MOLT Fetch

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MOLT Fetch moves data from a source database into CockroachDB as part of a database migration.

MOLT Fetch uses IMPORT INTO or COPY FROM to move the source data to cloud storage (Google Cloud Storage or Amazon S3), a local file server, or local memory. Once the data is exported, MOLT Fetch loads the data onto a target CockroachDB database. For details, see Usage.

Supported databases

The following source databases are currently supported:

• PostgreSQL
• MySQL
• CockroachDB

Installation

To install MOLT Fetch, download the binary that matches your system. To download the latest binary:

For previous binaries, refer to the MOLT version manifest. For release details, see the MOLT changelog.

Setup

Complete the following items before using MOLT Fetch:

• Follow the recommendations in Best practices and Security recommendations.

• Ensure that the source and target schemas are identical, unless you enable automatic schema creation with the 'drop-on-target-and-recreate' option. If you are creating the target schema manually, review the behaviors in Mismatch handling.

• Ensure that the SQL user running MOLT Fetch has SELECT privileges on the source and target CockroachDB databases, along with the required privileges to run IMPORT INTO or COPY FROM (depending on the fetch mode) on CockroachDB, as described on their respective pages.

• If you plan to use continuous replication (using either --ongoing-replication or the CDC cursor):

  • If you are migrating from PostgreSQL, enable logical replication. In postgresql.conf or in the SQL shell, set wal_level to logical.
  • If you are migrating from MySQL, enable GTID consistency. In mysql.cnf, in the SQL shell, or as flags in the mysql start command, set gtid-mode and enforce-gtid-consistency to ON and set binlog_row_metadata to full.

• Percent-encode the connection strings for the source database and CockroachDB. This ensures that the MOLT tools can parse special characters in your password.

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

    molt escape-password 'a$52&'
    

    Substitute the following encoded password in your original connection url string:
    a%2452%26
    

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

    postgres://postgres:a%2452%26@localhost:5432/replicationload
    

• If you plan to use cloud storage for the data migration, follow the steps in Secure cloud storage.

Best practices

• To prevent connections from terminating prematurely during data export, set the following to high values on the source database:

  • Maximum allowed number of connections: MOLT Fetch can export data across multiple connections. The number of connections it will create is the number of shards (--export-concurrency) multiplied by the number of tables (--table-concurrency) being exported concurrently.
  • Maximum lifetime of a connection: This is particularly important for MySQL sources, which can only use a single connection to move data. See the following note.

• If a MySQL database is set as a source, the --table-concurrency and --export-concurrency flags cannot be set above 1. If these values are changed, MOLT Fetch returns an error. This guarantees consistency when moving data from MySQL, due to MySQL limitations. MySQL data is migrated to CockroachDB one table and shard at a time, using WITH CONSISTENT SNAPSHOT transactions.

• To prevent memory outages during data export of tables with large rows, estimate the amount of memory used to export a table:

  --row-batch-size * --export-concurrency * average size of the table rows
  

  If you are exporting more than one table at a time (i.e., --table-concurrency is set higher than 1), add the estimated memory usage for the tables with the largest row sizes. Ensure that you have sufficient memory to run molt fetch, and adjust --row-batch-size accordingly.

• If a table in the source database is much larger than the other tables, filter and export the largest table in its own molt fetch task. Repeat this for each of the largest tables. Then export the remaining tables in another task.

• When using IMPORT INTO mode to load tables into CockroachDB, if the fetch process terminates before the import job completes, the hanging import job on the target database will keep the table offline. To make this table accessible again, manually resume or cancel the job. Then resume molt fetch using continuation, or restart the process from the beginning.

Security recommendations

Cockroach Labs strongly recommends the following:

Secure connections

• Use secure connections to the source and target CockroachDB database whenever possible.
• By default, insecure connections (i.e., sslmode=disable on PostgreSQL; sslmode not set on MySQL) are disallowed. When using an insecure connection, molt fetch returns an error. To override this check, you can enable the --allow-tls-mode-disable flag. Do this only for testing, or if a secure SSL/TLS connection to the source or target database is not possible.

Connection strings

• Avoid plaintext connection strings.
• Provide your connection strings as environment variables.

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

  For example, to export connection strings as environment variables:

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

  Afterward, to pass the environment variables in molt fetch commands:

  molt fetch \
  --source "$SOURCE" \
  --target "$TARGET" \
  --table-filter 'employees' \
  --bucket-path 's3://molt-test' \
  --table-handling truncate-if-exists
  

Secure cloud storage

• When using cloud storage for your intermediate store, ensure that access control is properly configured.

