urbit/base/pub/doc/arvo.md

arvo
====

Our operating system.

arvo is composed of modules called vanes:

<list dataPreview="true"></list>

<hr></hr>

At a high level `%arvo` takes a mess of unix io events and turns them
into something clean and structured for the programmer.

`%arvo` is designed to avoid the usual state of complex event networks:
event spaghetti. We keep track of every event's cause so that we have a
clear causal chain for every computation. At the bottom of every chain
is a unix io event, such as a network request, terminal input, file
sync, or timer event. We push every step in the path the request takes
onto the chain until we get to the terminal cause of the computation.
Then we use this causal stack to route results back to the caller.

`++ducts`
---------

The `%arvo` causal stack is called a `++duct`. This is represented
simply as a list of paths, where each path represents a step in the
causal chain. The first element in the path is the first letter of
whichever vane handled that step in the computation, or the empty span
for unix.

Here's a duct that was recently observed in the wild:

    ~[
      /g/a/~zod/4_shell_terminal/u/time
      /g/a/~zod/shell_terminal/u/child/4/main
      /g/a/~zod/terminal/u/txt
      /d/term-mess
      //term/1
    ]

This is the duct the timer vane receives when "timer" sample app asks
the timer vane to set a timer. This is also the duct over which the
response is produced at the specified time. Unix sent a terminal
keystroke event (enter), and arvo routed it to %dill(our terminal),
which passed it on to the %gall app terminal, which sent it to shell,
its child, which created a new child (with process id 4), which on
startup asked the timer vane to set a timer.

The timer vane saves this duct, so that when the specified time arrives
and unix sends a wakeup event to the timer vane, it can produce the
response on the same duct. This response is routed to the place we
popped off the top of the duct, i.e. the time app. This app produces the
text "ding", which falls down to the shell, which drops it through to
the terminal. Terminal drops this down to dill, which converts it into
an effect that unix will recognize as a request to print "ding" to the
screen. When dill produces this, the last path in the duct has an
initial element of the empty span, so this is routed to unix, which
applies the effects.

This is a call stack, with a crucial feature: the stack is a first-class
citizen. You can respond over a duct zero, one, or many times. You can
save ducts for later use. There are definitely parallels to Scheme-style
continuations, but simpler and with more structure.

Making Moves
------------

If ducts are a call stack, then how do we make calls and produce
results? Arvo processes "moves" which are a combination of message data
and metadata. There are two types of moves. A `%pass` move is analogous
to a call:

    [duct %pass return-path=path vane-name=@tD data=card]

Arvo pushes the return path (preceded by the first letter of the vane
name) onto the duct and sends the given data, a card, to the vane we
specified. Any response will come along the same duct with the path
`return-path`.

A `%give` move is analogous to a return:

    [duct %give data=card]

Arvo pops the top path off the duct and sends the given card back to the
caller.

Vanes
-----

As shown above, we use arvo proper to route and control the flow of
moves. However, arvo proper is rarely directly responsible for
processing the event data that directly causes the desired outcome of a
move. This event data is contained within a card, which is simply a
`(pair term noun)`. Instead, arvo proper passes the card off to one of
its vanes, which each present an interface to clients for a particular
well-defined, stable, and general-purpose piece of functionality.

As of this writing, we have seven vanes, which each provide the
following services:

-   `%ames` name of both our network and the vane that communicates over
    it
-   `%clay` version-controlled, referentially- transparent, and global
    filesystem
-   `%dill` terminal driver. Unix sends keyboard events to `%dill` from
    either the console or telnet, and `%dill` produces terminal output.
-   `%eyre` http server. Unix sends http messages to `%eyre`, and
    `%eyre` produces http messages in response
-   `%ford` handles resources and publishing
-   `%gall` manages our userspace applications.. `%gall` keeps state and
    manages subscribers
-   `%time` a simple timer

Cards
-----

Cards are the vane-specific portion of a move. Each vane defines a
protocol for interacting with other vanes (via arvo) by defining four
types of cards: kisses, gifts, notes, and signs.

When one vane is `%pass`ed a card in its `++kiss`, arvo activates the
`++call` gate with the card as its argument. To produce a result, the
vane `%give`s one of the cards defined in its `++gift`. If the vane
needs to request something of another vane, it `%pass`es it a `++note`
card. When that other vane returns a result, arvo activates the `++take`
gate of the initial vane with one of the cards defined in its `++sign`.

In other words, there are only four ways of seeing a move: (1) as a
request seen by the caller, which is a ++note. (2) that same request as
seen by the callee, a `++kiss`. (3) the response to that first request
as seen by the callee, a `++gift`. (4) the response to the first request
as seen by the caller, a `++sign`.

When a `++kiss` card is passed to a vane, arvo calls its `++call` gate,
passing it both the card and its duct. This gate must be defined in
every vane. It produces two things in the following order: a list of
moves and a possibly-modified copy of its context. The moves are used to
interact with other vanes, while the new context allows the vane to save
its state. The next time arvo activates the vane it will have this
context as its subject.

This overview has detailed how to pass a card to a particular vane. To
see the cards each vane can be `%pass`ed as a `++kiss` or return as a
`++gift` (as well as the semantics tied to them), each vane's public
interface is explained in detail in its respective overview.