1. Keep learning forward

    To be updated ...
  2. #TIL : Convert tabs to spaces

    This is my config to use 4 spaces instead tab

    filetype plugin indent on
    set tabstop=4
    set shiftwidth=4
    set expandtab

    To convert existing file from tabs to spaces, use this command

  3. #TIL : Transaction style in Redis

    In Redis, you can use transaction-style (mean queue commands then flush it once). It will improve performance in many case where latency or networking is slow.

    > SET hoge 2
    > MULTI
    > INCR foo
    > INCR hoge
    > EXEC
    1) (integer) 1
    2) (integer) 1

    MULTI command is begin transaction and EXEC command is commit transaction

    The result will be returned in order your command queue list.

  4. #TIL : Check vcl file syntax before restarting

    Like NginX, Varnish has a syntax checker function that helps us test the syntactic correctness.

    $ varnishd -C -f [vcl file path]

    Varnish will compile the file and output the result to stdout. If something goes wrong, it will throw a message like

    > Message from VCC-compiler:
    > Expected an action, 'if', '{' or '}'
    > ('input' Line 74 Pos 6)
    >     vcl_hash(req.http.Cookie);
    > -----########------------------
    > Running VCC-compiler failed, exit 1
  5. #TIL : Using mark to bookmark checkpoints in files

    Bookmarking a checkpoint will help you get back to it intermidately. Ex: your have to find some text to replace something but want to return back current position.

    Set a mark

    • [NORMAL MODE] , type m then follow by a letter from a-z (lowercase is filescope, uppercase for global scope - vim scope)

    Go to a mark

    • [NORMAL MODE] , type backstick ` then follow by the letter your marked above.

    List all current marks

    • [NORMAL MODE], :marks
    • It shows all marks included special ones :
    `.jump to position where last change occurred in current buffer
    `"jump to position where last exited current buffer
    `0jump to position in last file edited (when exited Vim)
    ''jump back (to line in current buffer where jumped from)
    ``jump back (to position in current buffer where jumped from)

    TIPS : Can use it as a motion with change, delete or yank

  6. #TIL : Basics of Elasticsearch

    Last days, I developed a EFK stack to centralize my system logging. I really like the concepts of FluentD, it's better than original stack ELK of elastic company.

    So I need to learn basics about Elasticsearch and Kibana

    This is what I learned :

    # Get all documents from elasticsearch node
    GET _search
      "query": {
        "match_all": {}
    # Check nodes statistics
    GET /_nodes/stats
    # Check health of cluster (I don't know why it is always yello status)
    GET _cluster/health
    # Get list of indices (indexes)
    GET /_cat/indices?v
    # Delete a index (with its data) with name
    DELETE /[index-name]
  7. #TIL : Use journalctl to check system logs

    Logging and Monitoring are important factor for system admin. Checking the log will help you have a closer look into the issue. One tool could help you will handy features is journalctl.

    Here are simple options :

    • -f : follow the log (tailf)
    • -u [service] : filter to show only [service] logs
    • --since=[date] : Show entries not older than the specified date
    • --until=[date] : Show entries not newer than the specified date

    Example :

    $ sudo journalctl -u nginx.service
    $ sudo journalctl -u nginx.service --since yesterday
    $ sudo journalctl -u nginx.service --since "2018-01-01" --until today
  8. #TIL : Ansible running host pattern

    Ansible supports pattern to select and filter running hosts from all hosts. Here is some common pattern

    • * : wildcard, standalone mean all
    • group1,group2 : run hosts belong to group1 or group2
    • *.web : run hosts belongs to group matches pattern. Ex: backend.web, frontend.web
    • all:!abc : run all hosts exclude hosts belongs to group abc

    Infrastructure by code ;)

  9. #TIL : Set up simple rate limiting on specified port using UFW

    Allow unmetrered connections on networking is so risky. Attacker can use the brute-force attacks to comprosise your service (or simple DOS).

