Advanced Restart Strategies

Welcome to a crucial topic in Erlang: restart strategies! These define how a supervisor reacts when one of its child processes crashes.

Understanding these strategies is key to building fault-tolerant and self-healing systems, which is a hallmark of Erlang/OTP.

Before diving in, let's quickly recap: a supervisor is a special process that monitors other processes (its children).

• If a child process dies, the supervisor detects it.
• Based on its configured restart strategy, the supervisor decides how to bring the system back to a healthy state.
• This ensures your application can recover from individual process failures automatically.

The one_for_one strategy is the simplest and most common. When a child process terminates:

• Only the failing child process is restarted.
• All other sibling processes remain unaffected and continue running.

This strategy is ideal for systems where child processes are largely independent of each other, such as individual client connections.

Let's see one_for_one. We'll have a supervisor managing a single worker. When the worker crashes, only it restarts.

To run:
1. Compile: c(worker_gen). c(supervisor_one_for_one).
2. Start supervisor: supervisor_one_for_one:start_link().
3. Crash worker: worker_gen:crash(worker_1).
Observe the output in your Erlang shell.

The one_for_all strategy takes a more drastic approach:

• If any child process terminates, all other child processes are first terminated.
• Then, all child processes (including the one that crashed) are restarted.

This is useful when your child processes are tightly coupled and require a consistent, synchronized state. A failure in one implies a need to reset the entire group.

Here, a supervisor manages two workers. Crash one, and both will restart.

To run:
1. Compile: c(worker_gen). c(supervisor_one_for_all).
2. Start supervisor: supervisor_one_for_all:start_link().
3. Crash worker: worker_gen:crash(worker_A).
Notice how both worker_A and worker_B restart.

The rest_for_one strategy offers a middle ground:

• If a child process terminates, it and all subsequent children (those defined after it in the supervisor's child list) are terminated.
• Then, the failed child and all subsequent children are restarted.
• Children defined before the failed child are left untouched.

This is useful when processes have sequential dependencies, where a failure in an earlier stage might invalidate the state of later stages.

We have three workers: X, Y, Z. If Y crashes, Y and Z restart, but X remains active.

To run:
1. Compile: c(worker_gen). c(supervisor_rest_for_one).
2. Start supervisor: supervisor_rest_for_one:start_link().
3. Crash worker: worker_gen:crash(worker_Y).
See worker_X stay running, while worker_Y and worker_Z restart.

Choosing the right strategy depends on your application's architecture and process dependencies:

• one_for_one: Use for independent processes, like individual client connections or request handlers.
• one_for_all: Best for tightly coupled processes that must always be in a consistent state together (e.g., a group of processes managing a single resource).
• rest_for_one: Suitable for sequential pipelines or layered systems where a failure in an earlier stage affects subsequent stages.

You are building a system where a primary worker fetches data, and two secondary workers process different aspects of that data. If the primary worker fails, the secondary workers cannot continue with stale data and must also restart. If a secondary worker fails, the others are unaffected. Which strategy is most appropriate for the primary worker and its dependent secondary workers?

You've explored Erlang's powerful restart strategies:

• one_for_one: Restarts only the failed child, keeping others running.
• one_for_all: Restarts all children if any one fails, ensuring full consistency.
• rest_for_one: Restarts the failed child and all subsequent children in the list.

These strategies are fundamental to building robust, self-healing applications in Erlang, allowing your system to recover from failures gracefully.