Knowledge Base

• Software Algorithms — In mathematics and computer science, an algorithm (/ˈælɡərɪðəm/ ( listen)) is an unambiguous specification of how to solve a class of problems. Algorithms can perform calculation, data processing, and automated reasoning tasks.
  • 3Sum — Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.
  • 4Sum — Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
  • Add Two Numbers — You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
  • Climbing Stairs ★ — You are climbing a stair case. It takes n steps to reach to the top. Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?
  • Find First and Last Position of Element in Sorted Array — Given an array of integers nums sorted in ascending order, find the starting and ending position of a given target value. Your algorithm's runtime complexity must be in the order of O(log n).
  • Flip Equivalent Binary Trees — For a binary tree T, we can define a flip operation as follows: choose any node, and swap the left and right child subtrees. A binary tree X is flip equivalent to a binary tree Y if and only if we can make X equal to Y after some number of flip operations. Write a function that determines whether two binary trees are flip equivalent. The trees are given by root nodes root1 and root2.
  • Generate Parentheses — Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.
  • Integer to Roman — Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.
  • Longest Common Prefix — Write a function to find the longest common prefix string amongst an array of strings. If there is no common prefix, return an empty string "".
  • Longest Palindromic Substring — Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
  • Longest Substring Without Repeating Characters — Given a string, find the length of the longest substring without repeating characters.
  • Maximum Width Ramp ★ — Given an array A of integers, a ramp is a tuple (i, j) for which i < j and A[i] <= A[j]. The width of such a ramp is j - i. Find the maximum width of a ramp in A. If one doesn't exist, return 0.
  • Median of Two Sorted Arrays — There are two sorted arrays nums1 and nums2 of size m and n respectively. Find the median of the two sorted arrays. The overall run time complexity should be O(log (m+n)).
  • Merge k Sorted Lists — Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.
  • Merge Two Sorted Lists — Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.
  • Next Permutation ★ — Implement next permutation, which rearranges numbers into the lexicographically next greater permutation of numbers. If such arrangement is not possible, it must rearrange it as the lowest possible order (ie, sorted in ascending order). The replacement must be in-place and use only constant extra memory.
  • Pow(x, n) — Implement pow(x, n), which calculates x raised to the power n (xn).
  • Remove Duplicates from Sorted Array — Given a sorted array nums, remove the duplicates in-place such that each element appear only once and return the new length. Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory.
  • Remove Element — Given an array nums and a value val, remove all instances of that value in-place and return the new length. Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory. The order of elements can be changed. It doesn't matter what you leave beyond the new length.
  • Remove Nth Node From End of List — Given a linked list, remove the n-th node from the end of list and return its head.
  • Reverse Integer — Given a 32-bit signed integer, reverse digits of an integer.
  • Reverse Nodes in k-Group — Given a linked list, reverse the nodes of a linked list k at a time and return its modified list. k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.
  • Roman to Integer — Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.
  • Safe Add function by checking integer overflow — When we implement add function by operator + in C++, sometimes we face over error - actually system does not show any error and that should be continuously increased, but there is a possibility to change into minus value suddenly.
  • String to Integer (atoi) — Implement atoi which converts a string to an integer.
  • Swap Nodes in Pairs — Given a linked list, swap every two adjacent nodes and return its head.
  • Two Sum — Given an array of integers, return indices of the two numbers such that they add up to a specific target.
  • Valid Parentheses — Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
  • ZigZag Conversion — The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)
• Software Architectures
  • AWS Architectures
    • Advertisement serving in AWS — Internet advertising services need to serve targeted advertising and must do so under limited time. These are just two of multiple technical challenges they face.
    • AWS Cost Calculator for estimating your required monthly budget — Whenever you design your software architecture based on AWS, you may need to know how must cost it will consume while serving your service - following service will help you to get your estimated cost.
    • Batch processing composed of job manager, job store and worker based on AWS — Batch processing on AWS allows for the on-demand provisioning of a multi-part job processing architecture that can be used for instantaneous or delayed deployment of a heterogeneous, scalable "grid" of worker nodes that can quickly crunch through large batch oriented applications in place today that can leverage this style of on-demand processing, including claims processing, large scale transformation, media transcoding and multi-part data processing work.
    • Checkout service architecture in AWS for e-commerce website — Managing the checkout process involves many steps, which have to be coordinated. Some steps such as credit card transactions, are subject to specific regulatory requirements. Other parts of the process involve manual labor, such as picking, packing, and shipping items from a warehouse.
    • Content Distribution for VOD streaming services in AWS — Below architectural diagram shows how to distribute video content to CDN (Amazon CloudFront) in AWS.
