NATSBridge/examples/tutorial.md

NATSBridge Tutorial

A step-by-step guide to get started with NATSBridge - a high-performance, bi-directional data bridge for Julia.

Table of Contents

1. Overview
2. Prerequisites
3. Installation
4. Quick Start
5. Basic Examples
6. Advanced Usage

---

Overview

NATSBridge enables seamless communication for Julia applications through NATS, with automatic transport selection based on payload size:

• Direct Transport: Payloads < 1MB are sent directly via NATS (Base64 encoded)
• Link Transport: Payloads >= 1MB are uploaded to an HTTP file server and referenced via URL

Supported Payload Types

Type	Description
text	Plain text strings
dictionary	JSON-serializable dictionaries
table	Tabular data (Arrow IPC format)
image	Image data (PNG, JPG bytes)
audio	Audio data (WAV, MP3 bytes)
video	Video data (MP4, AVI bytes)
binary	Generic binary data

---

Prerequisites

Before you begin, ensure you have:

1. NATS Server running (or accessible)
2. HTTP File Server (optional, for large payloads > 1MB)
3. Julia with required packages

---

Installation

Julia

using Pkg
Pkg.add("NATS")
Pkg.add("Arrow")
Pkg.add("JSON3")
Pkg.add("HTTP")
Pkg.add("UUIDs")
Pkg.add("Dates")

---

Quick Start

Step 1: Start NATS Server

docker run -p 4222:4222 nats:latest

Step 2: Start HTTP File Server (Optional)

# Create a directory for file uploads
mkdir -p /tmp/fileserver

# Use any HTTP server that supports POST for file uploads
python3 -m http.server 8080 --directory /tmp/fileserver

Step 3: Send Your First Message

Julia

using NATSBridge

# Send a text message
data = [("message", "Hello World", "text")]
env, env_json_str = smartsend("/chat/room1", data, broker_url="nats://localhost:4222")
# env: msg_envelope_v1 object with all metadata and payloads
# env_json_str: JSON string representation of the envelope for publishing
println("Message sent!")

# Or use is_publish=false to get envelope and JSON without publishing
env, env_json_str = smartsend("/chat/room1", data, broker_url="nats://localhost:4222", is_publish=false)
# env: msg_envelope_v1 object
# env_json_str: JSON string for publishing to NATS

Step 4: Receive Messages

Julia

using NATSBridge

# Receive and process message
env = smartreceive(msg; fileserver_download_handler=_fetch_with_backoff)
for (dataname, data, type) in env["payloads"]
    println("Received $dataname: $data")
end

---

Basic Examples

Example 1: Sending a Dictionary

Julia

using NATSBridge

config = Dict(
    "wifi_ssid" => "MyNetwork",
    "wifi_password" => "password123",
    "update_interval" => 60
)

data = [("config", config, "dictionary")]
env, env_json_str = smartsend("/device/config", data, broker_url="nats://localhost:4222")

Example 2: Sending Binary Data (Image)

Julia

using NATSBridge

# Read image file
image_data = read("image.png")

data = [("user_image", image_data, "binary")]
env, env_json_str = smartsend("/chat/image", data, broker_url="nats://localhost:4222")

Example 3: Request-Response Pattern

Julia (Requester)

using NATSBridge

# Send command with reply-to
data = [("command", Dict("action" => "read_sensor"), "dictionary")]
env, env_json_str = smartsend(
    "/device/command",
    data,
    broker_url="nats://localhost:4222",
    reply_to="/device/response",
    reply_to_msg_id="cmd-001"
)
# env: msg_envelope_v1 object
# env_json_str: JSON string for publishing to NATS

Julia (Responder)

using NATS, NATSBridge

# Configuration
const SUBJECT = "/device/command"
const NATS_URL = "nats://localhost:4222"

function test_responder()
    conn = NATS.connect(NATS_URL)
    NATS.subscribe(conn, SUBJECT) do msg
        env = smartreceive(msg, fileserver_download_handler=_fetch_with_backoff)
        
        # Extract reply_to from the envelope metadata
        reply_to = env["reply_to"]
        
        for (dataname, data, type) in env["payloads"]
            if dataname == "command" && data["action"] == "read_sensor"
                response = Dict("sensor_id" => "sensor-001", "value" => 42.5)
                # Send response to the reply_to subject from the request
                if !isempty(reply_to)
                    smartsend(reply_to, [("data", response, "dictionary")])
                end
            end
        end
    end
    
    sleep(120)
    NATS.drain(conn)
end

test_responder()

---

Advanced Usage

Example 4: Large Payloads (File Server)

For payloads larger than 1MB, NATSBridge automatically uses the file server:

Julia

using NATSBridge

# Create large data (> 1MB)
large_data = rand(UInt8, 2_000_000)

env, env_json_str = smartsend(
    "/data/large",
    [("large_file", large_data, "binary")],
    broker_url="nats://localhost:4222",
    fileserver_url="http://localhost:8080"
)

# The envelope will contain the download URL
println("File uploaded to: $(env.payloads[1].data)")

Example 5: Mixed Content (Chat with Text + Image)

NATSBridge supports sending multiple payloads with different types in a single message:

Julia

using NATSBridge

image_data = read("avatar.png")

data = [
    ("message_text", "Hello with image!", "text"),
    ("user_avatar", image_data, "image")
]

env, env_json_str = smartsend("/chat/mixed", data, broker_url="nats://localhost:4222")

Example 6: Table Data (Arrow IPC)

For tabular data, NATSBridge uses Apache Arrow IPC format:

Julia

using NATSBridge
using DataFrames

# Create DataFrame
df = DataFrame(
    id = [1, 2, 3],
    name = ["Alice", "Bob", "Charlie"],
    score = [95, 88, 92]
)

data = [("students", df, "table")]
env, env_json_str = smartsend("/data/students", data, broker_url="nats://localhost:4222")

---

Next Steps

1. Explore the test directory for more examples
2. Check the documentation for advanced configuration options

---

Troubleshooting

Connection Issues

• Ensure NATS server is running: docker ps | grep nats
• Check firewall settings
• Verify NATS URL configuration

File Server Issues

• Ensure file server is running and accessible
• Check upload permissions
• Verify file server URL configuration

Serialization Errors

• Verify data type matches the specified type
• Check that binary data is in the correct format (Vector{UInt8})

---

License

MIT