GitHub - tchatzig/devops-exercises: Linux, Jenkins, AWS, SRE, Prometheus, Docker, Python, Ansible, Git, Kubernetes, Terraform, OpenStack, SQL, NoSQL, Azure, GCP, DNS, Elastic, Network, Virtualization. DevOps Interview Questions

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DevOps	CI/CD	Git	Ansible	Network
Software Development	Python	Go	Shell Scripting	Kubernetes
Cloud	AWS	Azure	Google Cloud Platform	OpenStack
Operating System	Monitoring	Elastic	Virtualization	DNS
Testing	Databases	Regex	System Design	Hardware
Certificates	Containers	SQL	OpenShift	Storage
Terraform	Mongo	Puppet	Distributed	Questions you can ask
Circle CI	Argo	Kafka		Perl
Linux	Prometheus	Big Data	Soft Skills	Security
Chaos Engineering	Misc

Network

In general, what do you need in order to communicate?

A common language (for the two ends to understand)

A way to address who do you want to communicate with

A Connection (so the content of the communication can reach the recipients)

What is TCP/IP?

A set of protocols that define how two or more devices can communicate with each other.

To learn more about TCP/IP, read here

What is Ethernet?

Ethernet simply refers to the most common type of Local Area Network (LAN) used today. A LAN—in contrast to a WAN (Wide Area Network), which spans a larger geographical area—is a connected network of computers in a small area, like your office, college campus, or even home.

What is a MAC address? What is it used for?

A MAC address is a unique identification number or code used to identify individual devices on the network.

Packets that are sent on the ethernet are always coming from a MAC address and sent to a MAC address. If a network adapter is receiving a packet, it is comparing the packet’s destination MAC address to the adapter’s own MAC address.

When is this MAC address used?: ff:ff:ff:ff:ff:ff

When a device sends a packet to the broadcast MAC address (FF:FF:FF:FF:FF:FF), it is delivered to all stations on the local network. Ethernet broadcasts are used to resolve IP addresses to MAC addresses (by ARP) at the datalink layer .

What is an IP address?

An Internet Protocol address (IP address) is a numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.An IP address serves two main functions: host or network interface identification and location addressing.

Explain subnet mask and given an example

A Subnet mask is a 32-bit number that masks an IP address, and divides the IP address into network address and host address. Subnet Mask is made by setting network bits to all "1"s and setting host bits to all "0"s. Within a given network, out of the total usable host addresses, two are always reserved for specific purposes and cannot be allocated to any host. These are the first address, which is reserved as a network address (a.k.a network ID) and the last address used for network broadcast.

Example

What is a private IP address? In which scenarios/system designs, one should use it?

Private IP addresses are assigned to the hosts in the same network to communicate among one another. As the name "private" suggests, the devices having the private IP addresses assigned can't be reached by the devices from any external network. For example, if I am living in a hostel and I want my hostelmates to join the game server I have hosted, I will ask them to join via my server's private IP address, since the network is local to the hostel.

What is a public IP address? In which scenarios/system designs, one should use it?

A public IP address is the public facing IP address. In the event that you was hosting a game server that you want your friends to join, you will give your friends your public IP address to allow their computers to identify and locate your network and server in order for the connection to take place. One time that you would not need to use a public facing IP address is in the event that you was playing with friends who was connected to the same network as you, in that case, you would use a private ip address. In order for someone to be able to connect to your server that is located internally, you will have to setup a port forward to tell your router to allow traffic from the public domain into your network and vice versa.

Explain the OSI model. What layers there are? What each layer is responsible for?

Application: user end (HTTP is here)

Presentation: establishes context between application-layer entities (Encryption is here)

Session: establishes, manages and terminates the connections

Transport: transfers variable-length data sequences from a source to a destination host (TCP & UDP are here)

Network: transfers datagrams from one network to another (IP is here)

Data link: provides a link between two directly connected nodes (MAC is here)

Physical: the electrical and physical spec the data connection (Bits are here)

You can read more about the OSI model in penguintutor.com

For each of the following determines to which OSI layer it belongs:

Error correction
Packets routing
Cables and electrical signals
MAC address
IP address
Terminate connections
3 way handshake

Error correction - Data link

Packets routing - Network

Cables and electrical signals - Physical

MAC address - Data link

IP address - Network

Terminate connections - Session

3 way handshake - Transport

What delivery schemes are you familiar with?

Unitcast: One to one communication where there is one sender and one receiver.

