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NATSBridge/tutorial_julia.md
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NATSBridge.jl Tutorial

A comprehensive tutorial for learning how to use NATSBridge.jl for bi-directional communication between Julia and JavaScript services using NATS.

Table of Contents

  1. What is NATSBridge.jl?
  2. Key Concepts
  3. Installation
  4. Basic Usage
  5. Payload Types
  6. Transport Strategies
  7. Advanced Features
  8. Complete Examples

What is NATSBridge.jl?

NATSBridge.jl is a Julia module that provides a high-level API for sending and receiving data across network boundaries using NATS as the message bus. It implements the Claim-Check pattern for handling large payloads efficiently.

Core Features

  • Bi-directional communication: Julia ↔ JavaScript
  • Smart transport selection: Automatic direct vs link transport based on payload size
  • Multi-payload support: Send multiple payloads of different types in a single message
  • Claim-check pattern: Upload large files to HTTP server, send only URLs via NATS
  • Type-aware serialization: Different serialization strategies for different data types

Key Concepts

1. msgEnvelope_v1 (Message Envelope)

The msgEnvelope_v1 structure provides a comprehensive message format for bidirectional communication:

struct msgEnvelope_v1
  correlationId::String       # Unique identifier to track messages
  msgId::String               # This message id
  timestamp::String           # Message published timestamp
  
  sendTo::String              # Topic/subject the sender sends to
  msgPurpose::String          # Purpose (ACK | NACK | updateStatus | shutdown | chat)
  senderName::String          # Sender name (e.g., "agent-wine-web-frontend")
  senderId::String            # Sender id (uuid4)
  receiverName::String        # Message receiver name (e.g., "agent-backend")
  receiverId::String          # Message receiver id (uuid4 or nothing for broadcast)
  replyTo::String             # Topic to reply to
  replyToMsgId::String        # Message id this message is replying to
  brokerURL::String           # NATS server address
  
  metadata::Dict{String, Any}
  payloads::AbstractArray{msgPayload_v1}  # Multiple payloads stored here
end

2. msgPayload_v1 (Payload Structure)

The msgPayload_v1 structure provides flexible payload handling:

struct msgPayload_v1
  id::String                    # Id of this payload (e.g., "uuid4")
  dataname::String              # Name of this payload (e.g., "login_image")
  type::String                  # "text | dictionary | table | image | audio | video | binary"
  transport::String             # "direct | link"
  encoding::String              # "none | json | base64 | arrow-ipc"
  size::Integer                 # Data size in bytes
  data::Any                     # Payload data in case of direct transport or a URL in case of link
  metadata::Dict{String, Any}   # Dict("checksum" => "sha256_hash", ...)
end

3. Standard API Format

The system uses a standardized list-of-tuples format for all payload operations:

# Input format for smartsend (always a list of tuples with type info)
[(dataname1, data1, type1), (dataname2, data2, type2), ...]

# Output format for smartreceive (always returns a list of tuples)
[(dataname1, data1, type1), (dataname2, data2, type2), ...]

Important: Even when sending a single payload, you must wrap it in a list.

Installation

using Pkg
Pkg.add("NATS")
Pkg.add("JSON")
Pkg.add("Arrow")
Pkg.add("HTTP")
Pkg.add("UUIDs")
Pkg.add("Dates")
Pkg.add("Base64")
Pkg.add("PrettyPrinting")
Pkg.add("DataFrames")

Then include the NATSBridge module:

include("NATSBridge.jl")
using .NATSBridge

Basic Usage

Sending Data (smartsend)

using NATSBridge

# Send a simple dictionary
data = Dict("key" => "value")
env = NATSBridge.smartsend("my.subject", [("dataname1", data, "dictionary")])

Receiving Data (smartreceive)

using NATSBridge

# Subscribe to a NATS subject
NATS.subscribe("my.subject") do msg
    # Process the message
    result = NATSBridge.smartreceive(
        msg,
        max_retries = 5,
        base_delay = 100,
        max_delay = 5000
    )
    
