reshape/README.md

# Reshape

[![Test status badge](https://github.com/fabianlindfors/Reshape/actions/workflows/test.yaml/badge.svg)](https://github.com/fabianlindfors/Reshape/actions/workflows/test.yaml)

Reshape is an easy-to-use, zero-downtime schema migration tool for Postgres. It automatically handles complex migrations that would normally require downtime or manual multi-step changes. During a migration, Reshape ensures both the old and new schema are available at the same time, allowing you to gradually roll out your application. 

*Note: Reshape is **experimental** and should not be used in production. It can (and probably will) destroy your data and break your application.*

- [Getting started]()
	- [Installation]()
	- [Creating your first migration]()
	- [Preparing your application]()
	- [Running your migration]()
- [Writing migrations]()
	- [Basics]()
	- [Tables]()
		- [Create table]()
		- [Rename table]()
		- [Drop table]()
	- [Columns]()
		- [Create column]()
		- [Alter column]()
		- [Remove column]()
- [How it works]()

## Getting started

### Installation

On macOS:

```brew install reshape```

On Debian:

```apt-get install reshape```

### Creating your first migration

Each migration should be stored as a separate file under `migrations/`. The files can be in either JSON or TOML format. The name of the file will become the name of your migration and they will be sorted by file name. We recommend prefixing every migration with an incrementing number.

Let's create a simple migration to set up a new table `users` with two fields, `id` and `name`. We'll create a file called `migration/1_create_users_table.toml`:

```toml
[[actions]]
type = "create_table"
table = "users"

	[[actions.columns]]
	name = "id"
	type = "SERIAL"

	[[actions.columns]]
	name = "name"
	type = "TEXT"
```

This is the equivalent of running `CREATE TABLE users (id SERIAL, name TEXT)`.

### Preparing your application

Reshape relies on your application using a specific schema. When establishing the connection to Postgres in your application, you need to run a query to select the most recent schema. This query can be generated using: `reshape generate-schema-query`.

To pass it along to your application, you could use an environment variable in your build script: `RESHAPE_SCHEMA_QUERY=$(reshape generate-schema-query)`. Then in your application:

```python
# Example for Python
reshape_schema_query = os.getenv("RESHAPE_SCHEMA_QUERY")
db.execute(reshape_schema_query)
```

### Running your migration

To create your new `users` table, run:

```bash
reshape migrate
```

As this is the first migration, Reshape will automatically complete it. For subsequent migrations, you will need to first run `reshape migrate`, roll out your application and then complete the migration using `reshape complete`.

## Writing migrations

### Basics

Every migration consists of one or more actions. The actions will be run sequentially. Here's an example of a migration with two actions to create two tables, `customers` and `products`:

```toml
[[actions]]
type = "create_table"
table = "customers"

	[[actions.columns]]
	name = "id"
	type = "SERIAL"

[[actions]]
type = "create_table"
table = "products"

	[[actions.columns]]
	name = "sku"
	type = "TEXT"
```

Every action has a `type`. The supported types are detailed below.

### Create table

The `create_table` action will create a new table with the specified columns and indices.

*Example: creating a `customers` table with a few columns and a primary key*

```toml
[[actions]]
type = "create_table"
table = "customers"
primary_key = "id"

	[[actions.columns]]
	name = "id"
	type = "SERIAL"

	[[actions.columns]]
	name = "name"
	type = "SERIAL"

	# Columns default to nullable
	nullable = false 

	# default can be any valid SQL value, in this case a string literal
	default = "'PLACEHOLDER'" 
```

### Add column

The `add_column` action will add a new column to an existing table. You can optionally provide an `up` setting. This should be an SQL expression which will be run for all existing rows to backfill the new column.

*Example: add a new column `reference` to table `products`*

```toml
[[actions]]
type = "add_column"
table = "products"

	[actions.column]
	name = "reference"
	type = "INTEGER"
	nullable = false
	default = "10"
```

*Example: replace an existing `name` column with two new columns, `first_name` and `last_name`*

```toml
[[actions]]
type = "add_column"
table = "users"

# Extract the first name from the existing name column
up = "(STRING_TO_ARRAY(name, ' '))[1]"

	[actions.column]
	name = "first_name"
	type = "TEXT"


[[actions]]
type = "add_column"
table = "users"

# Extract the last name from the existing name column
up = "(STRING_TO_ARRAY(name, ' '))[2]"

	[actions.column]
	name = "last_name"
	type = "TEXT"


[[actions]]
type = "remove_column"
table = "users"
column = "name"

# Reconstruct name column by concatenating first and last name
down = "first_name || ' ' || last_name"
```


### Alter column

The `alter_column` action enables many different changes to an existing column, for example renaming, changing type and changing existing values.

When performing more complex changes than a rename, `up` and `down` must be provided. These should be set to SQL expressions which determine how to transform between the new and old version of the column. Inside those expressions, you can reference the current column value by the column name.

*Example: rename `last_name` column on `users` table to `family_name`*

```toml
[[actions]]
type = "alter_column"
table = "users"
column = "last_name"

	[actions.changes]
	name = "family_name"
```

*Example: change the type of `reference` column from `INTEGER` to `TEXT`*

```toml
[[actions]]
type = "alter_column"
table = "users"
column = "reference"

up = "CAST(reference AS TEXT)" # Converts from integer value to text
down = "CAST(reference AS INTEGER)" # Converts from text value to integer

	[actions.changes]
	type = "TEXT" # Previous type was 'INTEGER'
```

*Example: increment all values of a `index` column by one*

```toml
[[actions]]
type = "alter_column"
table = "users"

up = "index + 1" # Increment for new schema
down = "index - 1" # Decrement to revert for old schema

	[actions.changes]
	name = "index"

```


## How it works

Reshape works by creating views that encapsulate the underlying tables, which your application will interact with. During a migration, Reshape will automatically create a new set of views and set up triggers to translate inserts and updates between the old and new schema. This means that every migration is a two phase process:

1. **Start migration** (`reshape migrate`): Create new views to ensure both the new and old schema are usable at the same time.
	- After phase one is complete, you can start the roll out of your application. Once the roll out is complete, the second phase can be run.
2. **Complete migration** (`reshape complete`): Removes the old schema and any intermediate data.