# Request

> The payload of the channel request: the tag byte 4 and the request of the channel.

Canonical: https://provider.diverge.network/2.3.0/endpoints/containers-agents-run/client/postgres/request/
Specification revision: 2.3.0

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

- **`connection_id`.** The id the server announced on its own
  postgres channel, as four bytes, big-endian.
- **Malformed.** A channel request whose payload does not decode as
  stated is answered by a channel response finish that no channel
  response precedes, and the server does nothing else for it.

The channel request is defined by `diverge-provider-sdk/src/endpoints/containers/agents/run/client/channel_request/frame.rs`:

```rust
//! What a client's channel request frame carries for an agent container run.

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

use crate::decode::Decode;
use crate::encode::{Encode, Writer};
use crate::shared::containers::{enqueue, run_loop};
use crate::shared::containers::{postgres, read, write_path};

/// What a caller asks a provider for while an agent container runs.
///
/// A payload leads with one byte saying which, and the rest is that
/// variant's own bytes.
///
/// | tag | asks for |
/// |-----|----------|
/// | `0` | [`Stop`](Self::Stop) |
/// | `1` | [`Filetree`](Self::Filetree) |
/// | `2` | [`Read`](Self::Read) |
/// | `3` | [`Write`](Self::Write) |
/// | `4` | [`Postgres`](Self::Postgres) |
/// | `5` | [`RunLoop`](Self::RunLoop) |
/// | `6` | [`AgentSchema`](Self::AgentSchema) |
/// | `7` | [`Enqueue`](Self::Enqueue) |
/// | `8` | [`Dequeue`](Self::Dequeue) |
///
/// The first five are the same in every container scope, in the same
/// order, so a reader of one is a reader of all; what follows is this
/// family's own exchange. All of them but the first reach INTO the
/// container, which is the thing a caller cannot dial: it runs on the
/// provider. That is the whole reason these channels open outward
/// from the client rather than the other way.
#[derive(Debug, Clone, PartialEq)]
pub enum Frame {
    /// Stop the container. Tag `0`.
    ///
    /// # It has no answer, and does not need one
    ///
    /// Nothing comes back on this channel. What comes back is the end
    /// of the SCOPE — a
    /// [`ResponseFinish`](crate::frame::server::ServerFrame::ResponseFinish),
    /// which already means nothing bearing this scope follows on any
    /// channel. Finishing this one first would be a smaller way of
    /// saying the same thing, moments earlier.
    ///
    /// # What it adds over closing the connection
    ///
    /// The scope IS the container's life, so dropping the connection
    /// stops it too. The difference is that a provider cannot tell a
    /// deliberate exit from a network that stopped answering, and has
    /// to wait to find out. This is unambiguous and immediate: a
    /// caller that says so is not gone, it is finished.
    ///
    /// # What it does to everyone else
    ///
    /// Ends them. Connectors hold scopes on a container that no longer
    /// exists, so those scopes finish too — a connection cannot
    /// outlive the thing it joined.
    Stop,
    /// The container's filesystem, watched. Tag `1`.
    ///
    /// Carries nothing — the variant is bare — and the provider answers
    /// with a snapshot and then every change, for as long as the
    /// channel lives. See
    /// [`filetree`](crate::shared::containers::filetree).
    Filetree,
    /// One file, read out of the container. Tag `2`.
    ///
    /// See [`read`](crate::shared::containers::read) for why this is
    /// one file and never a directory.
    Read(read::request::Request),
    /// One file, written into the container. Tag `3`.
    ///
    /// Carries no content. The provider answers by opening a channel
    /// of its own asking for it — see
    /// [`write_path`](crate::shared::containers::write_path) for why
    /// it travels that direction, and
    /// [`write_bytes`](crate::shared::containers::write_bytes) for
    /// what comes back.
    Write(write_path::request::Request),
    /// The caller's half of a database connection. Tag `4`.
    ///
    /// Opened once the caller has taken the provider's half, quoting
    /// the same connection; what comes back is everything the
    /// container wrote. See
    /// [`postgres`](crate::shared::containers::postgres) for the pair.
    Postgres(postgres::request::Postgres),
    /// Run a loop. Tag `5`.
    ///
    /// The family's own exchange. Carries the prompt — the agent was
    /// on the request that made the container, and never changes —
    /// and answers with the loop's chunks. See
    /// [`run_loop`](crate::shared::containers::run_loop).
    RunLoop(run_loop::request::Request),
    /// What the agent may be. Tag `6`.
    ///
    /// Carries nothing — the variant is bare — and the provider answers
    /// with the JSON Schema of the agent value. See
    /// [`agent_schema`](crate::shared::containers::agent_schema).
    AgentSchema,
    /// A message for the running loop's queue. Tag `7`.
    ///
    /// Answered once — by an
    /// [`enqueue::response::Frame`](crate::shared::containers::enqueue::response::Frame)
    /// naming the message's fate, whenever that is known — and then
    /// the finish. See [`enqueue`](crate::shared::containers::enqueue).
    Enqueue(enqueue::request::Request),
