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When studying how computer networks work, sooner or later you will encounter the so-called OSI open network model. The OSI model is crucial for understanding network technologies, and it often presents unexpected challenges for beginners. In this article, we’ll go over the basic principles of the OSI model and will try to provide an “OSI model for dummies” kind of guide. The Concept of a Protocol Communication protocols (or simply protocols) are necessary so that participants in information exchange can understand each other. A wide variety of protocols are involved in the operation of computer networks, relating to different network layers. For example, a computer's network controller follows a protocol that describes how to convert digital data into an analog signal transmitted over wires. A browser connects to a website using the TCP transport protocol, and a server and a browser communicate using the HTTP protocol. In other words, a protocol is a set of agreements between software and hardware developers. It describes how programs and devices interact with other programs and devices that support the protocol. OSI OSI stands for Open Systems Interconnection. It does not refer to Open Source; in this context, "open systems" are systems built on open (publicly available) specifications that conform to established standards. You will often come across the term "Open Systems Interconnection (OSI) Reference Model." The reference model outlines the layers a network should have and the functions performed at each layer. The OSI model divides all protocols into the following seven layers: Physical Data Link Network Transport Session Presentation Application The OSI model does not include descriptions of the protocols themselves; these are defined in separate standards.  Today, the OSI model is not much used in practice. In the past, there were literal implementations with exactly seven layers, but over time, they were replaced by the less prescriptive TCP/IP protocol suite, which underpins the modern Internet. Nevertheless, the protocols in use today roughly correspond to the OSI layers, and the model is still used as a common language for describing how networks work. Physical Layer All layers are numbered, starting from the one closest to the data transmission medium. In this case, the first layer of the OSI model is the physical layer. This is where bits of information are converted into signals that are then transmitted through the medium. The physical protocol used depends on how the computer is connected to the network. For example, in a typical local area network (LAN) using twisted-pair cables, the 100BASE-TX specification (IEEE 802.3u standard) is employed. It defines the cables and connectors, wire characteristics, frequencies, voltage, encoding, and much more. Wi-Fi connections are more complex since data is transmitted over shared radio channels. The interaction of Wi-Fi devices is described by the IEEE 802.11 specification, which, like Ethernet, includes parts of both the physical and data link layers. When accessing the Internet via a cellular network, GSM specifications are utilized, which include specialized protocols (such as GPRS) that affect not only the first two layers but also the network layer. There are also relatively simple protocols, such as RS232, which is used when connecting two computers via a null-modem cable through COM ports. Data Link Layer Next is the data link layer of the OSI model. At this layer, entire messages (frames) are transmitted instead of just bits. The data link layer receives a stream of bits from the physical layer, identifies the start and end of the message, and packages the bits into a frame. Error detection and correction also take place here. In multipoint network connections, where multiple computers use the same communication channel, the data link layer additionally provides physical addressing and access control to the shared transmission medium. Some tasks theoretically handled by protocols at this layer are addressed in the Ethernet and Wi-Fi specifications; however, there is more. Network interfaces in multipoint connections recognize each other using special six-byte identifiers—MAC addresses. When configuring a network, network adapters must know which device is responsible for which network address (IP address) to send packets (blocks of data transmitted in a packet-switched mode) to their destinations correctly. The ARP (Address Resolution Protocol) is used to automatically build tables that map IP addresses to MAC addresses. In point-to-point connections, ARP is not needed. However, the PPP (Point-to-Point Protocol) is often used. In addition to frame structure and integrity checks, PPP includes rules for establishing a connection, checking line status, and authenticating participants. Network Layer The next level is the network layer of the OSI model. It is designed to build large, composite networks based on various networking technologies. At this level, differences