The Lifecycle of an HTTP Network Request
Tags: network, http, programming, beginners
Published: 2023-09-22 15:48:18

Before diving into each step, let's set the scene. Imagine you're in your web browser, and you click on a link. What's happening behind the scenes to make that webpage appear? It's a lengthy process involving multiple layers of the Internet infrastructure and various software components. The sequence of events is intricate but extremely vital for the world of web communications. Now, let's walk through the life cycle of an HTTP/HTTPS request and the role a Go application plays in this.
### Step 1: Client Prepares HTTP/HTTPS Request
![Client Prepares HTTP/HTTPS (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/ck4i7z3k8ohn00qsk328.png)
The very beginning of the journey is when the client, usually a web browser, prepares an HTTP or HTTPS request. The client formulates this request based on what resource it needs to fetch, such as an HTML page, an image, or some other file. The request contains essential information such as the URL, request method like GET or POST, and any additional headers. For example, headers may include user-agent details and accepted content types. Sometimes the request also contains a body, which is especially common in POST requests where additional data is sent to the server. When you type a URL into your browser and hit Enter, or when an application makes an API call, this is the process getting kickstarted. If the request is HTTPS, additional steps to secure the request are also initialized here, but we'll get into that in the next step. It's crucial to ensure the correctness of the request, as malformed requests are generally rejected by servers. Once the request is prepared, the client sends it out to find its way through the network labyrinth to the destined server. Essentially, this step initiates the client-server communication that forms the basis of the web.
### Step 2: SSL/TLS Handshake (if HTTPS) (Request)
![SSL/TLS Handshake (if HTTPS) (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/kosnulpf9tfzm3okoq58.png)
When a secure connection is needed, an SSL/TLS handshake kicks off. This process exchanges cryptographic parameters between the client and the server to establish a secure session. Initially, the client sends a "ClientHello" message to the server, specifying the encryption algorithms it supports, a randomly generated value, and other settings. The server replies with a "ServerHello," which confirms the settings both will use. Following this, server and client certificates are exchanged, and the encrypted session is established. Both parties have now ensured that they're talking to the right machine on the other end. In essence, this step locks down the communication channel, ensuring that the data exchanged is only accessible to the client and the server. This process is fundamental for the secure transmission of sensitive information like passwords, credit card numbers, and personal details.
### Step 3: Local DNS Resolver (Request)
![Local DNS Resolver (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/vye7mdtrvcpqlo3xp6hg.png)
The client's local DNS resolver plays a crucial role in converting domain names into IP addresses. Your computer maintains a local cache of domain-IP mappings it has recently encountered. If the domain you're requesting isn't in the cache, your query has to go further. The local DNS resolver is essentially a client-side library that manages DNS query resolution. It understands how to forward a DNS query to get the required information if it doesn't have it locally. This step is foundational to how the Internet works because IP addresses are the actual addresses used for routing on the Internet. DNS makes it human-readable and manageable. For programmers, this is abstracted away, but understanding it can help troubleshoot network issues effectively.
### Step 4: Recursive DNS Servers (Request)
![Recursive DNS Servers (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/itzhoz40nfkxucefyge5.png)
If the local DNS doesn't have the IP address you're looking for, the query is sent to a series of DNS servers. These are usually managed by your ISP or another third-party provider. The recursive DNS servers have their own caches, and if they can't satisfy the request, they'll talk to root DNS servers, followed by TLD (Top-Level Domain) servers and finally the authoritative DNS servers for the domain. This is a hierarchical structure and often takes several steps. Each query might also involve multiple DNS servers before finally getting the correct IP address. Once the IP address is found, it's returned to the client and stored in the local cache for future use. Recursive DNS servers are vital cogs in the DNS architecture, connecting local queries to the global DNS infrastructure.
### Step 5: Internet Service Provider (ISP) (Request)
![Internet Service Provider (ISP) (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/7xg5bxyngjjr7ayafgq3.png)
Your Internet Service Provider, or ISP, acts like the highway system for your data. After your computer knows the IP address it needs to reach, the ISP takes on the task of routing your request across the vast network of the internet. It does this through a series of interconnected high-speed data links and routing stations. The ISP has complex routing tables and algorithms to figure out the most efficient path for your request. At this stage, the ISP's primary role is to get your request to the destination IP address as fast and reliably as possible. If there are any issues or bottlenecks in the network, the ISP will dynamically change the route. It's a crucial link in the chain, making sure your data reaches its destination and gets back to you.
