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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. For a more thorough introduction to Reshape, check out the [introductory blog post](https://fabianlindfors.se/blog/schema-migrations-in-postgres-using-reshape/).
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.
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`:
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:
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](#connection-options) for details on how to change the connection settings.
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`.
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`:
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
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.
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.
The `remove_enum` action will remove an existing [enum type](https://www.postgresql.org/docs/current/datatype-enum.html). Make sure all usages of the enum has been removed before running the migration. The enum will only be removed once the migration is completed.
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](#connection-options)*
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 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.<br>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 |