between different data link layer technologies are reconciled, and global addressing is provided, allowing each computer on the network to be uniquely identified. Routing is also performed here, determining the path for packet forwarding through intermediate nodes. It’s sometimes said that in the Internet, the IP (Internet Protocol) functions as the network layer. This is true in a sense: IP defines the structure of individual packets transmitted through gateways, the system of network addresses, and some other functions. However, several other protocols can also be attributed to the network layer, even though they operate "on top" of the IP protocol. One of the most important of these is the Internet Control Message Protocol (ICMP). It enables communication between network participants regarding various normal and abnormal conditions, including link failures, the absence of a suitable route, and other delivery issues. Sometimes, ICMP messages contain recommendations for using alternative routes. Transport Layer Packets transmitted over a network using network layer protocols are typically limited in size. They may arrive out of order, be lost, or even duplicated. Application programs require a higher level of service that ensures reliable data delivery and ease of use. This is precisely the role of transport layer protocols in the OSI model. They monitor packet delivery by sending and analyzing acknowledgments, numbering packets, and reordering them correctly upon arrival. As mentioned earlier, network layer protocols do not guarantee packet delivery. A sent packet might be lost, duplicated, or arrive out of sequence. The content of such a packet is usually called a datagram. One of the simplest transport protocols is the User Datagram Protocol (UDP). Participants in network communication running on the same computer are identified by integers called port numbers (or simply ports). The UDP protocol requires that the data sent over the network be accompanied by the sender’s and receiver’s port numbers, the length of the datagram, and its checksum. All of this is “wrapped” into a packet according to the IP protocol's conventions. However, the responsibility for acknowledgments, retransmissions, splitting information into smaller pieces, and reassembling it in the correct order falls on the software developer. Therefore, UDP does not protect against packet loss, duplication, or disorder — only the integrity of data within a single datagram is ensured. There is also a second type of transport interaction — stream-based communication. Here, all issues related to packet loss and data reconstruction from fragments are handled by the transport protocol implementation itself, which makes it significantly more complex than datagram-based protocols. The corresponding transport protocol used on the Internet is TCP (Transmission Control Protocol). Unlike UDP, TCP stream communication requires establishing a connection. It guarantees that all bytes written to the stream will be available for reading on the other end and in the correct order. If this guarantee cannot be upheld, the connection will be terminated, and both parties will be informed. The TCP protocol includes a number of sophisticated agreements, but fortunately, all of these are handled by the operating system. The Remaining Layers Identifying which real-world protocols correspond to the remaining three layers is somewhat more difficult. Following the transport layer comes the session layer. According to the creators of the OSI model, its purpose is to establish communication sessions. This includes managing the order of message transmission during dialogues (such as in video conferences), handling concurrent access to critical operations, and providing protection against connection loss (synchronization function). The problem is that, in practice, all of these functions are either implemented by application-layer protocols or by even higher-level mechanisms that fall outside the scope of the OSI model. As a result, the session layer is not used in real networks. The next layer is the presentation layer. Its task is to present data in a form that is understandable to both the sender and the receiver. This includes various data formats and interpretation rules, such as text encoding protocols (like ASCII, UTF-8, and KOI8-R), specifications for different versions of HTML/XHTML, image formats (JPEG, GIF, PNG), the MIME specification set, and others. This is also the layer where encryption and decryption are implemented. The most popular examples are TLS (Transport Layer Security) and SSL (Secure Sockets Layer). The application layer is the most straightforward. It facilitates the interaction of user-facing applications. This includes email, the World Wide Web, social networks, video and audio communication, and so on. Pros and Cons  The OSI model was adopted by the International Organization for Standardization (ISO) in 1983, a time when networking technologies were rapidly developing. While the committee debated standards, the