Remote Procedure Calls Concepts

RPC RMI SOAP

• RPC (Remote Procedure Call) was introduced to improve communication between distributed processes, making it more uniform, reusable, and user-friendly.
• Early RPC implementations, such as Sun's RPC, were used with the Network File System (NFS).
• RPC technologies allow users to call functions on different physical machines as if they were local processes.

Introduction

• When distributed systems use processes to pass messages, these messages frequently contain only basic data.
• Interpretation of the message must be agreed upon between the sender and receiver.
• This makes it difficult to reuse components and promote interoperability between systems.

Review: Sockets

• A socket is an API (Application Programming Interface) that functions as a "door" between application processes and end-transport protocols.
• This permits application processes to communicate across a network.
• Applications using sockets utilize a protocol controlled by the operating system (OS), abstracting communication details from application developers.

Review: Sockets and Ports

• Sockets are assigned a free port by the operating system.
• Each process on a network can be identified through its listening port number.
• TCP and UDP ports function independently.
• Default server-side application ports are standardized (e.g., HTTP: 80, HTTPS: 443, SMTP: 25).

RPC Procedure

• RPC is a request-response protocol.
• A client initiates a procedure by sending a request message to a remote server.
• The request message includes parameters for the function to be executed.
• The remote server executes the specified function and sends a response message back to the client.
• The client waits for the response (synchronous request) until the reply is received.

RPC Events

• The client contacts the client stub. Parameters are usually pushed onto the stack.
• The client stub marshals the parameters into a message.
• The client's operating system sends the message to the server.
• The server's operating system sends the message to the server stub.
• The server stub unpacks (unmarshalls) the parameters.
• The server stub calls the remote procedure.
• The reply uses the same methodology in reverse.

Compilation

• The compilation process involves generating server and client stubs, using the interface specification.
• Common headers and the RPC library are also involved.

RPC Stub

• The stub acts as a gateway for distributed system objects.
• All outgoing requests to server-side objects are routed through the stub.
• Includes network logic to ensure reliable communication between the client and server.
• Responsible for initiating communication, handling marshalling/unmarshalling of messages, and informing the server about the procedure call.

Marshalling

• Marshalling transforms an object's in-memory representation into a format suitable for storage or transmission.
• Parameters in an RPC call are marshaled before being sent to the remote procedure.
• Marshalling is used in .NET and Mozilla Application Framework.

Marshalling Example

• Transitioning between user and kernel space often involves system calls.
• Minimizing system calls by maintaining a buffer in user and kernel space enhances performance.
• Commands waiting for execution are marshaled into the user space buffer.
• When the buffer nears emptiness, a system call is performed to transfer commands to kernel space.

Marshalling VS Serialisation

• In Python, marshalling and serialization are often treated as identical.
• In Java, marshalling involves recording codebases, differing from simple serialization.

RPC Failures

• Distributed systems, like RPC components, are susceptible to various failure types.
• Determining the root cause of an RPC failure can be complex.
• RPC failures might stem from lost replies, server crashes before starting, unreachable servers, or client crashes before response.

RPC Success Modes

• Exactly Once: Guarantees the procedure executes only once, no more or less. Complex implementation.
• At Most Once: Permits the procedure to be executed at most once. Not suitable for all situations.
• At Least Once: The operation executes the procedure repeatedly until an acknowledgment is received, overcoming potential lost acknowledgments.

XML-RPC

• XML-RPC utilizes XML to encode calls and as its transport mechanism.
• Multiple parameters and a single result can be sent through its calls.
• Parameters frequently use various data types, potentially more complex ones like array of integers.

JSON-RPC

• JSON-RPC is similar to XML-RPC, but uses JSON instead of XML.
• It supports notifications, which are calls to the server not requiring a response, and allows multiple calls with potentially out-of-order responses.

XML vs JSON

• XML is more verbose and difficult to parse than JSON.
• JSON represents data as a map, rather than a tree.
• XML-RPC has faced criticism due to its verbosity in recent years.

Communication Over the Years

• Presents a summary of various communication protocols and their respective timeframes

What do developers want?

• Developers prefer more application development and less communication overhead.

Simple Open Access Protocol (SOAP)

• SOAP was initially an Object Access Protocol in 1998.
• Designers focused on lightweight XML messaging compatible with various operating systems and platforms.
• It also promoted access to remote objects through non-HTTP traffic.

SOAP Characteristics

• Extensible: Extends the existing protocol with features like security and addressing.
• Neutral: Adaptable to various transport protocols (e.g., HTTP, SMTP, TCP, UDP, JMS).
• Independent: Uses any programming model.

SOAP Processing Model

• Describes the various steps in SOAP processing, including the use of HTTP, TCP, MSMQ and SMTP.

SOAP Extensibility Model

• Designed to allow extensions to its framework for altered processing models, with the inclusion of reliability, security, routing, and the like.
• Extensions require a URI, node-specific information, and implementation details, including handling communication failure and information transfer.

SOAP Envelope Example

• Provides code example showing different SOAP header elements.

SOAP Advantages and Disadvantages

• SOAP's advantage is its protocol neutrality, making it flexible to be used with various methods.
• However, reliance on HTTP and its use of verbose XML make it slow.

Modern SOAP Usage

• Though other protocols like REST are now widely used, SOAP is still useful where ACID transactions and robust security are needed, such as within WS-Security, WS-Transactions, and WS-Coordination.

Remote Method Invocation (RMI)

• RPC does not directly provide abstraction of object transfer, as RMI does.
• Java's RMI enables direct transfer of serialized classes, with support for garbage collection.
• Implementing the Remote or UnicastRemote interface is needed to convert objects into remote objects.

Java Remote Objects

• Remote objects are equivalent to local objects in Java.
• References identify objects; applications don't possess remote object references.
• A proxy object, called a stub, is representative locally.
• The stub handles marshalling messages and delivers them to the remote object.

Remote Method Invocation

• Describes process with diagrams (Client and Server, relationships between Object A, Object R, stub, proxies and RMI registry).

RMI Passing By Reference

• Values in Java are passed by reference. This is a challenge for RMI, where objects must be passed and transferred efficiently.
  • Java uses serialization to transfer objects across the network in an appropriate format.

Java Serialisation

• The process of converting an object into a byte stream, including class, member variable, and variable values.
• This enables the recreation of the target object after transfer across a network (e.g., to another computer).

Java Serialisation Example

• Provides code examples for serialisation with data structures.

Java RMI Remote Interface Example

• Demonstrates the structure of a Java RMI remote interface (defines methods for remote calls).

Java RMI Server Example

• A code example showcasing a Java RMI server implementation.

Java RMI Client Example

• A code example showcasing a Java RMI client implementation.

Apache Thrift

• Created by Facebook, now an Apache project, with a simple interface definition language (IDL).
• Offers optimized serialization.
• Supports various languages with code generators for glue code.
• Soft versioning allows for interface/data type evolution.

Apache Thrift

• This section shows diagrams of Thrift client-server interaction, highlighting protocol layers and code generation process.

gRPC

• Developed by Google in 2015, using Protocol Buffers for interface description language.
• Protocol buffers are known for their streamlined, efficient, and automated serialization mechanisms.
• Smaller structure than XML.

Google Protocol Buffers

• Shows examples of Google Protocol Buffer structure (message definitions for types like Person and PhoneNumber).

Google Protocol Buffers

• Provides examples of code usage for data serialization.

gRPC

• Describes diagrams showing gRPC client-server request/response flow via protocol buffers, including different client types (Ruby, Android-Java).