libresilient/plugins/alt-fetch.js

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/* ========================================================================= *\
|* === 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(LRPC){
// this never changes
const pluginName = "alt-fetch"
LRPC.set(pluginName, (LR, init={})=>{
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/*
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* plugin config settings
*/
// sane defaults
let defaultConfig = {
// 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: [],
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// 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
}
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// merge the defaults with settings from the init var
let config = {...defaultConfig, ...init}
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// 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
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}
/**
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* getting content using regular HTTP(S) fetch()
*/
let fetchContentFromAlternativeEndpoints = (url, init={}) => {
// 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 really want to make fetch happen, Regina!
// TODO: this change should *probably* be handled on the Service Worker level
init.cache = 'reload'
// we don't want to modify the original endpoints array
var sourceEndpoints = [...config.endpoints]
// if we have fewer than the configured concurrency or just as many, 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
)
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}
}
// debug log
LR.log(pluginName, `fetching from alternative endpoints:\n ${useEndpoints.join('\n ')}`)
return Promise.any(
useEndpoints.map(
u=>fetch(u, init)
))
.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
LR.log(pluginName, "fetched:", response.url);
// we need to create a new Response object
// with all the headers added explicitly,
// since response.headers is immutable
var responseInit = {
status: response.status,
statusText: response.statusText,
headers: {},
url: url
};
response.headers.forEach(function(val, header){
responseInit.headers[header] = val;
});
// add the X-LibResilient-* headers to the mix
responseInit.headers['X-LibResilient-Method'] = pluginName
// 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
responseInit.headers['X-LibResilient-ETag'] = response.headers.get('ETag')
if (responseInit.headers['X-LibResilient-ETag'] === null) {
// far from perfect, but what are we going to do, eh?
responseInit.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,
responseInit
)
})
})
}
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// return the plugin data structure
return {
name: pluginName,
description: 'HTTP(S) fetch() using alternative endpoints',
version: 'COMMIT_UNKNOWN',
fetch: fetchContentFromAlternativeEndpoints
}
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})
// done with not polluting the global namespace
})(LibResilientPluginConstructors)