MOLT Releases

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This page has details about each release of the following MOLT (Migrate Off Legacy Technology) tools:

• Fetch
• Verify

Cockroach Labs recommends using the latest available version of each tool. See Installation.

Installation

To install MOLT, download the binary that matches your architecture and source database:

The following binaries are included:

• molt
• replicator

Both molt and replicator must be in your current working directory. To use replication features, replicator must be located either in the same directory as molt or in a directory directly beneath molt. For example, either of the following would be valid:

/migration-project/          # Your current working directory
├── molt                     # MOLT binary
└── replicator               # Replicator binary

/migration-project/          # Your current working directory
├── molt                     # MOLT binary
└── bin/                     # Subdirectory
    └── replicator           # Replicator binary

To display the current version of each binary, run molt --version and replicator --version.

For previous binaries, refer to the MOLT version manifest.

Docker image

Docker multi-platform images containing both the AMD and ARM binaries are available. To pull the latest image for PostgreSQL and MySQL:

docker pull cockroachdb/molt

To pull a specific version (e.g., 1.1.3):

docker pull cockroachdb/molt:1.1.3

To pull the latest image for Oracle (note that only linux/amd64 is supported):

docker pull cockroachdb/molt:oracle-latest

September 25, 2025

MOLT Fetch/Verify 1.3.2 is available.

• MOLT Fetch replication modes will be deprecated in favor of a separate replication workflow in an upcoming release. This includes the data-load-and-replication, replication-only, and failback modes.
• Fixed a bug with stats-based sharding where PostgreSQL queries were generated incorrectly when the first primary key column did not have sufficient unique values.
• Fixed a bug in escape-password where passwords that start with a hyphen were not handled correctly. Users must now pass the --password flag when running escape-password. For example, molt escape-password --password 'a$52&'.
• Added support for assume role authentication during data export with MOLT Fetch.
• Added support to replicator for retrying unique constraint violations on the target database, which can be temporary in some cases.
• Added exponential backoff to replicator for retryable errors when applying mutations to the target database. This reduces load on the target database and prevents exhausting retries prematurely. The new replication flags --retryInitialBackoff, --retryMaxBackoff, and --retryMultiplier control backoff behavior. The new --maxRetries flag configures the maximum number of retries. To retain the previous "immediate retry" behavior, set --retryMaxBackoff 1ns --retryInitialBackoff 1ns --retryMultiplier 1.

• Added support to replicator for the source_lag_seconds, target_lag_seconds, apply_mutation_age_seconds, and source_commit_to_apply_lag_seconds metrics for replication from PostgreSQL and MySQL, and introduced histogram metrics source_lag_seconds_histogram and target_lag_seconds_histogram for replication from CockroachDB.

  source_lag_seconds measures the delay before data is ready to be processed by replicator, while target_lag_seconds measures the "end-to-end" delay until replicator has written data to the target. A steady increase in source_lag_seconds may indicate replicator cannot keep up with the source workload, while a steady increase in target_lag_seconds may indicate replicator cannot keep up with the source workload or that writes on the target database are bottlenecked.

August 21, 2025

MOLT Fetch/Verify 1.3.1 is available.

• MOLT Fetch now supports sharding of primary keys of any data type on PostgreSQL 11+ sources. This can be enabled with the --use-stats-based-sharding flag.
• Added the --ignore-replication-check flag to allow data loads with planned downtime and no replication setup. The --pglogical-ignore-wal-check flag has been removed.
• Added the --enableParallelApplies replication flag to enable parallel application of independent table groups during replication. By default, applies are synchronous. When enabled, this increases throughput at the cost of increased target pool and memory usage.
• Improved cleanup logic for scheduled tasks to ensure progress reporting and prevent indefinite hangs.
• Added parallelism gating to ensure the parallelism setting is smaller than the targetMaxPoolSize. This helps prevent a potential indefinite hang.
• Added new metrics that track start and end times for progress reports (core_progress_reports_started_count and core_progress_reports_ended_count) and error reports (core_error_reports_started_count and core_error_reports_ended_count). These provide visibility into the core sequencer progress and help identify hangs in the applier and progress tracking pipeline.
• Added new metrics target_apply_queue_utilization_percent and target_apply_queue_depth to track target apply queue utilization, indicating when the queue should be resized. These metrics apply only to PostgreSQL, Oracle, and CockroachDB sources.
• Oracle sources now support configurable backpressure between the source and target applier. The new target_apply_queue_depth metric tracks queue depth.
• Improved visibility into queue depth between each source frontend and the backend applier to the target. Updated logging to use lower log levels with clearer, less alarming messages.
• Improved throughput for tables in the replication stream that have no dependencies on one another. This increases parallelism and minimizes blocking of transactions that are mutually independent.
• The best effort window is now disabled by default to prevent unexpected mode switches that could cause consistency issues or stall replication due to failed target applies.

