# Explanation > Background on Everruns, why durable execution, why events are the primary store, how the agentic loop and capability layering fit together. Source: These pages discuss *why* Everruns is shaped the way it is. They don’t tell you how to do anything (see [How-to guides](https://everruns.com/docs/how-to/)) and they aren’t lookup tables (see [Reference](https://everruns.com/docs/api/)). They’re here to give you a mental model so the other docs make sense. Read these when: * You’re evaluating Everruns and want to understand the design. * A reference page tells you *what* something is but you want to know *why* it exists. * You’re about to make an architectural decision and want to know which guarantees you can rely on. ## Topics * [Concepts](https://everruns.com/docs/getting-started/concepts/), a glossary of the entity model: harnesses, agents, sessions, capabilities, and how they compose into a runtime. * [The agentic loop](https://everruns.com/docs/explanation/agentic-loop/), the reason–act cycle, execution phases, and why turns are bounded. * [Architecture](https://everruns.com/docs/explanation/architecture/), control plane, workers, and the API-first design. * [Durable execution](https://everruns.com/docs/explanation/durable-execution/), why agents survive crashes, and the trade-offs of a PostgreSQL-backed engine. * [Events as the primary store](https://everruns.com/docs/explanation/events/), why an append-only event log is the source of truth, not a side-channel.