    • Disaster recovery solution in AWS for local applications — Disaster recovery is about preparing for an recovering from any event that has a negative impact on your IT systems. A typical approach involves duplicating infrastructure to ensure the availability of spare capacity in the event of a disaster.
    • Fault Tolerance & High Availability based on multi Availability zones in AWS — Most of the higher-level services, such as Amazon Simple Storage Service(S3), Amazon SimpleDB, Amazon Simple Queue Service (SQS), and Amazon Elastic Load Balancing (ELB), have been built with fault tolerance and high availability in mind. Service that provide basic infrastructure, such as Amazon Elastic Compute Cloud (EC2) and Amazon Elastic Block Store (EBS), provide specific features, such as availability zones, elastic IP address, and snapshots, that a fault-tolerant and high available system
    • File synchronization service architecture in AWS — Given the straightforward, stateless client-server architecture in which web services are viewed as resources and can be identified by their URLs, development teams are free to create file sharing and syncing applications for their departments, for enterprises, or for consumers directly.
    • Financial services grid computing in AWS — Financial services grid computing on the cloud provides dynamic scalability and elasticity for operation when compute jobs are required, and utilizing services for aggregation that simplify the development of grid software.
    • Image Moderation Chatbot - a serverless chatbot on AWS that monitors chat channels and removes images suggestive or explicit image — The below architecture shows you how to build a serverless chatbot on AWS that monitors your chat channels and removes images containing suggestive or explicit content. 
    • Large scale computing based on huge data sets in AWS — These scenarios involve huge data sets collected from scientific equipment, measurement device, or other compute jobs. After collection, these data sets need to be analyzed by large-scale compute jobs to generate result data sets. Ideally, results will be available as soon as the data is collected. Often, these results are then made available to a larger audience.
    • Large scale messenger service architecture in AWS - Slack case study — Below slack architecture shows how to compose large scale messenger service based in AWS.
    • Live streaming / Static Content & Media Service based on AWS — AWS provides a suite of services specifically tailored to deliver high-performance media serving. Each service features pay as you go pricing on an elastic infrastructure - meaning that you can scale up and down according to your demand curve while paying for only the resources you use. BEcause this infrastructure is programmable, it can react quickly.
    • Media sharing service architecture in AWS — Media sharing is one of the hottest markets on the Internet. Customers have a staggering appetite for placing photos and videos on social networking sites, and for sharing their media in custom online photo albums.
    • Product Marketing & Recommendation Architecture in AWS for e-commerce website — Product Marketing & Recommendation is one of features that online service websites has to create more user engagement, so online company can try something new together with users.
    • Secure and Scalable LAMP Service Architecture in AWS — LAMP means Linux + Apache + MySQL + PHP that is commonly used to develop web service for Start-ups that does not require paying license fee, because all of technologies are from open source.
    • Synchronous online gaming architecutre in AWS - Highly Available, Scalable & Elastic to support Millions of Players — The below architecture is intended for online mobile & games. These workloads are a natural fit for running on Amazon Web Services, due to unexpected traffic patterns & high demaning request rates. 
    • Time series processing architecture in AWS — When data arrives as a succession of regular measurements, it is known as time series information. Processing of time series information poses systems scaling challenges that the elasticity of AWS services is uniquely positioned to address.
    • Web Application Hosting provides the reliable, scalable, secure, and high-performance infrastructure — High available and scalable web hosting can be complex and expensive. Dense peak periods and wild swing in traffic patterns result in low utilization of expensive hardware. Amazon Web Services provides the reliable, scalable, secure, and high-performance infrastructure required for web applications while enabling an elastic, scale-out and scale-down infrastructure to match IT costs in real time as customer traffic fluctuates.
    • Web front-end architecture in AWS for e-commerce website — Maintaining an e-commerce website with a large product catalog and global customer base can be challenging. The catalog should be searchable, and individual product pages should contain a rich information set that includes, for example, images, a PDF manual, and customer reviews.
  • Facebook Architectures
    • Facebook scalable architecture beyond LAMP — Facebook is known as using LAMP as their basic platform for their website. One of the outstanding things Facebook doing very well is that they have their own strategy/architecture based on LAMP. LAMP stands for Linux + Apache + MySQL + PHP that is entirely free and open sourced software, so they can compete with their competitors in competitive operation cost with high-avail-abilities. In person, I think their strategy is pretty much innovative and its status tells their strategy is on the right
    • Presto - a distributed SQL query engine based on pluggable backend at Facebook — Before elaboration about Presto architecture, you should know Presto is based in Java even though Facebook is known as using PHP as their major server software language.
  • Google Cloud Platform Architectural Patterns
    • Common Web Service Architecture with High Availability in GCP — Below architectural diagram is for common web service composed of web server and database with high availability.