    Linux has a cool firewall to hanlde this, via ip-tables. But it's so complicated to remember all the rule and syntax. That's why UFW was born to save us. :D

    You can use simple command to manage your firewall

    $ ufw default deny incoming # deny any incoming port, should be run before allow any port
    $ ufw default allow outgoing # allow any outgoing port
    $ ufw allow 80 # allow port 80
    $ ufw deny 53/udp # allow udp protocol to port 53
    $ ufw disable # disable firewall
    $ ufw enable # enable firewall
    $ ufw status # check all the rules
    $ ufw delete [num] # delete the rule by its order in status result
    $ ufw reload # reload all rule
    $ ufw limit ssh/tcp # finnaly, limit ssh (port 22 tcp), deny connections if an IP address has attempted to initiate 6 or more connections in the last 30 seconds
  10. #TIL : Disable IPv6 to stop getting stuck in network

    I know IPv6 will be future for networking, but at this moment "It's suck !" :(

    Some service will be failed when trying to connect IPv6 destination :

    • apt package manager
    • smtp
    • curl

    So I decided to disable IPv6 on every production server.

    $ echo "net.ipv6.conf.all.disable_ipv6 = 1" | sudo tee -a /etc/sysctl.conf
    $ echo "net.ipv6.conf.default.disable_ipv6 = 1" | sudo tee -a /etc/sysctl.conf
    $ echo "net.ipv6.conf.lo.disable_ipv6 = 1" | sudo tee -a /etc/sysctl.conf
    $ sudo sysctl -p

    I will re-enable it when everything works perfectly !

  11. #TIL : Tại sao biển xanh lại mặn ? :lol:


    Biển xanh lại mặn bởi vì đá ở mặt đất cọ xát với mưa trên trời (chứ không phải cá nó đái 😂 )

    Read more : https://oceanservice.noaa.gov/facts/whysalty.html

  12. #TIL : Trigger event after setting val in jQuery

    After setting value of an input via val method, we should call the change chaining method to trigger the onChange event of element.

    $('#selectCity').change(function() {
    $('#selectCity').val('HaNoi'); // No trigger
    $('#selectCity').val('HoChiMinh').change(); // Fire trigger
  13. #TIL : Tại sao biển xanh lại mặn ? :lol:


    Biển xanh lại mặn bởi vì đá ở mặt đất cọ xát với mưa trên trời (chứ không phải cá nó đái 😂 )

    Read more : https://oceanservice.noaa.gov/facts/whysalty.html

  14. #TIL : Tracking changes of cookie on webpage

    Using Object.defineProperty helper function as I wrote 3 days ago. We could track the changes of cookie on webpage.

    // Based on Vlad Shevchenko's script at https://stackoverflow.com/a/36826049
    var cookieSetterOrig = document.__lookupSetter__("cookie"); // get origin setter function
    var cookieGetterOrig = document.__lookupGetter__("cookie"); // get origin getter function
    Object.defineProperty(document, "cookie", {
        get: function () {
            return cookieGetterOrig.apply(document);
        set: function () {
            return cookieSetterOrig.apply(document, arguments);
        configurable: true

    Notice : This code only works if cookie is changed by javascript, not http header request !

  15. #TIL : Bypass CORS by using JSONP callback

    Sometimes you are blocked from request a cross-origin resource. Instead of adding our domain to allowed list of them, we can use another way to retrieve data from their API by using JSONP (in case they support it).

    The mechanism of JSONP is simple, instead of returning a JSON data. It will return a javascript text with passing your data into a function, whose name is declared in query string. So you just add a new script element with the URL and waiting the callback.

    Example :

    function callMeBaby(data) {
    var s = document.createElement("script");
    s.type = "text/javascript";
    s.src = "https://freegeoip.net/json/?callback=callMeBaby";

    or using jQuery (hide magic)

        url: "https://freegeoip.net/json/",
        jsonp: "callback",
        dataType: "jsonp",
        success: function( data ) {
            console.log( data );
  16. #TIL : Define property of an object in hacking way

    Sometimes, we want to define a property of an advanced object (has setter and getter function).

    Now, we could use this helper function Object.defineProperty to define property of an object in a cool way.

    Example :

    const foo = {};
    Object.defineProperty(a, 'bar', {
    	value: 'hogehoge',
    	writable: false,
    console.log(foo.bar); // 'hogehoge'
    foo.bar = 'foo bar'; // throw an error in strict mode
    console.log(foo.bar); // still be 'hogehoge'

    Modifying setter, getter function

    // Get callstack which function is getting or setting cookie value
    Object.defineProperty(document, 'cookie', {
    	get: function() {
    		console.log('get !');
    	set: function(val) {
    		console.log('set = ', val);
  17. #TIL : Debug js code using console.trace

    Browsers provide an useful function help you debug easier than using simple console.log function.

    That is console.trace, which prints a stack trace to called function.