    # result is a list of (dataname, data, type) tuples
    for (dataname, data, type) in result
        println("Received $dataname of type $type")
        println("Data: $data")
    end
end

Payload Types

NATSBridge.jl supports the following payload types:

Type Description Serialization
text Plain text UTF-8 encoding
dictionary JSON-serializable data (Dict, NamedTuple) JSON
table Tabular data (DataFrame, array of structs) Apache Arrow IPC
image Image data (Bitmap, PNG/JPG bytes) Binary
audio Audio data (WAV, MP3 bytes) Binary
video Video data (MP4, AVI bytes) Binary
binary Generic binary data Binary

Transport Strategies

NATSBridge.jl automatically selects the appropriate transport strategy based on payload size:

Direct Transport (< 1MB)

Small payloads are encoded as Base64 and sent directly over NATS.

# Small data (< 1MB) - uses direct transport
small_data = rand(1000)  # ~8KB
env = NATSBridge.smartsend("small", [("data", small_data, "table")])

Link Transport (≥ 1MB)

Large payloads are uploaded to an HTTP file server, and only the URL is sent via NATS.

# Large data (≥ 1MB) - uses link transport
large_data = rand(10_000_000)  # ~80MB
env = NATSBridge.smartsend("large", [("data", large_data, "table")])

Complete Examples

Example 1: Text Message

Sender:

using NATSBridge
using UUIDs

const SUBJECT = "/NATSBridge_text_test"
const NATS_URL = "nats://localhost:4222"
const FILESERVER_URL = "http://localhost:8080"

function plik_upload_handler(fileserver_url::String, dataname::String, data::Vector{UInt8})::Dict{String, Any}
    url_getUploadID = "$fileserver_url/upload"
    headers = ["Content-Type" => "application/json"]
    body = """{ "OneShot" : true }"""
    httpResponse = HTTP.request("POST", url_getUploadID, headers, body; body_is_form=false)
    responseJson = JSON.parse(String(httpResponse.body))
    uploadid = responseJson["id"]
    uploadtoken = responseJson["uploadToken"]
    
    file_multipart = HTTP.Multipart(dataname, IOBuffer(data), "application/octet-stream")
    url_upload = "$fileserver_url/file/$uploadid"
    headers = ["X-UploadToken" => uploadtoken]
    
    form = HTTP.Form(Dict("file" => file_multipart))
    httpResponse = HTTP.post(url_upload, headers, form)
    responseJson = JSON.parse(String(httpResponse.body))
    
    fileid = responseJson["id"]
    url = "$fileserver_url/file/$uploadid/$fileid/$dataname"
    
    return Dict("status" => httpResponse.status, "uploadid" => uploadid, "fileid" => fileid, "url" => url)
end

function test_text_send()
    small_text = "Hello, this is a small text message."
    large_text = join(["Line $i: " for i in 1:50000], "")
    
    data1 = ("small_text", small_text, "text")
    data2 = ("large_text", large_text, "text")
    
    env = NATSBridge.smartsend(
        SUBJECT,
        [data1, data2],
        nats_url = NATS_URL,
        fileserver_url = FILESERVER_URL,
        fileserverUploadHandler = plik_upload_handler,
        size_threshold = 1_000_000,
        correlation_id = string(uuid4()),
        msg_purpose = "chat",
        sender_name = "text_sender"
    )
end

Receiver:

using NATSBridge

const SUBJECT = "/NATSBridge_text_test"
const NATS_URL = "nats://localhost:4222"

function test_text_receive()
    conn = NATS.connect(NATS_URL)
    NATS.subscribe(conn, SUBJECT) do msg
        result = NATSBridge.smartreceive(
            msg,
            max_retries = 5,
            base_delay = 100,
            max_delay = 5000
        )
        
        for (dataname, data, data_type) in result
            if data_type == "text"
                println("Received text: $data")
                write("./received_$dataname.txt", data)
            end
        end
    end
    sleep(120)
    NATS.drain(conn)
end

