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Node gRPC Metadata options

  • Author(s): murgatroid99
  • Approver: wenbozhu
  • Status: Draft
  • Implemented in: Node
  • Last updated: 2019-03-01
  • Discussion at: (filled after thread exists)

Abstract

Add options to the Metadata class corresponding to start-of-call flags in the core.

Background

The gRPC core library defines a few "metadata" flags that can be passed to grpc_call_start_batch at the beginning of a call in a "send initial metadata" grpc_op. Currently, there is no way for a user of the Node gRPC library to set those flags.

Proposal

Currently the Metadata constructor takes no arguments. We will modify it to take one argument which is an "options" object mapping string keys to boolean values. These options correspond to the core's initial metadata flags The following options will be accepted:

  • idempotentRequest: Signal that the call is idempotent. Default: false
  • waitForReady: Signal that the call should not return UNAVAILABLE before it has started. Default: true
  • cacheableRequest: Signal that the call is cacheable. GRPC is free to use GET verb. Default: false
  • corked: Signal that the initial metadata should be corked. Default: false

We will also add a setOptions method to the Metadata class that accepts the same options object and replaces the previously set options.

Rationale

These flags need to be passed to the core in grpc_call_start_batch in a "send initial metadata" grpc_op, so they can be passed if and only if metadata is also passed. The simplest way to accomplish this is to make these flags part of the Metadata structure. It is useful to include a setter to allow users to easily use the same metadata with different calls that would want to use different flags.

Implementation

I (murgatroid99) will implement this in both grpc and @grpc/grpc-js. The initial implementation in @grpc/grpc-js will have no behavioral changes from the options.