  • If you are using Amazon S3 for cloud storage:

    • Ensure that the environment variable and access tokens are set appropriately in the terminal running molt fetch. For example:

      export AWS_REGION='us-east-1'
      export AWS_SECRET_ACCESS_KEY='key'
      export AWS_ACCESS_KEY_ID='id'
      

    • Alternatively, set the --use-implicit-auth flag to use implicit authentication.

    • Ensure the S3 bucket is created and accessible by CockroachDB.

  • If you are using Google Cloud Storage for cloud storage:

    • Ensure that your local environment is authenticated using Application Default Credentials:

      Using gcloud:

      gcloud init
      gcloud auth application-default login
      

      Using the environment variable:

      export GOOGLE_APPLICATION_CREDENTIALS={path_to_cred_json}
      

    • Alternatively, set the --use-implicit-auth flag to use implicit authentication.

    • Ensure the Google Cloud Storage bucket is created and accessible by CockroachDB.

• Do not use public cloud storage in production.

Perform a dry run

To verify that your connections and configuration work properly, run MOLT Fetch in a staging environment before moving any data in production. Use a test or development environment that is as similar as possible to production.

Commands

Subcommands

Flags

Global flags

tokens list flags

Usage

The following sections describe how to use the molt fetch flags.

Source and target databases

--source specifies the connection string of the source database.

PostgreSQL or CockroachDB:

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

MySQL:

--source 'mysql://{username}:{password}@{protocol}({host}:{port})/{database}'

--target specifies the CockroachDB connection string:

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

Fetch mode

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

By default, MOLT Fetch uses IMPORT INTO:

• IMPORT INTO mode achieves the highest throughput, but requires taking the tables offline to achieve its import speed. Tables are taken back online once an import job completes successfully. See Best practices.
• IMPORT INTO mode supports compression using the --compression flag, which reduces the amount of storage used.

--use-copy configures MOLT Fetch to use COPY FROM:

• COPY FROM mode enables your tables to remain online and accessible. However, it is slower than using IMPORT INTO.
• COPY FROM mode does not support compression.

Data path

MOLT Fetch can move the source data to CockroachDB via cloud storage, a local file server, or directly without an intermediate store.

Cloud storage

--bucket-path specifies that MOLT Fetch should write intermediate files to a path within a Google Cloud Storage or Amazon S3 bucket to which you have the necessary permissions. For example:

Google Cloud Storage:

--bucket-path 'gs://migration/data/cockroach'

Amazon S3:

--bucket-path 's3://migration/data/cockroach'

Cloud storage can be used with either the IMPORT INTO or COPY FROM modes.

Local file server

--local-path specifies that MOLT Fetch should write intermediate files to a path within a local file server. local-path-listen-addr specifies the address of the local file server. For example:

--local-path /migration/data/cockroach
--local-path-listen-addr 'localhost:3000'

In some cases, CockroachDB will not be able to use the local address specified by --local-path-listen-addr. This will depend on where CockroachDB is deployed, the runtime OS, and the source dialect.

For example, if you are migrating to CockroachDB Cloud, such that the Cloud cluster is in a different physical location than the machine running molt fetch, then CockroachDB cannot reach an address such as localhost:3000. In these situations, use --local-path-crdb-access-addr to specify an address for the local file server that is reachable by CockroachDB. For example:

--local-path /migration/data/cockroach
--local-path-listen-addr 'localhost:3000'
--local-path-crdb-access-addr '44.55.66.77:3000'

A local file server can be used with either the IMPORT INTO or COPY FROM modes.

Direct copy

--direct-copy specifies that MOLT Fetch should use COPY FROM to move the source data directly to CockroachDB without an intermediate store:

• Because the data is held in memory, the machine must have sufficient RAM for the data currently in flight:

  average size of each row * --row-batch-size * --export-concurrency * --table-concurrency
  

• Direct copy mode does not support compression or continuation.

• The --use-copy flag is redundant with --direct-copy.

Schema and table selection

By default, MOLT Fetch moves all data from the --source database to CockroachDB. Use the following flags to move a subset of data.

--schema-filter specifies a range of schema objects to move to CockroachDB, formatted as a POSIX regex string. For example, to move every table in the source database's public schema:

--schema-filter 'public'

--table-filter specifies a range of tables to move to CockroachDB, formatted as a POSIX regex string. For example, to move every table in the source database that has "user" in the title:

--table-filter '.*user.*'

Target table handling

--table-handling defines how MOLT Fetch loads data on the CockroachDB tables that match the selection.

