Comm Protocols: Server-Client Dynamics
Tags: communicationprotocols, restapi, http, beginers
Published: 2024-01-24 05:35:46

There are several types of communication between a client and a server in the context of web development. The choice of communication type depends on the requirements of the application. Here are some common types:

## HTTP (Hypertext Transfer Protocol):

HTTP is the foundation of data communication on the World Wide Web. It is the protocol used for transferring hypertext (text with links, images, etc.) over the internet. HTTP operates in a request-response model, where the client sends a request to the server, and the server responds with the requested data


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/7zpt0vym0iibisb9dcg5.png)

The key components in the HTTP (Hypertext Transfer Protocol) include:

### Uniform Resource Identifiers (URIs):

 URIs, such as URLs (Uniform Resource Locators), are used to identify and locate resources on the web. They specify the protocol, domain, and path to the resource.

### Request Methods (Verbs):

HTTP defines various request methods or verbs that indicate the desired action to be performed on a resource. Common methods include:

**GET:** Retrieve data from the server.

**POST:** Submit data to be processed by a specified resource.

**PUT:** Update a resource or create a new one if it doesn't exist.

**DELETE:** Request the removal of a resource.

**HEAD:** Retrieve only the headers of a response, without the actual data.

### Headers:

Both requests and responses in HTTP include headers. Headers provide additional information about the message, such as content type, content length, caching directives, and more.

### Status Codes:

HTTP responses include status codes that indicate the outcome of the request. Common status codes include:

2xx (Successful): The request was successful.

3xx (Redirection): Further action is needed to complete the request.

4xx (Client Error): The client made an error in the request.

5xx (Server Error): The server encountered an error while processing the request.

### Message Body:

For some HTTP methods (e.g., POST or PUT), a message body can be included in the request or response. The body contains the data being sent or received.

### Statelessness:

HTTP is a stateless protocol, meaning each request from a client to a server is independent. The server doesn't retain information about previous requests, but stateful interactions can be implemented using mechanisms like cookies.

### Version Number:

HTTP messages include a version number to indicate the version of the protocol being used (e.g., HTTP/1.1).

### Connection Handling:

The "Connection" header and related mechanisms are used to control the persistence and handling of connections between the client and server.

### Security (Optional):

While HTTP is not inherently secure, HTTPS (HTTP Secure) is a secure version that uses encryption (TLS/SSL) to protect data transmitted between the client and server.

Understanding these components is essential for developers working with HTTP to build and maintain web applications. Each component plays a crucial role in facilitating communication and data exchange between clients and servers on the web.



## REST (Representational State Transfer):

REST is an architectural style that uses standard HTTP methods (GET, POST, PUT, DELETE, etc.) for communication between clients and servers. RESTful APIs (Application Programming Interfaces) are designed to be stateless, scalable, and straightforward. They are commonly used for web services and mobile app backends.



![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/8a0go1v22qq6vsqudxzh.png)

_Key components in REST_

#### Resources

- Resources are the key abstractions in REST. They represent entities or services, such as data objects, services, or concepts.

#### Identification

- Each resource is uniquely identified by a URI (Uniform Resource Identifier), typically taking the form of a URL (Uniform Resource Locator).

#### HTTP Methods (Verbs)

- CRUD Operations: RESTful services use standard HTTP methods (verbs) to perform operations on resources:

	GET: Retrieve a resource.
	POST: Create a new resource.
	PUT or PATCH: Update a resource.
	DELETE: Remove a resource.

#### Representations

 - Data Format: Resources can have multiple representations, such as JSON or XML, used to represent the state of the resource.

#### Content Negotiation

- Clients and servers negotiate the format of data during communication using the "Accept" and "Content-Type" headers.
	

#### Stateless Communication

- Similar to HTTP, REST is stateless. Each request from a client to a server contains all the information needed to understand and fulfill the request. The server does not store information about the client's state.
	

#### Uniform Interface

- REST follows a uniform interface that includes several constraints, such as:

**Identification of Resources :** Resources are identified through URIs.

**Manipulation through Representations:** Resources are manipulated through representations.

**Self-Descriptive Messages:** Each message from the server to the client contains enough information for the client to understand and process it.

**Hypermedia as the Engine of Application State (HATEOAS):** Clients interact with the application entirely through hypermedia provided dynamically by application servers.
	

#### Stateless Server

- No Client State on the Server: Each request from a client must contain all the information necessary to understand and process the request. The server does not store any client state.

#### Hypermedia (HATEOAS)

- Dynamic Navigation: HATEOAS allows the server to provide hypermedia links within the response, enabling clients to navigate the application dynamically.