world gradually shifted to the TCP/IP stack, which began to displace other protocols. When the OSI protocol implementations were finally released, they were met with a wave of criticism. Critics pointed out their incompatibility with real technologies, incomplete specifications, and limited capabilities compared to existing protocols. Additionally, experts considered the division into seven layers to be unjustified. Some layers were rarely used, and the same tasks were often handled at multiple different layers. Specialists joke that the OSI model ended up with seven layers because the committee had seven subcommittees, and each proposed its own addition. Meanwhile, the TCP/IP protocol suite, which underpins the entire modern Internet, was developed by a small group of people in an ad hoc fashion—solving problems as they arose, with no committees involved. However, not everything is negative. A clear advantage of the OSI model is its strong theoretical foundation for network communication, making it a standard reference for documentation and education. Some believe that all is not lost and that the model may still find a role—for example, in cloud computing.

In today's tech landscape, the Kubernetes container orchestration platform is widely used across various projects. With its increasing popularity and widespread adoption, Kubernetes often comes up during interviews for certain IT roles, including DevOps, SRE, system administration, development, and operations. The questions can range from very simple ones about cluster components to more advanced topics like networking within the cluster and network policies. In this article, we’ll go over the top Kubernetes interview questions and provide detailed answers. What is Kubernetes? Kubernetes is an open-source platform for managing containerized applications. It enables the deployment, scaling, and management of containerized workloads and services. List the Main Components of a Kubernetes Cluster At the core of Kubernetes lies the Control Plane, which resides on the master node. The Control Plane includes the following components: kube-api-server – The API server processes REST requests and serves as the "brain" of the cluster. All interactions, including object creation and deletion, go through the API server, which also manages communication between cluster components. etcd – A highly available key-value store that saves configuration data and cluster state. It can be deployed externally for improved fault tolerance. etcd is an independent project maintained by a separate team. kube-scheduler – The component responsible for determining which nodes will run which pods. It monitors available resources on each node to balance workload distribution. kube-controller-manager – Runs controllers that monitor resources and ensure the cluster matches the desired state by making necessary changes. kube-proxy – A network service that acts as a load balancer. It distributes network traffic between pods and runs on every node in the cluster. What is a Pod in Kubernetes? A Pod is the smallest deployable unit in Kubernetes and serves as an abstraction for running containers. A pod usually contains one or more containers, its own IP address, and data storage. Kubernetes doesn’t interact directly with containers, but rather through pods. What is the difference between Deployment and StatefulSet? Both Deployment and StatefulSet are Kubernetes objects for managing applications, but they serve different purposes. Deployment: Used for managing stateless applications (e.g., web servers). Supports rolling updates for zero-downtime deployments. Pods are ephemeral with non-persistent names and IPs. No state persistence: when a pod is deleted, its data is lost. StatefulSet: Designed for stateful applications (e.g., databases). Pods have stable, unique names and identifiers that persist across restarts. Supports Persistent Volumes to retain data between restarts. Pods are created and terminated in a specific order, one at a time. In conclusion, data persistence is the main difference between a Deployment and a StatefulSet. Use Deployment if the application does not require state to be preserved. However, if the application needs to retain its state, then a StatefulSet is the appropriate choice. What is a Service in Kubernetes, and What are the Types? A Service in Kubernetes defines how to access a set of pods. It provides a stable IP and DNS name, allowing internal or external communication with pods. Types of Services: ClusterIP – The default type. Exposes the service on an internal IP, accessible only within the cluster. NodePort – Exposes the service on a specific port across all nodes. Allows external access via NodeIP:NodePort. LoadBalancer – Provisions an external load balancer (mainly in cloud environments) and assigns a public IP for external traffic distribution. ExternalName – Maps the service name to an external hostname or IP address using a DNS CNAME record. Works purely at the DNS level. What is Ingress in Kubernetes? Ingress is a Kubernetes