# Overview

## Built-in Engines

Currently, NocoBase supports the following built-in engine types:

- [Local Storage](./local.md)
- [Alibaba Cloud OSS](./aliyun-oss.md)
- [Amazon S3](./amazon-s3.md)
- [Tencent Cloud COS](./tencent-cos.md)

A local storage engine is automatically added during system installation and can be used directly. You can also add new engines or edit the parameters of existing ones.

## Common Engine Parameters

In addition to the specific parameters for different engine types, the following are common parameters (using local storage as an example):


![Example of File Storage Engine Configuration](https://static-docs.nocobase.com/20240529115151.png)


### Title

The name of the storage engine, used for human identification.

### System Name

The system name of the storage engine, used for system identification. It must be unique across the system. If left blank, it will be randomly generated by the system.

### Access base URL

The prefix of the URL address for external access to the file. This can be the base URL of a CDN, for example: "`https://cdn.nocobase.com/app`" (without the trailing "`/`").

### Path

The relative path used when storing files. This part will also be automatically concatenated to the final URL when accessed. For example: "`user/avatar`" (without the leading or trailing "`/`").

### File Size Limit

The size limit for files uploaded to this storage engine. Files exceeding this size cannot be uploaded. The system default limit is 20MB, and the maximum adjustable limit is 1GB.

### File Type

Limits the types of files that can be uploaded, using the [MIME](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types) syntax description format. For example, `image/*` represents image files. Multiple types can be separated by commas, such as `image/*, application/pdf` to allow both image and PDF files.

### Default Storage Engine

When checked, this is set as the system's default storage engine. When an attachment field or file collection does not specify a storage engine, uploaded files will be saved to the default storage engine. The default storage engine cannot be deleted.

### Keep Files When Destroying Records

When checked, uploaded files in the storage engine are retained even when the data records in the attachment or file collection are deleted. By default, this is unchecked, meaning files in the storage engine are deleted along with the records.

:::info{title=Tip}
After a file is uploaded, the final access path is constructed by concatenating several parts:

```
<Access base URL>/<Path>/<FileName><Extension>
```
For example: `https://cdn.nocobase.com/app/user/avatar/20240529115151.png`.
:::