#### Cacheability

- Caching Control: Responses from the server can include information about whether the response can be cached and, if so, for how long.

#### Layered System

- Hierarchical Layers: A RESTful system can be organiSed into hierarchical layers, with each layer having a specific role and responsibility.

#### Security

- Authentication and AuthoriSation: RESTful services often implement secure access through mechanisms like API keys, OAuth, or other authentication and authorisation methods.

Understanding and applying these key components is fundamental to designing RESTful services that follow the principles of the REST architectural style. This adherence contributes to scalable, interoperable, and maintainable web services.



## WebSocket:

WebSocket is a communication protocol that provides full-duplex communication channels over a single, long-lived connection. It is commonly used for real-time applications where low-latency bidirectional communication is crucial, such as chat applications, live updates, and online gaming.


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/vrugceie79kws42400ld.png)

_Here are the key components in WebSocket communication:_

#### Handshake:

**Upgrade Request:** The WebSocket connection begins with an HTTP handshake. The client initiates the handshake by sending an HTTP request that includes the "Upgrade" header, indicating a desire to upgrade the connection to a WebSocket.

**Upgrade Response:** If the server supports WebSocket, it responds with a confirmation containing the "101 Switching Protocols" status code and completes the handshake.

#### Persistent Connection:

Unlike traditional HTTP connections, WebSockets maintain a persistent connection, allowing bidirectional communication between the client and server.
	

#### WebSocket URI:

WebSocket URIs typically start with "ws://" (for unencrypted connections) or "wss://" (for encrypted connections), followed by the server's domain and port.
	

#### Frame-based Communication:
	
WebSocket communication is frame-based. Data is sent and received in frames, which can be text or binary. Each frame can be sent independently, allowing for efficient communication.

#### WebSocket Protocol:

WebSocket has its own protocol that defines the format of frames, how connections are established, and various control frames for managing the connection.

#### Full-duplex Communication:

WebSockets support full-duplex communication, meaning both the client and server can send messages independently without waiting for a response.

#### Low Latency:
	
WebSockets are designed for low-latency communication, making them suitable for real-time applications, such as chat applications, online gaming, and financial platforms.
	
#### Bi-directional Communication:

Clients and servers can send messages to each other at any time, allowing for bidirectional communication.

#### Subprotocols:

WebSocket supports subprotocols, allowing clients and servers to agree on a specific protocol for communication within the WebSocket connection. Common subprotocols include "chat" or "json."

#### Security:

WebSocket connections can be secured using the "wss://" protocol, which encrypts the communication using TLS/SSL, providing a secure channel.
	

#### Event-driven Model:

WebSockets follow an event-driven model, where the client and server can define event handlers to respond to various events, such as opening, closing, or receiving messages.

#### Close Handshake:

The WebSocket connection can be gracefully closed using a close handshake. This involves sending a special frame to signal the intention to close, followed by a response from the other party.

WebSockets offer an efficient and real-time communication solution, making them suitable for applications that require low-latency updates and bidirectional communication between clients and servers.


## AJAX (Asynchronous JavaScript and XML):

AJAX is a technique used in web development to send and receive data from a server asynchronously without reloading the entire web page. It typically involves using JavaScript to make asynchronous requests to the server, enabling dynamic updates to the user interface.

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/kdwdhkoc3i14q2pguobv.png)

Here's a breakdown of the key components and concepts associated with Ajax:

**Asynchronous:** Ajax allows web applications to make asynchronous requests to a web server. Asynchronous means that the browser can send a request to the server and continue to perform other tasks without waiting for the response. This is in contrast to synchronous requests, where the browser would have to wait for the server's response before proceeding.

**JavaScript:** Ajax relies heavily on JavaScript to handle asynchronous communication with the server. JavaScript is used to create dynamic content on the client side and manipulate the Document Object Model (DOM) to update the webpage without requiring a full page reload.

**XMLHttpRequest (XHR) Object:** XMLHttpRequest is a JavaScript object that provides the ability to make HTTP requests from the browser. It is the core technology behind Ajax. The XHR object can be used to send requests to the server, receive responses, and update the webpage dynamically.

**Data Format (XML or JSON):** While the "X" in Ajax originally stood for XML, modern applications often use JSON (JavaScript Object Notation) as the preferred data format for communication between the server and the client. JSON is more lightweight and easier to work with in JavaScript.

**Server-Side Technologies:** On the server side, technologies like PHP, Python, Ruby, or any other server-side scripting language can handle Ajax requests. These server-side scripts process the requests, perform necessary operations, and send back the response in the chosen format (XML or JSON).

