Protocol specification

Table of contents

1. Background
2. Defining the ECHO RPC

To begin, you will be modifying an existing protocol specification to define a new remote procedure.

Background

To be able to send RPC requests from the client to the server, we need to specify how to serialize and deserialize both client requests and server replies.

rpcgen makes this easier by taking a message protocol defined using the XDR specification language and autogenerating the necessary stubs.

Let’s take a look at the current protocol specification, which can be found in kv_store.x. As you can see, XDR syntax is very similar to C, but keep in mind that they are two different languages (hence the .x file extension). We will note any differences that are relevant to this lab.

 program KVSTORE {
	version KVSTORE_V1 {
		int EXAMPLE(int) = 1;
	} = 1;
} = 0x20000001;

The outer program KVSTORE block specifies the service that we would like to create (specifically, a key/value store). This service may have multiple versions, one of which is defined using the version KVSTORE_V1 block contained within. This versioning feature comes in handy since backward compatibility is critical in distributed systems, where different machines may be on different versions of the protocol at a given point in time.

However, the most important section of the specification is the innermost contents, which specify the API exposed by the RPC service. Specifically, this KVSTORE service currently only exposes an EXAMPLE RPC that takes in an integer and outputs and integer.

Defining the ECHO RPC

Modify kv_store.x to include an ECHO RPC, where a client sends a message string to the server, and receives that same message string back in the server’s reply. If you have not already, it may help to take a look at the rpcgen tutorial (carefully read up to, but not including, Example 3-2 for all of the information relevant to this section).

You will be implementing this RPC’s functionality in the next part.