    Example :

    function foo() {
    	var a = 1;
    function bar(x) {
  18. #TIL : Sleeping connections in MySQL

    When you check your MySQL process list via command show processlist;, it will show you a useful table which provide all current connection details.

    "Sleep" state connections are most connection pointer waiting for the timeout to terminate. Then they still count as a connection. (Can cause MySQL connection limit error, which default equal 150 connections)

    So next time, remember to close your connection before terminating your app.

    Every connection counts ;)

  19. #TIL : HSTS rule in browser

    HTTP Strict Transport Security (HSTS) is a web security policy mechanism which helps to protect websites against protocol downgrade attacks and cookie hijacking.

    Enabling HSTS on your web will make your browser validate every SSL issues more strictly :

    • User can not visit http version on browser
    • User can not add SSL exception for the domain to ignore the warning. (when SSL cert expire or invalid common name)

    Note : You can manually remove a domain from HSTS in Chrome by accessing this page URL chrome://net-internals/#hsts

    So remember to add HSTS to your website !

  20. #TIL : Create cross-platform downloading app URL

    You have a mobile app for both platforms iOS and Android, each platform has different download URL. But your user doesn't know which platform he using. Clicking wrong URL will lead to user bounce-rate.

    Solution is making only 1 URL to download your app, which can redirect to right place depends on using platform. So how we achieve this ??

    The key of problem is detecting user platform, which can be done by extracting the User-Agent header from http request.

    PlatformUser-Agent pattern
    iOScontains iPhone or iPad or iPod
    Androidcontains Android

    This is how I implement using Caddy web server, you can do same thing in Apache or NGINX

    app.yourcompany.com:443 {
        timeouts 1m
        redir 302 {
            if_op or
            if {>User-agent} has iPhone
            if {>User-agent} has iPod
            if {>User-agent} has iPad
    	/ [apple-store-url]
        redir 302 {
            if {>User-agent} has Android
    	/ [google-play-url]
        redir 302 {
            if {>User-agent} not_has iPhone
            if {>User-agent} not_has iPod
            if {>User-agent} not_has iPad
            if {>User-agent} not_has Android
            / [your-landing-page-which-user-visit-on-desktop-device]

    Then we get cool and easy to remember link, right ?

  21. #TIL : Using web proxy to bypass firewalls

    Someday, you will be blocked by a firewall while trying crawling or accessing some website. The reason is they block your IP address from accessing the server.

    One solution is using a web proxy (http proxy, socks4 or socks5) to bypass the firewall, by adding the middle-man server between you and target. It's a bit unsecured but you could use for https site only.

    Some HTTP Proxy supports https will stream TLS data from target to you (so don't worry about proxy server can read you data). Btw, it only knows which domain and IP address you're connecting.

    To find a free proxy from the internet, try this service : https://gimmeproxy.com/

    It provides a cool API to fetch new proxy from its database.

    Example this endpoint will return JSON response including proxy anonymous, supports HTTPS, from Japan and minimum speed more than 100KB


    In case you need more requests per day, try a subscription (cancelable and refundable). I tried last days, and really like their service (although I cancelled subscription b/c I don't need proxy anymore).

    Break the rules ! ;)

  22. #TIL : Fastly conflict detector script

    Last month, I built a CI solution for our project and adding a conflict detector to our build commands. This script runned so slow because it will check all application files (and our application codebase has many of css, js files).

    This was the script

    grep -rli --exclude=conflict_detector.sh --exclude-dir={.git,vendor,node_modules} "<<<<<<< HEAD" .
    if [ $? -eq 0 ]; then
        exit 1
        exit 0

    Today, I think why don't we just check recently updated files (in the latest commit) ??? Then I have this new script

    # New way :D
    CHANGED_FILES=$(git log --pretty=format: --name-only HEAD^..HEAD | sort | uniq)
    for f in $CHANGED_FILES
        if grep --exclude=conflict_detector.sh -q "<<<<<<< HEAD" $f
            exit 1
    exit 0

    conflict_detector.sh is the filename of script, we exclude it from check to make sure changing this file doesn't make it failed.

    You can use this approach to check linter, coding standard or run preprocessor ;)

    Result (in my context) :

    • Old script : 12 seconds
    • New script : ~ 50ms (200 times faster)

    be Automated, be Fast, but be Careful !