**DOM Manipulation:** Once the server responds with data, JavaScript is used to manipulate the DOM, updating specific elements on the webpage without requiring a full reload. This allows for a more dynamic and responsive user interface.


Ajax is widely used in modern web development to create interactive and user-friendly web applications. It plays a crucial role in enhancing the user experience by enabling real-time updates and reducing the need for full-page reloads, ultimately making web applications feel more fluid and responsive.

## GraphQL:

GraphQL is a query language and runtime for APIs that allows clients to request only the data they need. It provides a more flexible and efficient alternative to traditional REST APIs by enabling clients to specify the structure of the response they require

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/a1oodzhzcmmdee9m1ffx.png)

Here are the key components of GraphQL:

#### Schema:

The schema is a central component of GraphQL and defines the types of data that can be queried and the relationships between them. It serves as a contract between the client and the server.

Types - GraphQL schemas consist of types such as objects, interfaces, enums, and scalars. Types define the shape of the data that can be queried.

#### Query Language:

GraphQL has its own query language that clients use to request specific data. Queries are structured to request only the data needed, eliminating over-fetching or under-fetching issues.

#### Operations:

Query - A query is a read-only operation used to request data from the server.

Mutation -  A mutation is used to modify or create data on the server.

Subscription -  A subscription allows clients to receive real-time updates when specific events occur on the server.
Resolver Functions:

Execution - Resolvers are functions that determine how data is fetched or mutated. They are responsible for resolving the fields in a query and returning the requested data.

#### Introspection:

Meta-Query -  GraphQL provides a special query called introspection, allowing clients to query the schema itself. This enables dynamic exploration and documentation of the available types and their relationships.
Scalar Types:

Built-in Scalars - GraphQL has a set of built-in scalar types, such as Int, Float, String, Boolean, and ID. These represent basic data types like numbers, strings, and booleans.
Directives:

Control Execution -  Directives provide a way to control the execution of a query or mutation. They can be used to conditionally include or skip fields, apply transformations, or specify additional behaviour.

#### Variables:

Parameterisation - GraphQL supports variables, allowing clients to parameterise queries. This promotes reusability and avoids code duplication by allowing dynamic values to be injected into queries.

#### Aliases:

Field Renaming -  Aliases allow clients to request the same field with different names in a single query. This is useful when resolving conflicts or avoiding name collisions.
Fragments 

Reusable Units - Fragments are named sets of fields that can be reused in multiple queries. They help organise and maintain query structures, promoting modular and maintainable code.
Union and Interface Types:

Polymorphism - GraphQL supports union and interface types, allowing for polymorphic queries where a field can return multiple types based on certain conditions.

#### Batching and Caching:

Efficiency - GraphQL allows clients to request multiple resources in a single query, reducing the number of network requests. Additionally, clients can implement caching strategies based on their requirements.

#### Subscriptions:

Real-time Updates - Subscriptions enable real-time communication between clients and servers. Clients can subscribe to specific events, and the server pushes updates to subscribed clients when these events occur.

Understanding these key components helps developers build efficient, dynamic, and well-organised APIs using GraphQL. The flexibility and expressive power of GraphQL make it suitable for a wide range of applications and use cases.



## Server-Sent Events (SSE):

SSE is a simple and efficient mechanism for sending updates from the server to the client over a single HTTP connection. It is based on the EventSource API and is particularly useful for scenarios where the server needs to push real-time updates to the client.

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/bmua2phc2t7y2ua2ipgt.png)

_Key components_

SSE uses a simple and text-based format for communication. The server sends a stream of text-based events to the client over a single HTTP connection.

#### Event Stream MIME Type:

The MIME type for SSE is text/event-stream. This content type signals to the browser that the server will be sending a stream of events.

#### Event Format:

Each event within the stream is formatted as a series of lines. These lines include fields such as event, data, id, and retry, providing information about the event.

#### Event Types:

Events in SSE can have different types, specified by the "event" field. This allows clients to handle different types of events differently.

#### Data Field:

The "data" field contains the actual payload of the event. It can be a string of text or JSON data.

#### Event ID (Optional):

The "id" field is optional but can be used to assign an identifier to each event. This allows the client to keep track of the last event received, enabling reconnection without missing updates.

#### Retry Field (Optional):

The "retry" field is optional and indicates the reconnection time in milliseconds. If the connection is lost, the client can attempt to reconnect after the specified time.

#### Connection Establishment:

SSE is initiated by the client making a standard HTTP request to the server, but with the "text/event-stream" content type. The server responds with an HTTP stream that remains open for the duration of the connection.