Example 2: Dictionary (JSON) Message

Sender:

using NATSBridge
using UUIDs

const SUBJECT = "/NATSBridge_dict_test"
const NATS_URL = "nats://localhost:4222"
const FILESERVER_URL = "http://localhost:8080"

function plik_upload_handler(fileserver_url::String, dataname::String, data::Vector{UInt8})::Dict{String, Any}
    url_getUploadID = "$fileserver_url/upload"
    headers = ["Content-Type" => "application/json"]
    body = """{ "OneShot" : true }"""
    httpResponse = HTTP.request("POST", url_getUploadID, headers, body; body_is_form=false)
    responseJson = JSON.parse(String(httpResponse.body))
    uploadid = responseJson["id"]
    uploadtoken = responseJson["uploadToken"]
    
    file_multipart = HTTP.Multipart(dataname, IOBuffer(data), "application/octet-stream")
    url_upload = "$fileserver_url/file/$uploadid"
    headers = ["X-UploadToken" => uploadtoken]
    
    form = HTTP.Form(Dict("file" => file_multipart))
    httpResponse = HTTP.post(url_upload, headers, form)
    responseJson = JSON.parse(String(httpResponse.body))
    
    fileid = responseJson["id"]
    url = "$fileserver_url/file/$uploadid/$fileid/$dataname"
    
    return Dict("status" => httpResponse.status, "uploadid" => uploadid, "fileid" => fileid, "url" => url)
end

function test_dict_send()
    small_dict = Dict("name" => "Alice", "age" => 30)
    large_dict = Dict("ids" => collect(1:50000), "names" => ["User_$i" for i in 1:50000])
    
    data1 = ("small_dict", small_dict, "dictionary")
    data2 = ("large_dict", large_dict, "dictionary")
    
    env = NATSBridge.smartsend(
        SUBJECT,
        [data1, data2],
        nats_url = NATS_URL,
        fileserver_url = FILESERVER_URL,
        fileserverUploadHandler = plik_upload_handler,
        size_threshold = 1_000_000,
        correlation_id = string(uuid4()),
        msg_purpose = "chat"
    )
end

Receiver:

using NATSBridge

const SUBJECT = "/NATSBridge_dict_test"
const NATS_URL = "nats://localhost:4222"

function test_dict_receive()
    conn = NATS.connect(NATS_URL)
    NATS.subscribe(conn, SUBJECT) do msg
        result = NATSBridge.smartreceive(
            msg,
            max_retries = 5,
            base_delay = 100,
            max_delay = 5000
        )
        
        for (dataname, data, data_type) in result
            if data_type == "dictionary"
                println("Received dictionary: $data")
                write("./received_$dataname.json", JSON.json(data, 2))
            end
        end
    end
    sleep(120)
    NATS.drain(conn)
end

Example 3: DataFrame (Table) Message

Sender:

using NATSBridge
using DataFrames
using UUIDs

const SUBJECT = "/NATSBridge_table_test"
const NATS_URL = "nats://localhost:4222"
const FILESERVER_URL = "http://localhost:8080"

function plik_upload_handler(fileserver_url::String, dataname::String, data::Vector{UInt8})::Dict{String, Any}
    url_getUploadID = "$fileserver_url/upload"
    headers = ["Content-Type" => "application/json"]
    body = """{ "OneShot" : true }"""
    httpResponse = HTTP.request("POST", url_getUploadID, headers, body; body_is_form=false)
    responseJson = JSON.parse(String(httpResponse.body))
    uploadid = responseJson["id"]
    uploadtoken = responseJson["uploadToken"]
    
    file_multipart = HTTP.Multipart(dataname, IOBuffer(data), "application/octet-stream")
    url_upload = "$fileserver_url/file/$uploadid"
    headers = ["X-UploadToken" => uploadtoken]
    
    form = HTTP.Form(Dict("file" => file_multipart))
    httpResponse = HTTP.post(url_upload, headers, form)
    responseJson = JSON.parse(String(httpResponse.body))
    