    • Continuous Delivery with Spinnaker in GCP — CI(Continuous Integration) and CD(Continuous Delivery) are one of the important technologies in terms of Agile Development's point of view.
    • Continuous Integration by Jenkins, automated solution, in GCP — Jenkins is an open source automation server written in Java. Jenkins helps to automate the non-human part of the software development process, with continuous integration and facilitating technical aspects of continuous delivery. It is a server-based system that runs in servlet containers such as Apache Tomcat. Below architecture shows continuous integration based in GCP.
    • Scale testing with Kubernetes + Locust in GCP — Locust is an open source tool for quickly scaling up and down load tests of web frontend as REST APIs. It allows testers to quickly create test profiles that can be orchestrated by multiple nodes in order to stress an HTTP endpoint.
  • Programmatic Advertising Architecture — Traditionally programmatic advertising is also called as online advertising. Programmatic advertising includes real-time bidding as a part of its solution.
    • ATD(Agency Trading Desk) Architecture — ATD(Agency Trading Desk) is a kind of Advertiser solutions that uses DSP(Demand-Side Platform) as a tool. Trading desks are media buyers and re-sellers within an ad agency that help advertisers execute programmatic media buys.
    • DMP(Data Management Platform) Architecture — DMP(Data Management Platform) is used by digital advertising buyers and publishers to store and manage audience data, often from multiple sources.
    • DSP(Demand-Side Platform) Architecture — A Demand-Side Platform (DSP) is a real-time bidding system that connects media buyers with data exchanges and Supply-Side Platforms(SSPs) through a single interface.
    • False Positive Error — A false positive error, or in short a false positive, commonly called a "false alarm", is a result that indicates a given condition exists, when it does not.
    • ITD(Independent Trading Desk) Architecture — ITD(IndependentTrading Desk) is a kind of Advertiser solutions that uses DSP(Demand-Side Platform) as a tool.
    • SSP(Supply-Side Platform) Architecture — SSP(Supply-Side Platform) is a kind of publisher solutions as a centralized platform to sell/manage inventory, access many advertisers, DSPs, etc.
  • Software Architectural Patterns
    • Active-Active / Active-Stand-by Architectural Pattern — Active-Active / Active-Stand-by architectural patterns are commonly used when we consider fail-over in server deployment design.
    • Blackboard architectural pattern - used in NLU(Natural Language Understanding) and car recognition — Blackboard architectural pattern usually used in Artificial Intelligence Software to implement Non-Deterministic Algorithm.
    • Broker architectural pattern - usually used in Message Middlewares like Apache ActiveMQ, Apache Kafka, RabbitMQ — The Broker architectural pattern can be used to structure distributed software systems with decoupled components that interact by remote service invocations.
    • Client-Server architectural pattern — Client-server architecture (client/server) is a network architecture in which each computer or process on the network is either a client or a server.
    • Event-bus archiectural pattern - used in Android app development and Push Notification Service — Event-bus architecture pattern is a distributed asynchronous architecture pattern to create highly scalable reactive applications. The pattern suits for every level application stack from small to complex ones. The main idea is delivering and processing events asynchronously.
    • Interpreter architectural pattern - used in database query language such as SQL — This pattern is used for designing a component that interprets programs written in a dedicated language. It mainly specifies how to evaluate lines of programs, known as sentences or expressions written in a particular language. The basic idea is to have a class for each symbol of the language.
    • Layered architectural pattern — Layered Architecture is commonly using when design J2EE application - it is something like defacto standard and widely known by most architects, designers, and developers. The layered architecture pattern closely matches the traditional IT communication and organizational structures found in most companies, making it a natural choice for most business application development efforts. 
    • Master-Slave architectural pattern - useful for disaster recovery solutions — Master/slave is a model of communication where one device or process has unidirectional control over one or more other devices. In some systems a master is selected from a group of eligible devices, with the other devices acting in the role of slaves.
    • Microservice architectural pattern aka MSA - one of the trends of service architecture in 2018- — MSA (Microservice architecture), or simply microservices, is a kind of methodology to develop service by divide and conquer - a distinctive method of developing software systems that tries to focus on building single-function modules with well-defined interfaces and operations. The trend has grown popular in recent years as Enterprises look to become more Agile and move towards a DevOps and continuous testing. Microservices can help create scalable, testable software that can be delivered weekly
    • MVC(Model-view-controller) architectural pattern - used in web application development like Django and Rails — MVC(Model–view–controller) is an architectural pattern commonly used for developing user interfaces that divides an application into three interconnected parts. This is done to separate internal representations of information from the ways information is presented to and accepted from the user. The MVC design pattern decouples these major components allowing for efficient code reuse and parallel development.