Broadcast: Sending a message to everyone in the network. The address ff:ff:ff:ff:ff:ff is used for broadcasting. Two common protocols which use broadcast are ARP and DHCP.

Multicast: Sending a message to a group of subscribers. It can be one-to-many or many-to-many.

What is CSMA/CD? Is it used in modern ethernet networks?

CSMA/CD stands for Carrier Sense Multiple Access / Collision Detection. Its primarily focus it to manage access to shared medium/bus where only one host can transmit at a given point of time.

CSMA/CD algorithm:

Before sending a frame, it checks whether another host already transmitting a frame.

If no one transmitting, it starts transmitting the frame.

If two hosts transmitted at the same time, we have a collision.

Both hosts stop sending the frame and they send to everyone a 'jam signal' notifying everyone that a collision occurred

They are waiting for a random time before sending again

Once each host waited for a random time, they try to send the frame again and so the

Describe the following network devices and the difference between them:

router
switch
hub

What is a "Collision Domain"?

What is a "Broadcast Domain"?

three computers connected to a switch. How many collision domains are there? How many broadcast domains?

Three collision domains and one broadcast domain

How does a router works?

A router is a physical or virtual appliance that passes information between two or more packet-switched computer networks. A router inspects a given data packet's destination Internet Protocol address (IP address), calculates the best way for it to reach its destination and then forwards it accordingly.

What is NAT?

Network Address Translation (NAT) is a process in which one or more local IP address is translated into one or more Global IP address and vice versa in order to provide Internet access to the local hosts.

What is a proxy? How does it works? What do we need it for?

A proxy server acts as a gateway between you and the internet. It’s an intermediary server separating end users from the websites they browse.

If you’re using a proxy server, internet traffic flows through the proxy server on its way to the address you requested. The request then comes back through that same proxy server (there are exceptions to this rule), and then the proxy server forwards the data received from the website to you.

Proxy servers provide varying levels of functionality, security, and privacy depending on your use case, needs, or company policy.

What is TCP? How does it works? What is the 3 way handshake?

TCP 3-way handshake or three-way handshake is a process which is used in a TCP/IP network to make a connection between server and client.

A three-way handshake is primarily used to create a TCP socket connection. It works when:

A client node sends a SYN data packet over an IP network to a server on the same or an external network. The objective of this packet is to ask/infer if the server is open for new connections.

The target server must have open ports that can accept and initiate new connections. When the server receives the SYN packet from the client node, it responds and returns a confirmation receipt – the ACK packet or SYN/ACK packet.

The client node receives the SYN/ACK from the server and responds with an ACK packet.

What is round-trip delay or round-trip time?

From wikipedia: "the length of time it takes for a signal to be sent plus the length of time it takes for an acknowledgement of that signal to be received"

Bonus question: what is the RTT of LAN?

How does SSL handshake work?

What is the difference between TCP and UDP?

TCP establishes a connection between the client and the server to guarantee the order of the packages, on the other hand, UDP does not establish a connection between client and server and doesn't handle package order. This makes UDP more lightweight than TCP and a perfect candidate for services like streaming.

Penguintutor.com provides a good explanation.

What TCP/IP protocols are you familiar with?

Explain "default gateway"

A default gateway serves as an access point or IP router that a networked computer uses to send information to a computer in another network or the internet.

What is ARP? How does it works?

ARP stands for Address Resolution Protocol. When you try to ping an IP address on your local network, say 192.168.1.1, your system has to turn the IP address 192.168.1.1 into a MAC address. This involves using ARP to resolve the address, hence its name.

Systems keep an ARP look-up table where they store information about what IP addresses are associated with what MAC addresses. When trying to send a packet to an IP address, the system will first consult this table to see if it already knows the MAC address. If there is a value cached, ARP is not used.

What is TTL? What does it helps to prevent?

What is DHCP? How does it works?

It stands for Dynamic Host Configuration Protocol, and allocates IP addresses, subnet masks and gateways to hosts. This is how it works:

A host upon entering a network, broadcasts a message in search of a DHCP server (DHCP DISCOVER)

An offer message is sent back by the DHCP server as a packet containing lease time, subnet mask, IP addresses, etc (DHCP OFFER)

Depending on which offer accepted, the client sends back a reply broadcast letting all DHCP servers know (DHCP REQUEST)

Server sends an acknowledgment (DHCP ACK)

Read more here

Can you have two DHCP servers in the same network? How it works?

