Trait Backend 

pub trait Backend: Send + Sync {
    type Error: Error + Display + Send + Sync + From<BackendWalkError> + 'static;
    type Tensor: Clone + Send + Sync + 'static + Storage;

    // Provided methods
    fn add(
        &self,
        a: &Self::Tensor,
        b: &Self::Tensor,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn sub(
        &self,
        a: &Self::Tensor,
        b: &Self::Tensor,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn mul(
        &self,
        a: &Self::Tensor,
        b: &Self::Tensor,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn div(
        &self,
        a: &Self::Tensor,
        b: &Self::Tensor,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn neg(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error> { ... }
    fn abs(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error> { ... }
    fn sqrt(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error> { ... }
    fn pow(
        &self,
        a: &Self::Tensor,
        b: &Self::Tensor,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn exp(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error> { ... }
    fn log(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error> { ... }
    fn matmul(
        &self,
        a: &Self::Tensor,
        b: &Self::Tensor,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn reduce_sum(
        &self,
        a: &Self::Tensor,
        axes: &[i64],
        keepdims: bool,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn reduce_mean(
        &self,
        a: &Self::Tensor,
        axes: &[i64],
        keepdims: bool,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn reduce_max(
        &self,
        a: &Self::Tensor,
        axes: &[i64],
        keepdims: bool,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn reduce_min(
        &self,
        a: &Self::Tensor,
        axes: &[i64],
        keepdims: bool,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn reshape(
        &self,
        a: &Self::Tensor,
        shape: &[i64],
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn transpose(
        &self,
        a: &Self::Tensor,
        perm: &[i64],
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn concat(
        &self,
        inputs: &[&Self::Tensor],
        axis: i64,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn slice(
        &self,
        a: &Self::Tensor,
        starts: &[i64],
        ends: &[i64],
        axes: &[i64],
        steps: &[i64],
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn split(
        &self,
        a: &Self::Tensor,
        axis: i64,
        sizes: &[i64],
    ) -> Result<Vec<Self::Tensor>, Self::Error> { ... }
    fn squeeze(
        &self,
        a: &Self::Tensor,
        axes: &[i64],
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn unsqueeze(
        &self,
        a: &Self::Tensor,
        axes: &[i64],
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn identity(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error> { ... }
    fn cast(
        &self,
        a: &Self::Tensor,
        dtype: i32,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn equal(
        &self,
        a: &Self::Tensor,
        b: &Self::Tensor,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn greater(
        &self,
        a: &Self::Tensor,
        b: &Self::Tensor,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn less(
        &self,
        a: &Self::Tensor,
        b: &Self::Tensor,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn where(
        &self,
        cond: &Self::Tensor,
        t: &Self::Tensor,
        f: &Self::Tensor,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn constant(&self, value: TensorProto) -> Result<Self::Tensor, Self::Error> { ... }
    fn gather(
        &self,
        data: &Self::Tensor,
        indices: &Self::Tensor,
        axis: i64,
    ) -> Result<Self::Tensor, Self::Error> { ... }
    fn execute(
        &self,
        graph: &GraphProto,
        inputs: HashMap<String, Self::Tensor>,
        _attrs: BackendAttrs<'_>,
    ) -> Result<HashMap<String, Self::Tensor>, Self::Error> { ... }
    fn dispatch(
        &self,
        graph: &GraphProto,
        inputs: HashMap<String, Self::Tensor>,
        attrs: BackendAttrs<'_>,
        completion: CompletionHandle<HashMap<String, Self::Tensor>, Self::Error>,
    ) -> ContractResponse<HashMap<String, Self::Tensor>, Self::Error> { ... }
    fn materialize_from_wire(
        &self,
        type_hash: u64,
        bytes: Vec<u8>,
    ) -> Result<Self::Tensor, Self::Error> { ... }
}

User-facing Contract trait for a tensor compute backend.

The Tensor associated type lets backends dispatch over their native storage (Dense<f32>, an ndarray::ArrayD<f32>, an opaque GPU handle, …); the framework round-trips through the producer/consumer SlotValue carriers via the derive bridge in crate::roles::BackendRuntime.

Self::Tensor: Clone is required because the per-op default impls clone tensors into a temporary HashMap<String, _> to feed Backend::execute. Backends overriding the per-op methods directly never invoke this clone; backends overriding execute natively pay one clone per per-op call. ndarray’s ArrayD<f32> clones the shape + bumps an internal refcount — a few-hundred-nanosecond cost, not a memcpy.

