zlayer-agent

Container runtime agent for ZLayer, providing lifecycle management for containers, WebAssembly modules, and jobs.

Features

The agent supports multiple container runtimes through feature flags:

• youki (default on Linux): Linux-native container runtime using libcontainer. Preferred on Linux for performance (no daemon overhead).
• seatbelt (default on macOS): Native macOS sandbox runtime using Seatbelt (sandbox-exec). Preferred for native Darwin workloads.
• libkrun-vm (optional, macOS): Lightweight VM-based runtime using libkrun, used when full Linux compatibility is required on macOS.
• hcs-runtime (default on Windows): Native Windows containers via the Host Compute Service (HCS) APIs.
• wsl (default on Windows for Linux images): Delegates Linux-image workloads to a WSL2-hosted agent.
• docker: Cross-platform fallback using the Docker daemon. Works on Linux, Windows, and macOS.
• wasm: WebAssembly runtime using wasmtime for executing WASM workloads with WASI support.

Windows feature flags

The canonical Windows release build enables both HCS and WSL2 delegation:

cargo build --release -p zlayer --no-default-features --features hcs-runtime,wsl

On Windows, the agent uses a CompositeRuntime (src/runtimes/composite.rs) that inspects the OCI image manifest's OS field and dispatches each container to the appropriate backend: HcsRuntime for Windows images and Wsl2DelegateRuntime for Linux images. Docker remains available as a fallback when neither native runtime is suitable.

Installation

Add to your Cargo.toml:

[dependencies]
zlayer-agent = { path = "../zlayer-agent" }

# Or with specific features
zlayer-agent = { path = "../zlayer-agent", features = ["docker", "wasm"] }

Runtime Selection

Automatic Selection

Use RuntimeConfig::Auto to automatically select the best available runtime:

use zlayer_agent::{RuntimeConfig, create_runtime};

let runtime = create_runtime(RuntimeConfig::Auto).await?;

Selection logic:

• Linux: Prefers youki (libcontainer) if available, falls back to Docker.
• macOS: Prefers the Seatbelt sandbox runtime for native Darwin workloads, uses the libkrun VM runtime when full Linux compatibility is required, and falls back to Docker.
• Windows: Uses the CompositeRuntime to dispatch per image: HcsRuntime for Windows containers and Wsl2DelegateRuntime (requires the wsl feature) for Linux containers. Docker remains available as a fallback. Activated via --features hcs-runtime,wsl.
• WASM artifacts: Use RuntimeConfig::Wasm explicitly (auto-detected from the OCI manifest where applicable).

Runtime matrix

Explicit Selection

use zlayer_agent::{RuntimeConfig, YoukiConfig, create_runtime};

// Use youki runtime
let youki = create_runtime(RuntimeConfig::Youki(YoukiConfig::default())).await?;

// Use Docker runtime
#[cfg(feature = "docker")]
let docker = create_runtime(RuntimeConfig::Docker).await?;

// Use WASM runtime
#[cfg(feature = "wasm")]
{
    use zlayer_agent::WasmConfig;
    let wasm = create_runtime(RuntimeConfig::Wasm(WasmConfig::default())).await?;
}

WebAssembly Runtime Support

The wasm feature enables running WebAssembly modules with WASI support as an alternative to traditional containers.

Enabling WASM Support

[dependencies]
zlayer-agent = { path = "../zlayer-agent", features = ["wasm"] }

WASM Runtime Configuration

use zlayer_agent::{WasmConfig, WasmRuntime};
use std::path::PathBuf;
use std::time::Duration;

let config = WasmConfig {
    // Directory for caching WASM modules
    cache_dir: PathBuf::from("/var/lib/zlayer/wasm"),

    // Enable epoch-based interruption for timeouts
    enable_epochs: true,

    // Instructions before yield check (for cooperative scheduling)
    epoch_deadline: 1_000_000,

