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# Scopes

This page explains how scope stacks establish ownership, parentage, cleanup, and
isolation.

## Why Scopes Exist

Scopes are the ownership backbone of NeMo Relay. Every tool call, LLM call, and
mark event attaches to a scope hierarchy.

That hierarchy lets the runtime:

* Model nested agent work
* Preserve parent-child relationships
* Expose scope-local middleware and subscribers
* Clean up scope-owned runtime state automatically
* Isolate concurrent work

## What a Scope Represents

A scope represents a logical unit of work such as:

* An agent run
* A request
* A workflow step
* A background task
* A nested function or tool orchestration boundary

Scopes are not just labels. They define ownership and visibility for other
runtime behavior.

## Scope Hierarchy and Ownership

Scopes form a tree. A child scope inherits the active execution context from its
parent and contributes new nested work beneath it.

That hierarchy determines:

* Event parentage
* Lifetime boundaries
* Scope-local middleware visibility
* Scope-local subscriber visibility

## Scope Types

NeMo Relay includes standard scope types for common runtime semantics, including:

* `Agent`
* `Function`
* `Tool`
* `Llm`
* `Retriever`
* `Embedder`
* `Reranker`
* `Guardrail`
* `Evaluator`
* `Custom`
* `Unknown`

The specific type helps subscribers and downstream tracing systems understand
what the scope represents semantically.

## Scope Behavior

These scope behaviors define how root, child, and scope-local runtime state interact.

### Root Scope

A root scope is always present. Other scopes are pushed beneath that root as
work becomes more specific.

### Parent-Child Relationships

Nested scopes create the ownership tree used by emitted events. Tool and LLM
calls then attach beneath the active scope.

### Scope Lifetimes

Scopes have explicit lifetime boundaries. A scope starts when it becomes active
and ends when it is popped or closed.

### Scope-Local Cleanup

Scope-local middleware and subscribers are tied to the owning scope lifecycle.
When the scope closes, those registrations disappear automatically.

## Semantic Payloads

Scopes may expose semantic `input` and `output` payloads on their emitted start
and end events.

### Scope Input

Use scope `input` when the scope itself represents a request-style or task-style
unit of work whose starting payload matters semantically.

### Scope Output

Use scope `output` when the scope itself produces a meaningful semantic result.

Those payloads live on the emitted events rather than on the scope handle
itself.

## Context Isolation

Context isolation keeps concurrent requests, tenants, and agents from sharing scope-
local state accidentally.

### Why Isolation Matters

Concurrent requests must not share the same active scope stack accidentally.
Otherwise:

* Unrelated work can appear under the wrong parent
* Scope-local middleware can leak across requests
* Scope-local subscribers can observe the wrong execution tree

## Reuse an Existing Logical Trace

Reuse or propagate the active scope stack when detached work should continue the
same logical request or agent trace.

Use this when:

* Worker events should appear under the same parent request
* Scope-local middleware from the parent should still apply
* Subscribers should observe one continuous execution tree

## Start a Fresh Isolated Context

Create and bind a fresh stack when detached work should be independent.

Use this when:

* The worker is a separate job rather than part of the parent trace
* The boundary cannot safely carry a native stack handle
* You want a clean root scope with isolated scope-local registrations

### Fork Concurrent Work

Fork a scope stack before starting concurrent work when the child should remain
part of the parent's event tree without sharing its mutable stack. The fork
preserves the immediate parent but does not transfer scope-local middleware or
subscribers. This is Relay event-tree continuity: the rootless fork starts a
new OpenTelemetry trace from its first local span.

#### Python

```python
import asyncio

import nemo_relay

async def worker() -> None:
    with nemo_relay.scope.scope("worker", nemo_relay.ScopeType.Function):
        await asyncio.sleep(0)

async def main() -> None:
    with nemo_relay.scope.scope("parent", nemo_relay.ScopeType.Agent):
        await asyncio.create_task(
            worker(),
            context=nemo_relay.fork_asyncio_context(),
        )
```

#### Rust

```rust
use nemo_relay::api::runtime::{TASK_SCOPE_STACK, fork_scope_stack};

let stack = fork_scope_stack()?;
tokio::spawn(TASK_SCOPE_STACK.scope(stack, async {
    // Relay work in an isolated child task.
}));
```

#### Node.js

Use `withScopeStack()` to bind each asynchronous branch to its isolated stack.
The stack remains active until the branch's returned Promise settles, while the
calling context is restored immediately.

```javascript
const [first, second] = await Promise.all([
  withScopeStack(firstStack, async () => runWorker("first")),
  withScopeStack(secondStack, async () => runWorker("second")),
]);
```

`setThreadScopeStack()` mutates the current thread or async resource. Do not use
it to isolate concurrent branches that begin in the same synchronous callback.

## Cross-Process Propagation

When work crosses a process or remote-workflow boundary, applications can carry
the versioned Relay propagation context instead of a native stack handle. The
context contains an immediate `parent_uuid` and, when the application knows a
stable session root, an optional `root_uuid`.

The receiver creates a fresh isolated stack from that context and installs it
only for request handling. Its first local event becomes a child of
`parent_uuid`; scope-local middleware and subscribers are never transferred.
The transport is application-owned: authenticate and authorize inbound context
before importing it. Relay does not send headers, make IPC connections, or
trust remote identifiers automatically.

A rootless context, including the value returned by the default
`capture_propagation_context()` API, preserves this Relay event parentage but
does not assert OpenTelemetry trace continuity. The first local span starts a
new trace. Supply a stable `root_uuid` when the receiver should participate in
the Relay-derived trace rooted at that UUID.

Relay context is distinct from W3C propagation. An integration may carry
`traceparent` and `tracestate` alongside Relay's JSON context when it needs to
preserve OpenTelemetry sampling or vendor state.

Use the binding's JSON helpers at the transport boundary: Rust
`PropagationContext::to_json` and `PropagationContext::from_json`, Python
`context.to_json()` and `PropagationContext.from_json(...)`, Go
`context.ToJSON()` and `PropagationContextFromJSON(...)`, or Node.js
`propagationContextToJson(...)` and `propagationContextFromJson(...)`. The
helpers validate the version and UUIDs before a context is imported.

## Practical Guidance

Use these practices when applying the concept in application or integration code.

* Push a top-level scope at the entry point of a request, workflow, or agent
  run.
* Let nested helpers attach work beneath that scope whenever possible.
* Use scope-local registrations when the behavior should disappear with the
  owning scope.
* Emit mark events for retries, checkpoints, interrupts, or state transitions
  that are important for debugging but are not full spans.
* Prefer explicit isolation decisions when work crosses thread, task, or worker
  boundaries.