### Step 6: Firewalls (Request)
![Firewalls (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/ldzy1xzybfkph23ya3g1.png)
Before reaching the target server, the request has to pass through various firewalls. These are security systems designed to monitor and control incoming and outgoing network traffic. Firewalls act based on predetermined security rules and can be both hardware or software-based. The primary purpose is to establish a barrier between your secure internal network and untrusted external networks such as the Internet. Firewalls evaluate the header information in each data packet, like source IP, destination IP, and port numbers, to decide whether the packet is safe or potentially harmful. If the packet fails any of the checks, it's blocked or flagged for further inspection. Ensuring security is of paramount importance today, given the range of potential cyber-attacks and vulnerabilities.
### Step 7: Router & Network Nodes (Request)
![Router & Network Nodes (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/e5rhsliphoyeirk92xvj.png)
Once past the ISP and firewalls, the request enters the domain of routers and network nodes. Your request might pass through dozens of these on its journey to the target server. Each router it hits looks at the destination IP address and forwards the request along the most efficient route. These routes can be dynamically altered based on network congestion, failure, or other criteria. Network nodes act like the intersections in a road network, directing traffic to ensure it gets to its destination. Each of these nodes is also equipped with its own local routing table to direct packets. So, in essence, routers and network nodes serve as the internet's traffic cops, making sure data flows smoothly across the network.
### Step 8: Cache Servers (Request)
![Cache Servers	(Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/uc6xjp08xa7jre3mztlf.png)
Cache servers come into play in some network architectures, particularly those designed to improve performance and reduce latency. The primary role of a cache server is to store copies of responses from the web server. When a client requests a resource that has been previously requested, the cache server can serve that resource directly instead of fetching it again from the web server. This helps to reduce the workload on the web server and speeds up the response time for the client. Think of a cache server as a short-term memory for the network. It retains recently or frequently accessed data to quickly fulfill subsequent requests for the same data. This stage is especially useful for static resources like images or CSS files that don't change often. However, it's crucial to manage cache correctly to avoid serving outdated or incorrect data.
### Step 9: Server-side Load Balancer (Request) 
![Server-side Load Balancer	 (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/xwpfxhq1yeu33184eyd8.png)
When the request finally reaches the server's local network, it often encounters a server-side load balancer. This device or software is responsible for distributing incoming network traffic across multiple servers. The goal is to ensure that no single server is overwhelmed with too much work. Load balancers use various algorithms to determine how to distribute the load. These might include round-robin, least connections, or even custom rules. Load balancers also perform health checks on servers and will stop directing traffic to a server if it's down or not responding. This is a crucial component for high availability and reliability in any network that experiences variable traffic loads. By efficiently distributing the incoming requests, the load balancer ensures that all server resources are utilized optimally.
### Step 10: Operating System Network Stack (Request)
![Operating System Network Stack (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/acjb03oqjv2tq23v4w65.png)
Once the load balancer decides which server should handle the request, the data packets are forwarded to the operating system's network stack on that server. This is a set of software components that essentially act as the gateway between the hardware and the application layers on the server. The network stack is responsible for the end-to-end communication between the client and server. It takes care of tasks like packet segmentation, reordering, and error checking. Once the packets are received and validated, the network stack forwards them to the appropriate application for further processing. The stack works closely with the kernel, which is the core part of the operating system, to perform these tasks efficiently.
### Step 11: Go Application (Request)
![Go Application (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/c9hwd5i9va94196z9qcu.png)
At this stage, your Go application is finally in the spotlight. The operating system’s network stack forwards the incoming request to the application. The Go app starts by parsing the HTTP headers, method, and any data sent by the client. This could be form data, JSON payloads, or even files. Next, it often uses middleware for tasks like logging, request validation, or authentication. The core business logic of the app then takes over. For example, it may consult a configuration file, perform some calculations, or initiate a database query. Your Go code does the heavy lifting here, executing the logic that makes your application unique. The purpose of this stage is to process the incoming request and prepare an appropriate HTTP response. 
Ah, here comes the part where your Go application shines. Once the request makes it through the OS stack, it's handed off to the Go application. Your Go app parses the request headers, the body, and any parameters. Then, based on the defined routes and logic, the Go app knows what function to execute. It might be fetching data, performing some computations, or generating an HTML page—whatever it's designed to do.
