RPC过程理念详解

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RPC过程理念详解【待完善】

1.什么是进程通信？

IPC，以下引自wikipedia

In computer science, inter-process communication or interprocess communication (IPC) refers specifically to the mechanisms an operating system provides to allow the processes to manage shared data. Typically, applications can use IPC, categorized as clients and servers, where the client requests data and the server responds to client requests.

【在计算机科学中，进程内部通信（IPC）通常指的是一个操作系统提供允许进程管理共享数据的机制。典型的，应用能够使用IPC将机器分为客户机（client）和服务器（server），其中客户机请求数据并且服务器响应客户端的请求】

2.什么是RPC？

以下引自 wikipedia

In distributed computing, a remote procedure call (RPC) is when a computer program causes a procedure (subroutine) to execute in a different address space (commonly on another computer on a shared network), which is coded as if it were a normal (local) procedure call, without the programmer explicitly coding the details for the remote interaction. That is, the programmer writes essentially the same code whether the subroutine is local to the executing program, or remote.[1] This is a form of client–server interaction (caller is client, executor is server), typically implemented via a request–response message-passing system. In the object-oriented programming paradigm, RPC calls are represented by remote method invocation (RMI). The RPC model implies a level of location transparency, namely that calling procedures is largely the same whether it is local or remote, but usually they are not identical, so local calls can be distinguished from remote calls. Remote calls are usually orders of magnitude slower and less reliable than local calls, so distinguishing them is important.

在分布式计算中，一个远程调用（RPC）指的是：当一个程序调用一个在不同地址空间（典型的情况就是在另外的一台共享网络的计算机）的子程序，这个子程序被编码成好像是一个正常的本地（local）程序调用，没有经过程序员显式的为远程交互而做的编码细节。也就是说，无论子程序是在本地还是在远端执行，程序员在本质上编写的代码都是相同的。这是一个client-server互动的过程（caller 是client，executor是server），典型实现是通过一个request-response消息传递机制【划重点】。在面向对象的编程范式中，RPC调用由远程方法调用RMI【划重点】表示。RPC模型暗示了位置透明，即调用过程很大程度上是相同的（无论是本地还是远程调用），但是通常来说，他们又不相同，因此需要将本地调用和远程调用区分开。远程调用通常比本地调用慢几个数量级，并且远程调用更不可靠，所以将其区分开是很重要的。
RPCs是IPC(inter-process communication：进程间通信)的一种形式，因为不同的进程有着不同的地址空间：如果在相同的主机上，它们有着不同的虚拟地址空间，尽管物理地址空间是相同的，然而如果它们在不同的主机上，物理的地址空间就是不同的。许多不同的（经常是不兼容的）科技被用于实现此概念。

RPCs are a form of inter-process communication (IPC), in that different processes have different address spaces: if on the same host machine, they have distinct virtual address spaces, even though the physical address space is the same; while if they are on different hosts, the physical address space is different. Many different (often incompatible) technologies have been used to implement the concept.

Message passing

RPC is a request–response protocol. An RPC is initiated by the client, which sends a request message to a known remote server to execute a specified procedure with supplied parameters. The remote server sends a response to the client, and the application continues its process. While the server is processing the call, the client is blocked (it waits until the server has finished processing before resuming execution), unless the client sends an asynchronous request to the server, such as an XMLHttpRequest. There are many variations and subtleties in various implementations, resulting in a variety of different (incompatible) RPC protocols.

An important difference between remote procedure calls and local calls is that remote calls can fail because of unpredictable network problems. Also, callers generally must deal with such failures without knowing whether the remote procedure was actually invoked. Idempotent procedures (those that have no additional effects if called more than once) are easily handled, but enough difficulties remain that code to call remote procedures is often confined to carefully written low-level subsystems.

Sequence of events

1. The client calls the client stub. The call is a local procedure call, with parameters pushed on to the stack in the normal way.
2. The client stub packs the parameters into a message and makes a system call to send the message. Packing the parameters is called marshalling.
3. The client’s local operating system sends the message from the client machine to the server machine.
4. The local operating system on the server machine passes the incoming packets to the server stub.
5. The server stub unpacks the parameters from the message. Unpacking the parameters is called unmarshalling.
6. Finally, the server stub calls the server procedure. The reply traces the same steps in the reverse direction.

RPC是一个request-response协议。一个RPC是由client初始化，它发送一个请求消息给已知的远端服务器，让其执行一个具体的程序，使用提供的参数。远端服务器发送一个响应到客户端，并且应用继续处理它自己的进程。当服务器正在处理调用时，客户端是阻塞的（它一直等到服务器完成进程再开始执行），除非客户端发送一个匿名的请求给服务器，诸如XMLHttpRequest。在不同的实现中有许多微妙的变化，导致各种不同（不兼容）的RPC协议。
一个种农药的区别在RPC以及本地调用之间的是：远程调用可能会失败因为不可预知的网络问题。同样，调用者通常必须处理此类失败，尽管他们并不知道是否远程调用已经执行完了。幂等的过程是容易处理的，但是足够的困难仍然存在：编写远程调用过程时，经常被限制于在精心编写低层次的子系统。

调用过程

• 1.客户端调用客户端存根。这个调用是一个本地过程调用，将参数以正常的方式写入到栈中。
• 2.客户端存根打包参数到一个消息中并且让系统调用发送消息。打包参数被称之为集结。
• 3.客户端的本地操作系统发送这个消息从客户端机器到服务端机器。
• 4.远程主机的操作系统传递到达的包到server stub(服务器存根)
• 5.server stub服务器存根解包，并从消息中获取参数。解包参数的过程又被称之为unmarshalling
• 6.最终，server stub调用服务器程序。回复过程与此相反，但主要步骤相同。