object that defines rules for routing external HTTP/HTTPS traffic to internal services within the cluster. It enables fine-grained control over how traffic is handled and directed. What is an Ingress Controller? An Ingress Controller is a component that implements the Ingress rules. It typically consists of: A reverse proxy (e.g., Nginx, HAProxy) A controller that interacts with the Kubernetes API server to apply Ingress configuration and routing rules. The controller watches for changes to Ingress objects and configures the reverse proxy accordingly to handle incoming traffic. How to Store Sensitive Data (Secrets), Including Logins, Passwords, Tokens, and Keys? Kubernetes provides the Secret object for storing sensitive information. There are six types of secrets: Opaque – A general-purpose secret type used to store any data. Service Account Token – Used to work with service accounts by generating a JWT token. Typically, the token is automatically created when a service account object is created. Basic Auth – Stores login and password in Base64-encoded format. SSH Auth – Used for SSH authentication. The secret contains a pre-generated private key. TLS Certificates – Involves using certificates and their private keys, provided in the manifest's tls.crt and tls.key fields (Base64-encoded).  Bootstrap Token – A special token type used to add new nodes to the Kubernetes cluster safely. Secrets are usually injected into containers via volumeMount or secretKeyRef. You can also use external secret management tools like HashiCorp Vault. What Are Labels and Selectors, and What Are They Used For? Labels are key-value metadata that can be attached to any Kubernetes object. They help to identify attributes of objects that are not directly related to the running services but can provide useful information to users — for example, the purpose of a deployed application or the environment in which it will run. In other words, labels are intended to distinguish between different instances of objects. Selectors are used to filter or query objects based on their labels. A selector is a request to fetch objects that match specific label criteria. What Are Probes in Kubernetes, What Types Exist, and What Are They Used For? Probes in Kubernetes check the health and readiness of applications. There are three types: Liveness Probe: Checks whether a pod is running correctly. If the check fails, the pod is restarted automatically. Readiness Probe: Checks whether a pod is ready to receive network traffic. If it fails, the pod is excluded from load balancing, though it continues running. Startup Probe: Used for apps that take a long time to start. This probe checks the app's initial startup before liveness and readiness checks are activated. What Is Pod Disruption Budget (PDB) and What Is It Used For? Pod Disruption Budget is a Kubernetes feature used to ensure a minimum number of pods are available during voluntary disruptions (e.g., node maintenance or upgrades). Example: If you have an application with 3 replicas that can tolerate the loss of 1 pod, then the PDB should specify that no more than 1 pod can be unavailable at any time. This prevents disruptions that would make the application non-functional. How to Control Resource Usage in Containers? Use requests and limits in your pod definitions: Requests define the minimum amount of CPU and memory required for a pod to be scheduled. If the cluster doesn't have enough resources, the pod won't be scheduled. Limits define the maximum amount of CPU and memory a pod can consume. The pod will be throttled or terminated if it exceeds these limits. You can learn more about Kubernetes requests and limits in our article. How to Expose an Application Running in Kubernetes to the External Network? To provide external access to an application, you can use: Ingress Controller – A preferred method for managing HTTP/HTTPS access. It routes traffic to services based on defined rules. NodePort – Opens a specific port on all nodes for external access. LoadBalancer – Provisions an external IP through a cloud load balancer. What Is the CNI Interface? CNI (Container Network Interface) is a Kubernetes specification maintained by the Cloud Native Computing Foundation. It defines how network interfaces are managed in Linux containers. CNI is responsible for connecting pods to the network. CNI features are implemented through plugins, with popular ones including: Calico Weave Flannel Cilium What Is CRI? CRI (Container Runtime Interface) is the primary communication interface between the kubelet component in a Kubernetes cluster and the container runtime environment. Using CRI, Kubernetes interacts with the container engine responsible for creating and managing containers (Kubernetes itself does not create containers directly).  