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Core Concepts

foundations/net/docs/CORE_CONCEPTS.md

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Core Concepts

Understanding the fundamental concepts of Huly Virtual Network is essential for building robust distributed systems.

Table of Contents

Architecture Overview

Huly Network uses a hub-and-spoke architecture with three main components:

mermaid
graph TB
    subgraph "Network (Hub)"
        N[Network Server]
        R[Router]
        REG[Container Registry]
    end

    subgraph "Agent 1"
        A1[Agent]
        C1A[Container A]
        C1B[Container B]
    end

    subgraph "Agent 2"
        A2[Agent]
        C2A[Container C]
        C2B[Container D]
    end

    subgraph "Clients"
        CLIENT1[Client 1]
        CLIENT2[Client 2]
    end

    CLIENT1 -->|Request Container| N
    CLIENT2 -->|Request Container| N
    N -->|Route to Agent| A1
    N -->|Route to Agent| A2
    A1 --> C1A
    A1 --> C1B
    A2 --> C2A
    A2 --> C2B

Key Principles

  1. Centralized Coordination: The network server coordinates all agents and containers
  2. Distributed Execution: Containers run on agents, which can be distributed across multiple machines
  3. Dynamic Discovery: Clients discover and connect to containers through the network
  4. Automatic Lifecycle: The network manages container lifecycles based on client references
  5. Fault Tolerance: Automatic cleanup of failed agents and orphaned containers

Network

The Network is the central coordinator that:

  • Maintains a registry of all agents and their capabilities
  • Tracks all active containers and their locations
  • Routes client requests to appropriate containers
  • Manages container lifecycles (creation, reference counting, cleanup)
  • Provides service discovery and load balancing
  • Broadcasts events to clients about system changes

Network Responsibilities

typescript
interface Network {
  // Agent management
  register(record: AgentRecord, agent: NetworkAgentApi): Promise<ContainerUuid[]>
  unregister(agentId: AgentUuid): Promise<void>
  ping(agentId: AgentUuid): Promise<void>

  // Container management
  get(client: ClientUuid, kind: ContainerKind, options: GetOptions): Promise<[ContainerUuid, ContainerEndpointRef]>
  release(client: ClientUuid, uuid: ContainerUuid): Promise<void>
  list(kind?: ContainerKind): Promise<ContainerRecord[]>

  // Communication
  request(target: ContainerUuid, operation: string, data?: any): Promise<any>

  // Discovery
  agents(): AgentRecord[]
  kinds(): ContainerKind[]
}

Network Server

The network server listens on a TCP port (default: 3737) and:

  • Accepts connections from clients and agents
  • Uses ZeroMQ for high-performance messaging
  • Handles concurrent requests efficiently
  • Maintains connection health through ping/pong

Example: Starting a Network Server

typescript
import { NetworkImpl, TickManagerImpl } from '@hcengineering/network-core'
import { NetworkServer } from '@hcengineering/network-server'

const tickManager = new TickManagerImpl(1000) // 1000 ticks/sec
tickManager.start()

const network = new NetworkImpl(tickManager)
const server = new NetworkServer(
  network,
  tickManager,
  '*', // Bind to all interfaces
  3737 // Port
)

console.log('Network server running on port 3737')

Agents

Agents are worker processes that:

  • Host and manage containers
  • Register with the network to advertise capabilities
  • Create containers on-demand or pre-provision them (stateless)
  • Handle container lifecycle (start, stop, health checks)
  • Route requests between clients and containers

Agent Capabilities

Each agent declares which container kinds it can create:

typescript
// Note: For production code, use serveAgent() on the client
// This example uses AgentImpl directly for educational purposes
const agent = new AgentImpl('my-agent' as AgentUuid, {
  session: async (options) => {
    /* create session container */
  },
  workspace: async (options) => {
    /* create workspace container */
  },
  query: async (options) => {
    /* create query container */
  }
})

Agent Registration

Agents register with the network to announce availability:

typescript
import { NetworkAgentServer } from '@hcengineering/network-client'

const agentServer = new NetworkAgentServer(
  tickManager,
  'localhost', // Network host
  '*', // Bind address
  3738 // Agent port for container connections
)

await agentServer.start(agent)

// Register with network
await client.register(agent)

Agent Health

Agents must:

  • Send regular pings to the network (default: every 1 second)
  • Respond to health checks
  • Handle graceful shutdown
  • Clean up containers on termination

If an agent fails to ping within the aliveTimeout (default: 3 seconds), the network:

  1. Marks the agent as dead
  2. Removes all its containers
  3. Broadcasts removal events
  4. Allows standby agents to take over (for stateless containers)

Containers

Containers are the workhorses of the system. They:

  • Implement specific business logic
  • Process requests from clients
  • Maintain internal state
  • Broadcast events to connected clients
  • Are automatically created and destroyed based on demand

Container Interface

Every container must implement:

typescript
interface Container {
  // Handle incoming requests
  request(operation: string, data?: any, clientId?: ClientUuid): Promise<any>

