# Resume an Interrupted Migration Where it Left Off

#### NOTE
Savepoints (and automatic resume) are available for sources that track migration
progress: Apache Cassandra, DynamoDB (including Scylla Alternator), and Parquet. They are not
available for MySQL sources or the DynamoDB S3-export source, which must be restarted from
scratch if interrupted.

If, for some reason, the migration is interrupted (e.g., because of a networking issue, or if you need to manually stop it for some reason), the migrator is able to resume it from a “savepoint”.

Savepoints are configuration files that contain information about the already migrated items, which can be skipped when the migration is resumed. The savepoint files are automatically generated during the migration.

## Automatic resume

By default, the migrator resumes automatically. On startup it scans the configured `savepoints` location, finds the most recent savepoint, and merges its progress (the already-migrated token ranges, scan segments, or Parquet files) into the configuration it was launched with. This means that **re-running the same migration with the same configuration file continues where the previous run left off** — no manual editing or file selection is required. This is especially convenient for orchestrated environments (Kubernetes, Cloud Run, etc.) that restart the job with a fixed configuration.

Only the progress information is taken from the savepoint; every other setting — in particular the source and target credentials — comes from the configuration you launch with. You can disable this behavior by setting `savepoints.autoResume` to `false`, for example to force a full re-migration while reusing the same savepoints directory. See the [configuration reference](https://migrator.docs.scylladb.com/master/configuration.md#config-savepoints).

#### WARNING
Savepoint files are written with credentials **redacted** (secrets are replaced with `<redacted>`). They are therefore safe to keep in shared or long-lived storage such as an S3/GCS bucket, but a savepoint file can no longer be used directly as a standalone configuration file. Use automatic resume (above) instead, which supplies the real credentials from your launch configuration.

You can control the savepoints location and the interval at which they are generated in the configuration file under the top-level property `savepoints`. See the corresponding section of the [configuration reference](https://migrator.docs.scylladb.com/master/configuration.md#config-savepoints).

## Savepoint file names

During the migration, the savepoints are generated with file names like `savepoint_<epochMillis>_<counter>.yaml`, where:

* `<epochMillis>` is a 13-digit, zero-padded millisecond timestamp (monotonic within a migrator instance), and
* `<counter>` is a 10-digit, zero-padded per-instance sequence number that disambiguates dumps produced within the same millisecond.

Both components are zero-padded so that **lexicographical order matches chronological order**: listing the directory and picking the alphabetically last file (`ls <savepoints-dir> | tail -n 1`) always yields the most recent savepoint, which is the one automatic resume selects.

#### NOTE
Older versions of the migrator produced file names like `savepoint_1234567890.yaml` (a single Unix-epoch-seconds component). The migrator still reads those files, but any new savepoint written during a resumed migration follows the two-component format above.