To load the data without changing the existing data in the tables, use 'none':

--table-handling 'none'

To truncate tables before loading the data, use 'truncate-if-exists':

--table-handling 'truncate-if-exists'

To drop existing tables and create new tables before loading the data, use 'drop-on-target-and-recreate':

--table-handling 'drop-on-target-and-recreate'

When using the 'drop-on-target-and-recreate' option, MOLT Fetch creates a new CockroachDB table to load the source data if one does not already exist. To guide the automatic schema creation, you can explicitly map source types to CockroachDB types.

Mismatch handling

If either 'none' or 'truncate-if-exists' is set, molt fetch loads data into the existing tables on the target CockroachDB database. If the target schema mismatches the source schema, molt fetch will exit early in certain cases, and will need to be re-run from the beginning.

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.

Type mapping

If 'drop-on-target-and-recreate' is set, MOLT Fetch automatically creates a CockroachDB schema that is compatible with the source data. The column types are determined as follows:

• PostgreSQL types are mapped to existing CockroachDB types that have the same OID.

• The following MySQL types are mapped to corresponding CockroachDB types:

  MySQL type	CockroachDB type
  CHAR, CHARACTER, VARCHAR, NCHAR, NVARCHAR	VARCHAR
  TINYTEXT, TEXT, MEDIUMTEXT, LONGTEXT	STRING
  GEOMETRY	GEOMETRY
  LINESTRING	LINESTRING
  POINT	POINT
  POLYGON	POLYGON
  MULTIPOINT	MULTIPOINT
  MULTILINESTRING	MULTILINESTRING
  MULTIPOLYGON	MULTIPOLYGON
  GEOMETRYCOLLECTION, GEOMCOLLECTION	GEOMETRYCOLLECTION
  JSON	JSONB
  TINYINT, INT1	INT2
  BLOB	BYTES
  SMALLINT, INT2	INT2
  MEDIUMINT, INT, INTEGER, INT4	INT4
  BIGINT, INT8	INT
  FLOAT	FLOAT4
  DOUBLE	FLOAT
  DECIMAL, NUMERIC, REAL	DECIMAL (Negative scale values are autocorrected to 0)
  BINARY, VARBINARY	BYTES
  DATETIME	TIMESTAMP
  TIMESTAMP	TIMESTAMPTZ
  TIME	TIME
  BIT	VARBIT
  DATE	DATE
  TINYBLOB, MEDIUMBLOB, LONGBLOB	BYTES
  BOOL, BOOLEAN	BOOL
  ENUM	ANY_ENUM

• To override the default mappings for automatic schema creation, you can map source to target CockroachDB types explicitly. These are specified using a JSON file and --type-map-file. The allowable custom mappings are valid CockroachDB aliases, casts, and the following mappings specific to MOLT Fetch and Verify:

  • TIMESTAMP <> TIMESTAMPTZ
  • VARCHAR <> UUID
  • BOOL <> INT2
  • VARBIT <> TEXT
  • VARBIT <> BOOL
  • JSONB <> TEXT
  • INET <> TEXT

--type-map-file specifies the path to the JSON file containing the explicit type mappings. For example:

--type-map-file 'type-mappings.json'

The JSON is formatted as follows:

[
  {
    "table": "public.t1",
    "column-type-map": [
      {
        "column": "*",
        "type-kv": {
          "source-type": "int",
          "crdb-type": "INT2"
        }
      },
      {
        "column": "name",
        "type-kv": {
          "source-type": "varbit",
          "crdb-type": "string"
        }
      }
    ]
  }
]

• table specifies the table that will use the custom type mappings in column-type-map, written as {schema}.{table}.
• column specifies the column that will use the custom type mapping in type-kv. If * is specified, then all columns in the table with the matching source-type are converted.
• type-kv specifies the source-type that maps to the target crdb-type.

Fetch continuation

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. You can use this information to continue the process from the continuation point where it was interrupted. For an example, see Continue fetch after encountering an error.

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 to the target CockroachDB database is incomplete. This means that ongoing replication of source data has not begun.

To retry all data starting from the continuation point, reissue the molt fetch command and include the --fetch-id.

--fetch-id d44762e5-6f70-43f8-8e15-58b4de10a007

To retry a specific table that failed, include both --fetch-id and --continuation-token. The latter flag specifies a token string that corresponds to a specific table on the source database. A continuation token is written in the molt fetch output for each failed table. If the fetch process encounters a subsequent error, it generates a new token for each failed table. See List active continuation tokens.

