WASM and WASI
There are have been many attempts to create safer ways of exeucting untrusted code downloaded over the network. Java Applets, Adobe Flash, and more recently Javascript have all had their day. Each has had its upsides and downsides, but a persistent problem throughout is performance. The latest contender, WebAssembly, seeks to resolve this problem.
WebAssembly or WASM is a low-level assembly-style language that acts as a compilation target for high level languages. This means that compilers can take advantage of code optimizations that are not available in the Java Virtual Machine or Javascript runtimes. However WASM suffers from the drawback that low-level code can't operate on complex data structures and system APIs without adapter code. That's where WASI, the WebAssembly System Interface, comes into play.
WASI is a set of standardized operations for allowing WASM guests--compiled WASM meant to be and run in an isolated environment--to access common host functionality, like reading and writing to files or making network requests. How the host responds to these requests is up to the WASM runtime. For example, a host may allow guests to access the entire filesystem or just a single file. It may receive network requests and modify them before sending them, or simply let them pass through to the OS network interface.
Next-Gen Web Apps
Being able to customize how the host responds to WASI requests from the guest opens new doors to building web apps. Functionality that has traditionally been restricted to web browsers can now be cutomized to support specific kinds of guests.
As a concrete example, when WebRTC was standardized to support video and voice calls in the browser, browser vendors had to find ways to support these new standards. With WASI, WebRTC could have been implemented by creating an adapter to convert WebRTC API requests to WASI network requests. The adapter itself could be shipped as a WASM component to support WebRTC without any need to update the host code.
This idea is general enough to allow developers to build highly customized hosts that support specific operatiosn that would be challenging to implement in existing browsers. These hosts could provide better support for peer-to-peer applications and faster updates with no host downtime.
Conclusion
WASM and WASI are emerging standards that can be used to support next-gen web apps. These apps can be built upon and go beyond what is currently possible with web browsers, effectivly creating a more modular ecosystem for building and distributing apps.