:warning: You can use these for preparing for an interview but most of the questions and exercises don't represent an actual interview. Please read [FAQ page](faq.md) for more details
:stop_sign: If you are interested in pursuing a career as DevOps engineer, learning some of the concepts mentioned here would be useful, but you should know it's not about learning all the topics and technologies mentioned in this repository
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.
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 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 .
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.
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.
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.
<summary>What is a public IP address? In which scenarios/system designs, one should use it?</summary><br><b>
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.
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.
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.
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.
- 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.
From [wikipedia](https://en.wikipedia.org/wiki/Round-trip_delay): "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"
SSL handshake is a process that establishes a secure connection between a client and a server.
1. The client sends a Client Hello message to the server, which includes the client's version of the SSL/TLS protocol, a list of the cryptographic algorithms supported by the client, and a random value.
2. The server responds with a Server Hello message, which includes the server's version of the SSL/TLS protocol, a random value, and a session ID.
3. The server sends a Certificate message, which contains the server's certificate.
4. The server sends a Server Hello Done message, which indicates that the server is done sending messages for the Server Hello phase.
5. The client sends a Client Key Exchange message, which contains the client's public key.
6. The client sends a Change Cipher Spec message, which notifies the server that the client is about to send a message encrypted with the new cipher spec.
7. The client sends an Encrypted Handshake Message, which contains the pre-master secret encrypted with the server's public key.
8. The server sends a Change Cipher Spec message, which notifies the client that the server is about to send a message encrypted with the new cipher spec.
9. The server sends an Encrypted Handshake Message, which contains the pre-master secret encrypted with the client's public key.
10. The client and server can now exchange application data.
<summary>What is the difference between TCP and UDP?</summary><br><b>
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.
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.
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.
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).
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.
<summary>When performing a search query, what is more important, latency or throughput? And how to assure that what managing global infrastructure?</summary><br><b>
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.)
"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".
* 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)
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.
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.
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.<br>
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.
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.
Copy-on-write (COW) is a resource management concept, with the goal to reduce unnecessary copying of information. It is a concept which is implemented for instance within the POSIX fork syscall, which creates a duplicate process of the calling process.
The idea:
1. If resources are shared between 2 or more entities (for example shared memory segments between 2 processes) the resources don't need to be copied for every entity, but rather every entity has a READ operation access permission on the shared resource. (the shared segements are marked as read-only)
(Think of every entity having a pointer to the location of the shared resource which can be dereferenced to read its value)
2. If one entity would perform a WRITE operation on a shared resource a problem would arise since the resource also would be permanently changed for ALL other entities sharing it.
(Think of a process modifying some variables on the stack, or allocatingy some data dynamically on the heap, these changes to the shared resource would also apply for ALL other processes, this is definetly an undesirable behaviour)
3. As a solution only if a WRITE operation is about to be performed on a shared resource, this resource gets COPIED first and then the changes are applied.
<summary>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</summary><br><b>
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 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).
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."
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.
<summary>How the introduction of virtual machines changed the industry and the way applications were deployed?</summary><br><b>
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.
<summary>What is wrong with the old approach of watching for a specific value and trigger an email/phone alert while value is exceeded?</summary><br><b>
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.
With <code>var x int = 2</code> we are setting the variable type to integer while with <code>x := 2</code> we are letting Go figure out by itself the type.
Go's iota identifier is used in const declarations to simplify definitions of incrementing numbers. Because it can be used in expressions, it provides a generality beyond that of simple enumerations.
<br>
`x` and `y` in the first iota group, `z` in the second.
<br>
[Iota page in Go Wiki](https://github.com/golang/go/wiki/Iota)
<summary>What is better? Embedded documents or referenced?</summary><br><b>
</b></details>
<details>
<summary>Have you performed data retrieval optimizations in Mongo? If not, can you think about ways to optimize a slow data retrieval?</summary><br><b>
<summary>Which of the following queries would you use?
