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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 MinzPDOSQLite('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.