This document summarizes the various components involved with Azimuth and how they communicate with each other. This also constitutes an explanation for how Urbit implements the data flow of naive rollups.

Bridge

The primary way in which users interact with Azimuth is via Bridge. Bridge is responsible for collecting transactions from users, signing them, and forwarding them to a roller via an HTTP API.

Azimuth

Azimuth was originally defined as a set of smart contracts on Ethereum that defines the state and business logic of the PKI for layer 1. With the introduction of naive rollups, this has also come to include the set of components used for dealing with the PKI within Urbit, as now the complete PKI state is stored offchain (though this state is derived entirely from on-chain data). The following sections outline what each component is responsible for and how it communicates with the others.

The Gall agents involved with Azimuth are summarized as follows:

• %azimuth - obtains and holds PKI state.
• %azimuth-rpc - JSON RPC-API for %azimuth.
• %eth-watcher - Ethereum event log collector.
• %roller - submits batches of L2 transactions to Ethereum.
• %roller-rpc - JSON RPC-API for %roller.

The transaction processing library is /lib/naive.hoon.

Gall agents

%azimuth

%azimuth, located at /app/azimuth.hoon, is a Gall agent and thread handler responsible for finding Azimuth transactions gathered by %eth-watcher, keeping track of the PKI state, and exposing that data via scries.

The following diagram illustrates %azimuth's and %eth-watcher's role in the system.

The state held by %azimuth is the following.

++  app-state
  $:  %3
      url=@ta
      whos=(set ship)
      nas=^state:naive
      own=owners
      logs=(list =event-log:rpc:ethereum)
  ==

whos is the set of ships currently known by Azimuth. nas is the PKI state, as defined in naive.hoon. own is a jug of Ethereum addresses and the set of ships owned by that address. logs is a list of all Azimuth-related Ethereum event logs known by the ship.

Scries can be inferred from the +on-peek arm:

++  on-peek
  |=  =path
  ^-  (unit (unit cage))
  ?+  path  (on-peek:def path)
      [%x %logs ~]  ``noun+!>(logs.state)
      [%x %nas ~]   ``noun+!>(nas.state)
      [%x %dns ~]   ``noun+!>(dns.nas.state)
      [%x %own ~]   ``noun+!>(own.state)
  ==

%azimuth-rpc

%azimuth-rpc, located at app/azimuth-rpc.hoon, is a JSON RPC-API for getting point and dns data from the Azimuth PKI state kept by %azimuth.

%eth-watcher

%eth-watcher, located at /app/eth-watcher.hoon, is responsible for listening to an Ethereum node and collecting event logs from it. It is general-purpose and not particular to Azimuth. It sends collected transactions to +on-agent in %azimuth, which then obtains the resulting PKI state transitions by passing them to naive.hoon.

%roller

%roller, stored at /app/roller.hoon, is a Gall agent responsible for collecting and submitting batches of layer 2 transactions to the Ethereum blockchain. Among other things, it keeps track of a list of pending transactions to be sent, transactions it has sent that are awaiting confirmation, history of transactions sent organized by Ethereum address, and when the next batch of transactions will be sent. See also Rollers for more information on the roller.

The following diagram illustrates how the roller interacts with Bridge and Ethereum at a high level.

The relationship between the roller and other agents is outlined in the following diagram.

%roller has a number of scries available, intended primarily to display data to the end user in Bridge. They can be inferred from the +on-peek arm:

  ++  on-peek
    |=  =path
    ^-  (unit (unit cage))
    |^
    ?+  path  ~
      [%x %pending ~]       ``noun+!>(pending)
      [%x %pending @ ~]     (pending-by i.t.t.path)
      [%x %tx @ %status ~]  (status i.t.t.path)
      [%x %history @ ~]     (history i.t.t.path)
      [%x %nonce @ @ ~]     (nonce i.t.t.path i.t.t.t.path)
      [%x %spawned @ ~]     (spawned i.t.t.path)
      [%x %next-batch ~]    ``atom+!>(next-batch)
      [%x %point @ ~]       (point i.t.t.path)
      [%x %points @ ~]      (points i.t.t.path)
      [%x %config ~]        config
      [%x %chain-id ~]      ``atom+!>(chain-id)
    ==

This app is not responsible for communicating with Bridge via HTTP. Instead, that is handled by %roller-rpc. The scries are also communicated to Bridge via %roller-rpc.

%roller-rpc

%roller-rpc, stored at /app/roller-rpc.hoon, is a very simple Gall app responsible for receiving HTTP RPC-API calls, typically sent from other Urbit ID users via Bridge. It then translates these API calls from JSON to a format understood by %roller and forwards them to %roller. This app does not keep any state - its only purpose is to act as an intermediary between Bridge and %roller. See here for more information on the JSON RPC-API.

naive.hoon

/lib/naive.hoon consists of a gate whose sample is a verifier, chain-id=@ud, state, and input, which outputs a cell of [effects state]. This is the transition function which updates the state of the PKI stored in %azimuth which handles state transitions caused by both layer 1 and layer 2 transactions. A high-level overview of how naive.hoon functions can be found here.

A verifier is a gate whose sample is of the form [dat=octs v=@ r=@ s=@] and which returns (unit address):

+$  verifier  $-([dat=octs v=@ r=@ s=@] (unit address))

The verifier in use by naive.hoon runs the keccak hash function on dat to verify that dat is data signed by the ECDSA signature given by the [v r s] tuple, according to the format for signed transactions outlined in the bytestring format documentation.

chain-id is the ID used by the Ethereum blockchain, which is 1337. See bytestring format for more information. This is used so that e.g. transactions on the Ropsten test network cannot be replayed on the mainnet.

state is the current state of the PKI. This is structured similarly to the state held in Azimuth.eth, but will differ in general since state takes into account layer 2 transactions as well. See the Layer 2 Overview for more on how PKI state is handled.

+$  state
  $:  =points
      =operators
      dns=(list @t)
  ==
+$  points     (tree [ship point])
++  point
  $:  ::  domain
      ::
      =dominion
      ::
      ::  ownership
      ::
      $=  own
      $:  owner=[=address =nonce]
          spawn-proxy=[=address =nonce]
          management-proxy=[=address =nonce]
          voting-proxy=[=address =nonce]
          transfer-proxy=[=address =nonce]
      ==
      ::
      ::  networking
      ::
      $=  net
      $:  rift=@ud
          =keys
          sponsor=[has=? who=@p]
          escape=(unit @p)
      ==
  ==
+$  dominion  ?(%l1 %l2 %spawn)
+$  operators  (jug address address)

points should be self-explanatory if you are already familiar with the structure of Azimuth.eth. The only new addition is dominion, whose value says whether a ship is on layer 1, layer 2, or layer 1 with a layer 2 spawn proxy. See Layer 2 actions for an overview of how dominion determines the PKI actions available to a ship.

operators already existed on layer 1 and are defined as a part of the ERC-721 standard.

dns is a list of DNS entries by which galaxy IP addresses may be looked up. At present, this is always ~['urbit.org' 'urbit.org' 'urbit.org'].