subsetter 0.3.0

MySQL database subsetting CLI tool

Subsetter

Subsetter is a Python utility that can be used for subsetting portions of relational databases. Subsetting is the action extracting a smaller set of rows from your database that still maintain expected foreign-key relationships between your data. This can be useful for testing against a small but realistic dataset or for generating sample data for use in demonstrations. This tool also supports filtering that allows you to remove/anonymize rows that may contain sensitive data.

Similar tools include Tonic.ai's platform and condenser. This is meant to be a simple CLI tool that overcomes many of the difficulties in using `condenser.

Limitations

The subsetter tool takes an approach of "one table, one query". This means that the subsetter will sample each table using only a single query. It cannot support calculating a full transitive closure of foreign key relationships for schemas that contain cycles. In general, as long as your schema contains no foreign key cycles and no target is reachable from another target, the subsetter will be able to automatically generate a plan that can sample your data.

Usage

Create a sampling plan

The first step in subsetting a database is to generate a sampling plan. A sampling plan defines the queries that will be used to sample each table. You'll want to create a configuration file similar to planner_config.example.yaml that tells the planner what tables you want to sample along with any additional constraints that should be considered. Then you can create a plan with the below command:

subsetter plan -c my-config.yaml > plan.yaml

If you inspect the generated plan YAML document you will see a syntax tree that defines how each table will be sampled, potentially referencing other tables. Queries can reference either source tables or previously sampled tables. If you need to customize the way that tables are sampled beyond what the planner can automatically produce this is the place to do it. If needed, you can even write direct SQL here.

Sample a database with a plan

The sample sub-command will sample rows from the source database into your target output (either a database or as json files) using a plan generated using the plan sub-command. This tool will not copy schema from the source database. Any sampled tables must already exist in the destination database. Additionally you must pass --truncate if you wish to clear any existing data in the sampled tables that may interfere with the sampling process.

subsetter sample --config my-sample-config.yaml --plan my-plan.yaml --truncate

The sampling process proceeds in three phases:

1. If --truncate is specified any tables about to be sampled will be first truncated.
2. Any sampled tables that are referenced by other tables will first be materialized into temporary tables on the source database.
3. Data is copied for each table from the source to destination.

The sampler also supports filters which allow you to transform and anonymize your data using simple column filters. See sampler_config.sample.yaml for more details on what filters are available and how to configure them.

Plan and sample in one action

There's also a subset subcommand to perform the plan and sample actions together. This will automatically feed the generated plan into the sampler, in addition to ensuring the same source database configuration is used for each.

subsetter subset --plan-config my-plan-config.yaml --sample-config my-sample-config.yaml

Sampling Multiplicity

Sampling usually means condensing a large dataset into a semantically consistent small dataset. However, there are times that what you really want to do is create a semantically consistent large dataset from your existing data. The sampler has support for this by setting the multiplicity factor.

Multiplicity works by creating multiple copies of your sampled dataset in your output database. To ensure these datasets do not collide it remaps all foreign keys into a new key-space. Note that this process assumes your foreign keys are opaque integers identifiers.