  // Health check
  ping(): Promise<void>

  // Cleanup and shutdown
  terminate(): Promise<void>

  // Event broadcasting support
  connect(clientId: ClientUuid, broadcast: (data: any) => Promise<void>): void
  disconnect(clientId: ClientUuid): void

  // Optional termination callback
  onTerminated?(): void
}

Container Lifecycle

mermaid
stateDiagram-v2
    [*] --> Creating: Client requests container
    Creating --> Active: Factory creates instance
    Active --> Referenced: Client holds reference
    Referenced --> Active: Client releases reference
    Active --> Terminating: Timeout expires (no references)
    Referenced --> Terminating: All clients released
    Terminating --> [*]: Cleanup complete

Key lifecycle events:

  1. Creation: Factory method called when container is requested
  2. Active: Container is running and processing requests
  3. Referenced: At least one client holds a reference
  4. Idle Timeout: Container kept alive for containerTimeout after last reference released
  5. Termination: terminate() called, then removed from network

Container Types

Stateful Containers (Dynamic)

Created on-demand when requested:

typescript
// Note: For production code, use serveAgent() on the client
// This example uses AgentImpl directly for educational purposes
const agent = new AgentImpl('agent-1' as any, {
  'user-session': async (options: GetOptions) => {
    const uuid = options.uuid ?? generateUuid()
    const container = new UserSessionContainer(uuid, options)
    return {
      uuid,
      container,
      endpoint: `session://agent1/${uuid}` as any
    }
  }
})

Stateless Containers (Pre-provisioned)

Pre-created for high availability:

typescript
const leaderContainer = new LeaderContainer('leader-001' as ContainerUuid)

agent.addStatelessContainer(
  'leader-001' as ContainerUuid,
  'leader' as ContainerKind,
  'leader://agent1/leader-001' as ContainerEndpointRef,
  leaderContainer
)

Multiple agents can register the same stateless container UUID. The network accepts the first and rejects others, enabling automatic failover.

Container Example

typescript
import type { Container, ContainerUuid, ClientUuid } from '@hcengineering/network-core'

class DataStoreContainer implements Container {
  private data = new Map<string, any>()
  private connections = new Map<ClientUuid, (data: any) => Promise<void>>()

  constructor(readonly uuid: ContainerUuid) {}

  async request(operation: string, data?: any): Promise<any> {
    switch (operation) {
      case 'set':
        this.data.set(data.key, data.value)
        await this.broadcast({ type: 'dataChanged', key: data.key })
        return { success: true }

      case 'get':
        return { value: this.data.get(data.key) }

      default:
        return { error: 'Unknown operation' }
    }
  }

  async ping(): Promise<void> {}

  async terminate(): Promise<void> {
    this.data.clear()
    this.connections.clear()
  }

  connect(clientId: ClientUuid, broadcast: (data: any) => Promise<void>): void {
    this.connections.set(clientId, broadcast)
  }

  disconnect(clientId: ClientUuid): void {
    this.connections.delete(clientId)
  }

  private async broadcast(event: any): Promise<void> {
    const promises = Array.from(this.connections.values()).map((fn) => fn(event))
    await Promise.all(promises)
  }
}

Clients

Clients are applications or services that:

  • Connect to the network server
  • Request containers by kind and optional criteria
  • Send requests to containers
  • Receive events from containers
  • Manage container references (acquire/release)

Client Connection

typescript
import { createNetworkClient } from '@hcengineering/network-client'

const client = createNetworkClient(
  'localhost:3737', // Network address
  3600 // Alive timeout in seconds (optional)
)

await client.waitConnection(5000) // Wait up to 5 seconds

Requesting Containers

Clients request containers by kind and optional criteria:

typescript
// Get any container of this kind
const ref = await client.get('user-session' as ContainerKind, {})

// Get specific container by UUID
const ref = await client.get('user-session' as ContainerKind, {
  uuid: 'session-123' as ContainerUuid
})

// Get container with labels
const ref = await client.get('workspace' as ContainerKind, {
  labels: ['premium', 'us-west']
})

// Get container with extra data
const ref = await client.get('query-engine' as ContainerKind, {
  extra: { database: 'analytics', userId: 'user-456' }
})

Client API

typescript
interface NetworkClient {
  // Container management
  get(kind: ContainerKind, request: GetOptions): Promise<ContainerReference>
  list(kind?: ContainerKind): Promise<ContainerRecord[]>

  // Agent management
  register(agent: NetworkAgent): Promise<void>
  unregister(agentId: AgentUuid): Promise<void>

  // Discovery
  agents(): AgentRecord[]
  kinds(): ContainerKind[]

  // Events
  onUpdate(listener: NetworkUpdateListener): () => void

  // Connection
  close(): Promise<void>
}

Communication Patterns

Huly Network supports multiple communication patterns:

1. Request/Response (Synchronous)

Direct request to container with response:

typescript
const result = await containerRef.request('processData', {
  value: 42
})
console.log(result) // { processed: true, result: 84 }

2. Fire-and-Forget (Asynchronous)

Send data without waiting for response:

typescript
await containerRef.request('logEvent', {
  event: 'user_login',
  timestamp: Date.now()
})

