Phlow

Modular Flow Runtime for Composable Backends

Phlow is a high-performance, scalable, and Low Code flow runtime built with Rust — designed to revolutionize the way you build backends. With Phlow, you can create APIs, automations, and event-driven systems using just YAML, JSON, or TOML, composing logic like building blocks.

Thanks to its modular architecture and clear separation between control and behavior, Phlow lets you orchestrate complex flows without writing code — and when you need more power, just plug in lightweight scripts or Rust modules.

It also comes with native observability powered by OpenTelemetry, giving you full visibility into your flows, modules, and executions. Easily export traces and metrics to Jaeger, Grafana Tempo, or AWS X-Ray, all with simple environment variables.

If you're looking for speed, flexibility, and full insight into your backend — Phlow is the Low Code revolution you’ve been waiting for.

📚 Table of Contents

• 🎯 Philosophy
• 🔌 Module Types
• 🧱 Example: main.yaml for an HTTP Gateway
• 🧩 YAML Superpowers
• ⚙️ Install & Usage
• 🧠 Creating Your Own Module: log
• 📦 Project Structure
• 📡 Observability
• 🧪 OpenTelemetry + Jaeger (Local Dev Setup)
• 🌍 Environment Settings
• 📜 License

🎯 Philosophy

Phlow was built around the following principles:

1. Flow over Frameworks

Forget bulky frameworks. Phlow embraces flows. Each step is modular, each behavior is pluggable. You define what happens, when, and how — all through configuration and small, focused modules.

2. Composability

Phlow encourages building small pieces that fit together. Each module can:

• Run logic (step module)
• Start the system (main module)
• Interact via input and output
• Be swapped, reused, or extended easily.

3. Extensibility with Scripts

Need logic? Use phs (Phlow Script) or rhai. Define logic inline or in external files. You don't need to recompile to change behavior — just change the YAML.

4. Observability First

Every module, flow, and step can be traced using tracing and OpenTelemetry. You'll always know where, why, and how something happened.

5. Separation of Control and Behavior

Control lives in YAML (steps, conditions, includes). Behavior lives in modules and scripts. You can mix and match at will.

🔌 Module Types

🧱 Example: main.yaml for an HTTP Gateway

main: gateway

modules:
    - name: gateway
        module: rest_api
        with:
            host: 0.0.0.0
            port: 3000

    - name: request
        module: http_request
        with:
            timeout: 29000 # 29s

steps:
    - condition:
        assert: !eval main.path.start_with("/public")
        then:
            module: request
            input:
                method: !eval main.method
                url: !eval `public-service.local${main.uri}?` 
                headers:
                    x-forwarded-for: !eval main.client_ip
                    x-original-path: !eval main.path   
                body: !eval main.body
    - use: authorization
        id: auth
        input:
            api_key: !eval main.header.authorization
    - condition:
        assert: !eval steps.auth.authorized == true          
        then:
            module: request
            with:
                method: !eval main.method
                url: !eval `private-service.local${main.uri}?` 
                headers:
                    x-forwarded-for: !eval main.client_ip
                    x-original-path: !eval main.path   
                body: !eval main.body
    - return:
        status_code: 401
        body: {
            "message": "unauthorized",
            "code": 401
        }

🧩 YAML Superpowers

Phlow extends YAML with:

• !eval: execute inline expressions using Phlow Script (phs).
• !include: include other YAML files into the flow tree.
• !import: import external script files (.phs or .rhai) and evaluate them with !eval.

⚙️ Installation & Usage

Install Phlow globally using Cargo:

cargo install phlow-runtime

🔧 Running a Flow

By default, Phlow will look for a `main.yaml` in the current directory:

phlow

To run a specific file:

phlow path/to/your-flow.yaml

If you provide a directory path and it contains a `main.yaml`, Phlow will automatically run that:

phlow path/to/directory
# → runs path/to/directory/main.yaml

🆘 Help

For all available options and usage info:

phlow -h
# or
phlow --help

🧠 Creating Your Own Module: log

Phlow modules are written in Rust and compiled as shared libraries. Here’s a real example of a simple log module that prints messages at various log levels.