    // Maximum execution time for WASM modules
    max_execution_time: Duration::from_secs(3600),
};

let runtime = WasmRuntime::new(config).await?;

WASI Support

The WASM runtime supports WASI Preview 1 (wasip1) with the following capabilities:

• Environment variables
• Command-line arguments
• Stdout/stderr output
• Exit codes via proc_exit

WASM vs Container Runtime Comparison

WASM Limitations

• No exec support: WASM modules are single-process and don't support executing additional commands
• No cgroup stats: WASM runs in-process without kernel isolation, so resource stats return stub values
• Limited filesystem: Currently only environment variables are passed; filesystem mounts are not yet supported
• Single entry point: WASM modules must export _start or main function

Example: Running a WASM Workload

use zlayer_agent::{ContainerId, Runtime, WasmConfig, WasmRuntime};
use zlayer_spec::ServiceSpec;
use std::time::Duration;

async fn run_wasm_module(spec: &ServiceSpec) -> Result<i32, Box<dyn std::error::Error>> {
    // Create WASM runtime
    let runtime = WasmRuntime::new(WasmConfig::default()).await?;

    let id = ContainerId {
        service: "my-wasm-service".to_string(),
        replica: 1,
    };

    // Pull WASM artifact from registry
    runtime.pull_image(&spec.image.name.to_string()).await?;

    // Create instance
    runtime.create_container(&id, spec).await?;

    // Start execution
    runtime.start_container(&id).await?;

    // Wait for completion
    let exit_code = runtime.wait_container(&id).await?;

    // Cleanup
    runtime.remove_container(&id).await?;

    Ok(exit_code)
}

Service Manager

The ServiceManager provides higher-level orchestration for services:

use zlayer_agent::{ServiceManager, RuntimeConfig};
use zlayer_spec::DeploymentSpec;

// Load deployment spec
let spec: DeploymentSpec = serde_yaml::from_str(yaml)?;

// Create service manager with auto-selected runtime
let manager = ServiceManager::new(RuntimeConfig::Auto).await?;

// Deploy services
for (name, service_spec) in &spec.services {
    manager.deploy_service(name, service_spec).await?;
}

Testing

Running All Tests

# Unit tests
cargo test -p zlayer-agent

# With Docker runtime tests
cargo test -p zlayer-agent --features docker

# With WASM runtime tests
cargo test -p zlayer-agent --features wasm

# All features
cargo test -p zlayer-agent --all-features

WASM-Specific Tests

cargo test -p zlayer-agent --features wasm -- --nocapture

The WASM tests use inline WAT (WebAssembly Text Format) modules compiled at test time, so no external WASM binaries are required.

Module Structure

• runtime.rs: Abstract Runtime trait and ContainerState types
• runtimes/: Runtime implementations
  • youki.rs: Linux-native runtime using libcontainer
  • seatbelt.rs: macOS Seatbelt sandbox runtime
  • libkrun.rs: macOS libkrun VM runtime for full Linux compatibility (feature: libkrun-vm)
  • hcs.rs: Windows Host Compute Service runtime for Windows containers (feature: hcs-runtime)
  • wsl2_delegate.rs: Windows runtime that delegates Linux containers to a WSL2-hosted agent (feature: wsl)
  • composite.rs: CompositeRuntime that inspects the OCI image OS and dispatches to HcsRuntime or Wsl2DelegateRuntime on Windows
  • docker.rs: Docker daemon runtime (feature: docker)
  • wasm.rs: WebAssembly runtime using wasmtime (feature: wasm)
• service.rs: ServiceManager for higher-level orchestration
• container_supervisor.rs: Container health monitoring and restart logic
• job.rs: Job execution support for run-to-completion workloads
• cron_scheduler.rs: Cron-based job scheduling
• health.rs: Health check implementations
• init.rs: Init action orchestration
• env.rs: Environment variable resolution
• bundle.rs: OCI bundle creation for youki

Environment Variables

License

Apache-2.0