --replicas=3 ``` This ends downtime. ## See also * Migration Overview * Migration Considerations * Phased Bulk Load Migration * MOLT Fetch * MOLT Verify * MOLT Fetch Troubleshooting for PostgreSQL * MOLT Replicator Troubleshooting for PostgreSQL