--fetch-id d44762e5-6f70-43f8-8e15-58b4de10a007
--continuation-token 011762e5-6f70-43f8-8e15-58b4de10a007

To retry all data starting from a specific file, include both --fetch-id and --continuation-file-name. The latter flag specifies the filename of an intermediate file in cloud or local storage. All filenames are prepended with part_ and have the .csv.gz or .csv extension, depending on compression type (gzip by default). For example:

--fetch-id d44762e5-6f70-43f8-8e15-58b4de10a007
--continuation-file-name part_00000003.csv.gz

List active continuation tokens

To view all active continuation tokens, issue a molt fetch tokens list command along with --conn-string, which specifies the connection string for the target CockroachDB database. For example:

molt fetch tokens list \
--conn-string 'postgres://root@localhost:26257/defaultdb?sslmode=verify-full'

+--------------------------------------+--------------------------------------+------------------+----------------------+
|                  ID                  |               FETCH ID               |    TABLE NAME    |      FILE NAME       |
+--------------------------------------+--------------------------------------+------------------+----------------------+
| f6f0284c-d9c1-43c9-8fde-af609d0dbd82 | 66443597-5689-4df3-a7b9-9fc5e27180eb | public.employees | part_00000001.csv.gz |
+--------------------------------------+--------------------------------------+------------------+----------------------+
Continuation Tokens.

Replication

--ongoing-replication enables logical replication from the source database to the target CockroachDB database.

--ongoing-replication

When the --ongoing-replication flag is set, changes on the source database are continuously replicated on CockroachDB. This begins only after the fetch process succeeds—i.e., the initial source data is loaded into CockroachDB—as indicated by a fetch complete message in the output.

Before using this feature, configure the source PostgreSQL or MySQL database for continuous replication, as described in Setup.

If the source is a PostgreSQL database, you must also specify a replication slot name:

--ongoing-replication
--pglogical-replication-slot-name 'replication_slot'

To customize the replication behavior (an advanced use case), use --replicator-flags to specify one or more replication-specific flags (PostgreSQL or MySQL) to override.

--ongoing-replication
--replicator-flags "--applyTimeout '1h' --parallelism 64"

To cancel replication, enter ctrl-c to issue a SIGTERM signal. This returns an exit code 0. If replication fails, a non-zero exit code is returned.

CDC cursor

A change data capture (CDC) cursor is written to the output as cdc_cursor at the beginning and end of the fetch process. For example:

{"level":"info","type":"summary","fetch_id":"735a4fe0-c478-4de7-a342-cfa9738783dc","num_tables":1,"tables":["public.employees"],"cdc_cursor":"0/3F41E40","net_duration_ms":4879.890041,"net_duration":"000h 00m 04s","time":"2024-03-18T12:37:02-04:00","message":"fetch complete"}

You can use the cdc_cursor value with an external change data capture (CDC) tool to continuously replicate subsequent changes on the source data to CockroachDB.

Examples

The following examples demonstrate how to issue molt fetch commands to load data into CockroachDB. These examples assume that secure connections to the source and target database are used.

Load PostgreSQL data via S3 with ongoing replication

The following molt fetch command uses IMPORT INTO to load a subset of tables from a PostgreSQL database to CockroachDB.

molt fetch \
--source 'postgres://postgres:postgres@localhost/molt' \
--target 'postgres://root@localhost:26257/defaultdb?sslmode=verify-full' \
--table-handling 'truncate-if-exists' \
--table-filter 'employees' \
--bucket-path 's3://migration/data/cockroach' \
--cleanup \
--pglogical-replication-slot-name 'replication_slot' \
--ongoing-replication

• --table-handling specifies that existing tables on CockroachDB should be truncated before the source data is loaded.
• --table-filter filters for tables with the employees string in the name.
• --bucket-path specifies a directory on an Amazon S3 bucket where intermediate files will be written.
• --cleanup specifies that the intermediate files should be removed after the source data is loaded.
• --pglogical-replication-slot-name specifies a replication slot name to be created on the source PostgreSQL database. This is used in continuous replication.
• --ongoing-replication starts continuous replication of data from the source database to CockroachDB after the fetch process succeeds.