Bug fixes

• Fixed a panic that could occur with ENUM types.

July 24, 2025

MOLT Fetch/Verify 1.3.0 is available.

• Oracle is now supported on Fetch Docker images and standalone binaries.
• Oracle is now supported on Replicator Docker images and standalone binaries.
• Azure Blob Storage is now supported as an intermediate data store for Fetch.
• Added a --skip-pk-check flag that lets you run the initial data load even when the source or target table is missing a primary key, or the keys do not match. When this flag is set, Fetch treats every table as keyless, disables sharding, and exports each table in a single batch, ignoring export-concurrency and row-batch-size. Querying the entire table at once may lead to high memory usage or long-running queries. Duplicate source rows are automatically removed during import when the target has primary key or unique constraints.
• Replication now uses checkpoint polling by default. This is because polling (which periodically queries the staging table for updates) performs comparably to checkpoint streaming (which uses an internal changefeed from the staging table to broadcast real-time updates). To use checkpoint streaming, set --enableCheckpointStream. --disableCheckpointStream is deprecated and should be removed from replication commands.
• Replaced the --copierChannel and --stageCopierChannelSize replication flags with a single --targetApplyQueueSize flag, which controls downstream apply throughput and memory usage. This feature applies only to CockroachDB and PostgreSQL (pglogical) sources.
• Added support for compressed changefeed payloads, improving performance of changefeed-to-Replicator communication. This only affects failback workflows from CockroachDB v25.2 and later.
• Improved apply-side performance by switching to a faster JSON decoder.
• Improved batch-accumulation performance by removing unnecessary sorting.

Bug fixes

• Fixed a bug where some error logs were not displayed when replicating to the target database.

June 19, 2025

MOLT Fetch/Verify 1.2.7 is available.

• Updated the MOLT Grafana dashboard with the following timing metrics to better diagnose performance: Source Side Queries, Convert Source Side Queries to Datums, Writing Datums to Pipe, Preparing CSV files to be uploaded, Uploading CSV files to S3/GCP/local.
• Upgraded the MOLT parser to support new syntax that is valid in CockroachDB v25.2.
• Added more granular replication counter metrics to track data counts at each stage of the mutation pipeline, helping to diagnose data correctness issues.

Bug fixes

• MOLT Fetch failback now reliably creates changefeeds with a sorted list of table names so that create changefeed operations can be properly deduplicated.
• Fixed an issue where shard connections failed to recognize custom types (e.g., ENUM) in primary keys during table migration. This occurred because the type map from the original pgx.Conn was not cloned. The type map is now properly cloned and attached to each shard connection.
• Fixed a bug that could cause an integer overflow, which impacts retrieving the correct shards for exporting data.

May 22, 2025

MOLT Fetch/Verify 1.2.6 is available.

• Fixed a bug in --direct-copy mode that occurred when --case-sensitive was set to false (default). Previously, the COPY query could use incorrect column names in some cases during data transfer, causing errors. The query now uses the correct column names.
• Fixed a bug in how origin messages were handled during replication from PostgreSQL sources. This allows replication to successfully continue.
• ENUM types can now be replicated from MySQL 8.0 sources.

April 25, 2025

MOLT Fetch/Verify 1.2.5 is available.

• During data export, MOLT Fetch now treats empty STRING values on source Oracle databases as NULL values on the target database. This is because Oracle does not differentiate between empty STRING and NULL values.

April 7, 2025

MOLT Fetch/Verify 1.2.4 is available.

• MOLT Fetch now supports PostgreSQL 11.
• MOLT Fetch failback to CockroachDB is now disallowed.
• MOLT Verify can now compare tables that are named differently on the source and target schemas.
• The molt logging date format is now period-delimited for Windows compatibility.
• During replication, an index is now created on all tables by default, improving replication performance. Because index creation can cause the replication process to initialize more slowly, this behavior can be disabled using the --stageDisableCreateTableReaderIndex replication flag.
• Added a failback metric that tracks the time to write a source commit to the staging schema for a given mutation.
• Added a failback metric that tracks the time to write a source commit to the target database for a given mutation.