    • Peer-to-peer architectural pattern - usually used in file sharing like BitTorrent or multimedia protocols like P2PTV or PDTP — Peer-to-peer architectural pattern (P2P architecture) is a commonly used computer networking architecture in which each workstation, or node, has the same capabilities and responsibilities. It is often compared and contrasted to the classic client/server architecture, in which some computers are dedicated to serving others.
    • Pipe-filter architectural patterns - useful approach for analytics based on daisy chain — A very simple, yet powerful architecture, that is also very robust. It consists of any number of components (filters) that transform or filter data, before passing it on via connectors (pipes) to other components. The filters are all working at the same time. The architecture is often used as a simple sequence, but it may also be used for very complex structures.
• Sports
  • Golf
    • Golf Course Layout
    • Half bag for travel
    • Strong hook grip
• Tech companies
  • Aidditive Inc — Addictive develops total solutions for finance, public safety and surveillance, media and broadcasting, transportation, healthcare and etc.
  • AppLovin — AppLovin is a mobile marketing platform. The company was founded in 2012, but operated in stealth mode until 2014. 
  • Asurion - mobile protection by insurance — Asurion, LLC is a privately held company based in Nashville, Tennessee that provides insurance for smartphones, tablets, consumer electronics, appliances, satellite receivers and jewelry. In 2014, the company operated in 14 countries and had 49 offices with 17,000 employees serving 280 million consumers.
  • Confluent — Confluent is the event streaming platform powered by Apache Kafka®
  • Context Logic (dba. Wish) — "Context Logic" has a service, which is named as Wish - an online e-commerce platform that facilitates transactions between sellers and buyers. It was founded in 2010 by Peter Szulczewski (CEO) and Danny Zhang (CTO).
  • dunnhumby
  • Flexport
  • Freshworks — Freshworks has apps for small and medium businesses which cater those companies software need in customer support, Asset management, Sales & CRM and more.
  • GitHub — GitHub is an American company that provides hosting for software development version control using Git. It is a subsidiary of Microsoft, which acquired the company in 2018 for $7.5 billion.
  • GitLab - Cloud-Native CI — GitLab is a web-based DevOps lifecycle tool that provides a Git-repository manager providing wiki, issue-tracking and CI/CD pipeline features, using an open-source license, developed by GitLab Inc.
  • IDC (International Data Corporation)
  • Niantic
  • Robinhood
  • Roblox — Roblox is a massively multiplayer online and game creation system platform that allows users to design their own games and play a wide variety of different types of games created by other users. The platform hosts social network games constructed of Lego-like virtual blocks.
  • Social Finance (SoFi) — Social Finance, Inc. (commonly known as SoFi) is an online personal finance company that provides student loan refinancing, mortgages and personal loans.
  • Tanium
  • Unity Technologies
  • Uptake — Uptake is an industrial artificial intelligence (AI) software company that aims to help companies digitally transform with open, purpose-built software. Built around a foundation of data science and machine learning, Uptake’s core products include an Asset Performance Management application and a fully managed platform.
• Technologies
  • 5G — 5G is the fifth generation wireless technology for digital cellular networks that began wide deployment in 2019. As with previous standards, the covered areas are divided into regions called "cells", serviced by individual antennas.
    • ATSC 3.0 as Broadcast Standard in 5G - the next generation TV is coming
    • ONE Media announced Advanced SoC for next generation TV for Mobile and Broadcast Applications
  • Apache Spark — Apache Spark is an open-source distributed general-purpose cluster-computing framework.
  • Asuswrt Merlin Wifi Router — Asuswrt-Merlin is an enhanced version of Asuswrt, the firmware used by Asus's modern wifi routers. It is a third party alternative firmware for Asus routers, with a special emphasis on tweaks and fixes rather than radical changes or collecting as many features as possible.
    • How to add cron job on Asuswrt Merlin Wifi Router — cru enables you to check/add/delete cron job on your Asuswrt Merline Wifi Router. Below shows how to add cron job on your Asuswrt Merlin Wifi Router.
  • AutoHotkey - open-source custom scripting language a.k.a. RPA tool for Microsoft Windows — AutoHotkey is a free, open-source custom scripting language for Microsoft Windows, initially aimed at providing easy keyboard shortcuts or hotkeys, fast macro-creation and software automation that allows users of most levels of computer skill to automate repetitive tasks in any Windows application.
  • AutoML — Automated Machine Learning provides methods and processes to make Machine Learning available for non-Machine Learning experts, to improve efficiency of Machine Learning and to accelerate research on Machine Learning.