#### Single HTTP Connection:

Unlike WebSockets, SSE uses a single, long-lived HTTP connection for communication. This connection remains open, allowing the server to push events to the client as they occur.

#### Server Push:

SSE enables the server to push updates to the client without the need for the client to request them actively. This is especially useful for real-time applications where immediate updates are crucial

#### Browser Compatibility:

SSE is supported by most modern browsers. It uses the native EventSource API in JavaScript to handle the incoming stream of events.

#### Graceful Reconnection:

SSE supports graceful reconnection in case the connection is lost. The client can attempt to reconnect, and the server can continue sending updates from the last known event.

Server-Sent Events provide a lightweight and straightforward way to achieve real-time communication between servers and clients over HTTP. It is particularly well-suited for scenarios where a continuous stream of updates is needed, such as live feeds or real-time notifications.




## Message Queues:

Message queues are used for asynchronous communication between distributed components of an application. Systems like RabbitMQ or Apache Kafka facilitate communication by allowing components to send and receive messages, enabling decoupled and scalable architectures.


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/5dntdvkngn1o7efr29jn.png)

Here are the key components of message queues:

#### Message:

Payload: The actual data being sent between components. It can be in various formats such as JSON, XML, or plain text.
Metadata: Additional information associated with the message, including headers, priority, timestamp, and routing details.

#### Producer:

Message Sender: The component responsible for creating and sending messages to the message queue. Producers generate messages and enqueue them for further processing.

#### Consumer:

Message Receiver: The component that subscribes to a specific message queue and processes messages. Consumers dequeue messages from the queue and perform the necessary actions.

#### Queue:

Message Buffer: The storage mechanism that holds the messages until they are consumed by a recipient. Queues provide a temporary storage solution, decoupling producers and consumers.

#### Exchange (Optional):

Routing Mechanism: In some message queue systems, an exchange is used to route messages to one or more queues. Producers send messages to an exchange, and the exchange determines how to route them to the appropriate queues based on routing rules.

#### Routing Key (Optional):

Message Classification: In systems with exchanges, a routing key is often used to categorize messages. The exchange uses this key to determine which queues should receive the message.

#### Broker:

Middleware: The message broker is the middleware that manages the message queue. It is responsible for receiving messages from producers, storing them in the queue, and delivering them to consumers.

#### Acknowledgment:

Delivery Confirmation: Message queues often support acknowledgment mechanisms. After a consumer successfully processes a message, it sends an acknowledgment to the message broker, indicating that the message was received and processed.

#### Dead Letter Queue (DLQ) (Optional):

Error Handling: Some message queues have a Dead Letter Queue, which is a special queue where messages are sent if they cannot be processed successfully after a certain number of attempts. This allows for error handling and analysis.

#### Durability:

Persistence: Many message queues provide durability by persistently storing messages on disk. This ensures that messages are not lost even if the message broker or consumer restarts.

#### Priority Queue (Optional):

Message Priority: Some message queues support the concept of message priorities, allowing certain messages to be processed before others.

#### Message Filtering (Optional):

Selective Consumption: Advanced message queues may support message filtering, allowing consumers to subscribe to specific types of messages based on criteria such as content or metadata.

Message queues play a crucial role in building scalable, decoupled, and resilient distributed systems. They enable efficient communication between components while providing flexibility, reliability, and fault tolerance.


## Remote Procedure Call (RPC):

RPC is a protocol that allows a program to cause a procedure (subroutine) to execute in another address space, often on another machine. JSON-RPC and gRPC are examples of RPC protocols used for communication between services.



![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/krxlf474npexpup2clrb.png)



_Here are the key components in RPC:_

#### Client:

Initiates the Call: The client is the entity that initiates an RPC by calling a procedure that may be located on a remote server.

#### Server:

Hosts Procedures: The server is the entity that hosts the procedures that can be called remotely. It listens for incoming RPC requests and executes the corresponding procedures.

#### Interface Definition Language (IDL):

Contract Specification: IDL is a language-neutral specification used to define the interface of the remote procedures. It specifies the procedures, their parameters, and return types.

#### Stubs (Client and Server):

Proxy (Client Stub): The client stub is a proxy for the remote procedures on the client side. It marshals parameters, sends the request to the server, and unmarshals the results.

Skeleton (Server Stub): The server stub is responsible for unmarshaling the incoming parameters, invoking the local procedure, and marshaling the results back to the client.

#### Marshalling and Unmarshalling:

Data Serialization: Marshalling is the process of converting parameters and results into a format that can be transmitted over the network. Unmarshalling is the reverse process on the receiving end.

