PortunusD

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portunusd is a network application server inspired by OpenBSD's relayd and heirloom UNIX inetd. It listens for an incoming network connection, forwarding the incoming data over an illumos door to the intended application, and returning the response in a similar manner. portunusd maps each connected port to a door on the filesystem provided by the target application.

sequenceDiagram
    participant Client
    participant PortunusD
    participant Door
    participant Application
    Application->>Door: Create /var/run/app.door
    PortunusD->>Door: Open
    PortunusD->>PortunusD: listen on port 80
    loop Handle Requests
        Client->>+PortunusD: Send HTTP Request
        PortunusD->>+Application: Forward request via door_call
        Application->>-PortunusD: Send response via door_return
        PortunusD->>-Client: Send HTTP Response
    end

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The main goal of portunusd is to facilitate the scaling of single-threaded applications. Under the inetd model, a new process is created to handle every request. By leveraging doors, portunusd can create a new thread in the application process only when a new highwater mark of concurrency has been reached; otherwise, existing threads will be re-used to handle subsequent requests.

Problem Statement

We want our network-facing applications to scale according to user demand. We want to minimize the resource cost of our applications when they are idle, and we want to keep our costs linear in terms of demand. We want to minimize the degree to which the application developer is responsible for resource management, and we want to retain (so far as possible) the familiar development environment of UNIX command line tools.

Picking on Rails as an example, a single-threaded Ruby on Rails application can handle one user request at a time. Multiple simultaneous requests cannot be handled without multiple copies of the application resident in memory (on separate Ruby interpreters). This model consumes a great deal of memory even when there is little user demand, making it difficult for the host to run other workloads. Much paging and gnashing of disk will ensue.

Environments such as Node.js deal with this problem by making asynchronicity more transparent to the programmer. While it can be useful to embrace the asynchronous nature of computers, it has also introduced changes to languages that support it; this is not a mere change of syntax, but also a nontrivial change to the mental model one uses to read, write, and understand programs.

At the other end of the spectrum, CGI applications require a unique process and address space for each request. These applications can scale linearly with user demand, including scaling down to zero memory / cpu usage when idle, but the cost of invoking execv(2) for each request can hamper throughput.

The postmodern "Serverless" approach satisfies these criteria, but at the cost of abandoning the operating sytem. It is a wildly unfamiliar approach to developing software, and throws away many tools that could be used to observe and debug the application at runtime.

Thesis

Doors enable a new (old?) model of network application development wherein the developers are responsible for maintaining and understanding a linear, synchronous task, while the operating system + web server work together on the scaling problem

• When an application is idle, only a single copy of is needed in memory.
• When a request enters the system, it can be handled by an existing thread.
• New threads are created only when a new peak of concurrency is reached.

These qualities allow us to address our problem statement by developing network applications that feel like single-threaded UNIX command line tools, present a minimal expense when idle, and scale linearly on a per-request granularity.

Of course, doors alone will not handle scaling across the boundary of a single operating system instance, but a relayd-style collaboration with the firewall could facilitate this, assuming copies of the application are available on multiple hosts. This is where portunusd comes in.

Acknowledgements

The social media preview image is by Loudon dodd - Own work, CC BY-SA 3.0.

Many obscure illumos / Rust / Doors questions were answered by @jasonbking.