Popular container runtimes that implement CRI include containerd and CRI-O. What Is a Persistent Volume (PV)? A Persistent Volume (PV) is a Kubernetes object used to store data persistently across pod lifecycles. Volumes in Kubernetes are implemented via plugins, and the platform supports the following types: Container Storage Interface (CSI) Fibre Channel (FC) hostPath iSCSI Local Storage Network File System (NFS) What Is a Persistent Volume Claim (PVC)? A Persistent Volume Claim (PVC) is a user request for storage resources. It allows users to claim a portion of a Persistent Volume based on parameters such as requested size and access mode. PVCs enable dynamic provisioning of storage in Kubernetes, meaning the cluster can automatically create a volume that matches the claim. How to Assign Access Rights in a Kubernetes Cluster? Kubernetes manages access control using RBAC (Role-Based Access Control). RBAC allows administrators to define who can do what within the cluster using the following entities: Role – Defines a set of permissions within a specific namespace. RoleBinding – Assigns a Role to a user or group within a namespace. ClusterRole – Grants permissions across the entire cluster (not limited to a single namespace). ClusterRoleBinding – Binds a ClusterRole to users or groups across all namespaces. ServiceAccount – An identity used by Kubernetes workloads (pods) to interact with the API. Conclusion In this article, we covered a list of common interview questions that candidates might encounter when applying for IT roles involving Kubernetes. These questions span a range of foundational and advanced topics, including architecture, security, networking, and storage in Kubernetes.

A software development methodology is a set of principles, approaches, and tools used to organize and manage the software creation process. It defines how the team works, how members interact and divide responsibilities, how product quality is controlled, and more. A methodology aims to regulate the development process and ensure the project is delivered according to the requirements, timelines, and budget. Various software development methodologies exist, from the Waterfall model to Extreme Programming. One such methodology is DevOps. In this article, we’ll explore what DevOps is, why it’s needed in software delivery, what problems it solves, and the core concepts behind the methodology. We’ll also cover the role of the DevOps engineer and their responsibilities within a team and development process. What is DevOps? DevOps is a relatively new software development concept rapidly gaining popularity and gradually replacing traditional development methodologies. In 2020, the global DevOps market was valued at around $6 billion. By 2027, according to ResearchAndMarkets, it’s expected to grow to $25 billion. The definition of DevOps is broad and not easy to pin down, especially compared to other areas of IT. What is DevOps in simple terms? It’s a methodology where Development, Operations, and Testing intersect and merge. But such a definition raises several valid questions: Where do the boundaries of DevOps begin and end? Which parts of development, testing, and maintenance fall outside of DevOps? Why is it necessary to link these processes? We’ll try to answer those below. The Traditional Software Release Process Development, testing, and operations are the three main phases of the software release lifecycle. Let’s examine them more closely. Whenever we develop software, we aim to deliver a working product to end users. This goal is consistent across methodologies—whether it's Waterfall, Agile, or any other: the end goal is to create and deliver a product. Let’s consider the traditional Waterfall model for application development — from idea to deployment: A software idea is born. The idea turns into a list of business requirements for the product. Developers write code and build the application. Testers verify its functionality and return it for revisions if needed. Once ready, the application needs to be delivered to users. For a web app, this includes building, configuring the server and environment, and deploying. After deployment, users start using the app. Ongoing support ensures the app is user-friendly and performs well under load. After release comes the improvement phase — adding features, optimizing, and fixing bugs. This cycle repeats with each update. One of DevOps’ primary goals is to make this cycle faster and more reliable. Let’s look at the challenges it addresses and how. Problems with the Waterfall Model In the Waterfall model, teams may face several issues that slow down the process, require significant effort to overcome, or introduce errors. 1. Poor collaboration between developers, operations, and testers As mentioned earlier, the release cycle involves development, testing, and operations. Each has its own responsibilities. But without collaboration: Developers may write code that isn’t deployment-ready. Operations may lack insight into how the app works. Testers might face delays due to insufficient documentation. These gaps lead to increased Time to Market (TTM) and higher budgets. 