    /// Withdraw every message still waiting in the queue. Tag `8`.
    ///
    /// Carries nothing — the variant is bare. Answered once — by a
    /// [`dequeue::response::Frame`](crate::shared::containers::dequeue::response::Frame)
    /// saying whether the queue held anything — and then the finish.
    /// Each message it withdraws is ALSO answered, on its own enqueue
    /// channel. See [`dequeue`](crate::shared::containers::dequeue).
    Dequeue,
}

/// Tag for [`Frame::Stop`].
const STOP: u8 = 0;

/// Tag for [`Frame::Filetree`].
const FILETREE: u8 = 1;

/// Tag for [`Frame::Read`].
const READ: u8 = 2;

/// Tag for [`Frame::Write`].
const WRITE: u8 = 3;

/// Tag for [`Frame::Postgres`].
const POSTGRES: u8 = 4;

/// Tag for [`Frame::RunLoop`].
const RUN_LOOP: u8 = 5;

/// Tag for [`Frame::AgentSchema`].
const AGENT_SCHEMA: u8 = 6;

/// Tag for [`Frame::Enqueue`].
const ENQUEUE: u8 = 7;

/// Tag for [`Frame::Dequeue`].
const DEQUEUE: u8 = 8;

impl Encode for Frame {
    /// The ordinary JSON failure, from whichever half has one.
    type Error = serde_json::Error;

    fn encode(&self, out: &mut Writer<'_>) -> Result<(), Self::Error> {
        match self {
            Frame::Stop => {
                out.extend_from_slice(&[STOP]);
                Ok(())
            }
            Frame::Filetree => {
                out.extend_from_slice(&[FILETREE]);
                Ok(())
            }
            Frame::Read(request) => {
                out.extend_from_slice(&[READ]);
                request.encode(out)
            }
            Frame::Write(request) => {
                out.extend_from_slice(&[WRITE]);
                request.encode(out)
            }
            Frame::Postgres(request) => {
                out.extend_from_slice(&[POSTGRES]);
                request.encode(out).map_err(|error| match error {})
            }
            Frame::RunLoop(request) => {
                out.extend_from_slice(&[RUN_LOOP]);
                request.encode(out)
            }
            Frame::AgentSchema => {
                out.extend_from_slice(&[AGENT_SCHEMA]);
                Ok(())
            }
            Frame::Enqueue(request) => {
                out.extend_from_slice(&[ENQUEUE]);
                request.encode(out)
            }
            Frame::Dequeue => {
                out.extend_from_slice(&[DEQUEUE]);
                Ok(())
            }
        }
    }
}

impl Decode<'_> for Frame {
    /// Several ways to fail, and the parses among them name which.
    type Error = FrameError;

    fn decode(bytes: &[u8]) -> Result<Self, Self::Error> {
        let (tag, rest) = bytes.split_first().ok_or(FrameError::Empty)?;
        match *tag {
            STOP => Ok(Frame::Stop),
            FILETREE => Ok(Frame::Filetree),
            READ => read::request::Request::decode(rest)
                .map(Frame::Read)
                .map_err(FrameError::Read),
            WRITE => write_path::request::Request::decode(rest)
                .map(Frame::Write)
                .map_err(FrameError::Write),
            POSTGRES => postgres::request::Postgres::decode(rest)
                .map(Frame::Postgres)
                .map_err(FrameError::Postgres),
            RUN_LOOP => run_loop::request::Request::decode(rest)
                .map(Frame::RunLoop)
                .map_err(FrameError::RunLoop),
            AGENT_SCHEMA => Ok(Frame::AgentSchema),
            ENQUEUE => enqueue::request::Request::decode(rest)
                .map(Frame::Enqueue)
                .map_err(FrameError::Enqueue),
            DEQUEUE => Ok(Frame::Dequeue),
            tag => Err(FrameError::UnknownTag(tag)),
        }
    }
}

/// An agent container run 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.
    UnknownTag(u8),
    /// The read request did not parse.
    Read(serde_json::Error),
    /// The write request did not parse.
    Write(serde_json::Error),
    /// The connection id was not four bytes.
    Postgres(postgres::request::PostgresError),
    /// The loop's prompt did not parse as JSON.
    RunLoop(serde_json::Error),
    /// The enqueued message did not parse as JSON.
    Enqueue(serde_json::Error),
}

impl fmt::Display for FrameError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            FrameError::Empty => {
                f.write_str("agents run channel request frame is empty")
            }
            FrameError::UnknownTag(tag) => {
                write!(f, "unknown agents run channel request tag {tag}")
            }
            FrameError::Read(error) => {
                write!(f, "read request did not parse: {error}")
            }
            FrameError::Write(error) => {
                write!(f, "write request did not parse: {error}")
            }
            FrameError::Postgres(error) => write!(f, "{error}"),
            FrameError::RunLoop(error) => {
                write!(f, "run loop request did not parse: {error}")
            }
            FrameError::Enqueue(error) => {
                write!(f, "enqueue request did not parse: {error}")
            }
        }
    }
}

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

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

```rust
//! Naming a connection.

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

use crate::decode::Decode;
use crate::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 {}
```