Context

Context

Context propagation for metadata across service calls

---

Overview

The context object carries metadata that propagates through the entire call chain. When Service A calls Service B, and B calls Service C, context flows A→B→C.

┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│  Service A  │────▶│  Service B  │────▶│  Service C  │
│             │     │             │     │             │
│ tenant: X   │     │ tenant: X   │     │ tenant: X   │
│ caller: A   │     │ caller: B   │     │ caller: C   │
└─────────────┘     └─────────────┘     └─────────────┘

---

Context Object

{
  "context": {
    "caller": "checkout-service",
    "tenant_id": "tenant_acme",
    "user_id": "user_42"
  }
}

Standard Fields

Field	Type	Required	Description
caller	string	No	Identifier of the calling service

Custom Fields

Applications MAY add custom fields to context for domain-specific propagation:

{
  "context": {
    "caller": "checkout-service",
    "tenant_id": "tenant_acme",
    "user_id": "user_42",
    "feature_flags": ["new_checkout", "beta_pricing"],
    "correlation_id": "order_12345"
  }
}

Common custom fields:

• tenant_id — Multi-tenant isolation
• user_id — Acting user (for audit)
• feature_flags — Active feature flags
• correlation_id — Business correlation
• environment — Environment hint (staging, production)

---

Propagation Rules

Incoming Requests

When receiving a request, servers MUST:

1. Extract context from the request
2. Preserve custom fields unchanged
3. Update caller to current service name for downstream calls

Outgoing Requests

When making downstream calls, clients MUST:

1. Copy custom fields from current context
2. Set caller to current service name
3. Propagate custom fields as appropriate

Example Chain

Request A→B:

{
  "context": {
    "caller": "api-gateway",
    "tenant_id": "tenant_acme"
  }
}

Request B→C:

{
  "context": {
    "caller": "order-service",
    "tenant_id": "tenant_acme"
  }
}

---

Context vs Extensions

Context and extensions serve different purposes:

Aspect	Context	Extensions
Propagates	Yes, through call chain	Per-request (may influence downstream)
Purpose	Business metadata	Optional capabilities
Examples	tenant_id, user_id, caller	deadline, idempotency, tracing

Rule of thumb:

• If downstream services need it automatically → Context
• If it’s an optional capability → Extensions

---

Distributed Tracing

For distributed tracing (trace_id, span_id, parent_span_id), use the Tracing Extension. This keeps observability concerns separate from business context.

---

Missing Context

Servers MUST handle requests without context gracefully:

// Request without context
{
  "protocol": { "name": "mesh", "version": "0.1.0" },
  "id": "req_123",
  "call": {
    "function": "orders.get",
    "version": "1",
    "arguments": { "order_id": 42 }
  }
}

When context is missing:

• Servers SHOULD log a warning
• Servers MUST process the request normally

---

Examples

Basic Context

{
  "protocol": { "name": "mesh", "version": "0.1.0" },
  "id": "req_basic",
  "call": {
    "function": "orders.create",
    "version": "1",
    "arguments": { ... }
  },
  "context": {
    "caller": "web-frontend"
  }
}

Multi-Tenant Context

{
  "protocol": { "name": "mesh", "version": "0.1.0" },
  "id": "req_multi_tenant",
  "call": {
    "function": "reports.generate",
    "version": "1",
    "arguments": { ... }
  },
  "context": {
    "caller": "dashboard-service",
    "tenant_id": "tenant_acme_corp",
    "user_id": "user_42",
    "feature_flags": ["new_report_engine"]
  }
}

With Tracing Extension

For full distributed tracing, combine context with the tracing extension:

{
  "protocol": { "name": "mesh", "version": "0.1.0" },
  "id": "req_traced",
  "call": {
    "function": "orders.create",
    "version": "1",
    "arguments": { ... }
  },
  "context": {
    "caller": "checkout-service",
    "tenant_id": "tenant_acme"
  },
  "extensions": [
    {
      "urn": "urn:mesh:ext:tracing",
      "options": {
        "trace_id": "tr_abc123def456",
        "span_id": "sp_001"
      }
    }
  ]
}