splicer 🔍✂️

Plan and generate middleware splice operations for WebAssembly component composition graphs.

splicer reads:

• A composition graph (JSON)
• A splice configuration (YAML)

It produces a modified plan that injects middleware components according to declarative rules.

This tool is designed to work with component-based systems such as WASI HTTP services, but is interface-agnostic and can splice across any interface edge in a component graph.

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Why splicer?

When building component-based systems, middleware insertion often requires:

• Rewriting instantiation chains
• Re-threading handler references
• Maintaining correct edge ordering
• Traversing nested provider chains

splicer automates that planning step.

Instead of manually restructuring component wiring, you define:

• What interface to target
• Where to inject middleware
• What middleware components to insert

And splicer generates the modified composition plan.

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Installation

From source:

cargo build --release

Binary will be located at:

target/release/splicer

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Usage

splicer <JSON_GRAPH> <SPLICE_CFG> [--output <FILE>]

Arguments

Argument	Description
JSON_GRAPH	Path to the composition graph in JSON format
SPLICE_CFG	Path to the splice configuration YAML file
--output	Optional output file (defaults to stdout)

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Configuration Format

Splicing behavior is defined in a YAML configuration file.

See full specification:

docs/splice-config.md

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Example Configuration

version: 1

rules:
  - before:
      interface: wasi:http/handler
      provider:
        name: auth
    inject:
        - middleware-a
        - middleware-b

  - between:
      interface: wasi:http/handler
      inner:
        name: auth
      outer:
        name: handler
    inject:
        - tracing

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Splice Semantics

splicer operates on interface edges in the graph.

If no matches are found, the generated wac will produce an identity component (roundtrips to same component).

Two matching modes are supported:

1. Single-Target Injection

Inject middleware for a given interface, optionally scoped to a specific provider.

before:
  interface: wasi:http/handler
  provider:
    name: auth

If provider.name is omitted, all providers of that interface are matched.

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2. Between Injection

Inject middleware between two specific components connected via an interface edge.

between:
   interface: wasi:http/handler
   inner:
     name: auth
   outer:
     name: handler

This replaces:

handler → auth

With:

handler → middleware → auth

Middleware chains are traversed in reverse order during injection to preserve declared ordering.

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Rule Ordering

Rules are applied in file order.

Later rules operate on the graph after earlier modifications.

This allows intentional stacking:

auth → logging → metrics → handler

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Validation

The configuration will fail if:

• version is missing or unsupported
• Required fields are absent
• Middleware list is empty

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Project Structure

splicer/
├── src/
├── docs/
│   └── splice-config.md
├── README.md

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Design Principles

• Declarative configuration
• Deterministic ordering
• Interface-driven matching
• Graph-aware edge replacement
• Middleware-agnostic

splicer does not assume HTTP semantics — it operates on generic interface edges.

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Future Evolution

The configuration format is versioned:

version: 1

Breaking changes will increment the version number.