🔧 Code (src/lib.rs)

use phlow_sdk::{
    crossbeam::channel,
    modules::ModulePackage,
    prelude::*,
    tracing::{debug, error, info, warn},
};

plugin!(log);

enum LogLevel {
    Info,
    Debug,
    Warn,
    Error,
}

struct Log {
    level: LogLevel,
    message: String,
}

impl From<&Value> for Log {
    fn from(value: &Value) -> Self {
        let level = match value.get("level") {
            Some(level) => match level.to_string().as_str() {
                "info" => LogLevel::Info,
                "debug" => LogLevel::Debug,
                "warn" => LogLevel::Warn,
                "error" => LogLevel::Error,
                _ => LogLevel::Info,
            },
            _ => LogLevel::Info,
        };

        let message = value.get("message").unwrap_or(&Value::Null).to_string();

        Self { level, message }
    }
}

pub fn log(setup: ModuleSetup) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
    let (tx, rx) = channel::unbounded::<ModulePackage>();

    setup.setup_sender.send(Some(tx)).ok();

    for package in rx {
        let log = match package.context.input {
            Some(value) => Log::from(&value),
            _ => Log {
                level: LogLevel::Info,
                message: "".to_string(),
            },
        };

        match log.level {
            LogLevel::Info => info!("{}", log.message),
            LogLevel::Debug => debug!("{}", log.message),
            LogLevel::Warn => warn!("{}", log.message),
            LogLevel::Error => error!("{}", log.message),
        }

        sender_safe!(package.sender, Value::Null);
    }

    Ok(())
}

🛠️ Example usage in a flow

steps:
  - id: notify
    module: log
    with:
      level: info
      message: "Process started"

  - use: log
    with:
      level: error
      message: !eval "something went wrong: " + main.error

📦 Project Structure

you_project/
├── main.yaml
├── modules.yaml
├── assets/
│   └── body.yaml
├── scripts/
│   └── resolve_url.phs
├── phlow_packages/
│   ├── restapi/
│   │   └── module.so
│   ├── request/
│   │   └── module.so
│   └── log/
│       └── module.so

All compiled .so modules must be placed inside the phlow_packages/ directory.

To build all modules at once, this project includes a utility script:

📡 Observability

Phlow integrates with:

• OpenTelemetry (OTLP)
• Tracing (Spans and Logs)
• Prometheus Metrics
• Jaeger, Grafana Tempo, AWS X-Ray

Enable it with:

PHLOW_OTEL=true
PHLOW_LOG=DEBUG
PHLOW_SPAN=INFO

🧪 OpenTelemetry + Jaeger (Local Dev Setup)

To enable observability with Jaeger during development, you can run a full OpenTelemetry-compatible collector locally in seconds.

🔄 1. Run Jaeger with OTLP support

docker run -d \
  -p4318:4318 \  # OTLP HTTP
  -p4317:4317 \  # OTLP gRPC
  -p16686:16686 \  # Jaeger UI
  jaegertracing/all-in-one:latest

This container supports OTLP over HTTP and gRPC, which are both compatible with Phlow's OpenTelemetry output.

⚙️ 2. Configure environment variables

Set the following environment variables in your shell or .env file:

export OTEL_RESOURCE_ATTRIBUTES="service.name=phlow-dev,service.version=0.1.0"
export OTEL_SERVICE_NAME="phlow-dev"

🔍 3. Open the Jaeger UI

Once running, access the Jaeger web interface at:

http://localhost:16686

Search for your service using the name defined in OTEL_SERVICE_NAME.

✅ Tips

• Combine this with PHLOW_OTEL=true, PHLOW_SPAN=INFO, and PHLOW_LOG=DEBUG for full observability.
• You can also integrate with Grafana Tempo or AWS X-Ray by replacing the collector backend.

🌍 Environment Settings

Below is a list of all environment variables used by the application, combining those defined in both files, along with their descriptions, default values, and types.

Environment Variables Table

Notes

• If an environment variable is not set, the default value indicated in the table above will be used.
• Set the corresponding environment variables before running the application to override the defaults.
• The log level (PHLOW_LOG) and span level (PHLOW_SPAN) control different layers of logging:
  • PHLOW_LOG: Affects standard logging (e.g., error, warning, info messages).
  • PHLOW_SPAN: Affects tracing spans (useful for deeper telemetry insights with OpenTelemetry).
• The PHLOW_OTEL variable controls whether or not OpenTelemetry providers (for both tracing and metrics) are initialized.

📜 License

MIT © 2025 — Built with ❤️ and Rust.