  23. #TIL : Getting your external IP

    We can get our external IP address by following ways :

    1. Call http request : curl http://wtfismyip.com/text or curl http://ifconfig.me/ip
    2. Lookup A record for hostname nslookup myip.opendns.com resolver1.opendns.com (this only works when you use resolver of OpenDNS)

    Bonus : curl https://v6.ident.me/ for IPv6

  24. #TIL : using git hooks to improve working flow

    We can improve our team workflow by defining some git hooks that trigger on specified events.
    You can read all events and their usecases here : https://www.digitalocean.com/community/tutorials/how-to-use-git-hooks-to-automate-development-and-deployment-tasks

    This is what I implemented to my today-i-learned repo. I used pre-commit to update Table of Contents in the README.md file, so every content in my repo will be updated on Github repo page.

    $ ln pre-commit .git/hooks/pre-commit

    pre-commit file :

    echo 'Running pre-commit hook' 
    python til_update_readme.py
    git add README.md

    So it will run a Python script that update new TOC and then add the file to git.

    Automation ! Automation ! AND .... Automation !!! 🤖

  25. #TIL : Reduce init time MySQL docker image

    Original MySQL docker image uses a script to generate ssl certificates for service. Sometime we don't really need it (connect via a docker network link or need a fast enough database service to build a automated test).

    We can reduce init time by removing the script from original Docker image

    FROM mysql:5.7
    # Remove mysql_ssl_rsa_setup to ignore setup SSL certs
    RUN rm -f /usr/bin/mysql_ssl_rsa_setup

    FAST as a FEATURE !!! 🚀

  26. #TIL : Using watch command to tracking changes in period time

    watch to a good command to run a command every N seconds.

    And like its name, means you can watch something, its output changes with flag -d

    It's a great tool to help you learn a new language without hitting compile and run everytime you save a file.

    $ watch -n 1 -d go run learn.go

    This command will compile and run learn.go every 1 second

    More flags :

    • -t : no title
    • -b : beep on non-zero exit code
    • -e : stop loop on error and exit on a keypress
    • -g : exit on change
    • -c : support colors
    • -h : you know ! ;)
  27. #TIL : Using netcat to wait a TCP service

    When doing a CI/CD testing, you would need to connect a external service (RDBMS, HTTP server or generic TCP server service). So you need waiting the service before running your test app.

    One way to do right waiting instead of sleep for a specified time is using netcat tool

    $ while ! echo -e '\x04' | nc [service_host] [service_port]; do sleep 1; done;


    • MySQL service on port 3306
    $ while ! echo -e '\x04' | nc 3306; do sleep 1; done;
    $ ./run_test.sh

    Explanation :

    echo -e '\x04' will send an EOT (End Of Transmission) to the TCP every second to check if it's ready !

  28. #TIL : Indexes on multiple columns

    Let's say you have an indexes on 2 columns (A, B) of the table (X). So this is three use cases happen :

    1. You query data based on both of 2 columns => Indexes will be considered
    2. You query data based on (A) => Indexes will be considered
    3. You query data based on (B) => Indexes will be ignored because database indexes your data by B-tree algo. So it can't search node via a B => If you want, just create another indexes on B column

    I said will be considered because it depends on your query and your data (query optimizer will decide it !)

  29. #TIL : Using netcat as tiny TCP debug tool

    You can use netcat or nc as a debugging TCP tool. It can be a TCP sender and receiver with a short session (auto close when connection is closed)

    Examples :

    Scan ports

    $ nc -zv 20-80

    Check redis status

    $ echo 'info' | nc 6379

    Retrieve http response

    $ printf "GET /xinchao HTTP/1.1\r\n\r\n" | nc 8000 | tee xinchao.txt

    Change to IPv6 : nc -6

    Want more ??

    $ nc -h
  30. #TIL : Simple HTTP server function helper

    I use python3 (3.4+) to create a bash function to help me start quickly a simple http server on specified port

    function server() {
      local port="${1:-8000}" # Getting port number
      google-chrome "$port" # Open URL in browser, could change to firefox --new-tab "$port"
      python3 -m http.server $port --bind
  31. #TIL : TIME command output meaning

    When you want to know how long does it take to run a process, just use time command as a prefix

    $ time my_program arg1 arg2
    real        0m0.003s
    user        0m0.000s
    sys         0m0.004s
    • real : wall clock time, mean time to start to finish your process
    • user : CPUs-time outside the kernel
    • sys : CPUs-time within the kernel

    real+sys result is total multi CPUs time (so if you have a multi core CPUs, it is often bigger than real)