IPC Communication Patterns

Every way processes can communicate on a computer. This skill provides decision trees for selecting IPC mechanisms, failure diagnostics, and implementation patterns for AI agent coordination.

Decision Points

Primary IPC Selection Tree

Are processes related (parent-child)?
├── YES
│   ├── Need bidirectional? 
│   │   ├── YES → Unix Domain Socket (same machine) | TCP (may cross machines)
│   │   └── NO → Anonymous Pipe (stdin/stdout)
│   └── Simple one-way → Anonymous Pipe
│
└── NO (unrelated processes)
    ├── Same machine only?
    │   ├── YES
    │   │   ├── High throughput (>1GB/s) → Shared Memory + coordination
    │   │   ├── Low latency (<10us) → Unix Domain Socket
    │   │   ├── Typed RPC needed → gRPC over Unix Socket
    │   │   └── Simple messages → Named Pipe | Unix Domain Socket
    │   │
    │   └── NO (cross-machine capable)
    │       ├── Web-compatible → WebSocket | SSE | REST
    │       ├── Streaming data → gRPC streaming | WebSocket
    │       ├── Request-reply → REST/HTTP | gRPC unary
    │       └── Fire-and-forget → UDP
    │
    └── Browser client involved?
        ├── Server → Client only → SSE
        ├── Bidirectional → WebSocket  
        └── Request-reply → REST/HTTP

Performance-Driven Selection

If latency requirement:
├── <1us → Shared Memory (lock-free ring buffer)
├── <10us → Unix Domain Socket | Shared Memory (with locks)
├── <100us → TCP loopback | gRPC
└── <1ms → REST/HTTP acceptable

If throughput requirement:
├── >10GB/s → Shared Memory only option
├── >5GB/s → Unix Domain Socket
├── >1GB/s → TCP | gRPC
└── <1GB/s → Any mechanism works

Agent Architecture Decision Matrix

Agent Communication Pattern → Recommended IPC

Parent spawns child agents:
├── Simple task execution → stdin/stdout pipe
├── Progress reporting needed → Unix Domain Socket
└── Web UI monitoring → stdin/stdout + SSE to browser

Orchestrator + independent agents:
├── Same machine → Unix Domain Socket
├── May scale across machines → gRPC | WebSocket
└── Simple coordination → Named Pipe

Long-running agent services:
├── macOS → XPC Service
├── Linux desktop → D-Bus
└── Cross-platform → Unix Domain Socket | TCP

Failure Modes

Pipe Buffer Deadlock

Detection: Process hangs when writing to stdin while child's stdout buffer is full Symptoms: write() blocks indefinitely, process unresponsive, strace shows blocking on pipe write Root cause: Both stdin and stdout buffers full (~64KB Linux, ~16KB macOS), neither process can proceed Fix: Use async I/O to drain stdout while writing stdin, or separate threads for read/write operations

# Detect: strace shows blocked write to pipe
strace -p <pid> | grep -E 'write.*PIPE|read.*PIPE'

Unix Socket Permission Denied

Detection: EACCES error on connect(), "Permission denied" in logs Symptoms: Client process cannot connect to Unix domain socket, socket file exists with wrong permissions Root cause: Socket file permissions too restrictive, or client running as different user Fix: Set socket permissions to 0666 for multi-user access, or 0600 + proper ownership

# Fix socket permissions
chmod 666 /tmp/agent.sock
# Or set ownership
chown user:group /tmp/agent.sock

TCP Connection Refused Cascade

Detection: ECONNREFUSED errors, agents unable to reach orchestrator Symptoms: Multiple agents fail simultaneously, orchestrator shows no incoming connections Root cause: Orchestrator crashed/restarted, firewall blocking port, or port already in use Fix: Implement exponential backoff retry, health checks, and port conflict detection

// Detect port conflicts
const server = net.createServer();
server.on('error', (err: NodeJS.ErrnoException) => {
  if (err.code === 'EADDRINUSE') {
    console.error(`Port ${port} already in use`);
  }
});

Shared Memory Corruption

Detection: Garbage data reads, segfaults, inconsistent state between processes Symptoms: Data races, torn reads/writes, process crashes with SIGSEGV Root cause: Missing synchronization, incorrect memory barriers, or buffer overruns Fix: Add proper atomics for flags, use futex/semaphore for critical sections

// Fix: Use atomic operations
atomic_store(&shared->flag, 1);  // Not: shared->flag = 1;