  • bash
    • reboot_if_ping_does_not_work
    • Tail - useful when debug/monitor based on log file on Linux
  • C++
    • STL (STandard Library)
  • CentOS 7 — CentOS 7 is a Linux distribution that provides a free, enterprise-class, community-supported computing platform functionally compatible with its upstream source, Red Hat Enterprise Linux
    • Change MySQL port number — The default port that MySQL database server is running under Linux and Unix is 3306/TCP. In order to change the default MySQL/MariaDB database port in Linux, open MySQL server configuration file for editing by issuing the below command.
    • Check network port is open or not on CentOS 7 — To check listening port on CentOS 7, you can do it by "netstat" simply, or you can try it by "ss". I was easily able to check which port is open or not on CentOS 7.
    • Enabling a service on boot in Centos 7 — In case that you have service on /etc/init.d/ but it does not start automatically, you can simply enable it by run chkconfig function.
    • How do I send email on command line in Centos 7? — Unfortunately Centos 7 minimal version does not have mail client running on command line, but you can use it by installing mailx.
    • How to check CentOS version? — The simplest way to check CentOS version is reading release information on /etc/centos-release:
    • Increase Number of Open Files Limit in Linux — In Linux, there are some cases that application needs to open multiple files at the same time - probably database like MySQL is one of the cases accessing multiple files at the same time, because MySQL create files for table and its index on the file system, so it will cause opening multiple files at the same time.
    • Install the latest version of Python on CentOS 7 — This is the easier method of the two for installing Python on your machine. Here, we simply add a repository that has the pre-compiled version ready for us to install. In this case, we are adding the Inline with Upstream Stable repository, a community project whose goal is to bring new versions of software to RHEL-based systems.
    • List the current process by CPU occupation rates — ps is useful command to check process. It also supports to check CPU occupation rates.
    • Method to check if OS is 32bit or 64bit on CentOS — To know whether your system is 32-bit or 64-bit, type uname -m
    • My Centos does not have X Windows, how do I install it? — I haven't installed any graphical interface like X Window on Centos, and found its solution how to install it for my personal purpose. Please take a look below steps to install "Gnome Shell" as a main graphical interface.
    • Reboot server if Google DNS does not respond — The below bash script trigger rebooting server if Google DNS does not respond
    • Set locale to en_US.utf8 — If you need to change character set to en_US.utf8 or ko_KR.utf8 on CentOS 7, you can change it by localectl r changing locale.conf. Please take a look belows
    • Upgrade PHP on CentOS 7/RHEL 7
  • CentOS 8 — CentOS 8 is based on Fedora 28 and upstream kernel 4.18: Python 3.6 and limited support for Python 2.7, PHP 7.2, Ruby 2.5, Perl 5.26, MariaDB 10.3, MySQL 8.0, PostgreSQL 10, PostgreSQL 9.6, Redis 5, and etc,
    • Configure Firewall on CentOS 8 — On CentOS 8, the default firewall is firewalld, which is used to manage iptables rules on the server. To enable and start the firewalld service on the server, run the following commands.
    • Enable/disable network interface on CentOS 8 — Once you are done with network configuration, you need to run nmtui-connect to enable/disable network interface based on your need.
    • Install htop on CentOS 8 — htop is a good option to monitor your system interactively. In order to install htop, you will need to use dnf. If you cannot install htop by yum, you will need to follow following procedure on CentOS 8.
    • Install lynx on CentOS 8 — I usually use lynx to check website on my terminal, because  and it does not require to be on graphical environment - meaning I can see its content on terminal console. It can be installed by adding PowerTools repository in Yum.
    • Install MySQL on CentOS 8 — When you install MySQL on CentOS 8, you will need to install other associated packages as well. It can be easily solved by @ option when you call yum command.
    • Remove/Manage services on CentOS 8 — ss or netstat are useful command line tool can list up open ports combined with network services for TCP/UDP. And you can stop/disable/remove that service by systemctl and dnf. Finally you can see process tree by pstree or htop to discover and identify all the process running on your server.
    • Set Hostname on CentOS 8 — nmtui-hostname is useful command in CentOS 8 which promt you to enter your machine hostname and press OK to finish.
    • Set static IP address on CentOS 8 — CentOS provides a command line tool - mntui-edit - it will help you to change the necessary internet settings easily.
  • Chatbots
    • Kakao i OpenBuilder
  • Confluence — Confluence is a collaboration software program developed and published by Australian software company Atlassian.
    • Add a Jira Kanban board to Confluence — Jira Kanban board is super useful tool to manage tasks efficiently. The similar tool popular on internet might be Trello, but its concept may also similar to Kanban if you take a look in detail.
