gRPC & Kafka — Series 3

Preview — 3 of 10 questions

What do optional and oneof contribute?

javascript

message UpdateOrder {
  optional string note = 1;
  oneof target { string order_id = 2; string external_ref = 3; }
}

Aoptional makes the field nullable in the generated TypeScript, with no wire effect

BBoth restore field presence: without them a scalar set to its default ("", 0, false) is indistinguishable from an unset one, so a partial update cannot express "clear this note" versus "leave it alone" — and oneof additionally enforces that exactly one of its members is set

Coneof allows several members to be set simultaneously, unlike optional

DBoth are documentation hints consumed only by code generators

What is metadata, and how should the server treat it?

javascript

const metadata = new Metadata();
metadata.add('authorization', `Bearer ${token}`);
this.ordersService.findOne({ id }, metadata);

AA reserved channel for gRPC's own framing, which applications must not use

BA field of the request message, serialised alongside the payload

CPer-call state shared across every call on the same channel

DKey–value pairs sent with the call, the gRPC analogue of HTTP headers — carrying credentials, trace context and deadlines separately from the message; on the server they reach a guard through the execution context and should be validated exactly like an HTTP header, since a client controls them entirely

What is happening?

javascript

Long-lived gRPC channel through a load balancer.
After a few minutes of inactivity, the next call fails with UNAVAILABLE.

AAn intermediary closed the idle TCP connection without either endpoint noticing, so the client discovers it only when it tries to use it — configuring HTTP/2 keepalive pings keeps the connection observably alive and lets the client detect a broken one before the next request

BThe channel's credentials expired and must be refreshed

CgRPC channels are single-use and must be recreated per call

DThe server's deadline elapsed while the channel was idle

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