Required Associated Types

type Error: Error + Display + Send + Sync + From<BackendWalkError> + 'static

Library-maker-defined error type. The From<BackendWalkError> bound lets the default per-op / execute_graph_via_per_op walker surface graph-validation failures as typed errors instead of panicking on peer-supplied or malformed GraphProto bodies.

type Tensor: Clone + Send + Sync + 'static + Storage

Native tensor representation.

Provided Methods

fn add( &self, a: &Self::Tensor, b: &Self::Tensor, ) -> Result<Self::Tensor, Self::Error>

Element-wise a + b with NumPy broadcasting.

fn sub( &self, a: &Self::Tensor, b: &Self::Tensor, ) -> Result<Self::Tensor, Self::Error>

Element-wise a - b with NumPy broadcasting.

fn mul( &self, a: &Self::Tensor, b: &Self::Tensor, ) -> Result<Self::Tensor, Self::Error>

Element-wise a * b with NumPy broadcasting.

fn div( &self, a: &Self::Tensor, b: &Self::Tensor, ) -> Result<Self::Tensor, Self::Error>

Element-wise a / b with NumPy broadcasting.

fn neg(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error>

Element-wise unary negation.

fn abs(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error>

Element-wise absolute value.

fn sqrt(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error>

Element-wise square root.

fn pow( &self, a: &Self::Tensor, b: &Self::Tensor, ) -> Result<Self::Tensor, Self::Error>

Element-wise a ** b with NumPy broadcasting.

fn exp(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error>

Element-wise natural exponential.

fn log(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error>

Element-wise natural logarithm.

fn matmul( &self, a: &Self::Tensor, b: &Self::Tensor, ) -> Result<Self::Tensor, Self::Error>

Matrix multiplication (NumPy semantics: 2-D × 2-D + batched higher-rank broadcasting).

fn reduce_sum( &self, a: &Self::Tensor, axes: &[i64], keepdims: bool, ) -> Result<Self::Tensor, Self::Error>

Sum-reduce a along axes. keepdims = true preserves the reduced dims as length-1.

fn reduce_mean( &self, a: &Self::Tensor, axes: &[i64], keepdims: bool, ) -> Result<Self::Tensor, Self::Error>

Mean-reduce a along axes.

fn reduce_max( &self, a: &Self::Tensor, axes: &[i64], keepdims: bool, ) -> Result<Self::Tensor, Self::Error>

Max-reduce a along axes.

fn reduce_min( &self, a: &Self::Tensor, axes: &[i64], keepdims: bool, ) -> Result<Self::Tensor, Self::Error>

Min-reduce a along axes.

fn reshape( &self, a: &Self::Tensor, shape: &[i64], ) -> Result<Self::Tensor, Self::Error>

Reshape a to the given dims. Total element count must match.

fn transpose( &self, a: &Self::Tensor, perm: &[i64], ) -> Result<Self::Tensor, Self::Error>

Transpose axes. Empty perm reverses all dims.

fn concat( &self, inputs: &[&Self::Tensor], axis: i64, ) -> Result<Self::Tensor, Self::Error>

Concatenate inputs along axis.

fn slice( &self, a: &Self::Tensor, starts: &[i64], ends: &[i64], axes: &[i64], steps: &[i64], ) -> Result<Self::Tensor, Self::Error>

NumPy-style slice. Empty axes defaults to all dims; empty steps defaults to 1 per axis.

fn split( &self, a: &Self::Tensor, axis: i64, sizes: &[i64], ) -> Result<Vec<Self::Tensor>, Self::Error>

Split a along axis into parts of the given sizes. Empty sizes means equal-sized splits (count comes from the consumer side downstream).

fn squeeze( &self, a: &Self::Tensor, axes: &[i64], ) -> Result<Self::Tensor, Self::Error>

Remove dimensions of size 1. Empty axes removes all size-1 dims.

fn unsqueeze( &self, a: &Self::Tensor, axes: &[i64], ) -> Result<Self::Tensor, Self::Error>

Insert dimensions of size 1 at the given axes.

fn identity(&self, a: &Self::Tensor) -> Result<Self::Tensor, Self::Error>

Identity / clone — pass-through useful for graph rewrites.

fn cast( &self, a: &Self::Tensor, dtype: i32, ) -> Result<Self::Tensor, Self::Error>

Cast to the given ONNX DataType enum value (matches bb_ir::proto::onnx::tensor_proto::DataType).

fn equal( &self, a: &Self::Tensor, b: &Self::Tensor, ) -> Result<Self::Tensor, Self::Error>

