Great slide deck by Uwe Friedsrichsen on resilient patterns to use when designing applications
HOWTO
Running distributed cron jobs using Kubernetes
Good blog post by the engineering team at Stripe on using Kubernetes to run a distributed cron scheduler
https://stripe.com/blog/operating-kubernetes
HOW TO : Configure nginx for WordPress permalinks
Over the last week, I moved this blog from a LAMP (Linux, Apache, MySQL, PHP) stack to LEMP (Linux, Nginx, MySQL, PHP) stack. Have a blog post in the works with all the gory details, but wanted to quick document a quirk in the WordPress + Nginx combination that broke permalinks on this site.
Permalinks are user friendly permanent static URLs for a blog post. So for example this particular blog post’ URL is
https://kudithipudi.org/2017/02/24/how-to-configure…press-permalinks/
instead of
https://kudithipudi.org/?p=1762
This works by default in Apache because WordPress puts in the required rewrite rules.
To get it work in Nginx, you have to add the following config in the Nginx site configuration
Under the / location context, add the following
try_files $uri $uri/ /index.php?$args;
This is essentially telling Nginx to try to display the URI as is, and if it fails that, pass the URI as an argument to index.php.
HOW TO : Configure maven to work with a proxy server
We ran into an issue at work recently, when trying to deploy a spring boot based micro service. The application included some libraries that were downloaded by the application using maven during runtime.
The environment that this application was running in had controlled access to the Internet. We configured the jvm parameters using the following parameters
-Dhttp.useProxy=true -Dhttp.proxyHost=PROXY_SERVER_NAME -Dhttp.proxyPort=xxxx
when running the application, we were getting failed dependency errors that stated something like this
Caused by: org.eclipse.aether.resolution.ArtifactResolutionException: Could not transfer artifact com.xxxxx.boot:xxxx -boot-autoconfigure:pom:0.0.1-SNAPSHOT from/to central (http://repo1.maven.org/maven2/): Connection refused at org.eclipse.aether.internal.impl.DefaultArtifactResolver.resolve(DefaultArtifactResolver.java:444) at org.eclipse.aether.internal.impl.DefaultArtifactResolver.resolveArtifacts(DefaultArtifactResolver.java:246) at org.eclipse.aether.internal.impl.DefaultArtifactResolver.resolveArtifact(DefaultArtifactResolver.java:223) at org.apache.maven.repository.internal.DefaultArtifactDescriptorReader.loadPom(DefaultArtifactDescriptorReader.java:334)
Lot of head scratching, because the server running the application had connection to the proxy server and the JVM parameters were accurate.
Finally, we figured out Maven doesn’t use the JVM startup parameters and had to have proxy access configured specifically. There are two ways to do this
- Add an environment variable called “MAVEN_OPTS” and pass the proxy server settings.. something like this
set MAVEN_OPTS= -Dhttp.proxyHost=PROXY_SERVER_NAME -Dhttp.proxyPort=xxxx
- Configure the proxy server settings in Maven’s settings.xml… something like this
<proxies> <proxy> <id>genproxy</id> <active>true</active> <protocol>http</protocol> <!--username>proxyuser</username--> <!--password>proxypass</password--> <host>myproxy.mycompany.com</host> <port>8080</port> <nonProxyHosts>*.mycompany.com|127.0.0.1</nonProxyHosts> </proxy> </proxies>
HOW TO : Query varnishlogs to show backend server
Command parameters for varnishlog to view the backend server that is processing the request. In this particular case, I wanted to see the request URL and backend server for any responses with HTTP code 401 (unauthorized access)
sudo varnishlog -i BackendOpen,BereqURL -q "BerespStatus == 401"
HOW TO : Check websockets using curl
Found this good article by Mark Huot regarding using curl to check websocket servers at http://www.thenerdary.net/post/24889968081/debugging-websockets-with-curl
curl -i -N -H "Connection: Upgrade" -H "Upgrade: websocket" -H "Host: echo.websocket.org" -H "Origin: http://www.websocket.org" http://echo.websocket.org
HOW TO : Capture ftp passwords using tcpdump
On a server that is running either the FTP client on server, you can capture the ftp password using tcpdump by
tcpdump -A port ftp
HOW TO : Search for multiple words in a string using grep
Just for my records
grep "WORD\|ANOTHER\|NEITHER" string
searches for either “WORD” or “ANOTHER” or “NEITHER” in string.
HOW TO : Search which package contains a filename
If you are using a Linux system that uses yum for package management (like Fedora, Centos, RHEL), you can use the following command to find out which package contains a file. This is useful when you want to figure out which package to install. For example, dig (DNS utility) doesn’t come pre-installed on the system. And running “sudo yum install dig” doesn’t do anything.
sudo yum whatprovides '*/dig'
This returns
Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile 32:bind-utils-9.8.2-0.47.rc1.el6.x86_64 : Utilities for querying DNS name servers Repo : base Matched from: Filename : /usr/bin/dig
breaking down the command options
whatprovides : Is used to find out which package provides some feature or file. Just use a specific name or a file-glob-syntax wildcards to list the packages available or installed that provide that feature or file.
HOW TO : count lines in windows command line
Say you are using netstat to checl all established network connections on a windows machine (confirmed to work on windows 7+ and windows server 2008+) and want to find out how many connections you have, you can use
netstat -an | find "ESTABLISHED" | find /v /c ""
breaking down the command string
netstat -an : Uses netstat command to display all connections and listening ports (-a) and displays them in numerical form instead of resolving DNS or using common names (-n)
| : piping (passing) output of one command to the next one
find “ESTABLISHED” : Uses find command to filter out to just lines that contain the string “ESTABLISHED”‘
find /c /v “” : exclude blank lines (/v “”) and count the number of remaining lines (/c)
If you wanted to something similar in linux, you can use
netstat -an | grep "ESTABLISHED" | wc -l