A Technical I.T./DevOps Blog
Problem scenario
You have a MySQL database. Rather than use a SQL front-end application, you want to write a Java program that will run a SQL statement against the database. What should you do?
Solution
1. Install the Java compiler if it is not already installed. If you need assistance, see this posting.
2. Run this command:
curl -Lk https://www.javatpoint.com/src/jdbc/mysql-connector.jar >
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Continue reading “How Do You Run a Java Program to Run SQL Commands Against a MySQL Database?”
Problem scenario
You have configured a Docker container with a web service (e.g., Apache web server or Nginx). You configured the listening, external port to be 80 or a different port number. You find the web server (either Apache or Nginx) is not working from a web browser. How do you find what is wrong?
Solution
#1 Use nmap to test the port and IP address. If you are using Windows,
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Problem scenario
You cannot ping a server, but your nmap results show that port 22 on the server is open. You can reach the server with nmap, but not with ping. What is wrong?
Possible solution
Is the server in the AWS Security Group that you think it is? Go to the AWS console and verify the security group for the server is what you think is correct.
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Continue reading “Why Cannot You Ping a Server when Nmap Commands to The Server Work?”
Problem scenario
You want to create a Lambda function. What do you do?
Solution
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Problem scenario
You like adding logging messages manually in Linux/Unix. You do it from the command prompt with “# write a cool message here”. You review the messages with a “history” command. You also append messages to log files in /var/log/foorbar.d with ‘echo “note this” ‘. How do you introduce your own messages into Windows event log?
Solution
Run a command like this but change “Application” to “Security” or whichever log category you want:
Write-EventLog -LogName Application -EventId 3 -Message “Continual Integration helps!” -Source “Windows Error Reporting”
# Note that “Windows Error Reporting” is just one of several valid sources. …
Continue reading “How Do You Quickly Add a New Event to the Windows Logs Using PowerShell?”
Problem scenario
You want to change the main configuration file for Apache web server (e.g., httpd.conf). But you do not want to drop current web surfers viewing the website. How do you change a security module or listening port for an Apache web server and have the changes take effect without interrupting the service by doing a regular restart?
Solution
Use the “graceful” directive.
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Problem scenario
You are trying to install open source Accumulo on Linux. You have two GB of swap space. You have installed Java, Hadoop, and Zookeeper. You have run the bootstrap_config.sh script for Accumulo 1.9.2.
You run this (and expected it to work): /bin/accumulo-1.9.2/bin/accumulo init
But you get this error:
OpenJDK 64-Bit Server VM warning: If the number of processors is expected to increase from one, …
Continue reading “How Do You Troubleshoot the Installation of Apache Accumulo on Linux?”
Problem scenario
You want to set up Zookeeper with three nodes in AWS. What do you do?
Solution
1. Install Zookeeper on each of the servers. If you need assistance with this, see this posting.
2. Modify the zoo.cfg file on each of the servers. Add stanzas like these but substitute foobarX.amazonaws.com with the Public DNS name of each server:
server.1=foobar1.amazonaws.com:2888:3888
server.2=foobar2.amazonaws.com:2888:3888
server.3=foobar3.amazonaws.com:2888:3888
initLimit=5
syncLimit=5
3.
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Continue reading “How Do You Set Up a Multi-Node Cluster of Zookeeper?”
Problem scenario
You want to create a database in MongoDB using the CLI. How do you do this from a Linux server and then add a document to the database you just created?
Solution
Prerequisite
Install MongoDB on a Linux server; if you need assistance, see this posting.
Procedures
1.a. Enter the MongoDB shell: /usr/bin/mongo
1.b.
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Continue reading “How Do You Create a Database in MongoDB Not Using using Compass (the GUI)?”
Problem scenario
You have heard of A records and PTR records. But you do not know the difference. What is the difference?
Solution
The short answer is that they are opposites of each other in the context of DNS.
An A record maps a domain name to an IP address (pg 858 of A Practical Guide to Fedora and Red Hat Enterprise Linux).
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Continue reading “What Is The Difference between an A Record and a PTR Record in DNS?”