# Request

> The payload of the channel request: the tag byte 0 and the request of the channel.

Canonical: https://protocol.diverge.network/2.3.0/container-proxy-endpoints/tools-begin/server/postgres/request/
Specification revision: 2.3.0

```text
[0][connection_id: u32, big-endian]
```

- **`connection_id`.** Four bytes, big-endian, the id of the
  connection.

The channel request is defined by `diverge-sdk-rs/src/container_proxy/outside/endpoints/tools/begin/server/channel_request/frame.rs`:

```rust
//! What a server's channel request frame carries for a tool
//! container begin.

use std::convert::Infallible;
use std::fmt;

use crate::wire::decode::Decode;
use crate::wire::encode::{Encode, Writer};
use crate::shared::containers::{daemon, postgres};

/// What the proxy asks the server for once a tool container
/// has begun.
///
/// A payload leads with one byte saying which, and the rest is that
/// variant's own bytes.
///
/// | tag | asks for |
/// |-----|----------|
/// | `0` | [`Postgres`](Self::Postgres) |
/// | `1` | [`Daemon`](Self::Daemon) |
///
/// Two: everything a tool container asks of the world outside — its
/// database connections, its daemon connection — which the server
/// relays to the caller as the provider protocol's own channel
/// requests. No tool call outward: a tool container makes none, and
/// a proxy serving one answers its program's MCP client with an
/// error; an agent container's begin has those asks and no daemon
/// connection. The fuse asks are not here; each rides the scope of
/// the mount it belongs to.
#[derive(Debug, Clone, PartialEq)]
pub enum Frame {
    /// The proxy's half of a database connection the container's
    /// driver opened. Tag `0`.
    ///
    /// An id the proxy minted. What comes back is everything the
    /// database says; the server opens the other half, or declines by
    /// finishing this one with nothing before it. See
    /// [`postgres`](crate::shared::containers::postgres).
    Postgres(postgres::request::Postgres),
    /// A daemon connection the program opened, by the id the proxy
    /// minted. Tag `1`.
    ///
    /// This half is the proxy's own. What comes back on it is
    /// everything the DAEMON says, as
    /// [`daemon::server::Frame`](crate::shared::containers::daemon::server::Frame)s;
    /// its finish is the connection closed, and an empty finish is the
    /// connection declined. See
    /// [`daemon`](crate::shared::containers::daemon) for the pair.
    Daemon(daemon::request::Daemon),
}

/// Tag for [`Frame::Postgres`].
const POSTGRES: u8 = 0;

/// Tag for [`Frame::Daemon`].
const DAEMON: u8 = 1;

impl Encode for Frame {
    /// [`Infallible`]: both asks are an id, bytes copied.
    type Error = Infallible;

    fn encode(&self, out: &mut Writer<'_>) -> Result<(), Infallible> {
        match self {
            Frame::Postgres(request) => {
                out.extend_from_slice(&[POSTGRES]);
                request.encode(out)
            }
            Frame::Daemon(request) => {
                out.extend_from_slice(&[DAEMON]);
                request.encode(out)
            }
        }
    }
}

impl Decode<'_> for Frame {
    /// Four ways to fail, and each names which ask it was.
    type Error = FrameError;

    fn decode(bytes: &[u8]) -> Result<Self, Self::Error> {
        let (tag, rest) = bytes.split_first().ok_or(FrameError::Empty)?;
        match *tag {
            POSTGRES => postgres::request::Postgres::decode(rest)
                .map(Frame::Postgres)
                .map_err(FrameError::Postgres),
            DAEMON => daemon::request::Daemon::decode(rest)
                .map(Frame::Daemon)
                .map_err(FrameError::Daemon),
            tag => Err(FrameError::UnknownTag(tag)),
        }
    }
}

/// A tools begin channel request that could not be read.
#[derive(Debug)]
pub enum FrameError {
    /// No bytes at all, so not even a tag.
    Empty,
    /// A tag that is none of this frame's two.
    UnknownTag(u8),
    /// The connection id was not four bytes.
    Postgres(postgres::request::PostgresError),
    /// The daemon connection's id was not four bytes.
    Daemon(daemon::request::DaemonError),
}

impl fmt::Display for FrameError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            FrameError::Empty => {
                f.write_str("tools begin channel request frame is empty")
            }
            FrameError::UnknownTag(tag) => {
                write!(f, "unknown tools begin channel request tag {tag}")
            }
            FrameError::Postgres(error) => write!(f, "{error}"),
            FrameError::Daemon(error) => write!(f, "{error}"),
        }
    }
}

impl std::error::Error for FrameError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            FrameError::Daemon(error) => Some(error),
            FrameError::Postgres(error) => Some(error),
            FrameError::Empty | FrameError::UnknownTag(_) => None,
        }
    }
}
```

and by `diverge-sdk-rs/src/shared/containers/postgres/request/postgres.rs`:

```rust
//! Naming a connection.

use std::error::Error;
use std::fmt;

use crate::wire::decode::Decode;
use crate::wire::encode::{Encode, Writer};

/// Which connection a channel is one half of.
///
/// The provider opens its half carrying this with an id it minted,
/// and the caller opens the other half quoting the same id. That is
/// the whole of the correlation: a caller with several connections in
/// flight learns which one it is being asked to feed.
///
/// # It is the provider's to choose and the provider's to keep unique
///
/// Unique among the connections this provider has open in this scope
/// — reusing one that is still live makes two connections
/// indistinguishable, and the provider is the only party that could
/// have prevented it. A number that counts up is the obvious way and
/// nothing requires it. Reusing one after both channels have finished
/// is fine; nothing here remembers.
///
/// # Why not the channel it arrived on
///
/// Because channels are numbered per SENDER. The provider opens its
/// half on a channel of its own, the caller opens the other on a
/// channel of its own, and neither side's header can name the other's
/// — so a payload quoting a channel number would be quoting one out of
/// a namespace its reader does not share. An id invented for the
/// connection belongs to neither channel, which is what makes it
/// readable on both sides — the same argument a
/// [`write_id`](crate::shared::containers::write_path::request::Request::write_id)
/// makes.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
pub struct Postgres {
    /// What this connection is called, chosen by the provider.
    pub connection_id: u32,
}

/// The bytes a connection id occupies.
const CONNECTION_ID_LEN: usize = 4;

/// Four big-endian bytes. No serialization, because a fixed-width
/// integer does not need one.
impl Encode for Postgres {
    /// [`Infallible`](std::convert::Infallible): four known bytes.
    type Error = std::convert::Infallible;

    fn encode(&self, out: &mut Writer<'_>) -> Result<(), Self::Error> {
        out.extend_from_slice(&self.connection_id.to_be_bytes());
        Ok(())
    }
}

impl Decode<'_> for Postgres {
    /// One way to fail: the wrong number of bytes.
    type Error = PostgresError;

    fn decode(bytes: &[u8]) -> Result<Self, Self::Error> {
        <[u8; CONNECTION_ID_LEN]>::try_from(bytes)
            .map(|bytes| Postgres {
                connection_id: u32::from_be_bytes(bytes),
            })
            .map_err(|_| PostgresError::Length(bytes.len()))
    }
}

/// A Postgres channel request that could not be read.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum PostgresError {
    /// A payload that was not four bytes, carrying however many there
    /// were.
    Length(usize),
}

impl fmt::Display for PostgresError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            PostgresError::Length(len) => {
                write!(f, "postgres connection request is {len} bytes, not 4")
            }
        }
    }
}

impl Error for PostgresError {}
```