3. Event Broadcasting (Publish/Subscribe)

Container broadcasts events to all connected clients:

typescript
// Client side
const connection = await containerRef.connect()
connection.on = async (event) => {
  console.log('Received:', event)
}

// Container side
connect(clientId: ClientUuid, broadcast: (data: any) => Promise<void>): void {
  this.clients.set(clientId, broadcast)
}

// Broadcast to all clients
for (const broadcast of this.clients.values()) {
  await broadcast({ type: 'update', data: changes })
}

4. Bidirectional Streaming

Establish persistent connection for streaming:

typescript
const connection = await containerRef.connect()

// Receive stream
connection.on = async (chunk) => {
  console.log('Chunk:', chunk)
}

// Send requests
await connection.request('subscribe', { topic: 'updates' })
await connection.request('getData', { range: [0, 100] })

Lifecycle Management

Reference Counting

The network uses reference counting to manage container lifecycles:

  • Each client.get() increments the reference count
  • Each containerRef.close() decrements it
  • Containers with zero references are kept alive for containerTimeout
  • After timeout, containers are automatically terminated
typescript
// Acquire reference (ref count = 1)
const ref1 = await client.get('service' as any, { uuid: 'svc-1' })

// Acquire another reference to same container (ref count = 2)
const ref2 = await client.get('service' as any, { uuid: 'svc-1' })

// Release first reference (ref count = 1)
await ref1.close()

// Release second reference (ref count = 0)
await ref2.close()

// Container kept alive for containerTimeout, then terminated

Health Monitoring

The network continuously monitors health:

Agent Health:

  • Agents must ping every pingInterval (default: 1 second)
  • Network marks agent as dead after aliveTimeout (default: 3 seconds)
  • Dead agents' containers are removed

Container Health:

  • Containers respond to ping() calls
  • Failed pings may trigger termination
  • Unhealthy containers are removed from registry

Graceful Shutdown

Proper cleanup on shutdown:

typescript
// Container cleanup
async terminate(): Promise<void> {
  // 1. Notify connected clients
  await this.broadcastShutdown()

  // 2. Close external connections
  await this.database.close()

  // 3. Clear internal state
  this.data.clear()

  // 4. Release resources
  this.connections.clear()
}

// Agent cleanup
await agentServer.close()
tickManager.stop()

// Client cleanup
await containerRef.close()
await client.close()

// Server cleanup
await server.close()
tickManager.stop()

Endpoint References

Containers are addressed by endpoint references:

typescript
type ContainerEndpointRef = string & { _containerEndpointRef: true }

Endpoint Types

  1. Direct Endpoint: Direct connection to container

    tcp://host:port/uuid

  2. Routed Endpoint: Connection through agent

    agent://host:port:agentId/uuid

  3. No-Connect Endpoint: Request-only, no persistent connection

    noconnect://host:port/uuid

Parsing Endpoints

typescript
import { parseEndpointRef, EndpointKind } from '@hcengineering/network-core'

const parsed = parseEndpointRef(endpoint)
console.log(parsed.kind) // EndpointKind.direct | routed | noconnect
console.log(parsed.host) // Host address
console.log(parsed.port) // Port number
console.log(parsed.uuid) // Container UUID
console.log(parsed.agentId) // Agent ID (for routed)

Container Kinds and Labels

Container Kinds

Containers are categorized by kind (string type):

typescript
type ContainerKind = string & { _containerKind: true }

Examples:

  • 'user-session' - User session management
  • 'workspace' - Workspace containers
  • 'query-engine' - Query processing
  • 'transactor' - Transaction handling

Agents declare which kinds they support:

typescript
// Note: For production code, use serveAgent() on the client
// This example uses AgentImpl directly for educational purposes
const agent = new AgentImpl('agent-1' as any, {
  'user-session': sessionFactory,
  workspace: workspaceFactory,
  'query-engine': queryFactory
})

Labels

Containers can have labels for fine-grained selection:

typescript
// Create container with labels
await client.get('workspace' as any, {
  labels: ['premium', 'us-west', 'production']
})

// Labels enable:
// - Multi-tenancy (tenant ID as label)
// - Geographic routing (region labels)
// - Tier-based selection (free, premium, enterprise)
// - Environment separation (dev, staging, production)

Get Options

typescript
interface GetOptions {
  uuid?: ContainerUuid // Specific container UUID
  extra?: Record<string, any> // Additional parameters for factory
  labels?: string[] // Labels for selection
}

Summary

Key concepts to remember:

  1. Network = Central coordinator
  2. Agent = Worker node hosting containers
  3. Container = Service instance with business logic
  4. Client = Application requesting containers
  5. Reference Counting = Automatic lifecycle management
  6. Endpoint = Address for reaching a container
  7. Kind = Container type/category
  8. Labels = Fine-grained selection criteria

These concepts form the foundation for building scalable, fault-tolerant distributed systems with Huly Network.

Next Steps

Need more help? Check the Troubleshooting Guide or Examples.