Element-wise a == b. Result is boolean-typed.

fn greater( &self, a: &Self::Tensor, b: &Self::Tensor, ) -> Result<Self::Tensor, Self::Error>

Element-wise a > b. Result is boolean-typed.

fn less( &self, a: &Self::Tensor, b: &Self::Tensor, ) -> Result<Self::Tensor, Self::Error>

Element-wise a < b. Result is boolean-typed.

fn where( &self, cond: &Self::Tensor, t: &Self::Tensor, f: &Self::Tensor, ) -> Result<Self::Tensor, Self::Error>

Element-wise ternary: where cond { t } else { f }. Named r#where to dodge the reserved Rust keyword.

fn constant(&self, value: TensorProto) -> Result<Self::Tensor, Self::Error>

Materialize a constant from an ONNX TensorProto. The value attribute on the ONNX Constant op carries the data; rank, dtype, raw bytes all come from the proto.

fn gather( &self, data: &Self::Tensor, indices: &Self::Tensor, axis: i64, ) -> Result<Self::Tensor, Self::Error>

Gather slices of data along axis indexed by indices.

fn execute( &self, graph: &GraphProto, inputs: HashMap<String, Self::Tensor>, _attrs: BackendAttrs<'_>, ) -> Result<HashMap<String, Self::Tensor>, Self::Error>

Execute every NodeProto in graph.node against the value env inputs. Returns the subset of values named in graph.output.

graph.node is topologically ordered per the ONNX spec, so the default walker (a linear scan) suffices for any GraphProto whose ops are all in bb_ir::tensor_primitives::TENSOR_PRIMITIVES_OPS. A backend overriding this method may detect fused patterns, compile to GPU, or any other strategy.

fn dispatch( &self, graph: &GraphProto, inputs: HashMap<String, Self::Tensor>, attrs: BackendAttrs<'_>, completion: CompletionHandle<HashMap<String, Self::Tensor>, Self::Error>, ) -> ContractResponse<HashMap<String, Self::Tensor>, Self::Error>

Dispatch a BackendSubgraph carrier — the engine-facing entry point for whole-subgraph execution.

The default falls through to Self::execute synchronously, keeping existing backends’ behaviour identical. Backends with per-subgraph caching, JIT compilation, or async device execution override this to:

• Cache the compiled subgraph by identity (e.g. graph name or hash).
• Return ContractResponse::Later and retain completion while the device runs. The engine schedules other work; the backend completes the handle from whatever runtime it uses — std::thread, tokio task, custom event loop, single- thread no-std loop.
• Fall through to Self::execute on compile failure or unsupported op.

The completion parameter in the default impl is intentionally discarded (let _ = completion) because ContractResponse::Now does not retain the handle. This is correct — only overriders that return ContractResponse::Later must hold it.

fn materialize_from_wire( &self, type_hash: u64, bytes: Vec<u8>, ) -> Result<Self::Tensor, Self::Error>

Materialise an inbound tensor SlotFill into this backend’s native tensor representation.

The framework has already (a) capped bytes.len() against the envelope’s EnvelopeCaps::max_per_fill_bytes, (b) charged the length against NodeConfig::ingress_byte_budget, and (c) moved ownership of the wire bytes into this call. The backend may adopt the Vec<u8> directly (zero-copy via ArrayD::from_shape_vec when alignment permits), pull a buffer from a pool and copy in, or allocate fresh. The framework will not touch bytes after this call returns.

The default delegates to the global wire-decoder registry: it looks up the decoder for type_hash, runs it on the bytes, then downcasts the resulting boxed SlotValue to Self::Tensor via the registry’s Box<dyn Any> repackaging. Backends that have not implemented tensor pooling continue to work through this path; backends that override pay the registry hop only at override time.

On Err, the engine drops the fill, releases the byte charge, and emits WireReceiveError { kind: BackendMaterializeFailed }.

Ownership note: bytes: Vec<u8> by value (not &[u8] or Cow). This is the framework→backend handoff, NOT an external boundary — the backend lives inside the framework ecosystem and plays by the runtime contract. Principle 1a (ephemeral borrowed slices at external boundaries) does not apply here: the framework copied or owned the bytes already, and a backend that wants to adopt them (zero-copy) needs ownership.

Dyn Compatibility

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors

impl Backend for CpuBackend

bb::Backend Contract impl. Overrides execute to run through graph_walker::execute_graph rather than the default per-op walker.

type Error = BackendError

type Tensor = CpuTensor