# abstract migrate

Abstract migrate is a data-agnostic tool for running scripts (e.g., migrations) once.

## Usage

Install with `yarn`:
 
    $ yarn global add abstract-migrate
 
 or `npm`:
     
    $ npm i -g abstract-migrate

Now you can execute the provided CLI interface:

    $ abstract-migrate --help

or

    $ am --help

### Create an Engine

This library was written to be storage agnostic. You will need to write some code to tell
`abstract-migrate` how to store details about things it has ran. Your engine is expected to export
five methods:

```js
module.exports = {
  // Returns all of the loaded migrations from your storage in the format:
  // [{ name, timestamp }, ...] 
  load: function(/* cb */) {
    // ...
  },
  
  // Persist a collection of migrations to your storage. Migrations will be an array in the same
  // format as the `load` function: [{ name, timestamp }, ...]
  add: function(migrations/*, cb */) {
    // ...
  },
  
  // Remove a collection of migrations to your storage. Migrations will be an array in the same
  // format as the `load` and `add` function: [{ name, timestamp }, ...]
  remove: function(migrations/*, cb */) {
    // ...
  },
  
  // Ask the storage for a lock to prevent running migrations multiple times. This function should
  // return `true` if it was able to acquire a lock or `false` otherwise.
  acquireLock: function(/* cb */) {
    // ...
  },
  
  // This will denote that the process is done and the storage engine can release whatever lock it
  // acquired earlier.
  releaseLock: function(/* cb */) {
    // ...
  },
};
```

All of the above functions can return a promise that resolves to the desired value, or use the
callback that is passed as the last argument to every engine function.

There is a basic [filesystem engine](src/engines/fileEngine.js) that can be used as an example
engine implementation.

**Caution:** Pay extra attention to make your `acquireLock` function is atomic so that it does not
create a race condition where two simultaneous executions can acquire a lock at the same time. If
there is no case where this command will be run multiple times, you can "ignore" locking by
immediately resolving your `acquireLock` function to `true`.

### Configuration

Now that you have your engine, you will need to tell `abstract-migrate` how to use it. You can
configure this library via the command line interface (see the CLI help output `am --help`) or via
a JSON file. Using the JSON file is the recommended option. By default, this library will look for
a file named `.abstract-migrate.json`, but you can configure this location using the CLI. The
configuration file accepts the following options:

 - **engine**: The migration storage engine to use
 - **require**: A JavaScript file (or array of files) to require before running
 - **noColor**: Disables color output (default: `false`)
 - **migrationPath**: The directory where migration files are stored (default: `migrations`)

#### Example `.abstract-migrate.json`

```json
{
  "engine": "./src/postgresEngine.js",
  "require": "babel-register",
  "migrationPath": "db/migrations",
  "noColor": true
}
```

### Commands

#### create 

    $ am create my-cool-migration

This command will create a migration of the given name in the migrations folder. The migration will
be a simple file that exports an `up` and `down` function. The `up` and `down` functions can either
return a promise or call the provided callback function.

#### list (alias: ls)

    $ am list

This command will list all of the migrations in the migration directory and denote whether or not a
given migration has been run.

#### up

    $ am up

This command will run all of the unran migrations. You can pass the `--ignore-past` (`-p`) to not
run migrations that are older than the most recently successful migration. 

You can specify a migration name to migrate _up to and including_ the named migration:

    $ am up 1492708337968-my-cool-migration

or specifying the number of migrations to apply:

    $ am up 2

or specifying exactly one migration to apply:

    $ am up --only 1492708337968-my-cool-migration

You can pass the `--dry-run` (`-d`) flag to preview which migrations will be run.

#### down

    $ am down 2

This command will roll back the specified number of migrations. You can also specify a migration
name to migrate _down to and including_ the named migration:

    $ am down 1492708337968-my-cool-migration

or specifying exactly one migration to run down:

    $ am down --only 1492708337968-my-cool-migration

You can pass the `--dry-run` (`-d`) flag to preview which migrations will be run down.

#### rollback

    $ am rollback

This command will undo the most recently applied set of migrations. You can pass the `--dry-run`
(`-d`) flag to preview which migrations will be run down.