    fileid = responseJson["id"]
    url = "$fileserver_url/file/$uploadid/$fileid/$dataname"
    
    return Dict("status" => httpResponse.status, "uploadid" => uploadid, "fileid" => fileid, "url" => url)
end

function test_table_send()
    small_df = DataFrame(id = 1:10, name = ["Alice", "Bob", "Charlie"], score = [95, 88, 92])
    large_df = DataFrame(id = 1:50000, name = ["User_$i" for i in 1:50000], score = rand(1:100, 50000))
    
    data1 = ("small_table", small_df, "table")
    data2 = ("large_table", large_df, "table")
    
    env = NATSBridge.smartsend(
        SUBJECT,
        [data1, data2],
        nats_url = NATS_URL,
        fileserver_url = FILESERVER_URL,
        fileserverUploadHandler = plik_upload_handler,
        size_threshold = 1_000_000,
        correlation_id = string(uuid4()),
        msg_purpose = "chat"
    )
end

Receiver:

using NATSBridge
using DataFrames

const SUBJECT = "/NATSBridge_table_test"
const NATS_URL = "nats://localhost:4222"

function test_table_receive()
    conn = NATS.connect(NATS_URL)
    NATS.subscribe(conn, SUBJECT) do msg
        result = NATSBridge.smartreceive(
            msg,
            max_retries = 5,
            base_delay = 100,
            max_delay = 5000
        )
        
        for (dataname, data, data_type) in result
            if data_type == "table"
                data = DataFrame(data)
                println("Received DataFrame with $(size(data, 1)) rows")
                display(data[1:min(5, size(data, 1)), :])
            end
        end
    end
    sleep(120)
    NATS.drain(conn)
end

Example 4: Mixed Content (Chat with Text, Image, Audio)

Sender:

using NATSBridge
using DataFrames
using UUIDs

const SUBJECT = "/NATSBridge_mix_test"
const NATS_URL = "nats://localhost:4222"
const FILESERVER_URL = "http://localhost:8080"

function plik_upload_handler(fileserver_url::String, dataname::String, data::Vector{UInt8})::Dict{String, Any}
    url_getUploadID = "$fileserver_url/upload"
    headers = ["Content-Type" => "application/json"]
    body = """{ "OneShot" : true }"""
    httpResponse = HTTP.request("POST", url_getUploadID, headers, body; body_is_form=false)
    responseJson = JSON.parse(String(httpResponse.body))
    uploadid = responseJson["id"]
    uploadtoken = responseJson["uploadToken"]
    
    file_multipart = HTTP.Multipart(dataname, IOBuffer(data), "application/octet-stream")
    url_upload = "$fileserver_url/file/$uploadid"
    headers = ["X-UploadToken" => uploadtoken]
    
    form = HTTP.Form(Dict("file" => file_multipart))
    httpResponse = HTTP.post(url_upload, headers, form)
    responseJson = JSON.parse(String(httpResponse.body))
    
    fileid = responseJson["id"]
    url = "$fileserver_url/file/$uploadid/$fileid/$dataname"
    
    return Dict("status" => httpResponse.status, "uploadid" => uploadid, "fileid" => fileid, "url" => url)
end

function test_mix_send()
    # Text data
    text_data = "Hello! This is a test chat message. 🎉"
    
    # Dictionary data
    dict_data = Dict("type" => "chat", "sender" => "serviceA")
    
    # Small table data
    table_data_small = DataFrame(id = 1:10, name = ["msg_$i" for i in 1:10])
    
    # Large table data (link transport)
    table_data_large = DataFrame(id = 1:150_000, name = ["msg_$i" for i in 1:150_000])
    