```
SELECT count(*) SELECT count(*)
FROM shawarma_purchases FROM shawarma_purchases
WHERE vs. WHERE
YEAR(purchased_at) == '2017' purchased_at >= '2017-01-01' AND
purchased_at <= '2017-31-12'
```
</summary><br><b>
```
SELECT count(*)
FROM shawarma_purchases
WHERE
purchased_at >= '2017-01-01' AND
purchased_at <= '2017-31-12'
```
When you use a function (`YEAR(purchased_at)`) it has to scan the whole database as opposed to using indexes and basically the column as it is, in its natural state.
* Management Network - used for internal communication between OpenStack components. Any IP address in this network is accessible only within the datacetner
* Guest Network - used for communication between instances/VMs
* API Network - used for services API communication. Any IP address in this network is publicly accessible
* External Network - used for public communication. Any IP address in this network is accessible by anyone on the internet
* glance-api - responsible for handling image API calls such as retrieval and storage. It consists of two APIs: 1. registry-api - responsible for internal requests 2. user API - can be accessed publicly
* glance-registry - responsible for handling image metadata requests (e.g. size, type, etc). This component is private which means it's not available publicly
* metadata definition service - API for custom metadata
* database - for storing images metadata
* image repository - for storing images. This can be a filesystem, swift object storage, HTTP, etc.
</b></details>
#### OpenStack - Swift
<details>
<summary>Explain Swift in detail</summary><br><b>
* Swift is Object Store service and is an highly available, distributed and consistent store designed for storing a lot of data
* Swift is distributing data across multiple servers while writing it to multiple disks
* One can choose to add additional servers to scale the cluster. All while swift maintaining integrity of the information and data replications.
</b></details>
<details>
<summary>Can users store by default an object of 100GB in size?</summary><br><b>
Not by default. Object Storage API limits the maximum to 5GB per object but it can be adjusted.
</b></details>
<details>
<summary>Explain the following in regards to Swift:
<summary>Explain Cinder in detail</summary><br><b>
* Cinder is OpenStack Block Storage service
* It basically provides used with storage resources they can consume with other services such as Nova
* One of the most used implementations of storage supported by Cinder is LVM
* From user perspective this is transparent which means the user doesn't know where, behind the scenes, the storage is located or what type of storage is used
* Puppet has a primary-secondary node architecture. The clients are distributed across the network and communicate with the primary-secondary environment where Puppet modules are present. The client agent sends a certificate with its ID to the server; the server then signs that certificate and sends it back to the client. This authentication allows for secure and verifiable communication between the client and the master.
Elasticserach, Logstash and Kibana are also known as the ELK stack.
</b></details>
<details>
<summary>Explain what is Elasticsearch</summary><br><b>
From the official [docs](https://www.elastic.co/guide/en/elasticsearch/reference/current/documents-indices.html):
"Elasticsearch is a distributed document store. Instead of storing information as rows of columnar data, Elasticsearch stores complex data structures that have been serialized as JSON documents"
From the [blog](https://logit.io/blog/post/the-top-50-elk-stack-and-elasticsearch-interview-questions):
"Logstash is a powerful, flexible pipeline that collects, enriches and transports data. It works as an extract, transform & load (ETL) tool for collecting log messages."
"Kibana is an open source analytics and visualization platform designed to work with Elasticsearch. You use Kibana to search, view, and interact with data stored in Elasticsearch indices. You can easily perform advanced data analysis and visualize your data in a variety of charts, tables, and maps."
<summary>Describe what happens from the moment an app logged some information until it's displayed to the user in a dashboard when the Elastic stack is used</summary><br><b>
* Track the status of all the nodes in the cluster
* Verify replicas are working and the data is available from every data node.
* No hot nodes (no data node that works much harder than other nodes)
While there can be multiple master nodes in reality only of them is the elected master node.
</b></details>
<details>
<summary>What is an ingest node?</summary><br><b>
A node which responsible for parsing the data. In case you don't use logstash then this node can recieve data from beats and parse it, similarly to how it can be parsed in Logstash.