What is SSL tunneling? How does it works?

What is a socket? Where can you see the list of sockets in your system?

What is IPv6? Why should we consider using it if we have IPv4?

What is VLAN?

What is MTU?

MTU stands for Maximum Transmission Unit. It's the size of the largest PDU (protocol Data Unit) that can be sent in a single transaction.

What happens if you send a packet that is bigger than the MTU?

With IPv4 protocol, router can fragment the PDU then sending all the fragmented PDU through the transaction.

With IPv6 protocol, it issues a error to the user's computer.

True or False?. Ping is using UDP because it doesn't care about reliable connection

What is SDN?

What is ICMP? What is it used for?

What is NAT? How does it work?

NAT stands for network address translation. It’s a way to map multiple local private addresses to a public one before transferring the information. Organizations that want multiple devices to employ a single IP address use NAT, as do most home routers. For example, your computer's private IP could be 192.168.1.100, but your router maps the traffic to it's public IP (e.g. 1.1.1.1). Any device on the internet would see the traffic coming from your public IP (1.1.1.1) instead of your private IP (192.168.1.100).

Which factors affect network performances

Which port number is used in each of the following protocols?:

SSH
SMTP
HTTP
DNS
HTTPS
FTP
SFTP

SSH - 22

SMTP - 25

HTTP - 80

DNS - 53

HTTPS - 443

FTP - 21

SFTP - 22

Which factors affect network performances

What is APIPA?

APIPA is a set of it addresses that devices are allocated when the main DHCP server is not reachable

What ip range does APIPA use?

APIPA uses the ip range: 169.254.0.1 - 169.254.255.254.

Control Plane and Data Plane

What "control plane" refers to?

The control plane is the part of the network that decides how to route and forward packets to a different location.

What "data plane" refers to?

The data plane is the part of the network that actually forwards the data/packets.

What "management plane" refers to?

Refers to monitoring and management functions.

To which plane (data, control, ...) is creating routing tables belongs to?

Control Plane.

Explain Spanning Tree Protocol (STP)

What is link aggregation? Why is it used?

What is Asymmetric Routing? How do deal with it?

What overlay (tunnel) protocols are you familiar with?

What is GRE? How does it works?

What is VXLAN? How does it works?

What is SNAT?

Explain OSPF

What is latency?

Latency is the time taken for an information to reach its destination from the source.

What is bandwidth?

Bandwidth is the capacity of a communication channel to measure how much data the latter can handle over a specific time period. More bandwidth would imply more traffic handling and thus more data transfer.

What is throughput?

Throughput refers to the measurement of the real amount of data transferred over a certain period of time across any transmission channel.

When performing a search query, what is more important, latency or throughput? And how to assure that what managing global infrastructure?

Latency. To have a good latency, a search query should be forwarded to the closest datacenter.

When uploading a video, what is more important, latency or throughput? And how to assure that?

Throughput. To have a good throughput, the upload stream should be routed to an underutilized link.

What other considerations (except latency and throughput) are there when forwarding requests?

Keep caches updated (which means the request could be forwarded not to the closest datacenter)

Explain Spine & Leaf

What is Network Congestion? What can cause it?

What can you tell me about UDP packet format? What about TCP packet format? How is it different?

What is the exponential backoff algorithm? Where is it used?

Using Hamming code, what would be the code word for the following data word 100111010001101?

00110011110100011101

Give examples of protocols found in the application layer

Hypertext Transfer Protocol (HTTP) - used for the webpages on the internet

Simple Mail Transfer Protocol (SMTP) - email transmission

Telecommunications Network - (TELNET) - terminal emulation to allow client access to telnet server

File Transfer Protocol (FTP) - facilitates transfer of files between any two machines

Domain Name System (DNS) - domain name translation

Dynamic Host Configuration Protocol (DHCP) - allocates IP addresses, subnet masks and gateways to hosts

Simple Network Management Protocol (SNMP) - gathers data of devices on the network

Give examples of protocols found in the network Layer

Internet Protocol (IP) - assists in routing packets from one machine to another

Internet Control Message Protocol (ICMP) - lets one know what is going such as error messages and debugging information

What is HSTS?

HTTP Strict Transport Security is a web server directive that informs user agents and web browsers how to handle its connection through a response header sent at the very beginning and back to the browser. This forces connections over HTTPS encryption, disregarding any script's call to load any resource in that domain over HTTP.

