RpcParamsFactory

Kind: Class

Source: packages/microservices/factories/rpc-params-factory.ts

Part of: Microservices

RpcParamsFactory resolves RPC handler parameter values from the arguments supplied by a microservice transport. Its exchangeKeyForValue() method maps an RPC parameter type—such as payload or context—to the appropriate value, including selecting a named property from the payload.

Methods

Where it refuses work

• RpcParamsFactory stops the work with an early return when !args.

Diagram

mermaid
graph LR
  A[Transport invokes RPC handler] --> B[Handler arguments array]
  B --> C[RpcParamsFactory.exchangeKeyForValue]
  C --> D{RPC parameter type}
  D -->|PAYLOAD| E[Full payload or payload property]
  D -->|CONTEXT| F[Transport context]
  D -->|Unknown type| G[null]

Usage

ts
import { RpcParamsFactory } from '@nestjs/microservices/factories/rpc-params-factory';
import { RpcParamtype } from '@nestjs/microservices/enums/rpc-paramtype.enum';

const paramsFactory = new RpcParamsFactory();

const payload = { id: 'user-123', action: 'created' };
const context = { pattern: 'users.create' };
const args = [payload, context];

// Resolve the complete message payload.
const message = paramsFactory.exchangeKeyForValue(
  RpcParamtype.PAYLOAD,
  undefined,
  args,
);

// Resolve a property from the message payload.
const userId = paramsFactory.exchangeKeyForValue(
  RpcParamtype.PAYLOAD,
  'id',
  args,
);

// Resolve the RPC transport context.
const rpcContext = paramsFactory.exchangeKeyForValue(
  RpcParamtype.CONTEXT,
  undefined,
  args,
);

AI Coding Instructions

• Pass handler arguments in their expected order: payload at index 0 and RPC context at index 1.
• Use RpcParamtype.PAYLOAD with a property key to extract a specific payload field; omit the key to return the full payload.
• Use RpcParamtype.CONTEXT only when the transport provides context as the second handler argument.
• Preserve the null fallback for unsupported parameter types or missing argument arrays so parameter resolution fails safely.

How it works

RpcParamsFactory is a class with one method, exchangeKeyForValue(type, data, args), that selects an RPC handler parameter value from an invocation-argument array. Its parameters are a numeric type, optional string data, and unknown[] arguments. [packages/microservices/factories/rpc-params-factory.ts:3-8]

• For RpcParamtype.PAYLOAD, it returns args[0] when data is falsy; when data is truthy, it returns the property named by data from args[0]. Optional chaining means a missing or nullish first argument yields undefined for property extraction. [packages/microservices/factories/rpc-params-factory.ts:12-15]
• For RpcParamtype.CONTEXT, it returns args[1]; data is ignored. [packages/microservices/factories/rpc-params-factory.ts:12-17]
• For RpcParamtype.GRPC_CALL, it returns args[2]; data is ignored. [packages/microservices/factories/rpc-params-factory.ts:12-19]
• The enum defines these three kinds as PAYLOAD, CONTEXT, and GRPC_CALL. [packages/microservices/enums/rpc-paramtype.enum.ts:3-7]
• If args is falsy, or if type does not match one of those enum values, the method returns null. [packages/microservices/factories/rpc-params-factory.ts:9-11] [packages/microservices/factories/rpc-params-factory.ts:19-20]
• It does not check whether the selected array slot or requested payload property exists. Thus, with a truthy args array, missing slots or properties can result in undefined. [packages/microservices/factories/rpc-params-factory.ts:14-18]
• The method only reads from args and does not write to it or other state. [packages/microservices/factories/rpc-params-factory.ts:9-21]

RpcContextCreator owns an instance of this class and creates extraction callbacks that pass the callback’s received arguments into exchangeKeyForValue. [packages/microservices/context/rpc-context-creator.ts:41-45] [packages/microservices/context/rpc-context-creator.ts:237-241] The default non-gRPC metadata maps the payload parameter to argument slot 0; the gRPC defaults additionally map context and gRPC call parameters to slots 1 and 2. [packages/microservices/context/rpc-metadata-constants.ts:3-10]