</b></details>
<details>
<summary>What is Coordinating node?</summary><br><b>
A Coordinating node responsible for routing requests out and in to the cluser (data nodes).
</b></details>
<details>
<summary>How data is stored in elasticsearch?</summary><br><b>
* Data is stored in an index
* The index is spread across the cluster using shards
Index in Elastic is in most cases compared to a whole database from the SQL/NoSQL world.<br>
You can choose to have one index to hold all the data of your app or have multiple indices where each index holds different type of your app (e.g. index for each service your app is running).
The official docs also offer a great explanation (in general, it's really good documentation, as every project should have):
"An index can be thought of as an optimized collection of documents and each document is a collection of fields, which are the key-value pairs that contain your data"
An index is split into shards and documents are hashed to a particular shard. Each shard may be on a different node in a cluster and each one of the shards is a self contained index.<br>
This allows Elasticsearch to scale to an entire cluster of servers.
Continuing with the comparison to SQL/NoSQL a Document in Elastic is a row in table in the case of SQL or a document in a collection in the case of NoSQL.
As in NoSQL a Document is a JSON object which holds data on a unit in your app. What is this unit depends on the your app. If your app related to book then each document describes a book. If you are app is about shirts then each document is a shirt.
<summary>You check the health of your elasticsearch cluster and it's red. What does it mean? What can cause the status to be yellow instead of green?</summary><br><b>
Red means some data is unavailable.
Yellow can be caused by running single node cluster instead of multi-node.
<summary>True or False? Elasticsearch indexes all data in every field and each indexed field has the same data structure for unified and quick query ability</summary><br><b>
False.
From the official docs:
"Each indexed field has a dedicated, optimized data structure. For example, text fields are stored in inverted indices, and numeric and geo fields are stored in BKD trees."
In a network/cloud environment where failures can be expected any time, it is very useful and highly recommended to have a failover mechanism in case a shard/node somehow goes offline or disappears for whatever reason.
To this end, Elasticsearch allows you to make one or more copies of your index’s shards into what are called replica shards, or replicas for short.
<summary>Can you explain Term Frequency & Document Frequency?</summary><br><b>
Term Frequency is how often a term appears in a given document and Document Frequency is how often a term appears in all documents. They both are used for determining the relevance of a term by calculating Term Frequency / Document Frequency.
<summary>What this command does? <code>curl -X PUT "localhost:9200/customer/_doc/1?pretty" -H 'Content-Type: application/json' -d'{ "name": "John Doe" }'</code></summary><br><b>
It creates customer index if it doesn't exists and adds a new document with the field name which is set to "John Dow". Also, if it's the first document it will get the ID 1.
</b></details>
<details>
<summary>What will happen if you run the previous command twice? What about running it 100 times?</code></summary><br><b>
1. If name value was different then it would update "name" to the new value
2. In any case, it bumps version field by one
</b></details>
<details>
<summary>What is the Bulk API? What would you use it for?</code></summary><br><b>
Bulk API is used when you need to index multiple documents. For high number of documents it would be significantly faster to use rather than individual requests since there are less network roundtrips.
<summary>Explain what is Relevance Score</summary><br><b>
</b></details>
<details>
<summary>Explain Query Context and Filter Context</summary><br><b>
From the official docs:
"In the query context, a query clause answers the question “How well does this document match this query clause?” Besides deciding whether or not the document matches, the query clause also calculates a relevance score in the _score meta-field."
"In a filter context, a query clause answers the question “Does this document match this query clause?” The answer is a simple Yes or No—no scores are calculated. Filter context is mostly used for filtering structured data"
<summary>Describe how would an architecture of production environment with large amounts of data would be different from a small-scale environment</summary><br><b>
There are several possible answers for this question. One of them is as follows:
A small-scale architecture of elastic will consist of the elastic stack as it is. This means we will have beats, logstash, elastcsearch and kibana.<br>
A production environment with large amounts of data can include some kind of buffering component (e.g. Reddis or RabbitMQ) and also security component such as Nginx.