    • Change port number for collaborative editing — If you need to change the port number for collaborative editing of Confluence, you need to add following line on <confluence home directory>/bin/setenv.sh
    • Force links to open in a new tab or window on Confluence — By default, all links are opened in the same window on confluence. In order to open that link in new tab or window, we can open it by CTRL+Click Link on new tab or window.
    • Hide the Space Tools menu for Anonymous Users in Confluence — When you want to hide some menu for anonymous users, Custom HTML is useful.
    • How do I check confluence java settings on Centos 7? — If you want to check your java settings on Centos 7, you can simply check by "ps" command.
    • How to solve when Confluence editor page loading takes too long? — I am running multiple Tomcat instances for confluence, and faced that confluence page loading takes too long. In addition, my server suddenly faced huge amount of CPU consumption including memory.
    • Integrate Jira and Confluence together — Jira project chart like Kanban can be easily embedded on Confluence, Please take a look following steps.
    • Limit access permission to Confluence by IP address — I wanted to setup a confluence for me and some limited organization, but confluence does not have that feature at the moment, and I was able to make it by modifying server.xml.
    • My confluence server has no response without any errors — If you found that your server shows "Proxy Error" - "Error reading remote server", it might be associated with the system resources or settings.
    • Page break on confluence content — In case that you need a page break between para graphs, you will need to add following HTML tags
    • Restart confluence on Centos 7
    • Restrict the ability to export a Space in Confluence
    • Stylesheet for exporting PDF
  • Data Visualization Tools
    • Google Data Studio
  • Delphi
    • Accessing Registry Keys and Items on Windows — TRegistry is a useful class can access Windows Registry which can check if the key exists or not, create a key, write a key, and etc.
    • Adding a secret key in a program file — I had some projects needs to put its own secret key when they needed, and the secret key should not be exist as an alternative file or stored in any secret place in order to have mobility - meaning that application having a secret key should be able to use in any PC/Laptop. The easiest approach can do that is adding an additional information at the end of the binary file.
    • Associate a Delphi application with a file extension — When user register a application by user choice, there are some registry entries automatically created by windows. By doing the same behaviors like windows, application can be launched automatically by clicking associated files.
    • Capture Console Output To Memo in RealTime — The below code captures console outputs and send it in real time to callback function, so the application can handle it such as displaying its progress by adding to memo, so end user may not think the application is hanging while program performing the command.
    • Combo box in readonly mode — Sometimes you may want to make user to select item on combo box without any editing permission. For your information, Combo Box is basically allowed user to edit its internal content.
    • Downloading a web page as string from web site in Delphi — One of the easiest way can get web page as string is to use TIdHTTP.Get function.
    • Enable internet proxy on windows by changing registry key — The code here simply set EnableProxy property to checked on Control Panel like below
    • Encrypting a text string based on a key in Delphi — There are lots of way can encrypt a text string based on a key. By the approach, you can use a single numerical key, or text string. In terms of security's point of view, text string-based key has better security performance.
    • Exec and Wait with new Thread
    • Execute a Console Program and Capture Its Output
    • ExtractFileExt — Delphi provides useful filename functions for file drive, file dir, file path, file name, and file ext.
    • Find Files in Delphi — The custom procedure FindFiles locates files in a given directory (folder) and its subdirectories, and adds their complete path to a stringlist. Note that recursion is used: FindFiles calls itself at the end of the procedure!
    • Get Current Directory
    • Get local IP address in Delphi — Introducing three different approaches to get local IP address in Delphi: Winsock, TIdStack and TIdIPWatch.
    • Get My Document Folder Location by TPath
    • Get remote filename from URL in delphi — To get the filename from a URL, you can retrieve the HEAD information and check Content Disposition header field. For this task, you can use the TIdHTTP indy component. If the Content Disposition does not have the file name, you can try parsing the URL.
    • Get Timezone Information in local machine in Delphi — GetTimeZoneInformation() enables you to get timezone information in standard name, standard bias, daylight name, and daylight bias.
    • Get Windows Temp Directory — GetEnvironmentVariable() enables to get get environmental variables like TEMP.
    • Get year, month, day from current date time — When getting system date time information, DecodeDate() is useful.
    • Kill a process directly by program name — In order to terminate process on the current process list, you will need to refer Process Entry - by comparing process name and process entry, you can terminate it by comparing process name.
    • Put administrator permission to the Delphi application
    • Read/Write data in Windows Registry — TRegistry enables you to read registry entry.
    • RunAsAdminAndWaitForCompletion - replacement of ShellExecute
    • Running with Delphi application with parameters — If you are considering taking parameters when starting your app, you can take it by ParamCount and ParamStr.
    • Save/Load using file in Windows Registry — When application use a certain file, and there are some needs to use the previous file in many reason. In that case, Windows Registry is useful.