2. Conflicting priorities Development and operations don’t work closely in the Waterfall model. Developers want to innovate, while operations want stability. Since operations aren’t part of the development phase, they need more time to assess changes, creating friction and slowing down releases. 3. Idle teams One of the key characteristics of the waterfall model is its sequential nature. First, developers write the code, then testers check it, and only after that does the operations team deploy and maintain the application. Because of this step-by-step structure, there can be idle periods for different teams. For example, while testers check the application, developers wait for feedback and issues to fix. At the deployment stage, testers might review the entire product rather than a small update, which takes significantly more time. As a result, some teams may find themselves without tasks to work on. All these issues lead to longer release cycles and inflated budgets. Next, we’ll look at how DevOps helps address these problems—and how it does so. How DevOps Solves Waterfall Problems DevOps aims to minimize the above issues through automation, collaboration, and process standardization, making it easier and faster to integrate improvements. DevOps combines approaches, practices, and tools to streamline and accelerate product delivery. Because the concept is broad, different companies implement DevOps differently. Over time, common toolsets and practices have emerged across the industry. One common practice is introducing a DevOps engineer, responsible for creating communication and alignment between teams, and ensuring smooth product releases. What Does a DevOps Engineer Do? A DevOps engineer aims to create and maintain an optimized application release pipeline. Here's how they do that: Automation and CI/CD DevOps's cornerstone is the development, testing, and deployment automation. This forms a CI/CD pipeline — Continuous Integration and Continuous Deployment. Key DevOps stages and tools: Code: Managed in a shared repository (e.g., GitLab), facilitating automation and collaboration. Testing: Code changes are automatically tested using predefined test suites. If successful, the code moves to the build stage. Build: Code is compiled into a deployable application using tools like npm (JavaScript), Maven or Gradle (Java). Containerization & Orchestration: Apps are containerized (commonly with Docker) for consistent environments.For small setups, use Docker Compose; for large-scale setups, use Kubernetes. Artifacts are stored in repositories like Nexus or Docker Hub. Deployment: Tools like Jenkins automate app deployment. The result is a process where code changes are continually tested, integrated, and delivered to users. Infrastructure Management Thanks to CI/CD, teams can automatically deploy apps and updates to servers. Cloud platforms are often preferred over physical servers, offering better automation, scaling, and environment management. Monitoring Real-time monitoring ensures application availability and performance. Tools like Prometheus and Nagios track system metrics and availability. Infrastructure as Code (IaC) Instead of manually configuring infrastructure, DevOps uses IaC tools like Terraform to automate and standardize environments. Scripts Scripts automate adjacent processes like backups. Tools: OS-specific: Bash (Linux), PowerShell (Windows) Cross-platform: Python, Go, Ruby (Python is most popular) Version Control DevOps uses version control for application code and infrastructure (e.g., Terraform configs). Important: Terraform stores sensitive data (e.g., passwords) in state files; these must not be stored in public repositories. Cross-Team Collaboration A major DevOps goal is to improve collaboration between departments. Shared tools, standards, and processes enable better communication and coordination. For example, DevOps acts as a bridge between development and operations, unifying workflows and expectations. Why Businesses Should Implement DevOps Benefits of DevOps: Speed: Automated testing, building, and deployment enable faster release cycles without sacrificing quality. This improves agility and market responsiveness. Predictability & Quality: Frequent, automated releases mean more reliable delivery timelines and better budget control. Lower Maintenance Costs: Automated infrastructure management and monitoring reduce downtime and labor, improving SLA compliance. Challenges: Organizational Change: Implementing DevOps may require cultural and structural shifts, along with training and adaptation. Automation Risks: Poorly implemented automation can introduce new problems — misconfigured scripts, faulty pipelines — so thorough testing is essential. Investment Required: DevOps needs upfront investment in tools, technologies, and training. Conclusion DevOps enables an automated, collaborative environment for development, testing, and deployment. It helps teams release apps faster, with higher quality and reliability. If you’re considering integrating DevOps into your development process, Hostman offers services like cloud servers and Kubernetes, which can reduce your workload and streamline operations.