State machines: a practical reading guide

A state machine makes a system’s lifecycle explicit. It records the current state and defines which events can lead to another state. This is useful when the correct response to an action depends on what has already happened.

The Road to State Machines series builds that model gradually in Python, using an order that can be paid, shipped, or canceled. Read the five parts in order:

1. Road to State Machines Part 1 - Designing Systems When the Past Determines What Is Allowed Next

   Start with an order lifecycle: its current state summarizes the past and determines which actions should be allowed next.

2. Road to State Machines Part 2 - But How Do We Prevent Impossible Changes?

   Protect valid configurations with invariants and reject invalid changes before mutating the order.

3. Road to State Machines Part III - But How Do We Represent the Entire Lifecycle as One Behavioral Contract?

   Represent states, events, and transitions in one explicit finite state machine, shared by operations and the read path.

4. Road to State Machines Part IV - But How Do We Let Data Influence Transitions Without Turning Every Value into Another State?

   Use guards and supporting data to influence transitions without turning every value into another state.

5. Road to State Machines Part V - But How Does the Machine Wait for Something That Happens Later?

   Give asynchronous work a waiting state, then handle replies, correlation identifiers, timeouts, and queued events.