# EPF C4 - Tomás Arjovsky - Week 11

These are the updates for the week of 2023/09/25.

## First update

The first update is a bit short as monday was a holiday here. I've mostly been writing documentation so that new collaborators can better navigate the ethspecs when entering the project. I'll be publishing those asap!

I also had a technical discussion with Tomás Grüner on how to implement the request-response pattern in ports, as the PoC I built is mostly used for pubsub style communication.

I came up with a skeleton of an idea (without taking care of the encoding/decoding) that can be used to use the pubsub as a request response using the serialized pids as request ids:

def request(message) do
    GenServer.cast(__MODULE__, {:request, self(), message})
    receive do
        response -> response
    end
end

def handle_cast({:request, pid, message}, state) do  
    send_port(Map.put(message, :id, :erlang.term_to_binary(pid)))
    {:noreply, state}
end

def handle_info(response, state) do
    send(:erlang.binary_to_term(response.id), response)
end

Right now the PoC already uses UUIDs as request ids, so we might as well use this without much code change

## Second Update

### Nix

I updated nix so that it was compatible with OTP 26 and elixir 15, which are the versions that the latest OSX version requires. I tried the environment from scratch and tested that the app worked. [My PR](https://github.com/lambdaclass/lambda_ethereum_consensus/pull/246) was merged, so people that don't want to install all languages manually and globally, should now be able to bootstrap their environment very quickly with nix.

### Documentation updates

Updated the architecture diagrams to include a more realist interaction with levelDB, more similar to what we'll use on the next milestone.

graph LR
subgraph "Consensus Node"
  engine[Engine API Client]
  BAPI[Beacon API]
  TICK[Slot processor]
	blk_db[Block DB]
	BS_db[Beacon State DB]
  brod[Broadway]
	FCTree[Fork choice tree - Genserver]
  BAPI -->|Beacon state queries| BS_db
	brod -->|Save blocks| blk_db
	brod -->|Blocks and attestations| FCTree
	TICK -->|New ticks| FCTree
  BAPI --> engine
	BAPI --> |head/slot requests| FCTree 
	brod --> |Save new states|BS_db
end
GOS[Gossip Protocols]
exec[Execution Client]
VALIDATOR[Validator]
engine <--> |payload validation, execution| exec
GOS -->|blocks, attestations| brod
VALIDATOR --> BAPI

This now reflects:

- There's no more "state transition server". Gossip protocol communicates with Broadway, which in turn calls `on_block` and `on_attestation` handlers. At a higher level, this means that state transition happens in broadway tasks and are not sequentialized unnecessarily. This is also supported by leveldb being thread-safe.
- Fork choice updates now go to the fork choice tree, which is a genserver holding the fork-choice data in memory. Eventually we'll add persistence for most of the fork-choice state.
- The DBs are logically separated in Beacon state and block DBs.