Add the protocol-comparison module

protocol-comparison/docs/03-payload.md

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+# 3. Payload: the one comparison that needs no benchmark
+
+Previous: [2. Benchmarks](02-benchmarks.md) · Next: [4. Back-pressure](04-backpressure.md)
+
+---
+
+Five fields — a symbol, a sequence number, two doubles and a timestamp — encoded three
+ways ([`payload-sizes.txt`](output/payload-sizes.txt)):
+
+```
+protobuf :  35 bytes  0a044141504c102a197b14ae47e11a5940215c8fc2f5281c5940288080abb4fef39203
+JSON     :  83 bytes  {"symbol":"AAPL","seq":42,"bid":100.42,"ask":100.44,"epochMicros":1772000000000000}
+CBOR     :  67 bytes  bf6673796d626f6c644141504c63736571182a63626964fb40591ae147ae147b6361736bfb40591c28f5c28f5c6b65706f63684d6963726f731b00064b9fe68ac000ff
+JSON is 2.37x protobuf; CBOR is 1.91x protobuf
+```
+
+This is a property of the formats, not of a machine, so it is the one row of the comparison that
+transfers to your hardware unchanged.
+
+**Protobuf is 35 bytes because field names are integers.** `0a04 41 41 50 4c` is field 1, length
+4, `AAPL`. There is no `"symbol"` on the wire at all — the name lives in the `.proto` file
+that both sides compiled against. That is where the size comes from, and it is also the whole
+argument about schemas: the saving and the coupling are the same fact.
+
+**CBOR is 67 bytes and still writes the field names.** `6673796d626f6c` is a 6-character text
+string, `symbol`. CBOR is binary JSON, not a schema format: it saves the punctuation and encodes
+numbers compactly, and it keeps every key. So its advantage over JSON is real but modest, and it
+does not require the coupling.
+
+Concretely, on a stream of a million quotes: 35 MB, 67 MB, 83 MB. On a loopback
+that difference disappears into memory bandwidth — which is exactly why the benchmarks in
+[chapter 2](02-benchmarks.md) do not reward it and a real link would.
+
+Two practical notes:
+
+- **Spring's RSocket default is CBOR**, not JSON, which is why
+  `spring-boot-starter-rsocket` brings `jackson-dataformat-cbor`. If you are debugging with
+  `tcpdump` and expecting to read your payloads, that is why you cannot. The mechanism is codec
+  ordering, printed by
+  [`RSocketDefaultsTest`](../src/test/java/com/ankurm/protocols/RSocketDefaultsTest.java):
+
+  ```
+  encoders : [CharSequenceEncoder, ByteBufferEncoder, ByteArrayEncoder, DataBufferEncoder, JacksonCborEncoder, JacksonJsonEncoder]
+  bare RSocketStrategies.create() : [CharSequenceEncoder, ByteBufferEncoder, ByteArrayEncoder, DataBufferEncoder]
+  ```
+
+  CBOR sits *ahead of* JSON in the Boot-configured list, which is the whole of why it wins. Note
+  also that a bare `RSocketStrategies.create()` carries neither — an `RSocketRequester` built
+  without injecting Boot's strategies cannot encode your objects at all. And the class names
+  follow the Boot 4 rule: `JacksonCborEncoder`, not `Jackson2…`.
+- **You can put protobuf on RSocket.** The encoding and the protocol are independent choices;
+  the row above is what each stack does *by default*, not what it is capable of.
+
+---
+
+Previous: [2. Benchmarks](02-benchmarks.md) · Next: [4. Back-pressure](04-backpressure.md)