libresilient/plugins/alt-fetch.js

/* ========================================================================= *\
|* === HTTP(S) fetch() from alternative endpoints                        === *|
\* ========================================================================= */

/**
 * this plugin does not implement any push method
 * 
 * NOTICE: this plugin uses Promise.any()
 *         https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/any
 *         the polyfill is implemented in LibResilient's service-worker.js
 */

// no polluting of the global namespace please
(function () {

    /*
     * plugin config settings
     */
    
    // sane defaults
    let defaultConfig = {
        // name of this plugin
        // should not be changed
        name: "alt-fetch",
        // endpoints to use
        // 
        // they have to respond to requests formatted like:
        // <endpoint-url>/<path>
        // 
        // let's say the endpoint is:
        // https://example.com/api/endpoint/
        // ...and that we are trying to get:
        // <original-domain>/some/path/img.png
        // 
        // the endpoint is supposed to return the expected image
        // when this URL is requested:
        // https://example.com/api/endpoint/some/path/img.png
        // 
        // this has to be explicitly configured by the website admin
        endpoints: [],
 
        // how many simultaneous connections to different endpoints do we want
        // 
        // more concurrency means higher chance of a request succeeding
        // but uses more bandwidth and other resources;
        // 
        // 3 seems to be a reasonable default
        concurrency: 3
    }

    // merge the defaults with settings from LibResilientConfig
    let config = {...defaultConfig, ...self.LibResilientConfig.plugins[defaultConfig.name]}

    // reality check: endpoints need to be set to an array of non-empty strings
    if (typeof(config.endpoints) !== "object" || !Array.isArray(config.endpoints)) {
        let err = new Error("endpoints not confgured")
        console.error(err)
        throw err
    }
    

    /**
     * getting content using regular HTTP(S) fetch()
     */
    let fetchContentFromAlternativeEndpoints = (url) => {
        
        // we're going to try a random endpoint and building an URL of the form:
        // https://<endpoint_address>/<pubkey>/<rest_of_URL>
        var path = url.replace(/https?:\/\/[^/]+\//, '')
        
        // we don't want to modify the original endpoints array
        var sourceEndpoints = [...config.endpoints]
        
        // if we have fewer than the configured concurrency, use all of them
        if (sourceEndpoints.length <= config.concurrency) {
            var useEndpoints = sourceEndpoints
        // otherwise get `config.concurrency` endpoints at random
        } else {
            var useEndpoints = new Array()
            while (useEndpoints.length < config.concurrency) {
                // put in the address while we're at it
                useEndpoints.push(
                    sourceEndpoints
                        .splice(Math.floor(Math.random() * sourceEndpoints.length), 1)[0] + path
                )
            }
        }
        
        // debug log
        self.log(config.name, `fetching from alternative endpoints:\n  ${useEndpoints.join('\n  ')}`)
        
        return Promise.any(
            useEndpoints.map(
                u=>fetch(u, {cache: "reload"})
            ))
            .then((response) => {
                // 4xx? 5xx? that's a paddlin'
                if (response.status >= 400) {
                    // throw an Error to fall back to other plugins:
                    throw new Error('HTTP Error: ' + response.status + ' ' + response.statusText);
                }
                // all good, it seems
                self.log(config.name, "fetched:", response.url);
                
                // we need to create a new Response object
                // with all the headers added explicitly,
                // since response.headers is immutable
                var init = {
                    status:     response.status,
                    statusText: response.statusText,
                    headers: {}
                };
                response.headers.forEach(function(val, header){
                    init.headers[header] = val;
                });
                
                // add the X-LibResilient-* headers to the mix
                init.headers['X-LibResilient-Method'] = config.name
                
                // we will not have it most of the time, due to CORS rules:
                // https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_response_header
                init.headers['X-LibResilient-ETag'] = response.headers.get('ETag')
                if (init.headers['X-LibResilient-ETag'] === null) {
                    // far from perfect, but what are we going to do, eh?
                    init.headers['X-LibResilient-ETag'] = response.headers.get('last-modified')
                }
                
                // return the new response, using the Blob from the original one
                return response
                        .blob()
                        .then((blob) => {
                            return new Response(
                                blob,
                                init
                            )
                        })
            })
    }

    // and add ourselves to it
    // with some additional metadata
    self.LibResilientPlugins.push({
        name: config.name,
        description: 'HTTP(S) fetch() using alternative endpoints',
        version: 'COMMIT_UNKNOWN',
        fetch: fetchContentFromAlternativeEndpoints
    })
    
// done with not poluting the global namespace
})()