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# How to manage migrations with Minz
Migrations are the way to modify the database or the structure of files under the `data/` path.
## How to write a migration?
Migrations are placed under the `app/migrations` folder.
Good practice is to prepend the filename by the current date and explain what does the migration do in few words (e.g. `2020_01_11_CreateFooTable.php`).
The files must contain a class which name starts with `FreshRSS_Migration_`, followed by the basename of the file (e.g. `FreshRSS_Migration_2020_01_11_CreateFooTable`).
The class must declare a `migrate` static function. It must return `true` or a string to indicate the migration is applied, or `false` otherwise. It can also raise an exception: the message will be used to detail the error.
Example:
```php
// File: app/migrations/2020_01_11_CreateFooTable.php
class FreshRSS_Migration_2020_01_11_CreateFooTable {
public static function migrate() {
$pdo = new Minz_PdoSqlite('sqlite:/some/path/db.sqlite');
$result = $pdo->exec('CREATE TABLE foos (bar TEXT)');
if ($result === false) {
$error = $pdo->errorInfo();
raise Exception('Error in SQL statement: ' . $error[2]);
}
return true;
}
}
```
## How to apply migrations?
They are automatically applied one by one when a user accesses FreshRSS.
Before being applied, migrations are sorted by filenames (see the [`strnatcmp`](https://php.net/strnatcmp) function). Already applied migrations are skipped (the list can be found in the `data/applied_migrations.txt` file).
To ensure migrations are not applied several times if two users access FreshRSS at the same time, a folder named `data/applied_migrations.txt.lock` is created, then deleted at the end of the process.
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