February 26, 2025

MOLT Fetch/Verify 1.2.3 is available.

• MOLT Fetch users can now set --table-concurrency and --export-concurrency to values greater than 1 for MySQL sources.
• MOLT Fetch now supports case-insensitive comparison of table and column names by default. Previously, case-sensitive comparisons could result in no matching table on target errors. To disable case-sensitive comparisons explicitly, set --case-sensitive=false. If = is not included (e.g., --case-sensitive false), this is interpreted as --case-sensitive (i.e., --case-sensitive=true).

February 5, 2025

MOLT Fetch/Verify 1.2.2 is available.

• Added an --import-region flag that is used to set the AWS_REGION query parameter explicitly in the s3 URL.
• Fixed the truncate-if-exists schema mode for cases where there are uppercase table or schema names.
• Fixed an issue with unsigned BIGINT values overflowing in replication.
• Added a --schemaRefresh replication flag that is used to configure the schema watcher refresh delay in the replication phase. Previously, the refresh delay was set to a constant value of 1 minute. Set the flag as follows: --replicator-flags "--schemaRefresh {value}".

December 13, 2024

MOLT Fetch/Verify 1.2.1 is available.

• MOLT Fetch users now can use --assume-role to specify a service account for assume role authentication to cloud storage. --assume-role must be used with --use-implicit-auth, or the flag will be ignored.
• MySQL 5.7 and later are now supported with MOLT Fetch replication modes. For details on setup, refer to the MOLT Fetch documentation.
• Fetch replication mode now defaults to a less verbose INFO logging level. To specify DEBUG logging, pass in the --replicator-flags '-v' setting, or --replicator-flags '-vv' for trace logging.
• MySQL columns of type BIGINT UNSIGNED or SERIAL are now auto-mapped to DECIMAL type in CockroachDB. MySQL regular BIGINT types are mapped to INT type in CockroachDB.
• The pglogical replication workflow was modified in order to enforce safer and simpler defaults for the data-load, data-load-and-replication, and replication-only workflows for PostgreSQL sources. Fetch now ensures that the publication is created before the slot, and that replication-only defaults to using publications and slots created either in previous Fetch runs or manually.
• Fixed scan iterator query ordering for BINARY and TEXT (of same collation) PKs so that they lead to the correct queries and ordering.
• For a MySQL source in replication-only mode, the --stagingSchema replicator flag can now be used to resume streaming replication after being interrupted. Otherwise, the --defaultGTIDSet replicator flag is used to start initial replication after a previous Fetch run in data-load mode, or as an override to the current replication stream.

October 29, 2024

MOLT Fetch/Verify 1.2.0 is available.

• Added failback mode to MOLT Fetch, which allows the user to replicate changes on CockroachDB back to the initial source database. Failback is supported for MySQL and PostgreSQL databases.
• The --pprof-list-addr flag, which specifies the address of the pprof endpoint, is now configurable. The default value is '127.0.0.1:3031'.
• Fetch modes involving replication now state that MySQL 8.0 and later are supported for replication between MySQL and CockroachDB.
• Partitioned tables can now be moved to CockroachDB using IMPORT INTO.
• Improved logging for the Fetch schema check phases under the trace logging level, which is set with --logging trace.
• Added a sample Grafana dashboard for monitoring MOLT tasks.
• Fetch now logs the name of the staging database in the target CockroachDB cluster used to store metadata for replication modes.
• String primary keys that use C collations on PostgreSQL can now be compared to the default en_US.utf8 on CockroachDB.
• MOLT is now distributed under the Cockroach Labs Product License Agreement, which is bundled with the binary.

August 26, 2024

MOLT Fetch/Verify 1.1.7 is available.

• When a Fetch transformation rule is used to rename a table or map partitioned tables, a script in the format partitionTableScript.{timestamp}.ts is now automatically generated to ensure that replication works properly if enabled.

August 19, 2024

MOLT Fetch/Verify 1.1.6 is available.

• Fixed a Fetch bug where --table-exclusion-filter was ignored when --table-filter and --schema-filter were set to the default ('.*').

August 15, 2024

MOLT Fetch/Verify 1.1.5 is available.

• Deprecated the --ongoing-replication flag in favor of --mode data-load-and-replication, using the new --mode flag. Users should replace all instances of --ongoing-replication with --mode data-load-and-replication.
• Fetch can now be run in an export-only mode by specifying --mode export-only. This will export all the data in csv or csv.gz format to the specified cloud or local store.
• Fetch can now be run in an import-only mode by specifying --mode import-only. This will load all data in the specified cloud or local store into the target CockroachDB database, effectively skipping the export data phase.
• Strings for the --mode flag are now word-separated by hyphens instead of underscores. For example, replication-only instead of replication_only.