Read more [here](https://www.globalsign.com/en/blog/what-is-hsts-and-how-do-i-use-it#:~:text=HTTP%20Strict%20Transport%20Security%20(HSTS,and%20back%20to%20the%20browser.)

Network - Misc

What is the Internet? Is it the same as the World Wide Web?

The internet refers to network of networks, transferring huge amounts of data around the globe.
The World Wide Web is an application running on millions of server, on top of the internet, accessed through what is know as the web browser

What is the ISP?

ISP (Internet Service Provider) is the local internet company provider.

Operating System

Operating System Exercises

Name	Topic	Objective & Instructions	Solution	Comments
Fork 101	Fork	Link	Link
Fork 102	Fork	Link	Link

Operating System - Self Assessment

What is an operating system?

From the book "Operating Systems: Three Easy Pieces":

"responsible for making it easy to run programs (even allowing you to seemingly run many at the same time), allowing programs to share memory, enabling programs to interact with devices, and other fun stuff like that".

Operating System - Process

Can you explain what is a process?

A process is a running program. A program is one or more instructions and the program (or process) is executed by the operating system.

If you had to design an API for processes in an operating system, what would this API look like?

It would support the following:

Create - allow to create new processes

Delete - allow to remove/destroy processes

State - allow to check the state of the process, whether it's running, stopped, waiting, etc.

Stop - allow to stop a running process

How a process is created?

The OS is reading program's code and any additional relevant data

Program's code is loaded into the memory or more specifically, into the address space of the process.

Memory is allocated for program's stack (aka run-time stack). The stack also initialized by the OS with data like argv, argc and parameters to main()

Memory is allocated for program's heap which is required for dynamically allocated data like the data structures linked lists and hash tables

I/O initialization tasks are performed, like in Unix/Linux based systems where each process has 3 file descriptors (input, output and error)

OS is running the program, starting from main()

True or False? The loading of the program into the memory is done eagerly (all at once)

False. It was true in the past but today's operating systems perform lazy loading which means only the relevant pieces required for the process to run are loaded first.

What are different states of a process?

Running - it's executing instructions

Ready - it's ready to run but for different reasons it's on hold

Blocked - it's waiting for some operation to complete. For example I/O disk request

What are some reasons for a process to become blocked?

I/O operations (e.g. Reading from a disk)

Waiting for a packet from a network

What is Inter Process Communication (IPC)?

What is "time sharing"?

Even when using a system with one physical CPU, it's possible to allow multiple users to work on it and run programs. This is possible with time sharing where computing resources are shared in a way it seems to the user the system has multiple CPUs but in fact it's simply one CPU shared by applying multiprogramming and multi-tasking.

What is "space sharing"?

Somewhat the opposite of time sharing. While in time sharing a resource is used for a while by one entity and then the same resource can be used by another resource, in space sharing the space is shared by multiple entities but in a way where it's not being transferred between them.
It's used by one entity until this entity decides to get rid of it. Take for example storage. In storage, a file is yours until you decide to delete it.

What component determines which process runs at a given moment in time?

CPU scheduler

Operating System - Memory

What is "virtual memory" and what purpose it serves?

Virtual memory combines your computer's RAM with temporary space on your hard disk. When RAM runs low, virtual memory helps to move data from RAM to a space called a paging file. Moving data to paging file can free up the RAM so your computer can complete its work. In general, the more RAM your computer has, the faster the programs run. https://www.minitool.com/lib/virtual-memory.html

What is demand paging?

What is copy-on-write or shadowing?

What is a kernel, and what does it do?

The kernel is part of the operating system and is responsible for tasks like:

Allocating memory

Schedule processes

Control CPU

True or False? Some pieces of the code in the kernel are loaded into protected areas of the memory so applications can't overwritten them

True

What is POSIX?

Explain what is Semaphore and what its role in operating systems

What is cache? What is buffer?

Buffer: Reserved place in RAM which is used to hold data for temporary purposes Cache: Cache is usually used when processes reading and writing to the disk to make the process faster by making similar data used by different programs easily accessible.

Virtualization

What is Virtualization?

Virtualization uses software to create an abstraction layer over computer hardware that allows the hardware elements of a single computer—processors, memory, storage and more - to be divided into multiple virtual computers, commonly called virtual machines (VMs).

What is a hypervisor?

Red Hat: "A hypervisor is software that creates and runs virtual machines (VMs). A hypervisor, sometimes called a virtual machine monitor (VMM), isolates the hypervisor operating system and resources from the virtual machines and enables the creation and management of those VMs."

