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# Developer Usage Guide
This guide explains how developers can use the moz-cached-ohttp protocol in Firefox code.
## Basic Usage
The moz-cached-ohttp protocol is designed to be used like any other resource loading mechanism in Firefox,
with the key differences being that it provides privacy-preserving OHTTP-based loading with automatic caching,
and cannot be used for regular content, so only privileged contexts can use the scheme.
## Creating Resource Requests
To load a resource via the moz-cached-ohttp protocol, construct a URL in the following format:
```javascript
const encodedURL = encodeURIComponent(targetImageURL);
```
**Important**: The host must be `newtab-image` - other hosts are not currently supported and will result in an error.
:::{note}
Target URLs must use the HTTPS protocol. HTTP URLs will be rejected.
:::
### Using with Image Elements
The protocol can be used directly in HTML image elements within privileged contexts.
```html
```
### Using with Fetch API
In privileged contexts, you can use the Fetch API:
```javascript
try {
const response = await fetch(ohttpImageURL);
if (response.ok) {
const blob = await response.blob();
// Use the image blob
}
} catch (error) {
console.error("Failed to load image:", error);
}
```
## Performance Best Practices
### Cache Utilization
The protocol automatically handles caching, but you can optimize usage:
- **Consistent URLs**: Use the same encoded URL format for the same target resource
- **Have the server provide appropriate cache lifetimes**: The longer the lifetime, the less we may need to refetch the resource on the same client.
- **Respect cache headers**: Don't bypass cache unnecessarily with cache-busting parameters
- **Monitor cache hits**: In development, verify that repeated requests hit the cache