Relay

The wippy/relay module routes WebSocket connections through a central hub and per-user hubs. User hubs manage client connections and dispatch messages to prefixed plugins.

This page is a partial integration recipe and protocol reference, not a standalone WebSocket application. The setup and plugin blocks assume an existing Wippy project, a real security scope at the configured user_security_scope, and an HTTP WebSocket endpoint connected to the relay as described in WebSocket Relay. Protocol payloads and lifecycle blocks are reference shapes.

Architecture

Central Hub
├── User Hub (alice)
│   ├── Plugin: session_
│   ├── Plugin: ai_
│   ├── WebSocket Client 1
│   └── WebSocket Client 2
├── User Hub (bob)
│   ├── Plugin: session_
│   └── WebSocket Client 1
└── ...

The central hub runs as a service. When a WebSocket client connects, it finds or creates a hub for that user. The user hub manages the connection lifecycle and routes messages by command prefix.

Setup

Add the module to your project:

wippy add wippy/relay
wippy install

Declare the dependency with required parameters:

version: "1.0"
namespace: app

entries:
  - name: os_env
    kind: env.storage.os

  - name: processes
    kind: process.host
    lifecycle:
      auto_start: true

  - name: dep.relay
    kind: ns.dependency
    component: wippy/relay
    version: "*"
    parameters:
      - name: application_host
        value: app:processes
      - name: env_storage
        value: app:os_env
      - name: user_security_scope
        value: app.security:user_scope

Configuration Parameters

Client Connection Flow

1. WebSocket client connects with user_id in metadata
2. Central hub validates the connection and checks per-user limits
3. Central hub creates or reuses a user hub for the user
4. User hub sends a welcome message to the client:

{
    "user_id": "alice",
    "client_count": 1,
    "plugins": [
        { "prefix": "session_", "process_id": "...", "status": "running" },
        { "prefix": "ai_", "process_id": "...", "status": "pending" }
    ]
}

Plugin status can be "not_started" (registered but never spawned), "pending" (spawn in progress), "running", "failed", or "stopped".

Message Routing

Clients send JSON messages with a type field. The user hub matches the type prefix against registered plugins and routes the message:

{ "type": "session_get_state", "data": { "key": "value" } }

The session_ prefix selects the session plugin. The hub removes the prefix and sends the message to the plugin process, using the remaining type as the topic:

-- process topic: "get_state"
-- payload:
{
    conn_pid = client_pid,
    type = "session_get_state",  -- original full type preserved
    data = { key = "value" },
    request_id = "...",
    session_id = "..."
}

Plugins respond by sending messages back to conn_pid.

Plugins

Plugins are process.lua entries with meta.type: relay.plugin:

entries:
  - name: session_plugin
    kind: process.lua
    meta:
      type: relay.plugin
      command_prefix: session_
      auto_start: true
    source: file://session_plugin.lua
    modules: [json, time, logger]
    method: run

Plugin Metadata

Plugin Lifecycle

The user hub spawns each plugin with these startup arguments:

function run(args)
    local user_id = args.user_id
    local user_metadata = args.user_metadata
    local user_hub_pid = args.user_hub_pid
    local config = args.config
end

The session_ plugin receives lifecycle messages:

Plugins get 1 automatic restart on crash. After a second crash, the plugin is marked as "failed" and not restarted.

Plugin Implementation

Plugins receive messages through their process inbox. Each message has a topic derived from the command type and a payload containing the original message data and conn_pid for responses.

local json = require("json")

local function handle_message(topic, payload)
    if topic == "get_state" then
        if not payload.conn_pid then
            return nil, "Relay message is missing conn_pid"
        end

        local encoded, encode_err = json.encode({
            type = "session_state",
            data = { status = "active" }
        })
        if encode_err then
            return nil, encode_err
        end

        local sent, send_err = process.send(payload.conn_pid, "ws.message", encoded)
        if not sent then
            return nil, send_err or "Relay response was not sent"
        end
    end

    return true
end

local function run(args)
    local user_id = args.user_id
    local inbox = process.inbox()
    local events = process.events()

    while true do
        local result = channel.select({
            inbox:case_receive(),
            events:case_receive()
        })
        if not result.ok then break end

        if result.channel == inbox then
            local msg = result.value
            local topic = msg:topic()
            local payload = msg:payload():data()

            if topic == "resume" then
                -- first client connected
            elseif topic == "shutdown" then
                -- last client disconnected
            else
                local ok, err = handle_message(topic, payload)
                if not ok then
                    error("Failed to handle relay message: " .. tostring(err))
                end
            end
        elseif result.channel == events then
            local event = result.value
            if event.kind == process.event.CANCEL then
                break
            end
        end
    end
end

return { run = run }

Error Handling

The relay reports client errors using these codes:

Hub Lifecycle

User Hub Creation

The first client connection for a user creates that user's hub. The hub runs with the user's security actor and scope.

Garbage Collection

The central hub periodically checks for inactive user hubs. A hub with no connected clients for longer than user_hub_inactivity_timeout (default 2 hours) is gracefully terminated with a 10-second cancel timeout.

The GC check interval is automatically derived: inactivity_timeout / 2.5.

Security

The central hub runs under its own security group (wippy.relay.security:root) with full access. Each user hub spawns with the configured user_security_scope, isolating user-level operations.

Internal Topics

See Also

• WebSocket Relay — HTTP WebSocket endpoint configuration
• Process Model — Process lifecycle and messaging
• Security — Security actors and scopes
• Framework Overview — Install and import framework modules