    • Save Text in TStringList in UTF8 in Delphi — Delphi provides a straightforward way when saving text in TStringList in Unicode. Please see following example.
    • Select Directory in Delphi — Below code is an example to select directory by TFileOpenDialog:
    • Simple function can select directory in Delphi — Instead of using TOpenDialog, Delphi provides super simple function can select directory as following:
    • Simple Thread Example in Delphi
    • String replace by regular expression in Delphi — TRegEx is a useful class for regular expression. You can define a instance with a regular expression, and match the case by TMatch. If the case exists, the search can be continued by TMatch.NextMatch.
    • StringReplace in Delphi — Delphi provides a straight forward function can replace string - StringReplace
    • Update domain name at DNSZI in Delphi — To update IP address at DNSZI(http://dnszi.com) http://dnszi.com), you need user ID, auth key, your domain name, and its hostname. 
  • Difference between Host and Guest Operating System — The main difference between Host and Guest Operating System is that the host operating system runs directly on the hardware while the guest operating system runs on the virtual machine. You can run hypervisor like QEMU, VMWare, and etc, and setup virtual machines based on your design.
  • Docker
    • Installing docker on Centos 7
    • Installing docker on Mac
    • Installing docker on RedHat
    • Installing docker on windows — Just tested it on windows 10, so other OS version may not able to run as I have done.
  • e-mail
    • Add New Address to gmail without SMTP (forwarding-only-address)
    • Free domain email forwarding service
  • Escort
    • Write files on USB stick under Escort environment
  • ffmpeg — ffmepg is a integrated tool for video encoding/decoding processing as open source project - http://ffmpeg.org http://ffmpeg.org
    • Splitting video file by size in bash
  • Firebase — Firebase is a mobile and web application development platform developed by Firebase, Inc. in 2011, then acquired by Google in 2014.
  • GraphQL — GraphQL is a query language for APIs and a runtime for fulfilling those queries with your existing data.
  • gRPC — gRPC (gRPC Remote Procedure Calls) is an open source remote procedure call (RPC) system initially developed at Google. The first g seems like Google.
  • G Suite
    • Getting an email in a single mailbox with multiple domain name at G Suite — I have lots of domain name and use G Suite for a single domain. I needed to create a mail account for one of another domain names, and tried to find a way to combine with the existing G Suite domain. Following procedure enables you to combine your G Suite with multiple domain names.
  • HTML5
    • Adding HTML content dynamically by innerHTML
    • Drawing circle
    • Drawing rectangle
    • HTML5 Canvas
    • Playback Video in HTML5
    • Viewport for optimizing web pages for computer and tablets
  • Java
    • Spring Framework
  • Javascript
    • Javascript code snipets
    • jQuery
    • node.js
    • react.js
    • tensorflow.js
  • Mac
    • How do I fix "Java Command-Line Tool Pop-Up" on Mac OS
    • How to fix when Mac book pro fan is too loud(noisy)? — When your Mac book pro fan is loud without any dependency in running application, you should have a doubt. Usually the fan could be noisy when your Mac book needs to cool system's heat. But if that is not the case and fan is working loud, it might be caused by some kind of complex conditions triggering that in SMC(System Management Control) associated with temperature sensor in Mac book pro.
  • Microsoft EXCEL — Microsoft Excel is a spreadsheet developed by Microsoft for Windows, macOS, Android and iOS. It features calculation, graphing tools, pivot tables, and a macro programming language called Visual Basic for Applications.
    • Adding combo box on the cell in EXCEL by Data Validation
  • Microsoft Windows — Microsoft Windows is a group of several graphical operating system families, all of which are developed, marketed and sold by Microsoft. Each family caters to a certain sector of the computing industry.
    • Batch file can kill explorer running on background and start new instance
    • Disable windows update on Windows 10
    • How to stop kicking user out while remote desktop connection is established on server
    • How to view saved Wi-Fi passwords in Windows 10
    • Install OpenSSH on Windows 7
    • Install SSH on Windows 10
    • Kill chrome.exe to escape slow computer performance
    • List tasks running on the system
    • Logon automatically when power is on
    • Port forwarding in Microsoft Windows by PortProxy in NetSh
    • Virtual desktop on windows 10
    • Windows 10 built-in alarm clock - you can set alarm easily
  • MikroTik Router — MikroTik is a Latvian network equipment manufacturer. The company develops and sells wired and wireless network routers, network switches, access points, as well as operating systems and auxiliary software. The company was founded in 1996 with the focus of selling equipment in emerging markets.
  • MySQL
    • Increase max_open_files for Mysql max-connections in Ubuntu 15
    • Repair / Optimize MySQL Database
  • PHP — PHP: Hypertext Preprocessor is a general-purpose programming language originally designed for web development. It was originally created by Rasmus Lerdorf in 1994; the PHP reference implementation is now produced by The PHP Group.