    # Small image data (direct transport)
    image_data = UInt8[rand(1:255) for _ in 1:100]
    
    # Large image data (link transport)
    large_image_data = UInt8[rand(1:255) for _ in 1:1_500_000]
    
    # Small audio data (direct transport)
    audio_data = UInt8[rand(1:255) for _ in 1:100]
    
    # Large audio data (link transport)
    large_audio_data = UInt8[rand(1:255) for _ in 1:1_500_000]
    
    # Small video data (direct transport)
    video_data = UInt8[rand(1:255) for _ in 1:150]
    
    # Large video data (link transport)
    large_video_data = UInt8[rand(1:255) for _ in 1:1_500_000]
    
    # Small binary data (direct transport)
    binary_data = UInt8[rand(1:255) for _ in 1:200]
    
    # Large binary data (link transport)
    large_binary_data = UInt8[rand(1:255) for _ in 1:1_500_000]
    
    # Create payloads list - mixed content
    payloads = [
        # Small data (direct transport)
        ("chat_text", text_data, "text"),
        ("chat_json", dict_data, "dictionary"),
        ("chat_table_small", table_data_small, "table"),
        
        # Large data (link transport)
        ("chat_table_large", table_data_large, "table"),
        ("user_image_large", large_image_data, "image"),
        ("audio_clip_large", large_audio_data, "audio"),
        ("video_clip_large", large_video_data, "video"),
        ("binary_file_large", large_binary_data, "binary")
    ]
    
    env = NATSBridge.smartsend(
        SUBJECT,
        payloads,
        nats_url = NATS_URL,
        fileserver_url = FILESERVER_URL,
        fileserverUploadHandler = plik_upload_handler,
        size_threshold = 1_000_000,
        correlation_id = string(uuid4()),
        msg_purpose = "chat",
        sender_name = "mix_sender"
    )
end

Receiver:

using NATSBridge
using DataFrames

const SUBJECT = "/NATSBridge_mix_test"
const NATS_URL = "nats://localhost:4222"

function test_mix_receive()
    conn = NATS.connect(NATS_URL)
    NATS.subscribe(conn, SUBJECT) do msg
        result = NATSBridge.smartreceive(
            msg,
            max_retries = 5,
            base_delay = 100,
            max_delay = 5000
        )
        
        println("Received $(length(result)) payloads")
        
        for (dataname, data, data_type) in result
            println("\n=== Payload: $dataname (type: $data_type) ===")
            
            if data_type == "text"
                println("  Type: String")
                println("  Length: $(length(data)) characters")
                
            elseif data_type == "dictionary"
                println("  Type: JSON Object")
                println("  Keys: $(keys(data))")
                
            elseif data_type == "table"
                data = DataFrame(data)
                println("  Type: DataFrame")
                println("  Dimensions: $(size(data, 1)) rows x $(size(data, 2)) columns")
                
            elseif data_type == "image"
                println("  Type: Vector{UInt8}")
                println("  Size: $(length(data)) bytes")
                write("./received_$dataname.bin", data)
                
            elseif data_type == "audio"
                println("  Type: Vector{UInt8}")
                println("  Size: $(length(data)) bytes")
                write("./received_$dataname.bin", data)
                
            elseif data_type == "video"
                println("  Type: Vector{UInt8}")
                println("  Size: $(length(data)) bytes")
                write("./received_$dataname.bin", data)
                
            elseif data_type == "binary"
                println("  Type: Vector{UInt8}")
                println("  Size: $(length(data)) bytes")
                write("./received_$dataname.bin", data)
            end
        end
    end
    sleep(120)
    NATS.drain(conn)
end

Best Practices

  1. Always wrap payloads in a list - Even for single payloads: [("dataname", data, "type")]
  2. Use appropriate transport - Let NATSBridge handle size-based routing (default 1MB threshold)
  3. Customize size threshold - Use size_threshold parameter to adjust the direct/link split
  4. Provide fileserver handler - Implement fileserverUploadHandler for link transport
  5. Include correlation IDs - Track messages across distributed systems
  6. Handle errors - Implement proper error handling for network failures
  7. Close connections - Ensure NATS connections are properly closed using NATS.drain()

Conclusion

NATSBridge.jl provides a powerful abstraction for bi-directional communication between Julia and JavaScript services. By understanding the key concepts and following the best practices, you can build robust, scalable applications that leverage the full power of NATS messaging.

For more information, see:

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