#### Transport Layer:

Communication Channel: The transport layer provides the underlying communication mechanism between the client and server. This could be TCP/IP, HTTP, or another protocol depending on the implementation.

#### Binding:

Connection Establishment: Binding is the process of associating a client with a specific server. It involves specifying the location and communication details of the server.

#### Procedure Call:

Invocation: The client initiates an RPC by invoking a procedure. The client stub packages the parameters, and the server stub unpackages them for the local procedure to execute.

#### Remote Object/Service:

Procedure Execution: The remote object or service is the actual entity that performs the computation or action specified by the remote procedure.

#### Synchronous and Asynchronous RPC:

Synchronous: In synchronous RPC, the client sends a request to the server and waits for a response before continuing execution.

Asynchronous: In asynchronous RPC, the client does not wait for the response. Instead, it continues with its work, and the server responds when ready.

#### Error Handling:

Fault Tolerance: RPC mechanisms include error handling to manage failures or errors that may occur during the remote procedure execution or communication.

#### Security:

Authentication and Authorization: RPC systems often include mechanisms for authentication and authorization to ensure that only authorized clients can invoke specific procedures.

#### Naming Service (Optional):
Location Independence: In distributed systems, a naming service may be used to provide location-independent addressing. Clients can refer to remote procedures by a logical name, and the naming service resolves it to the actual network address.

Understanding these key components is essential for developing and working with RPC systems, which are widely used in distributed computing for building scalable and modular applications.


## FTP (File Transfer Protocol) and SFTP (Secure File Transfer Protocol):

FTP and SFTP are protocols designed for transferring files between a client and a server. While FTP is older and less secure, SFTP adds a layer of encryption for secure file transfers.


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/noet91bshg07q8soayw4.png)

Here are the key components for both FTP and SFTP:

### FTP (File Transfer Protocol):

#### Client:

The client initiates the FTP session and interacts with the server to request file transfers or directory listings.

#### Server:

The server hosts the files and responds to client requests. It manages file storage and permissions.

#### User Authentication:

FTP requires user authentication for access. Authentication can be done with a username and password, but it may also involve anonymous access.

#### Control Connection:

The control connection is established between the client and server to send commands and receive responses. It typically uses TCP on port 21.

#### Data Connection:

FTP uses separate data connections for file transfers. Depending on the mode (active or passive), data connections may be initiated by the server or the client.

#### FTP Commands:

FTP commands are used to perform various operations such as changing directories, listing files, uploading, and downloading files. Common commands include RETR (retrieve), STOR (store), LIST (list), and CWD (change working directory).

#### File Types:

FTP supports different file types, including ASCII and binary. The file type must be specified to ensure proper file transfer.

#### Passive Mode:

Passive mode is used when the client is behind a firewall. In passive mode, the server opens a random port for data transfer, and the client connects to that port.

#### Active Mode:

In active mode, the client opens a random port for data transfer, and the server connects to that port. Active mode may encounter issues with firewalls.

#### Anonymous FTP:

FTP allows anonymous access, where users can log in with the username "anonymous" or "ftp" and provide an email address as the password.

### SFTP (Secure File Transfer Protocol):

#### Client:

Similar to FTP, the client initiates the SFTP session and interacts with the server for secure file transfers.

#### Server:

The server in SFTP is responsible for hosting files securely and responding to client requests.

#### User Authentication:

SFTP, being secure, uses secure authentication methods such as public-key authentication, password-based authentication, or other secure methods.

#### Secure Connection:

SFTP operates over a secure channel, typically using SSH (Secure Shell), ensuring that data transferred between the client and server is encrypted.

#### Single Connection:

SFTP typically uses a single connection for both control and data transfer, simplifying the connection setup compared to traditional FTP.

#### Commands:

SFTP commands are similar to FTP but operate within the secure channel established by SSH. Common commands include GET (retrieve), PUT (upload), LS (list), and CD (change directory).

#### Public-Key Authentication:

SFTP commonly uses public-key authentication for added security. Clients authenticate with a private key, and the server verifies using the corresponding public key.

#### Security Features:

SFTP provides encryption, data integrity, and optional compression for secure and efficient file transfers.

#### Port Number:

SFTP typically operates on port 22 by default, the same port used by SSH. This port can be configured for different use cases.

#### Subsystem of SSH:

SFTP operates as a subsystem of SSH, leveraging the security features provided by the SSH protocol.

Understanding these components is crucial for users and administrators working with FTP and SFTP to facilitate file transfers securely over a network.


## Conclusion

The choice of communication type depends on factors such as the nature of the application, the need for real-time updates, the type of data being transferred, and security considerations. Many modern applications use a combination of these communication types to meet their specific requirements.