"Many processes running on many machines...only message-passing via an unreliable network with variable delays, and the system may suffer from partial failures, unreliable clocks, and process pauses."
<summary>What is "Shared-Nothing" architecture?</summary><br><b>
It's an architecture in which data is and retrieved from a single, non-shared, source usually exclusively connected to one node as opposed to architectures where the request can get to one of many nodes and the data will be retrieved from one shared location (storage, memory, ...).
<summary>What happens when you type in a URL in an address bar in a browser?</summary><br><b>
1. The browser searches for the record of the domain name IP address in the DNS in the following order:
* Browser cache
* Operating system cache
* The DNS server configured on the user's system (can be ISP DNS, public DNS, ...)
2. If it couldn't find a DNS record locally, a full DNS resolution is started.
3. It connects to the server using the TCP protocol
4. The browser sends an HTTP request to the server
5. The server sends an HTTP response back to the browser
6. The browser renders the response (e.g. HTML)
7. The browser then sends subsequent requests as needed to the server to get the embedded links, javascript, images in the HTML and then steps 3 to 5 are repeated.
I like this definition from [blog.christianposta.com](https://blog.christianposta.com/microservices/api-gateways-are-going-through-an-identity-crisis):
"An explicitly and purposefully defined interface designed to be invoked over a network that enables software developers to get programmatic access to data and functionality within an organization in a controlled and comfortable way."
<summary>What is an API specification?</summary><br><b>
From [swagger.io](https://swagger.io/resources/articles/difference-between-api-documentation-specification):
"An API specification provides a broad understanding of how an API behaves and how the API links with other APIs. It explains how the API functions and the results to expect when using the API"
</b></details>
<details>
<summary>True or False? API Definition is the same as API Specification</summary><br><b>
False. From [swagger.io](https://swagger.io/resources/articles/difference-between-api-documentation-specification):
"An API definition is similar to an API specification in that it provides an understanding of how an API is organized and how the API functions. But the API definition is aimed at machine consumption instead of human consumption of APIs."
<summary>What is Automation? How it's related or different from Orchestration?</summary><br><b>
Automation is the act of automating tasks to reduce human intervention or interaction in regards to IT technology and systems.<br>
While automation focuses on a task level, Orchestration is the process of automating processes and/or workflows which consists of multiple tasks that usually across multiple systems.
<summary>Explain what is a firmware</summary><br><b>
[Wikipedia](https://en.wikipedia.org/wiki/Firmware): "In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide hardware abstraction services to higher-level software such as operating systems."
[Avinetworks](https://avinetworks.com/glossary/layer-7/): HTTP stands for Hypertext Transfer Protocol. HTTP uses TCP port 80 to enable internet communication. It is part of the Application Layer (L7) in OSI Model.
<summary>When you publish a project, you usually publish it with a license. What types of licenses are you familiar with and which one do you prefer to use?</summary><br><b>
<summary>Explain what is "X-Forwarded-For"</summary><br><b>
[Wikipedia](https://en.wikipedia.org/wiki/X-Forwarded-For): "The X-Forwarded-For (XFF) HTTP header field is a common method for identifying the originating IP address of a client connecting to a web server through an HTTP proxy or load balancer."
A load balancer accepts (or denies) incoming network traffic from a client, and based on some criteria (application related, network, etc.) it distributes those communications out to servers (at least one).
* Scalability - using a load balancer, you can possibly add more servers in the backend to handle more requests/traffic from the clients, as opposed to using one server.
* Redundancy - if one server in the backend dies, the load balancer will keep forwarding the traffic/requests to the second server so users won't even notice one of the servers in the backend is down.
<summary>What are the drawbacks of round robin algorithm in load balancing?</summary><br><b>
* A simple round robin algorithm knows nothing about the load and the spec of each server it forwards the requests to. It is possible, that multiple heavy workloads requests will get to the same server while other servers will got only lightweight requests which will result in one server doing most of the work, maybe even crashing at some point because it unable to handle all the heavy workloads requests by its own.