Read more here

What types of hypervisors are there?

Hosted hypervisors and bare-metal hypervisors.

What are the advantages and disadvantges of bare-metal hypervisor over a hosted hypervisor?

Due to having its own drivers and a direct access to hardware components, a baremetal hypervisor will often have better performances along with stability and scalability.

On the other hand, there will probably be some limitation regarding loading (any) drivers so a hosted hypervisor will usually benefit from having a better hardware compatibility.

What types of virtualization are there?

Operating system virtualization Network functions virtualization Desktop virtualization

Is containerization is a type of Virtualization?

Yes, it's a operating-system-level virtualization, where the kernel is shared and allows to use multiple isolated user-spaces instances.

How the introduction of virtual machines changed the industry and the way applications were deployed?

The introduction of virtual machines allowed companies to deploy multiple business applications on the same hardware while each application is separated from each other in secured way, where each is running on its own separate operating system.

Virtual Machines

Do we need virtual machines in the age of containers? Are they still relevant?

Monitoring

Explain monitoring. What is it? What its goal?

Google: "Monitoring is one of the primary means by which service owners keep track of a system’s health and availability".

What is wrong with the old approach of watching for a specific value and trigger an email/phone alert while value is exceeded?

This approach require from a human to always check why the value exceeded and how to handle it while today, it is more effective to notify people only when they need to take an actual action. If the issue doesn't require any human intervention, then the problem can be fixed by some processes running in the relevant environment.

What types of monitoring outputs are you familiar with and/or used in the past?

Alerts
Tickets
Logging

What is the difference between infrastructure monitoring and application monitoring? (methods, tools, ...)

Application Performance Management

What is Application Performance Management?

IT metrics translated into business insights

Practices for monitoring applications insights so we can improve performances, reduce issues and improve overall user experience

Name three aspects of a project you can monitor with APM (e.g. backend)

Frontend

Backend

Infra

...

What can be collected/monitored to perform APM monitoring?

Metrics

Logs

Events Traces

Prometheus

What is Prometheus? What are some of Prometheus's main features?

In what scenarios it might be better to NOT use Prometheus?

From Prometheus documentation: "if you need 100% accuracy, such as for per-request billing".

Describe Prometheus architecture and components

Can you compare Prometheus to other solutions like InfluxDB for example?

What is an Alert?

Describe the following Prometheus components:

Prometheus server
Push Gateway
Alert Manager

Prometheus server is responsible for scraping and storing the data
Push gateway is used for short-lived jobs
Alert manager is responsible for alerts ;)

What is an Instance? What is a Job?

What core metrics types Prometheus supports?

What is an exporter? What is it used for?

Which Prometheus best practices are you familiar with?. Name at least three

How to get total requests in a given period of time?

What HA in Prometheus means?

How do you join two metrics?

How to write a query that returns the value of a label?

How do you convert cpu_user_seconds to cpu usage in percentage?

Go

What are some characteristics of the Go programming language?

Strong and static typing - the type of the variables can't be changed over time and they have to be defined at compile time

Simplicity

Fast compile times

Built-in concurrency

Garbage collected

Platform independent

Compile to standalone binary - anything you need to run your app will be compiled into one binary. Very useful for version management in run-time.

Go also has good community.

What is the difference between var x int = 2 and x := 2?

The result is the same, a variable with the value 2.

With var x int = 2 we are setting the variable type to integer while with x := 2 we are letting Go figure out by itself the type.

True or False? In Go we can redeclare variables and once declared we must use it.

False. We can't redeclare variables but yes, we must used declared variables.

What libraries of Go have you used?

This should be answered based on your usage but some examples are:

fmt - formatted I/O

What is the problem with the following block of code? How to fix it?

func main() {
    var x float32 = 13.5
    var y int
    y = x
}

The following block of code tries to convert the integer 101 to a string but instead we get "e". Why is that? How to fix it?

package main

import "fmt"

func main() {
    var x int = 101
    var y string
    y = string(x)
    fmt.Println(y)
}

It looks what unicode value is set at 101 and uses it for converting the integer to a string. If you want to get "101" you should use the package "strconv" and replace y = string(x) with y = strconv.Itoa(x)

What is wrong with the following code?:

package main

func main() {
    var x = 2
    var y = 3
    const someConst = x + y
}

Constants in Go can only be declared using constant expressions. But x, y and their sum is variable.
const initializer x + y is not a constant