What's the order of execution within a trigger transaction?

When a record is saved, Apex runs in this order:

1. System validation — required fields, max length, foreign keys.
2. All `before` triggers fire (one trigger per object recommended; if multiple exist, ordering is undefined).
3. Custom validation rules evaluate.
4. Before-save flows run (record-triggered flows configured for fast field updates).
5. Database commit-1 — record is saved but not yet committed.
6. All `after` triggers fire. Record's Id is now populated.
7. Assignment rules (Lead/Case).
8. Auto-response rules (Lead/Case).
9. After-save record-triggered flows.
10. Workflow rules — legacy field updates and outbound messages.
11. Process Builder processes.
12. If a workflow field update happened, before/after triggers and validation rules fire AGAIN on those affected fields (re-entry).
13. Escalation rules (Case).
14. Roll-up summary recalculation.
15. Sharing rules evaluation.
16. Database commit-2 — transaction is now durable.
17. Async logic — @future, queueable, batch, outbound messages, platform events that publish on commit.

Implications for trigger code:

• before triggers can mutate Trigger.new directly without DML.
• after triggers run on records that are already in the DB but pre-commit.
• Outbound messages, async Apex, and platform events fire only after commit-2 — they don't see intermediate state.
• Recursion: if your trigger fires another DML on the same object, the trigger fires again — guard with a static recursion flag.