    • Checking XML element by preg_match — Below code shows how to check XML element by preg_match
    • Clearing a PHP value/array — Sometimes, you will need to reset variables, and unset() will help you to do it.
    • Client IP address in PHP
    • Codes need to be changed when upgraded from PHP 5.6 to PHP 7.3
    • Display the result of running python code by shell command (shell_exec) in PHP — I usually use shell_exec when I need to run python code. The benefit of shell_exec is returning the result as string, so I can use it for other purpose.
    • Downgrade PHP from version 7.3 to 7.2 on CentOS — You can downgrade PHP from version 7.3. to 7.2 by yum and yum-config-manager.
    • Get Client's IP Address in PHP — I had got client's IP address by $REMOTE_ADDR or $_SERVER['REMOTE_ADDR'], but it does not work on latest version of PHP.
    • Get DB field name in PHP PDO from MySQL query result — PDO provides a method to get database field name (column name) from the query result.
    • PHP_SELF — PHP_SELF is a variable that returns the current script being executed. This variable returns the name and path of the current file (from the root folder). You can use this variable in the action field of the FORM or any place when you write a code dynamically. 
    • PHP configuration I commonly use
    • PHP does not show any errors
    • preg functions as replacement of ereg/eregi functions in PHP — In PHP 7.x, ereg functions are no longer supported. Based on my search on internet, ereg functions use a very limited regex flavor called POSIX ERE - meaning pref functions can cover more REGEX syntax as designed.
    • Restart server daemon by checking dead processes in PHP — PHP is a good language to manage remote/local server. Below code shows how to restart daemons like httpd, mysqld, Confluence and Jira by PHP.
    • Run shell commands on remote server in SSH — I wanted to run a specific command through ssh protocol on my server. And this is super helpful to do some kind of remote processing by PHP easily, so I do not need to log on remote server.
    • Set default timezone by date_default_timezone_set — When you want to set the default timezone used by all date/time functions, you can take one of two options: 1) put date_default_timezone_set() on your code, 2) put value in php.ini
    • Simple function appending strings to a text file
    • Stacked bar chart for PHP
  • Python — Python is an interpreted, high-level, general-purpose programming language. Created by Guido van Rossum and first released in 1991, Python's design philosophy emphasizes code readability with its notable use of significant white space.
    • _tkinter.TclError: no display name and no $DISPLAY environment variable — If you face _tkinter.TclError when you run your python application based on MatPlotLib on CentOS 7, it might be caused by MatPlotLib choose Xwindows backend by default.
    • Django
    • Install Pandas, NumPy and MatPlotLib on CentOS 7 — For data processing in Python, Pandas and NumPy are super useful.  Before installing Pandas and NumPy, you should have pip on your CentOS 7.
    • Install Tkinter on CentOS 7 — Tkinter is a Python binding to the Tk GUI toolkit. It is the standard Python interface to the Tk GUI toolkit, and is Python's de facto standard GUI.
    • Python Code Snippets
  • QEMU — QEMU is a generic and open source machine emulator and virtualizer a.k.a. hypervisor. QEMU works like VMWare, VirtualBox and etc.
  • Raspberry Pi
    • Add hidden SSID in Raspberry Pi
    • Enable Remote Desktop Server in Raspberry Pi
    • Install Korean font in Raspberry Pi
    • RDP client in Raspberry Pi
  • Redis (Remote Dictionary Server) — Redis is an open-source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability. Redis supports different kinds of abstract data structures, such as strings, lists, maps, sets, sorted sets, hyperloglogs, bitmaps, streams and spatial indexes.
  • Ruby — Ruby is a dynamic, interpreted, reflective, object-oriented, general-purpose programming language. It was designed and developed in the mid-1990s by Yukihiro "Matz" Matsumoto in Japan. According to the creator, Ruby was influenced by Perl, Smalltalk, Eiffel, Ada, and Lisp.
    • Ruby on Rails, or Rails
  • SEO (Search Engine Optimization)
    • Home page registration to increase traffics
    • How to remove search link?
    • robots.txt for confluence for SEO(Search Engine Optimization)
  • SSL
  • SSO (Single Sign On)
    • OAuth
    • SAML (Security Assertion Markup Language)
  • Sublime Text
    • Set F1 key to reindent in Sublime Text
  • Tensorflow
    • Installing TensorFlow
    • Tensorflow version
  • Tmux
  • Vim
  • Visual Studio Code
    • Writing codes on remote server via SSH/SFTP by SSH FS plugin on Visual Studio Code
  • VPN

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