gRPC, Protobuf & Streaming RPC | Communication & Data Transfer

Why gRPC for internal services

HTTP/2 multiplexing (see Networking) — many RPCs on one connection
Protobuf — compact, fast encode/decode, schema in .proto
Codegen — stubs in 10+ languages from one IDL
First-class streaming — not bolted on like chunked HTTP
Deadlines/cancellation propagated in metadata

Default at Google, widely adopted in K8s ecosystems (etcd, Kubernetes API).

Four RPC types

Type	Pattern	Example
Unary	1 req → 1 resp	`GetUser`
Server streaming	1 req → N resp	`ListLogs`, stock ticks
Client streaming	N req → 1 resp	upload chunks, aggregate
Bidirectional	N ↔ N	chat, collaborative edit

Streaming uses HTTP/2 DATA frames; flow control per stream.

Protobuf essentials

message Stock {
  string sku = 1;
  int32 quantity = 2;
  google.protobuf.Timestamp updated_at = 3;
}

service Inventory {
  rpc GetStock(GetStockRequest) returns (Stock);
  rpc WatchStock(WatchRequest) returns (stream Stock);
}

Field numbers never reuse
optional / repeated / oneof / map<>
Well-known types — Timestamp, Struct, Any
Package versioning: package inventory.v2;

gRPC status codes

Maps to google.rpc.Status — not HTTP status (though HTTP/2 carries them):

OK, INVALID_ARGUMENT, NOT_FOUND, ALREADY_EXISTS
DEADLINE_EXCEEDED, UNAVAILABLE, RESOURCE_EXHAUSTED
Rich errors: grpc-status-details-bin with ErrorInfo, RetryInfo

Metadata

Headers for auth (authorization), tracing (traceparent), routing, custom baggage. Binary metadata for encoded tokens.

Deadlines

Client sets deadline — server should abort work when exceeded. Propagate to downstream calls. Critical for preventing thread pile-up.

Load balancing

L7 gRPC-aware LB required — connection reuse means naive round-robin on TCP connections skews load. xDS, lookaside, or proxyless gRPC LB. Underlying HTTP/2 multiplexing is Networking; pool sizing on the server is Concurrency → Thread Pools.

vs REST

gRPC: performance, typing, streaming, internal mesh. REST: browsers, CDNs, public APIs, curl debugging.

Bridge: gRPC-Gateway generates REST from proto; Envoy translates gRPC-Web.

Hands-On Tasks (Optional)

API design drills and whiteboard exercises — protocol selection, contract design, and bulk-transfer architecture. Assumes Networking and sibling tracks on the hub page (Distributed Systems, Databases, Concurrency, LLD).

Design a protobuf service
Define `InventoryService` with unary `GetStock`, server-streaming `WatchStock`, and client-streaming `BulkUpdate`. Justify streaming mode per method.
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