Project 10: Session and Window Hierarchy

Model sessions, windows, and panes as persistent state with commands to create and switch.

Quick Reference

Attribute Value
Difficulty Level 4: Expert
Time Estimate 2 weeks
Main Programming Language C (Alternatives: Rust)
Alternative Programming Languages Rust
Coolness Level Level 4: State Wrangler
Business Potential 2: The “Session Manager”
Prerequisites data structures, client-server state
Key Topics session model, lists/maps, active pointers

1. Learning Objectives

By completing this project, you will:

  1. Build a working implementation of session and window hierarchy and verify it with deterministic outputs.
  2. Explain the underlying Unix and terminal primitives involved in the project.
  3. Diagnose common failure modes with logs and targeted tests.
  4. Extend the project with performance and usability improvements.

2. All Theory Needed (Per-Concept Breakdown)

State Graphs for Sessions, Windows, and Panes

  • Fundamentals State Graphs for Sessions, Windows, and Panes is the core contract that makes the project behave like a real terminal tool. It sits at the boundary between raw bytes and structured state, so you must treat it as both a protocol and a data model. The goal of the fundamentals is to understand what assumptions the system makes about ordering, buffering, and ownership, and how those assumptions surface as user-visible behavior. Key terms include: session, window, pane, active pointer. In practice, the fastest way to gain intuition is to trace a single input through the pipeline and note where it can be delayed, reordered, or transformed. That exercise reveals why State Graphs for Sessions, Windows, and Panes needs explicit invariants and why even small mistakes can cascade into broken rendering or stuck input.

  • Deep Dive into the concept A deep understanding of State Graphs for Sessions, Windows, and Panes requires thinking in terms of state transitions and invariants. You are not just implementing functions; you are enforcing a contract between producers and consumers of bytes, and that contract persists across time. Most failures in this area are caused by violating ordering guarantees, dropping state updates, or misunderstanding how the operating system delivers events. This concept is built from the following pillars: session, window, pane, active pointer. A reliable implementation follows a deterministic flow: Create session object -> Add windows list -> Track active window -> Link panes to windows. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness. When you design the project, treat each key term as a source of constraints. For example, if a term implies buffering, decide the buffer size and how overflow is handled. If a term implies state, decide how that state is initialized, updated, and reset. Finally, validate your assumptions with deterministic fixtures so you can reproduce bugs. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness.

  • How this fit on projects This concept is the backbone of the project because it defines how data and control flow move through the system.

  • Definitions & key terms

    • session -> top-level grouping of windows in tmux
    • window -> a collection of panes arranged in a layout
    • pane -> a single terminal running a program
    • active pointer -> a reference to the current window or pane

  • Mental model diagram (ASCII)

[Input] -> [State Graphs for Sessions, Windows, and Panes] -> [State] -> [Output]

  • How it works (step-by-step, with invariants and failure modes)

    1. Create session object
    2. Add windows list
    3. Track active window
    4. Link panes to windows

  • Minimal concrete example

Session{ name="work", windows=[w0,w1], active=1 }

  • Common misconceptions

    • “State is just lists” -> active pointers and invariants matter.

  • Check-your-understanding questions

    • How do you keep clients in sync?
    • What happens when active window is removed?

  • Check-your-understanding answers

    • Broadcast state changes; recompute active pointer.

  • Real-world applications

    • tmux session model
    • window managers

  • Where you’ll apply it

  • References

    • tmux 3 Ch. 1

  • Key insights State Graphs for Sessions, Windows, and Panes works best when you treat it as a stateful contract with explicit invariants.

  • Summary You now have a concrete mental model for State Graphs for Sessions, Windows, and Panes and can explain how it affects correctness and usability.

  • Homework/Exercises to practice the concept

    • Design structs for Session, Window, Pane.

  • Solutions to the homework/exercises

    • Use stable IDs and pointers for panes.

3. Project Specification

3.1 What You Will Build

A state model with commands to create/list/rename sessions and switch windows without losing references.

3.2 Functional Requirements

  1. Requirement 1: Create/list/rename sessions
  2. Requirement 2: Create/switch windows
  3. Requirement 3: Track active window per session
  4. Requirement 4: Persist pane references

3.3 Non-Functional Requirements

  • Performance: Avoid blocking I/O; batch writes when possible.
  • Reliability: Handle partial reads/writes and cleanly recover from disconnects.
  • Usability: Provide clear CLI errors, deterministic output, and helpful logs.

3.4 Example Usage / Output

    $ ./mytmux new-session -s work
$ ./mytmux new-window -n logs
$ ./mytmux list-sessions
work: 2 windows
[exit code: 0]

$ ./mytmux new-session -s work
[error] session already exists
[exit code: 1]

3.5 Data Formats / Schemas / Protocols

    Session table: name -> list of windows with active index.

3.6 Edge Cases

  • Deleting active window
  • Duplicate session names

3.7 Real World Outcome

This section defines a deterministic, repeatable outcome. Use fixed inputs and set TZ=UTC where time appears.

3.7.1 How to Run (Copy/Paste)

make
./mytmux new-session -s work

3.7.2 Golden Path Demo (Deterministic)

The “success” demo below is a fixed scenario with a known outcome. It should always match.