### Step 12: Database/File System (Request)
![Database/File System (Request)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/y5ozvm4czackmsv03gza.png)
Many web applications interact with some form of storage, be it a database, a file system, or a cloud-based storage service. At this point in the process, your Go app may need to query a SQL database to fetch user data, read a configuration file from the file system, or access images stored in an Amazon S3 bucket. The app does this to gather all the necessary data it needs to compile a complete and accurate response to the client's request. This is often where bottlenecks occur, especially if database queries are not optimized. Thus, this is a critical stage for the performance and reliability of your application.
### Step 13: Go Application (Response)
![Go Application (Response)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/akugbazg5gv5t6yk2js7.png)
Once your Go application has all the data and has executed the necessary business logic, it's time to construct an HTTP response. The app will build an HTTP header, set cookies if needed, and populate the response body, often using templates for HTML content. Your Go code will define what the client receives, whether that's JSON data, HTML, or a plain text message. The response is then passed back to the operating system’s network stack, which will handle the task of sending it back through the network to the client. This is the final stage of your application's involvement in this journey; from here, the focus shifts back to networking components.
### Step 14: Server-side Load Balancer (Response)
![Server-side Load Balancer (Response)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/ao43dmy1dpimmtqcbfkn.png)
As the HTTP response is ready to be sent back, it first goes through the server-side load balancer once again. Just like with incoming requests, the load balancer examines the outgoing responses. This often involves routing the response back through the same path it came in to maintain session consistency, a concept known as 'session persistence.' The load balancer's role is just as crucial for outgoing data as it is for incoming data. It ensures that all responses are directed correctly and optimizes the network resources in the process.
### Step 15: Cache Servers (Response)
![Cache Servers (Response)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/dv2vbzvf6kutmi0mqng8.png)
Now that the response is on its way back, it hits the cache servers again. If the data in the response is cacheable, these servers store a copy. This is highly useful for subsequent requests for the same resource, saving the need to go through the entire process. Caching strategies can be complex, involving expiration times and validation mechanisms to make sure the cache doesn't serve stale or incorrect data. Businesses optimize these settings to balance between performance and data accuracy. If done right, cache servers can significantly speed up data delivery and reduce load on the application servers.
### Step 16: Router & Network Nodes (Response)
![Router & Network Nodes (Response)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/rmci82mir21kf514e153.png)
The response continues its journey, hopping between network nodes and routers. Each hop is a physical or virtual device that routes the packets to the next point in the path. This is like the reverse of what happened when the request was coming in, but it's just as crucial to get it right. Routing algorithms ensure the most efficient path, and routers look at the destination IP to decide where to send the packets next. Failures or slowdowns can happen, but redundancy and routing algorithms are in place to minimize impact.
### Step 17: Firewalls (Response)
![Firewalls (Response)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/co8i0oy63adllr0tm66b.png)
Before the response reaches the client, it has to pass through various firewalls again. Just like on the way in, these firewalls scan the packets for malicious content or signs of an attack. This is a two-way street; security is as important for outgoing data as it is for incoming data. Firewalls will check against predefined rules and security policies. If everything looks good, the packets are allowed to proceed. Otherwise, they might be dropped or flagged for further inspection, depending on the firewall's settings.
### Step 18: Internet Service Provider (ISP) (Response)
![Internet Service Provider (ISP) (Response)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/5m08imzaon8fjqkx3rrf.png)
We're getting close! The Internet Service Provider takes the data packets of the HTTP response and routes them back through its network to the client's local network. The ISP has a big role here, not just as a facilitator but also as a potential bottleneck. Network congestion, maintenance, or routing issues at the ISP level can impact the speed at which the response is delivered. This is the last network-level hurdle the response has to clear before reaching the client.
### Step 19: Client Receives HTTP/HTTPS Response
![Client Receives HTTP/HTTPS (Response)](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/3v562v9fs0s3huabz8j8.png)
Finally, the client's web browser gets the HTTP or HTTPS response. The browser starts by reading the HTTP headers to understand the type of content and any other settings or cookies. Then, it renders the HTML, CSS, and runs JavaScript to display the web page or handle the received data. For HTTPS, a final decryption step is performed to convert the data back into a readable format. At this point, the whole journey concludes. The user can now see the web page, download a file, or interact with the received data in any other way the application intended.