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Process Model

sbsh's process model ensures terminals are independent, isolated, and durable. Unlike traditional terminal multiplexers, sbsh has no central server process.

No-Server Architecture

Traditional Multiplexers (screen/tmux)

┌─────────────────┐
│  Server Process │  Single process manages all sessions
│  (screen/tmux) │  Sessions are children of server
└────────┬────────┘  Server crash affects all sessions
         │
    ┌────┴────┬─────────┐
    ▼         ▼         ▼
 Session  Session  Session

Problems:

  • Single point of failure
  • Shared failure domain
  • Server crash affects all sessions

sbsh Architecture

┌─────────────┐  ┌─────────────┐  ┌─────────────┐
│ Supervisor  │  │ Supervisor  │  │ Supervisor  │
│     1       │  │     2       │  │     3       │
└──────┬──────┘  └──────┬──────┘  └──────┬──────┘
       │                │                │
       └────────┬───────┴────────────────┘
                │
         ┌──────┴──────┐
         │   Terminal  │  Independent process
         │  (Process)  │  Not a child of supervisor
         └─────────────┘  Survives supervisor exit

Benefits:

  • No single point of failure
  • Isolated failure domains
  • Supervisor crashes don't affect terminals
  • True process independence

Process Independence

Terminal Processes

Terminals run as independent processes:

  • Own process group
  • Not children of supervisors
  • Continue running if supervisor exits
  • Can be attached to by new supervisors

Supervisor Processes

Supervisors are lightweight:

  • Manage terminal attachment
  • Forward I/O
  • Can exit without affecting terminals
  • New supervisors can attach to existing terminals

Process Lifecycle

Terminal Lifecycle

Terminal Process
├── Spawned (independent)
├── Initializing (onInit hooks)
├── Ready (accepting input)
├── Running (executing commands)
└── Exited (or restarted per policy)

Supervisor Lifecycle

Supervisor Process
├── Created
├── Attaching (connecting to terminal)
├── Attached (forwarding I/O)
├── Detached (disconnected)
└── Exited (terminal continues)

Isolation Benefits

Failure Isolation

  • Terminal crash doesn't affect other terminals
  • Supervisor crash doesn't affect terminals
  • No cascading failures
  • Each terminal is self-contained

Resource Isolation

  • Each terminal has its own process
  • Independent resource limits
  • No shared memory or state
  • Clean process boundaries

Multi-Attach Process Model

Multiple supervisors can attach to the same terminal:

Terminal Process
├── Supervisor 1 (process A)
├── Supervisor 2 (process B)
└── Supervisor 3 (process C)

All supervisors connect via the same Unix domain socket. The terminal broadcasts output to all attached supervisors.

Process Groups

Terminals run in their own process groups:

  • Isolated from parent process
  • Can be managed independently
  • Proper signal handling
  • Clean process tree