> For the complete documentation index, see [llms.txt](https://docs.fact-finder.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.fact-finder.com/integration/tracking-interface/user-login.md).
# User login
If you use the Personalization module, FactFinder needs to know which session belongs to which user. To do that, send a `login` event whenever a user logs in to your shop.
| Parameter | | Description |
| --------- | --------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `sid` | essential | Use this to pass the user’s session identifier. It is only needed to link the session to the user and must be anonymized. |
| `userId` | essential | Use this to pass a user identifier. Like the session ID, it must be anonymized. Unlike sid, it stays the same when users return. This parameter is required for this event.
If your shop does not use permanent user accounts, provide another identifier with a longer lifespan than a session, such as a cookie ID. That allows personalization to reuse what it learned in the user’s next shopping session. You can also first assign the session to the cookie through the login event and later reassign it to the actual user ID once it becomes known.
|
## REST
The REST API documentation is part of the search application and is available at \[your NG server]/fact-finder/swagger-ui.html. You can also access it here:
Swagger Link:
In addition to the parameter documentation, you can test requests and inspect the results directly. Because the documentation is built with Swagger, you can also use Swagger Codegen to generate a client.
Login event documentation is available through the `login` method in the `tracking` resource.
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