Signal Handler Race Condition

Detection: SIGTERM not handled cleanly, processes leave stale state Symptoms: Lock files not cleaned up, connections not closed, zombie processes Root cause: Signal handler interrupted critical section, or handler not async-signal-safe Fix: Use self-pipe trick or signalfd (Linux) for safe signal handling

// Fix: Only set flag in signal handler, do cleanup in main loop
volatile sig_atomic_t shutdown_requested = 0;
void sigterm_handler(int sig) { shutdown_requested = 1; }

Worked Examples

Multi-Agent Task Orchestration

Scenario: Orchestrator manages 5 agent processes, each running different AI models. Need bidirectional communication for task assignment and progress reporting.

Decision process:

1. Communication pattern: Bidirectional, structured messages
2. Performance needs: Low latency preferred, moderate throughput
3. Platform: Same machine initially, may scale later
4. Initial choice: Unix Domain Socket for low latency
5. Message format: Newline-delimited JSON for simplicity

Implementation walkthrough:

// 1. Create Unix domain socket server (orchestrator)
const SOCKET_PATH = '/tmp/windags-orchestrator.sock';
if (fs.existsSync(SOCKET_PATH)) fs.unlinkSync(SOCKET_PATH); // Clean stale socket

const server = net.createServer((connection) => {
  const agentId = `agent-${Date.now()}`;
  agents.set(agentId, connection);
  
  let buffer = '';
  connection.on('data', (chunk) => {
    buffer += chunk.toString();
    
    // Process complete messages (newline-delimited)
    let newlineIdx;
    while ((newlineIdx = buffer.indexOf('\n')) !== -1) {
      const line = buffer.slice(0, newlineIdx);
      buffer = buffer.slice(newlineIdx + 1);
      
      try {
        const message = JSON.parse(line);
        handleAgentMessage(agentId, message);
      } catch (err) {
        console.error(`Invalid JSON from ${agentId}:`, err);
      }
    }
  });
  
  connection.on('close', () => agents.delete(agentId));
});

server.listen(SOCKET_PATH);
fs.chmodSync(SOCKET_PATH, 0o600); // Owner-only access

// 2. Agent connection pattern
function connectAgent(): Promise<net.Socket> {
  return new Promise((resolve, reject) => {
    const socket = net.connect(SOCKET_PATH);
    socket.on('connect', () => resolve(socket));
    socket.on('error', reject);
  });
}

// 3. Message sending with error handling
function sendMessage(socket: net.Socket, msg: object): Promise<void> {
  return new Promise((resolve, reject) => {
    const line = JSON.stringify(msg) + '\n';
    socket.write(line, (err) => err ? reject(err) : resolve());
  });
}

Expert vs novice differences:

• Novice mistake: Forget to clean up stale socket file on restart → EADDRINUSE error
• Expert catches: Always unlink() socket path before binding
• Novice mistake: Parse JSON on every data chunk → crashes on partial messages
• Expert catches: Buffer incomplete messages, parse only complete lines
• Novice mistake: No error handling on write() → silent failures
• Expert catches: Promises/callbacks for write completion and error detection

Quality Gates

• IPC mechanism matches communication pattern (unidirectional vs bidirectional)
• Message framing implemented for stream protocols (length-prefix or delimiter)
• SIGPIPE handler prevents process termination on broken connections
• Stale socket/pipe files cleaned up on process startup
• Connection errors handled with appropriate retry/backoff logic
• Buffer sizes configured for expected message volumes
• Permissions set correctly for multi-process access (socket files, shared memory)
• Graceful shutdown sequence: SIGTERM → cleanup → exit (with SIGKILL timeout)
• Deadlock potential analyzed for pipe-based communication
• Performance validated under realistic load (latency and throughput SLAs met)

NOT-FOR Boundaries

This skill should NOT be used for:

• Distributed systems across machines → Use distributed-systems skill instead
• Message broker setup (Kafka, RabbitMQ) → Use data-pipeline-engineer skill instead
• Database connection patterns → Use database-specific skills instead
• Network protocol design (custom wire formats) → Use networking skill instead
• HTTP API design patterns → Use api-design skill instead

Delegate to other skills when:

• Client needs WebSocket streaming patterns → Use websocket-streaming skill
• Building daemon/service architecture → Use daemon-development skill
• Coordinating multiple AI agents → Use multi-agent-coordination skill
• Designing data formats for IPC → Use agent-interchange-formats skill