Skip to main content
CDN applications built on the Proxy-Wasm ABI interact with HTTP requests and responses through a property system. Since each hook (onRequestHeaders, onRequestBody, onResponseHeaders, onResponseBody) runs in its own isolated WASM instance, properties provide the mechanism for reading request/response data and modifying CDN behavior throughout the request lifecycle.

​
How properties work

Properties are accessed through two core methods:
  • get_property(path): Reads data from the current request context
  • set_property(path, value): Modifies writable properties to customize CDN behavior
Each property is identified by a path (e.g., request.path, response.status) and has specific access permissions based on the hook where it’s accessed.

​
Property access patterns

Read-only properties: Can be read but not modified. These typically represent immutable request data or CDN internal state. Read-write properties: Can be both read and modified. Use these to customize headers, adjust routing, or modify response behavior. Hook-specific availability: Some properties are only accessible in certain hooks. For example, response properties are unavailable in request hooks.

​
Available properties

The following table lists all available properties and their access permissions. These properties can only be written to within the onRequestHeaders hook:
Property PathTypeAccessDescription
request.urlStringRead-writeOriginal URL path before modifications
request.hostStringRead-writeHost header value
request.pathStringRead-writeRequest URL path
request.schemeStringRead-onlyProtocol scheme (http/https)
request.methodStringRead-onlyHTTP method (GET, POST, etc.)
request.extensionStringRead-onlyRequest path extension
request.queryStringRead-writeQuery string parameters
request.countryStringRead-onlyCountry Code - deciphered from IP
request.cityStringRead-onlyCity name - deciphered from IP
request.asnStringRead-onlyASN of the network/ISP associated with the request IP
request.geo.latStringRead-onlyLatitude - deciphered from IP
request.geo.longStringRead-onlyLongitude - deciphered from IP
request.regionStringRead-onlyRegion - deciphered from IP
request.continentStringRead-onlyContinent - deciphered from IP
request.country.nameStringRead-onlyCountry name - deciphered from IP
nginx.log_field1StringWrite-onlyAdds value to nginx access logs
response.statusIntegerRead-onlyHTTP status code

​
nginx.log_field1

This field is write-only (onRequestHeaders). Adding a value here will ensure it is written to CDN access log, available through the Log uploader feature.

​
Custom properties

You can also create custom properties within hooks. This allows you to track data between hook runs. e.g. custom property markdown These custom hooks are all read-write, however they are only available from onRequestBody onwards. Anything added within onRequestHeaders will not be available, in future hooks.

​
Example: Response modification

// In onResponseHeaders
fn on_response_headers() {
    // Add Custom markdown property
    if let Some(content_type) = self.get_http_response_header("Content-Type") {
        if content_type.starts_with("text/plain") || content_type.starts_with("text/markdown") {
            self.set_http_response_header("Content-Length", None);
            self.set_http_response_header("Transfer-Encoding", Some("Chunked"));
            self.set_http_response_header("Content-Type", Some("text/html"));
            self.set_property(vec!["response.markdown"], Some(b"true"));
            println!("Response is markdown, on_response_body can now convert to HTML");
        }
    }
}

// In onResponseBody
fn on_http_response_body(&mut self, body_size: usize, end_of_stream: bool) -> Action {
    // only process markdown
    if None == self.get_property(vec!["response.markdown"]) {
        return Action::Continue;
    }
}

​
Property modification best practices

Validate before setting: Always validate property values before modification to prevent errors. Use appropriate hooks: Modify request properties only in onRequestHeaders. Handle missing properties: Not all properties are available in all contexts. Use error handling when accessing optional properties.
NoteProperty paths are case-sensitive. Header names follow HTTP conventions (lowercase with hyphens).