Scopes

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

Why Scopes Exist

Scopes track where work belongs in 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 workflow

Scopes are not just labels. They determine event parentage and which local middleware and subscribers are visible.

Scope Hierarchy and Ownership

Scopes form a tree. A child scope inherits the active 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.

Worked Scope Lifetime

Consider an agent that calls one tool and then one LLM:

1. The application or framework integration pushes an Agent scope beneath the root and emits its start event.
2. The application registers middleware and a subscriber on the agent scope. Both are visible while the agent or any of its nested scopes are active.
3. A managed tool wrapper emits the tool start event, runs the tool, and emits the tool end event. The agent remains the active scope.
4. The application emits a mark under the active agent scope. The mark does not change the stack.
5. A managed LLM wrapper emits the LLM start event, runs the model call, and emits the LLM end event. The agent remains the active scope.
6. The application or framework integration emits the agent end event and pops the agent scope. Its local middleware and subscriber are removed.

The active scope stack changes over time. The emitted events remain in a parent-linked tree after their scopes close. A mark event attaches beneath the active scope but does not push another scope onto the stack.

Agent-local middleware and subscribers are visible during the first four stack states. They are no longer visible after the agent scope closes.

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.

Choose the context behavior based on whether the work belongs to the same logical trace and whether it runs concurrently:

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.

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import asyncio
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import nemo_relay
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async def worker() -> None:
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    with nemo_relay.scope.scope("worker", nemo_relay.ScopeType.Function):
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        await asyncio.sleep(0)
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async def main() -> None:
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    with nemo_relay.scope.scope("parent", nemo_relay.ScopeType.Agent):
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        await asyncio.create_task(
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            worker(),
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            context=nemo_relay.fork_asyncio_context(),
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        )

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.

For outbound-only W3C propagation, use capture_traceparent() to obtain the current Relay traceparent directly, or convert a rooted PropagationContext with PropagationContext::to_traceparent (and the equivalent binding methods). Rootless contexts cannot be converted because they intentionally start a new local OpenTelemetry trace. Managed LLM execution automatically adds one runtime-owned traceparent header to the provider request; a standalone request-intercept call adds it only when a real Relay trace context exists.

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.