* Each request from the client creates a whole new session. This might be a problem for certain scenarios where you would like to perform multiple operations where the server has to know about the result of operation so basically, being sort of aware of the history it has with the client. In round robin, first request might hit server X, while second request might hit server Y and ask to continue processing the data that was processed on server X already.
<summary>Explain use case for connection draining?</summary><br><b>
To ensure that a Classic Load Balancer stops sending requests to instances that are de-registering or unhealthy, while keeping the existing connections open, use connection draining. This enables the load balancer to complete in-flight requests made to instances that are de-registering or unhealthy.
The maximum timeout value can be set between 1 and 3,600 seconds on both GCP and AWS.
<summary>Explain the differences between copyleft and permissive licenses</summary><br><b>
In Copyleft, any derivative work must use the same licensing while in permissive licensing there are no such condition. GPL-3 is an example of copyleft license while BSD is an example of permissive license.
These are only a suggestion, use them carefully. Not every interviewer will be able to answer these (or happy to) which should be perhaps a red flag warning for your regarding working in such place but that's really up to you.
Not only this will tell you what is expected from you, it will also provide big hint on the type of work you are going to do in the first months of your job.
<summary>You find out your database became a bottleneck and users experience issues accessing data. How can you deal with such situation?</summary><br><b>
Not much information provided as to why it became a bottleneck and what is current architecture, so one general approach could be<br>
to reduce the load on your database by moving frequently-accessed data to in-memory structure.
<summary>What is a connection pool?</summary><br><b>
Connection Pool is a cache of database connections and the reason it's used is to avoid an overhead of establishing a connection for every query done to a database.
</b></details>
<details>
<summary>What is a connection leak?</summary><br><b>
A connection leak is a situation where database connection isn't closed after being created and is no longer needed.
</b></details>
<details>
<summary>What is Table Lock?</summary><br><b>
</b></details>
<details>
<summary>Your database performs slowly than usual. More specifically, your queries are taking a lot of time. What would you do?</summary><br><b>
* Query for running queries and cancel the irrelevant queries
* Check for connection leaks (query for running connections and include their IP)
* Check for table locks and kill irrelevant locking sessions
</b></details>
<details>
<summary>What is a Data Warehouse?</summary><br><b>
"A data warehouse is a subject-oriented, integrated, time-variant and non-volatile collection of data in support of organisation's decision-making process"
<summary>What is an index in a database?</summary><br><b>
A database index is a data structure that improves the speed of operations in a table. Indexes can be created using one or more columns, providing the basis for both rapid random lookups and efficient ordering of access to records.
<summary>What data types are there in relational databases?</summary><br><b>
</b></details>
<details>
<summary>Explain Normalization</summary><br><b>
Data that is used multiple times in a database should be stored once and referenced with a foreign key.<br>
This has the clear benefit of ease of maintenance where you need to change a value only in a single place to change it everywhere.
</b></details>
<details>
<summary>Explain Primary Key and Foreign Key</summary><br><b>
Primary Key: each row in every table should a unique identifier that represents the row.<br>
Foreign Key: a reference to another table's primary key. This allows you to join table together to retrieve all the information you need without duplicating data.
</b></details>
<details>
<summary>What types of data tables have you used?</summary><br><b>
* Primary data table: main data you care about
* Details table: includes a foreign key and has one to many relationship
* Lookup values table: can be one table per lookup or a table containing all the lookups and has one to many relationship
* Multi reference table
</b></details>
<details>
<summary>What is ORM? What benefits it provides in regards to relational databases usage?</summary><br><b>
[Wikipedia](https://en.wikipedia.org/wiki/Object%E2%80%93relational_mapping): "is a programming technique for converting data between incompatible type systems using object-oriented programming languages"
In regards to the relational databases:
* Database as code
* Database abstraction
* Encapsulates SQL complexity
* Enables code review process
* Enables usage as a native OOP structure
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<summary>What is DDL?</summary><br><b>
[Wikipedia](https://en.wikipedia.org/wiki/Data_definition_language): "In the context of SQL, data definition or data description language (DDL) is a syntax for creating and modifying database objects such as tables, indices, and users."