3.7.3 If CLI: provide an exact terminal transcript

    $ ./mytmux new-session -s work
$ ./mytmux new-window -n logs
$ ./mytmux list-sessions
work: 2 windows
[exit code: 0]

Failure Demo (Deterministic)

    $ ./mytmux new-session -s work
[error] session already exists
[exit code: 1]

3.7.8 If TUI

At least one ASCII layout for the UI:

    +------------------------------+
    | Session and Window Hierarchy           |
    | [content area]               |
    | [status / hints]             |
    +------------------------------+

4. Solution Architecture

4.1 High-Level Design

    +-----------+     +-----------+     +-----------+
    |  Client   | <-> |  Server   | <-> |  PTYs     |
    +-----------+     +-----------+     +-----------+

4.2 Key Components

| Component | Responsibility | Key Decisions | |-----------|----------------|---------------| | Session store | Maps names to session objects. | Use hash map for lookup. | | Window list | Ordered windows per session. | Use linked list or vector. | | Command handlers | Mutate state safely. | Validate inputs before mutation. |

4.4 Data Structures (No Full Code)

    struct Session { char name[64]; int active_idx; Window *windows; };

4.4 Algorithm Overview

Key Algorithm: Active window update

  1. Find target window
  2. Update active index
  3. Notify clients

Complexity Analysis:

  • O(n) windows

5. Implementation Guide

5.1 Development Environment Setup

    cc --version
make --version

5.2 Project Structure

    session-model/
|-- src/
|   |-- session.c
|   `-- command.c
`-- Makefile

5.3 The Core Question You’re Answering

“How do you model sessions, windows, and panes as data?”

5.4 Concepts You Must Understand First

  1. session model
    • Why it matters and how it impacts correctness.
  2. lists/maps
    • Why it matters and how it impacts correctness.
  3. active pointers
    • Why it matters and how it impacts correctness.

5.5 Questions to Guide Your Design

  • How do you index sessions and windows?
  • How do you prevent dangling pane pointers?

    5.6 Thinking Exercise

Draw a diagram of sessions -> windows -> panes.

5.7 The Interview Questions They’ll Ask

  • How do you handle deleting active window?
  • How do you maintain state across clients?

    5.8 Hints in Layers

  • Use a list per session and a pointer to active.

5.9 Books That Will Help

| Topic | Book | Chapter | |——-|——|———| | tmux architecture | tmux 3 | Ch. 1 |

5.10 Implementation Phases

Phase 1: Foundation (2 weeks)

Goals:

  • Establish the core data structures and loop.
  • Prove basic I/O or rendering works.

Tasks:

  1. Implement the core structs and minimal main loop.
  2. Add logging for key events and errors.

Checkpoint: You can run the tool and see deterministic output.

Phase 2: Core Functionality (2 weeks)

Goals:

  • Implement the main requirements and pass basic tests.
  • Integrate with OS primitives.

Tasks:

  1. Implement remaining functional requirements.
  2. Add error handling and deterministic test fixtures.

Checkpoint: All functional requirements are met for the golden path.

Phase 3: Polish & Edge Cases (2 weeks)

Goals:

  • Handle edge cases and improve UX.
  • Optimize rendering or I/O.

Tasks:

  1. Add edge-case handling and exit codes.
  2. Improve logs and documentation.

Checkpoint: Failure demos behave exactly as specified.

5.11 Key Implementation Decisions

Decision Options Recommendation Rationale
I/O model blocking vs non-blocking non-blocking avoids stalls in multiplexed loops
Logging text vs binary text for v1 easier to inspect and debug

6. Testing Strategy

6.1 Test Categories

Category Purpose Examples
Unit Tests Validate components parser, buffer, protocol
Integration Tests Validate interactions end-to-end CLI flow
Edge Case Tests Handle boundary conditions resize, invalid input

6.2 Critical Test Cases

  1. Creating sessions updates list
  2. Deleting active window updates pointer

    6.3 Test Data

text Create session with 2 windows, delete active; expect active moves to neighbor.

7. Common Pitfalls & Debugging

7.1 Frequent Mistakes

| Pitfall | Symptom | Solution | |———|———|———-| | Active window lost | Pointer not updated | Always update active on delete. |

7.2 Debugging Strategies

  • Dump state graph on each command.

    7.3 Performance Traps

  • Linear scans for every lookup; use hash maps for sessions.

8. Extensions & Challenges

8.1 Beginner Extensions

  • Add rename-session command.
  • Add list-windows output.

    8.2 Intermediate Extensions

  • Add move-window between sessions.
  • Add window IDs.

    8.3 Advanced Extensions

  • Add per-client active window overrides.

9. Real-World Connections

9.1 Industry Applications

  • Terminal multiplexers
  • desktop window managers
  • tmux

    9.3 Interview Relevance

  • Event loops, terminal I/O, and state machines are common interview topics.

10. Resources

10.1 Essential Reading

11. Self-Assessment Checklist

11.1 Understanding

  • I can explain the core concept without notes
  • I can explain how input becomes output in this tool
  • I can explain the main failure modes

11.2 Implementation

  • All functional requirements are met
  • All test cases pass
  • Code is clean and well-documented
  • Edge cases are handled

11.3 Growth

  • I can identify one thing I’d do differently next time
  • I’ve documented lessons learned
  • I can explain this project in a job interview

12. Submission / Completion Criteria

Minimum Viable Completion:

  • Tool runs and passes the golden-path demo
  • Deterministic output matches expected snapshot
  • Failure demo returns the correct exit code

Full Completion:

  • All minimum criteria plus:
  • Edge cases handled and tested
  • Documentation covers usage and troubleshooting

Excellence (Going Above & Beyond):

  • Add at least one advanced extension
  • Provide a performance profile and improvement notes