August 8, 2024

MOLT Fetch/Verify 1.1.4 is available.

• Added a replication-only mode for Fetch that allows the user to run ongoing replication without schema creation or initial data load. This requires users to set --mode replication_only and --replicator-flags to specify the defaultGTIDSet (MySQL) or slotName (PostgreSQL).
• Partitioned tables can now be mapped to renamed tables on the target database, using the Fetch transformations framework.
• Added a new --metrics-scrape-interval flag to allow users to specify their Prometheus scrape interval and apply a sleep at the end to allow for the final metrics to be scraped.
• Previously, there was a mismatch between the errors logged in log lines and those recorded in the exceptions table when an IMPORT INTO or COPY FROM operation failed due to a non-PostgreSQL error. Now, all errors will lead to an exceptions table entry that allows the user to continue progress from a certain table's file.
• Fixed a bug that will allow Fetch to properly determine a GTID if there are multiple source_uuids.

July 31, 2024

MOLT Fetch/Verify 1.1.3 is available.

• 'infinity'::timestamp values can now be moved with Fetch.
• Fixed an issue where connections were not being closed immediately after sharding was completed. This could lead to errors if the maximum number of connections was set to a low value.
• Fetch users can now exclude specific tables from migration using the --table-exclusion-filter flag.

July 18, 2024

MOLT Fetch/Verify 1.1.2 is available.

• Fetch users can now specify columns to exclude from table migrations in order to migrate a subset of their data. This is supported in the schema creation, export, import, and direct copy phases.
• Fetch now automatically maps a partitioned table from a PostgreSQL source to the target CockroachDB schema.
• Fetch now supports column exclusions and computed column mappings via a new transformations framework.
• The new Fetch --transformations-file flag specifies a JSON file for schema/table/column transformations, which has validation utilities built in.

July 10, 2024

MOLT Fetch/Verify 1.1.1 is available.

• Fixed a bug that led to incorrect list continuation file behavior if a trailing slash was provided in --bucket-path.
• Fixed a bug with extracting the filename from a failed import URL. Previously, an older filename was being used, which could result in duplicated data. Now, the filename that is used in import matches what is stored in the exceptions log table.
• Added a --use-implicit-auth flag that determines whether implicit authentication is used for cloud storage import URIs.

July 8, 2024

MOLT Fetch/Verify 1.1.0 is available.

• Fixed a bug where integer values for JSONB/JSON types were not correctly handled when migrating from a PostgreSQL or CockroachDB source.
• Optimized the logic for clearing continuation tokens. Now, the clearing option is only presented in the relevant modes and when there are tokens to clear.
• CockroachDB sources now exclusively accept connections from admin or root users.
• The statement timeout can now be adjusted during the export phase. This guards against hanging queries.

May 31, 2024

MOLT Fetch/Verify 1.0.0 is available.

• Renamed the --table-splits flag to --concurrency-per-table, which is more descriptive.
• Increased the default value of --import-batch-size to 1000. This leads to better performance on the target post-migration. Each individual import job will take longer, since more data is now imported in each batch, but the sum total of all jobs should take the same (or less) time.

May 29, 2024

MOLT Fetch/Verify 0.3.0 is available.

• Added an --import-batch-size flag, which configures the number of files to be imported in each IMPORT job.
• In some cases on the previous version, binaries would not work due to how molt was being built. Updated the build method to use static linking, which creates binaries that should be more portable.
• VARBIT <> BOOL conversion is now allowed for Fetch and Verify. The bit array is first converted to UINT64. A resulting 1 or 0 is converted to true or false accordingly. If the UINT64 is another value, an error is emitted.

May 20, 2024

MOLT Fetch/Verify 0.2.1 is available.

• MOLT tools now enforce secure connections to databases as a default. The --allow-tls-mode-disable flag allows users to override that behavior if secure access is not possible.
• When using MySQL as a source, --table-concurrency and --export-concurrency are strictly set to 1.
• Fixed a bug involving history retention for DECIMAL values.

May 3, 2024

MOLT Fetch/Verify 0.2.0 is available.

