# Database files

## Database files

Starcounter database consists of the following items:

| Name                                  | Number | Description                      | Required | Platform Specific |
| ------------------------------------- | ------ | -------------------------------- | -------- | ----------------- |
| `📁 archive`                          | 0 - 1  | Archived transaction log files.  | No       | No                |
| `📜 DatabaseName.000000000000.log`    | 0 - ∞  | Transaction log file.            | Yes      | No                |
| `📜 DatabaseName.000000000000.optlog` | 0 - ∞  | Compressed transaction log file. | Yes      | No                |
| `📜 DatabaseName.cfg`                 | 1      | Database configuration file.     | Yes      | Yes               |
| `📜 starcounter.000000000000.log`     | 0 - ∞  | Database events log.             | No       | No                |

* **Required**: specifies whether this item is a required database part; for example, when moving or backing up a database.
* **Platform Specific**: `No` - the file is platform independent; `Yes` - the file is platform specific and not cross-compatible between Windows / Linux versions of Starcounter.

**Note**: due to the `.cfg` file being platform specific, it is not currently possible to copy databases between Windows and Linux.

## Transaction log files

The very first database transaction log file is created during the first database startup. A created, but never started database does not contain any transaction log files.

When database grows, Starcounter:

* Creates new transaction log files.
* Compresses existing transaction log files into `.optlog` files.
* Moves compressed transaction log files into the `archive` folder.

**\*Note**: due to the current limitations, it is important to start the database at least once before using it from multiple different processes.\*

## Database configuration file

This file contains all required information about the database.

## Database events log

Starcounter logs database events into this file. Sample content:

```
20191118T090843 Debug DatabaseName Starcounter - Database load started.
20191118T090843 Debug DatabaseName Starcounter - Database load completed. 0 transactions recovered.
```

## Deleting database files while `scdata` is running

On Windows, it's not possible to delete database files while the `scdata` process is running. On Linux operating systems, however, it is, which may lead to potential issues. Consider the following scenario:

* A database is created at `/home/database/TestDatabase`.
* An `scdata` process is started for the database.
* The database files are deleted.
* New database is created in exactly the same location.
* An app tries to connect to the new database.
* Starcounter detects, that there is a running `scdata` process associated with this database path.
* The app connects to the existing `scdata` process, which still operates on the deleted files. The filesystem will keep those files around for the `scdata` process, as long as it's running.
* The app continues to run as expected, but the next time `scdata` is restarted, for example due to a system restart, the database files will be removed from the system completely, taking all the app's database data with them.


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.starcounter.io/3.0.0/database-files.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.