# Validate the Migration

In addition to the monitoring user interface provided by Spark, you can run another program, after running a migration, to check whether the destination table contains exactly the same items as the source table.

Running this program consists of submitting the same Spark job as the migration, but with a different entry point.

For MySQL-to-ScyllaDB validations, run the validator only after writes have stopped on both the
MySQL source table and the ScyllaDB target table. The validator may scan both sides using multiple
statements over time, so it does not provide a single global point-in-time snapshot while live
writes are still happening.

Before running the validator, adjust the corresponding configuration in the top-level `validation` property of the file `config.yml`:

```yaml
validation:
  # Should WRITETIMEs and TTLs be compared?
  compareTimestamps: true
  # What difference should we allow between TTLs?
  ttlToleranceMillis: 60000
  # What difference should we allow between WRITETIMEs?
  writetimeToleranceMillis: 1000
  # How many differences to fetch and print
  failuresToFetch: 100
  # What difference should we allow between floating point numbers?
  floatingPointTolerance: 0.001
  # What difference in ms should we allow between timestamps?
  timestampMsTolerance: 0
```

The exact way to run the validator depends on the way you set up the Spark cluster.

## Run the Validator in the Ansible-based Setup

Submit the job `submit-cql-job-validator.sh`.

## Run the Validator in a Manual Spark Setup

Pass the argument `--class com.scylladb.migrator.Validator` to the `spark-submit` invocation:

```bash
spark-submit --class com.scylladb.migrator.Validator \
  --master spark://<spark-master-hostname>:7077 \
  --conf spark.scylla.config=<path to config.yaml> \
  <path to scylla-migrator-assembly.jar>
```

## Run the Validator in AWS EMR

Use the following arguments for the Cluster Step that runs a custom JAR:

```text
spark-submit --deploy-mode cluster --class com.scylladb.migrator.Validator --conf spark.scylla.config=/mnt1/config.yaml /mnt1/scylla-migrator-assembly.jar
```

## Run the Validator in a Docker-based Spark Setup

Pass the argument `--class com.scylladb.migrator.Validator` to the `spark-submit` invocation:

```bash
docker compose exec spark-master /spark/bin/spark-submit --class com.scylladb.migrator.Validator \
  --master spark://spark-master:7077 \
  --conf spark.driver.host=spark-master \
  --conf spark.scylla.config=/app/config.yaml \
  /jars/scylla-migrator-assembly.jar
```