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Design Alternatives

This document outlines the various designs that have been proposed for cross-origin communication between ServiceWorkers. It also includes ideas for features/optimizations of the implementation that are of interest, but perhaps not core to the fundamental design.

This document is meant to be a work in progress that should be updated as new designs and ideas are brought to the table.

Designs for Cross-Origin Communication Between Service Workers

MessagePorts

  • navigator.connect("https://example.com/services/v1") can be accepted/rejected by a Service Worker that has registered to handle this URL
  • A MessagePort is opened between the origins on accept
  • Explicit connection attempt to a Service Worker allows potential for browser to pull down the Service Worker if it is not installed

Local look-aside fetch()

  • A local_fetch() request to https://example.com/services/v1 is routed to a local Service Worker if one is installed
  • The local_fetch() does not then go to the network if no Service Worker is installed for the relevant scope
  • This is also a somewhat explicit connection attempt to a Service Worker, so the browser could potentially pull down the Service Worker if it is not installed

Universal look-aside fetch()

  • Same as the local look-aside fetch(), except if no Service Worker is installed for the relevant scope, then the `fetch()`` goes to the network just as it would in today's world
  • This new mode for fetch() would need to be enabled explicitly, either as an option in the Request or as a new kind of scope the cross-origin Service Worker could opt into.

Features & Optimizations

MessagePort lifetime management

Stashed MessagePorts would allow a Service Worker to transfer a MessagePort to or from "external" ownership, in order to allow any MessagePort to outlive the JavaScript context of a Service Worker.

A variant of this idea might allow a Service Worker to keep using the MessagePort as an object, but record that it should be stashed when the Service Worker is destroyed. Then the top-level script could reconstitute its object graph, including MessagePorts when it receives a message.