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RPC 请求语义 RPC Semantics

Definition​

RPC (Remote Procedure Call)

Semantics = meaning

client() {
 RPC_deposit(server, 42, $50.00); 
}

What should we expect when RPC returns?​

image-20240518161333063

At-least-once​

Procedure is executed on the server one or multiple times.

It is useful when the operation is Idempotent.

It is not useful when the operation is non-idempotent.

An idempotent operation produces the same result whether it is executed once or multiple times.

Examples include setting a specific value, deleting a resource, or incrementing a counter by a fixed amount.

  • call getResult(record_id=1) API, if it always returns same result, it is idempotent
  • call paid(order_id=1) API, it should only paid whenever how many times it is executed

Implementation​

Client keep retrying until receiving the response.

At-most-once​

Procedure is either executed once or not executed at all

Implementation​

  1. Client includes a unique ID in every request
  2. Server keeps a history of requests it has already answered, their ID, and the result
  3. If server encounters request with duplicate ID, server resends history result

When should sesver garbage collect RPC history?​

There are 3 options

Option 1:

  1. Never

Option 2:

  1. Unique clientID
  2. Client includes a seqID and its ClientID for each message
  3. Client include seen all msg <= X with every RPC, server can discard the RPC history with seqID <= X for this ClientID

Option 3:

  1. Each client only allow one outstanding request at a time
  2. Server discards the request with SeqNum <= CurSeq+1

Exactly-once​

Real-world usage​

launch a missile, need to ensure the command is sent exactly once so that the boom will not be triggered twice

Impossibility​

It is Impossible because the server can crash, either

  1. The server crashed before the execution arrived
  2. The server crashes after the execution arrives

Implementation with workaround​

We can ensure the same executed result with allowing retries and db record. Which is At-most-once + client retries until success.

More​

The Exactly Once Semantics in Kafka

Chauhan, A. (2024, March 5). Demystifying Kafka Exactly Once Semantics (EOS) - HelloTech. Medium. https://engineering.hellofresh.com/demystifying-kafka-exactly-once-semantics-eos-390ae1c32bba

Kafka 是如何实现精确一次（exactly once）语义的？

Reference​

  1. NUS CS5223 Distributed System Course
  2. Xu, A. (2022, February 24). At most once, at least once, exactly once. ByteByteGo Newsletter. https://blog.bytebytego.com/p/at-most-once-at-least-once-exactly
  3. Chauhan, A. (2024, March 5). Demystifying Kafka Exactly Once Semantics (EOS) - HelloTech. Medium. https://engineering.hellofresh.com/demystifying-kafka-exactly-once-semantics-eos-390ae1c32bba