velo-ext 0.1.0

Extension trait surface for Velo. External crates implement Transport, FrameTransport, and PeerDiscovery against this stable contract without depending on the Velo runtime.
Documentation 
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// SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

//! Transport observability extension surface.
//!
//! Out-of-tree transport implementors integrate with the runtime's metrics
//! pipeline through the [`TransportObservability`] trait. The runtime hands
//! each transport a pre-bound observability handle via
//! [`Transport::set_observability`](crate::transport::Transport::set_observability);
//! the transport then calls [`record_frame`](TransportObservability::record_frame),
//! [`record_rejection`](TransportObservability::record_rejection), etc. to
//! emit data into the same `velo_transport_*` metric series as the in-tree
//! transports — without depending on the runtime crate or its concrete
//! metrics implementation.

/// Direction of a transport frame.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Direction {
    /// Inbound frame (received from peer).
    Inbound,
    /// Outbound frame (sent to peer).
    Outbound,
}

impl Direction {
    /// Prometheus label value.
    pub fn as_str(self) -> &'static str {
        match self {
            Self::Inbound => "inbound",
            Self::Outbound => "outbound",
        }
    }
}

/// Reason a transport rejected or dropped a frame.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum TransportRejection {
    /// Send operation failed.
    SendError,
    /// Message rejected during graceful drain.
    DrainRejected,
    /// Frame decode or preamble parse failed.
    DecodeError,
    /// Failed to route frame to adapter channel.
    RouteFailed,
    /// NATS message missing required headers.
    MissingHeaders,
    /// Velo-Type header missing (NATS).
    MissingType,
    /// Invalid Velo-Type value (NATS).
    InvalidType,
    /// Invalid Velo-HLen header (NATS).
    InvalidHeaderLength,
    /// Frame shorter than declared header length (NATS).
    TruncatedFrame,
    /// Failed to build ShuttingDown response (gRPC).
    DrainReplyBuildFailed,
}

impl TransportRejection {
    /// Prometheus label value.
    pub fn as_str(self) -> &'static str {
        match self {
            Self::SendError => "send_error",
            Self::DrainRejected => "drain_rejected",
            Self::DecodeError => "decode_error",
            Self::RouteFailed => "route_failed",
            Self::MissingHeaders => "missing_headers",
            Self::MissingType => "missing_type",
            Self::InvalidType => "invalid_type",
            Self::InvalidHeaderLength => "invalid_header_length",
            Self::TruncatedFrame => "truncated_frame",
            Self::DrainReplyBuildFailed => "drain_reply_build_failed",
        }
    }
}

/// Observability hook handed to a [`Transport`](crate::transport::Transport)
/// during [`set_observability`](crate::transport::Transport::set_observability).
///
/// Implementors of `Transport` invoke these methods to publish metrics into
/// the runtime's collectors. The runtime's concrete handle pre-binds the
/// transport's `key` label so the trait surface stays free of label-management
/// concerns.
///
/// All methods take `&self` and have implementations that are typically lock-free
/// — they are safe to call from any hot path. Default impls are intentionally
/// not provided: every method represents an observable signal a real
/// implementation would care about.
pub trait TransportObservability: Send + Sync {
    /// Record an accepted frame.
    ///
    /// `message_type` is one of the well-known
    /// [`MessageType`](crate::transport::MessageType) label strings:
    /// `"message"`, `"response"`, `"ack"`, `"event"`, or `"shutting_down"`.
    fn record_frame(&self, direction: Direction, message_type: &str, bytes: usize);

    /// Record a rejected or dropped frame.
    fn record_rejection(&self, reason: TransportRejection);

    /// Set the gauge for the number of registered peers on this transport.
    fn set_registered_peers(&self, count: usize);

    /// Set the gauge for the number of active connections on this transport.
    fn set_active_connections(&self, count: usize);

    /// Record a send that hit the bounded per-peer channel's `Full` branch and
    /// fell through to the [`SendBackpressure`](crate::transport::SendBackpressure) await path.
    fn record_send_backpressure(&self);
}