CDN (Content Delivery Network) responsible for distributing content geographically. Part of it, is what is known as edge locations, aka cache proxies, that allows users to get their content quickly due to cache features and geographical distribution.
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<summary>Explain Multi-CDN</summary><br><b>
In single CDN, the whole content is originated from content delivery network.<br>
In multi-CDN, content is distributed across multiple different CDNs, each might be on a completely different provider/cloud.
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<summary>What are the benefits of Multi-CDN over a single CDN?</summary><br><b>
* Resiliency: Relying on one CDN means no redundancy. With multiple CDNs you don't need to worry about your CDN being down
* Flexibility in Costs: Using one CDN enforces you to specific rates of that CDN. With multiple CDNs you can take into consideration using less expensive CDNs to deliver the content.
* Performance: With Multi-CDN there is bigger potential in choosing better locations which more close to the client asking the content
* Scale: With multiple CDNs, you can scale services to support more extreme conditions
<summary>What is the difference between high availability and Disaster Recovery?</summary><br><b>
[wintellect.com](https://www.wintellect.com/high-availability-vs-disaster-recovery): "High availability, simply put, is eliminating single points of failure and disaster recovery is the process of getting a system back to an operational state when a system is rendered inoperative. In essence, disaster recovery picks up when high availability fails, so HA first."
The load on the producers or consumers may be high which will then cause them to hang or crash.<br>
Instead of working in "push mode", the consumers can pull tasks only when they are ready to handle them. It can be fixed by using a streaming platform like Kafka, Kinesis, etc. This platform will make sure to handle the high load/traffic and pass tasks/messages to consumers only when the ready to get them.
A central processing unit (CPU) performs basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions in the program. This contrasts with external components such as main memory and I/O circuitry, and specialized processors such as graphics processing units (GPUs).
RAM (Random Access Memory) is the hardware in a computing device where the operating system (OS), application programs and data in current use are kept so they can be quickly reached by the device's processor. RAM is the main memory in a computer. It is much faster to read from and write to than other kinds of storage, such as a hard disk drive (HDD), solid-state drive (SSD) or optical drive.
An embedded system is a computer system - a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is embedded as part of a complete device often including electrical or electronic hardware and mechanical parts.
DataOps seeks to reduce the end-to-end cycle time of data analytics, from the origin of ideas to the literal creation of charts, graphs and models that create value.
DataOps combines Agile development, DevOps and statistical process controls and applies them to data analytics.
<summary>What is Data Architecture?</summary><br><b>
An answer from [talend.com](https://www.talend.com/resources/what-is-data-architecture):
"Data architecture is the process of standardizing how organizations collect, store, transform, distribute, and use data. The goal is to deliver relevant data to people who need it, when they need it, and help them make sense of it."
<summary>What is Packer? What is it used for?</summary><br><b>
In general, Packer automates machine images creation.
It allows you to focus on configuration prior to deployment while making the images. This allows you start the instances much faster in most cases.
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<summary>Packer follows a "configuration->deployment" model or "deployment->configuration"?</summary><br><b>
A configuration->deployment which has some advantages like:
1. Deployment Speed - you configure once prior to deployment instead of configuring every time you deploy. This allows you to start instances/services much quicker.
2. More immutable infrastructure - with configuration->deployment it's not likely to have very different deployments since most of the configuration is done prior to the deployment. Issues like dependencies errors are handled/discovered prior to deployment in this model.
If you are looking for a way to prepare for a certain exam this is the section for you. Here you'll find a list of certificates, each references to a separate file with focused questions that will help you to prepare to the exam. Good luck :)
Thanks to all of our amazing [contributors](https://github.com/bregman-arie/devops-exercises/graphs/contributors) who make it easy for everyone to learn new things :)
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