If the fetch process succeeds, the output displays a fetch complete message like the following:

{"level":"info","type":"summary","fetch_id":"f5cb422f-4bb4-4bbd-b2ae-08c4d00d1e7c","num_tables":1,"tables":["public.employees"],"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"}

Continuous replication begins immediately afterward:

{"level":"info","time":"2024-05-13T14:33:07-04:00","message":"starting replicator"}
{"level":"info","time":"2024-05-13T14:36:22-04:00","message":"creating publication"}

To cancel replication, enter ctrl-c to issue a SIGTERM signal.

Load MySQL data via GCP with ongoing replication

The following molt fetch command uses COPY FROM to load a subset of tables from a MySQL database to CockroachDB.

molt fetch \
--source 'mysql://root:password@localhost/molt?sslcert=.%2fsource_certs%2fclient.root.crt&sslkey=.%2fsource_certs%2fclient.root.key&sslmode=verify-full&sslrootcert=.%2fsource_certs%2fca.crt' \
--target 'postgres://root@localhost:26257/defaultdb?sslmode=verify-full' \
--table-handling 'truncate-if-exists' \
--table-filter 'employees' \
--bucket-path 'gs://migration/data/cockroach' \
--use-copy \
--cleanup

• --source specifies the MySQL connection string and the certificates in URL-encoded format. Secure connections should be used by default. Refer to Best practices.
• --table-handling specifies that existing tables on CockroachDB should be truncated before the source data is loaded.
• --table-filter filters for tables with the employees string in the name.
• --bucket-path specifies a directory on an Google Cloud Storage bucket where intermediate files will be written.
• --use-copy specifies that COPY FROM is used to load the tables, keeping the source tables online and queryable but loading the data more slowly than IMPORT INTO.
• --cleanup specifies that the intermediate files should be removed after the source data is loaded.
• --ongoing-replication starts continuous replication of data from the source database to CockroachDB after the fetch process succeeds.

If the fetch process succeeds, the output displays a fetch complete message like the following:

{"level":"info","type":"summary","fetch_id":"f5cb422f-4bb4-4bbd-b2ae-08c4d00d1e7c","num_tables":1,"tables":["public.employees"],"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"}

Continuous replication begins immediately afterward:

{"level":"info","time":"2024-05-13T14:33:07-04:00","message":"starting replicator"}

To cancel replication, enter ctrl-c to issue a SIGTERM signal.

Load CockroachDB data via direct copy

The following molt fetch command uses COPY FROM to load all tables directly from one CockroachDB database to another.

molt fetch \
--source 'postgres://root@localhost:26257/defaultdb?sslmode=disable' \
--target 'postgres://root@localhost:26258/defaultdb?sslmode=disable' \
--table-handling 'none' \
--direct-copy \
--allow-tls-mode-disable

• --source specifies sslmode=disable to establish an insecure connection. By default, insecure connections are disallowed and should be used only for testing or if a secure SSL/TLS connection to the source or target database is not possible.
• --table-handling specifies that existing tables on the target CockroachDB database should not be modified before the source data is loaded.
• --direct-copy specifies that COPY FROM is used to load the tables directly, without creating intermediate files.
• --allow-tls-mode-disable enables insecure connections to the source and target databases. Refer to Secure connections.

Continue fetch after encountering an error

If the fetch process encounters an error, it exits with an error message, fetch ID, and continuation token for each table that failed to load on the target database. You can use these values to continue the fetch process from where it was interrupted.

{"level":"info","table":"public.tbl1","file_name":"shard_01_part_00000001.csv","message":"creating or updating token for duplicate key value violates unique constraint \"tbl1_pkey\"; Key (id)=(22) already exists."}
{"level":"info","table":"public.tbl1","continuation_token":"5e7c7173-101c-4539-9b8d-28fad37d0240","message":"created continuation token"}
{"level":"info","fetch_id":"87bf8dc0-803c-4e26-89d5-3352576f92a7","message":"continue from this fetch ID"}

To retry a specific table, reissue the initial molt fetch command and include the fetch ID and a continuation token:

molt fetch \
... \
--fetch-id '87bf8dc0-803c-4e26-89d5-3352576f92a7' \
--continuation-token '5e7c7173-101c-4539-9b8d-28fad37d0240'

To retry all tables that failed, exclude --continuation-token from the command. When prompted, type y to clear all active continuation tokens. To avoid the prompt (e.g., when running molt fetch in a job), include the --non-interactive flag:

molt fetch \
... \
--fetch-id '87bf8dc0-803c-4e26-89d5-3352576f92a7' \
--non-interactive

See also

• MOLT Verify
• Migration Overview
• Migrate from PostgreSQL
• Migrate from MySQL
• Migrate from CSV

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