#postgres #migrations

bin+lib reshape

An easy-to-use, zero-downtime schema migration tool for Postgres

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#152 in Database interfaces

Download history 14/week @ 2021-12-31 37/week @ 2022-01-07 37/week @ 2022-01-14

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Reshape

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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. It will also perform all changes without excessive locking, avoiding downtime caused by blocking other queries.

Reshape is designed for Postgres 12 and later.

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

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 deployment is a three-phase process:

  1. Start migration (reshape migrate): Sets up views and triggers to ensure both the new and old schema are usable at the same time.
  2. Roll out application: Your application can be gradually rolled out without downtime. The existing deployment will continue using the old schema whilst the new deployment uses the new schema.
  3. Complete migration (reshape complete): Removes the old schema and any intermediate data and triggers.

If the application deployment fails, you should run reshape abort which will roll back any changes made by reshape migrate without losing data.

Getting started

Installation

Binaries

Binaries are available for macOS and Linux under Releases.

Cargo

Reshape can be installed using Cargo:

cargo install reshape

Docker

Reshape is available as a Docker image on Docker Hub.

docker run -v $(pwd):/usr/share/app fabianlindfors/reshape reshape migrate

Creating your first migration

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

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 migrations/1_create_users_table.toml:

[[actions]]
type = "create_table"
name = "users"
primary_key = ["id"]

	[[actions.columns]]
	name = "id"
	type = "INTEGER"
	generated = "ALWAYS AS IDENTITY"

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

This is the equivalent of running CREATE TABLE users (id INTEGER GENERATED ALWAYS AS IDENTITY, 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 can for example use an environment variable in your run script: RESHAPE_SCHEMA_QUERY=$(reshape generate-schema-query). Then in your application:

# 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:

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.

If nothing else is specified, Reshape will try to connect to a Postgres database running on localhost using postgres as both username and password. See Connection options for details on how to change the connection settings.

Using during development

When adding new migrations during development, we recommend running reshape migrate but skipping reshape complete. This way, the new migrations can be iterated on by updating the migration file and running reshape abort followed by reshape migrate.

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:

[[actions]]
type = "create_table"
name = "customers"
primary_key = ["id"]

	[[actions.columns]]
	name = "id"
	type = "INTEGER"
	generated = "ALWAYS AS IDENTITY"

[[actions]]
type = "create_table"
name = "products"
primary_key = ["sku"]

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

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

Tables

Create table

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

Example: create a customers table with a few columns and a primary key

[[actions]]
type = "create_table"
name = "customers"
primary_key = ["id"]

	[[actions.columns]]
	name = "id"
	type = "INTEGER"
	generated = "ALWAYS AS IDENTITY"

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

	# Columns default to nullable
	nullable = false 

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

Example: create users and items tables with a foreign key between them

[[actions]]
type = "create_table"
name = "users"
primary_key = ["id"]

	[[actions.columns]]
	name = "id"
	type = "INTEGER"
	generated = "ALWAYS AS IDENTITY"

[[actions]]
type = "create_table"
name = "items"
primary_key = ["id"]

	[[actions.columns]]
	name = "id"
	type = "INTEGER"
	generated = "ALWAYS AS IDENTITY"

	[[actions.columns]]
	name = "user_id"
	type = "INTEGER"

	[[actions.foreign_keys]]
	columns = ["user_id"]
	referenced_table = "users"
	referenced_columns = ["id"]

Rename table

The rename_table action will change the name of an existing table.

Example: change name of users table to customers

[[actions]]
type = "rename_table"
table = "users"
new_name = "customers"

Remove table

The remove_table action will remove an existing table.

Example: remove users table

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

Columns

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

[[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

[[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 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

[[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

[[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 an index column by one

[[actions]]
type = "alter_column"
table = "users"
column = "index"

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

	[actions.changes]
	name = "index"

Example: change default value of created_at column to current time

[[actions]]
type = "alter_column"
table = "users"
column = "created_at"

	[actions.changes]
	default = "NOW()"

Remove column

The remove_column action will remove an existing column from a table. You can optionally provide a down setting. This should be an SQL expression which will be used to determine values for the old schema when inserting or updating rows using the new schema. The down setting must be provided when the removed column is NOT NULL or doesn't have a default value.

Example: remove column name from table users

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

# Use a default value of "N/A" for the old schema when inserting/updating rows
down = "'N/A'"

Indices

Add index

The add_index action will add a new index to an existing table.

Example: create a users table with a unique index on the name column

[[actions]]
type = "create_table"
table = "users"
primary_key = "id"

	[[actions.columns]]
	name = "id"
	type = "INTEGER"
	generated = "ALWAYS AS IDENTITY"

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

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

	[actions.index]
	name = "name_idx"
	columns = ["name"]

	# Defaults to false
	unique = true

Example: add GIN index to data column on products table

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

	[actions.index]
	name = "data_idx"
	columns = ["data"]

	# One of: btree (default), hash, gist, spgist, gin, brin
	type = "gin"

Remove index

The remove_index action will remove an existing index. The index won't actually be removed until the migration is completed.

Example: remove the name_idx index

[[actions]]
type = "remove_index"
index = "name_idx"

Commands and options

reshape migrate

Starts a new migration, applying all migrations under migrations/ that haven't yet been applied. After the command has completed, both the old and new schema will be usable at the same time. When you have rolled out the new version of your application which uses the new schema, you should run reshape complete.

Options

See also Connection options

Option Default Description
--complete, -c false Automatically complete migration after applying it.
--dirs migrations/ Directories to search for migration files. Multiple directories can be specified using --dirs dir1 dir2 dir3.

reshape complete

Completes migrations previously started with reshape complete.

Options

See Connection options

reshape abort

Aborts any migrations which haven't yet been completed.

Options

See Connection options

reshape generate-schema-query

Generates the SQL query you need to run in your application before using the database. This command does not require a database connection. Instead it will generate the query based on the latest migration in the migrations/ directory (or the directories specified by --dirs).

The query should look something like SET search_path TO migration_1_initial_migration.

Options

Option Default Description
--dirs migrations/ Directories to search for migration files. Multiple directories can be specified using --dirs dir1 dir2 dir3.

Connection options

The options below can be used with all commands that communicate with Postgres. Use either a connection URL or specify each connection option individually.

Option Default Description
--url URI to connect to your Postgres database.
Can also be provided with the environment variable DATABASE_URL.
--host localhost Hostname to use when connecting to Postgres
--port 5432 Port which Postgres is listening on
--database postgres Database name
--username postgres Postgres username
--password postgres Postgres password

License

Reshape is released under the MIT license.

Dependencies

~14MB
~291K SLoC

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