• Fetch now supports CockroachDB multi-region tables.
• Fetch now supports continuous replication for PostgreSQL and MySQL source databases via the --ongoing-replication flag. When Fetch finishes the initial data load phase, it will start the replicator process as a subprocess, which runs indefinitely until the user ends the process with a SIGTERM (ctrl-c).
• Replicator flags for (PostgreSQL and MySQL) are now supported, allowing users to further configure the --ongoing-replication mode for their use case.
• Added the --type-map-file flag, which enables custom type mapping for schema creation.
• Fixed a bug where primary key positions could be missed when creating a schema with multiple primary keys.
• Added a default mode for MySQL sources that ensures consistency and does not leverage parallelism. New text is displayed that alerts the user and links to documentation in cases where fetching from MySQL might not be consistent.
• Logging for continuation tokens is now omitted when data export does not successfully complete.

April 10, 2024

MOLT Fetch/Verify 0.1.4 is available.

• Fixed an inconsistency where inserting 'null' as a string into JSON/JSONB columns did not align with PostgreSQL's case-sensitive null handling.

March 22, 2024

MOLT Fetch/Verify 0.1.2 is available.

• Fixed a bug where running Verify with a PostgreSQL or CockroachDB source would cause an error when there are dropped columns on the table.

March 13, 2024

MOLT Fetch/Verify 0.1.1 is available.

• An error is now displayed if the original column is a PostgreSQL nested array.
• Added sharded data export functionality for faster performance.
• Added support for the --table-handling 'drop-on-target-and-recreate' flag for MySQL sources, enabling automatic schema recreation.
• Added support for ENUM data types in schema creation.
• Deprecated the --s3-bucket and --gcp-bucket flags in favor of a single --bucket-path flag.

February 22, 2024

MOLT Fetch/Verify 0.1.0 is available.

• Added implicit authentication for GCP sources.
• Resolved a CSV write deadlock issue for large file exports.
• Added new dashboard metrics: molt_verify_num_tables (categorizes tables as verified, missing, or extraneous) and molt_verify_duration_ms (how long a Verify run took to complete, in milliseconds).
• Previously, if a comparison between heterogeneous column types occurred, Verify would panic and stop execution. Now, Verify no longer panics, but records the error and reports it to logging, and also marks the row as having mismatching columns. The error message is logged at most once per shard to avoid log spewing. Additionally, introduced the concept of conditional success, which means that all comparable row columns were compared and succeeded with the same value. Upon conditional success, the user is instructed to check all warnings and verify the data.
• Marked the --live and --continuous flags as dependencies for other sub-configuration flags. If a flag is set without its dependency, MOLT will indicate which dependency flag is not correctly set.
• Added the RFC for MOLT Fetch, which explains the architectural decisions and considerations of how Fetch was built. The RFC also outlines limitiations of the current system and further improvements around usability, performance, and integration with replication tools.
• Added logging annotations for data-level errors and information that surfaced during the Verify process, as well as summary status logs about the overall functioning of the job (e.g., 10000 truth rows seen, 5000 mismatches, etc.). These can be filtered so that exact tables and rows can be triaged in one place.
• Changed the output for the running summary statistics so that they are now more easily indexable by log processing tools. Previously, it was more human-readable, but fields were not easily indexable out of the box. Now, there is a balance between readability and downstream processing.
• Added tagging for summary and data logs so that users can filter the logs in a variety of log processing tools, or grep in the command line. This solves the pain point of all the logs going to the same location without any differentiation. Now, it is possible to view types of logs one at a time. Summary logs report aggregated results on either a task or step in a task (e.g., task took 10 minutes; 500 tables processed overall). Data logs report on individual or row-level operations and quantitative markers (e.g., 1000 rows completed so far; failed to verify item with PK 1000)
• Marked the --s3-bucket, --gcp-bucket, and --local-path flags as mutually exclusive because they all represent different intermediary stores.
• Adjusted settings for the number of rows and the size of data to be flushed so that only one rule is applied at any given time.
• Fixed a bug where outputs containing special characters were not formatted correctly.
• When running the import query for Fetch, the import statement is now printed if the logger is in debug mode.
• Updated the MOLT README with an OS + Docker-based edge case with Fetch.
• Logging for Fetch and Verify now defaults to structured JSON logging, which contains at least the message, level, and formatted time string. The --use-console-writer flag enables prettier console writing that has colored and human-readable log output. Structured logging is the default because it is easier to parse with log ingestion tools and is more efficient for the application to output.
• Task/data logs can now be written to specified log files using --log-file. If the flag is left empty, the MOLT tool will only write to stdout, which is more efficient.