> ## Documentation Index > Fetch the complete documentation index at: https://iii.dev/docs/llms.txt > Use this file to discover all available pages before exploring further. # Overview > Every new capability in iii is added by writing a Worker, never by extending the Engine. Every new capability in a iii system is added by creating a Worker. A queue, a scheduler, an HTTP edge, a browser tab, an agent, a CRM integration, or a sandbox. Each is a Worker that connects to the Engine and registers Triggers and Functions. The Engine itself never needs to change. ## Workers are the services A Worker is the unit of capability in iii. It is the service. When you need new behaviour, you write a new Worker (or install an existing one from the registry); you do not patch the Engine, add a plugin to it, or fork it. The Engine is a fixed coordinator, it routes invocations between Workers and maintains the live registry of what each Worker provides. Everything that makes a iii system do something useful runs in a Worker. This is why "add a Worker" is the answer to almost every "how do I add X to iii?" question. If a capability is missing, the gap is filled by a Worker, not by a change to iii itself. For the mental model behind Workers, Triggers, Functions, and the Engine, see [Understanding iii / Overview](../understanding-iii). To build Workers continue with this section. ## Build on existing Workers Most Workers you create will use one or more existing Workers for their basic functionality, an HTTP endpoint, a key-value store, a queue, scheduling, observability, a sandbox, rather than building those from scratch. Reach for what already exists before writing your own. The ones you reach for most often are documented here under **Common Workers**: * [HTTP](./http), expose Functions as HTTP endpoints. * [Queues](./queues), async job processing with retries and a dead-letter queue. * [Observability](./observability), traces, metrics, and logs across Workers. * [Sandboxes](./sandboxes), run untrusted or short-lived code in an isolated microVM. * [Access Control](./worker-manager), accept untrusted connections through RBAC-gated listeners. The full catalog of Workers is published at [workers.iii.dev](https://workers.iii.dev). If you need common functionality for your Worker, look in these two places first. ## What's in this section Connect a Worker to the Engine and deploy it. Declare what causes those Functions to run. Register the Functions a Worker contributes.