Postgresql common configuration files are the postgresql.conf and pg_hba.conf. In many cases, its possible to edit without the need to restart the database. Here are the 2 options on Centos Linux;
# systemctl reload postgresql-11
or
# systemctl reload postgresql
Login and and access psql as admin or user postgres then run this sql.
SELECT pg_reload_conf();
Postgresql database provides 2 main configurations to limit the incoming connections. This is done in the file postgresql.conf
Following are common sql commands to monitor connections.
There are many cases where remote git repositories provide a central location to store files such as source codes. The command 'clone' is used to retrieve a remote git repository. Depends on how a user is provided access to the server, by http, ssh or other methods.
Here is how its done on Centos Linux, where it defaults to create a folder which is the same name as the repository.
cd /var/www
git clone ssh://nicholas@remoteserver:22/repo/tboxmy.git
Another approach is to create in our empty folder myproject
git clone ssh://nicholas@remoteserver:22/repo/tboxmy.git myproject
How to add user to the remote
git remote add newuser ssh://nicholas@remoteserver:22/repo/tboxmy.git
View remote users
git remote -v
Postgresql database version 11 uses write ahead log (WAL) to ensure data integrity. It contains a log of all activity within the database, and helps with recovery of data. WAL records are saved to a WAL file. The insert position is a log sequence number (LSN) that is a byte offset into the logs, increasing with each new record.
Here, I am provided a Postgresql database that have been configured as a MASTER database. A user has been created on the Linux server and a role within the Postgresql called "replication". Now a new linux server have been provided for purpose of running a standby node, this has the same version of Postgresql database as in the MASTER server.
For simplicity the IP of MASTER will be 10.1.1.100, and for STANDBY will be 10.1.1.101.
Following information is provided by the MASTER slave;
Linux username: replication
with password: password
Replication standby server is assigned name: replication
Folder of the installation: /var/lib/pgsql/11/data
Within MASTER, the STANDBY server IP has been added to its pg_hba.conf.
Lets configure the STANBY server
sudo su postgres
(enter your password)
cd
cd 11
mv data data.original
We will use the option -R to generate the default recovery.conf file.
pg_basebackup -h 10.1.1.100 -U replication -D /var/lib/pgsql/11/data -P --password --slot replication -R
chmod go-rw data
cd data
Ensure firewalld allows the database access.
Review contents of recovery.conf, standby_mode should be 'on'.
sudo systemctl start postgresql-11
In normal case of a query to Postgresql database, it would take a very small amount of time to be processed. However, when query takes a long time to process then it will be a problem as resources such as connection, CPU and RAM will be locked to process that query.
Problem queries may lead to more queries in waiting until the database maximum connection is reached. Default Postgresql is set to maximum 100 connections. This will cause users to fail when trying to login to the application or trying to access the database. These 2 are common examples of when you will need to kill queries.
Lets see break this to 2 parts.
Part 1, list max connection, and the current settings. Part 2, find the problem query, then to end it.
Login to postgresql terminal as admin (postgres) and use the commands.
SELECT count(*) used FROM pg_stat_activity;
Compare values, to determine if this is an issue.
SELECT datid, pid, usename, client_addr, backend_start FROM pg_stat_activity WHERE state = 'active';
SELECT pg_cancel_backend(PID);
Where PID is the value identified in Step 1 above.
How to access to Postgresql where only reading of data is required?
Lets create a new user called "reader" and password as "password" who will be given access to the database mydatabase with READ only access. The following sign denotes the sql prompt, which you do not need to type.
=#
The steps listed here are rather generic on PostgreSQL 11 onwards. There are other considerations for security and more complex database designs that you should look into in the future.
Connect to the database as administrator.
psql -U postgres -H localhost
=# \c mydatabase
Create the new user
=# CREATE USER reader WITH encrypted PASSWORD 'password';
=# GRANT CONNECT ON DATABASE mydatabase TO reader;
=# GRANT SELECT ON ALL TABLES IN SCHEMA public TO reader;
Determine the user exist with the following command
=# \du
Verify this by login as that user. Additionally, the user access can be shown through the commands.
=# \c mydatabase
=# SELECT table_catalog, table_name from information_schema.role_table_grants WHERE grantee = "reader";
=# SELECT table_catalog, table_schema, table_name from information_schema.table_privileges WHERE grantee = "reader";
In the case where that user is no longer required, it is only prudent to remove that user from access to the database. It is common to maintain that user within the system, in case they need to assign the user to another database access.
Adding user to an existing READ only role would allow faster management of users.
=# CREATE ROLE reader
=# CREATE USER dave IN ROLE reader
Document your steps for this intermediate exercise.
Create a new user with login access name "dave" and password "exercise".
Fill this table "users" with 2 rows of data.
Assign user "dave" to be able to connect to this database and read contents of any table in this database.
Connect as user "dave" and verify if the user can only read data, without being able to update its data.
Its what is known as a runtime environment, where programmes written in JavaScript can run.
Python in a programming language.
Its a node package manager, which is an application and repository for developing and sharing JavaScript code. Many developers write applications and allow the NPM access to their programmes.
Its a package manager for windows that runs in Windows PowerShell. Which means, when installing a programme, and there are other dependencies, it will go search and download for you.
Download the executable for your version of Windows, at https://nodejs.org/en/download/
Example, download and Save the Windows Installaer (.msi) for 64-bit.
Run the installer. Make sure to CHECK box to Automatically install the necessary tools. This will add to path the Node.js and npm programmes.
At end of installation, a Windows prompt will appear. This runs several scripts, press "Space" and follow the instructions which will lead you to the Windows PowerShell prompt.
Open a command prompt and type the following;
npm -version
node -v
My output was
8.1.0
v16.13.0
Lets use an available helloworld package. At the windows terminal;
npm init hello-world helloworld
cd helloworld
npm run start
Use a web browser with the URL that is displayed at the terminal to see the results. In my case, the URL is http://localhost:3000
The Windows PowerShell waits for a long time with the last message, and task manager show lots of memory+disk activity+Ethernet.
Created a UnelevatedInstallerTelemetryDecorator
Could it be a slow PC with slow network connectivity? If there are no errors in windows logs, just wait until its done processing and the text appears. My PC took almost 30 mins.
Press Enter to exit.
On Linux Centos 8, with Apache Tomcat that would start but almost immediately fail. There is 4 Gb of Ram assigned, and without any load, the service kept crashing. It was really strange.
The Java installed was Openjdk version "1.8.0_312", and Apache Tomcat version 9.0.46.
Each attempt to start Tomcat, it fails then from the logs:
How can this be? There is 4Gb assigned to the server, with lots of free memory. As can be seen in the configuration, the maximum memory of 1024Mb is assigned to Tomcat. The configuration file is <TOMCAT>/bin/setenv.sh
export CATALINA_OPTS="-server -Xms1024M -Xmx1024M"
JAVA_OPTS="$JAVA_OPTS -XX:+UnlockExperimentalVMOptions -XX:+UseTLAB -XX:+UseG1GC -XX:G1MaxNewSizePercent=30 -XX:InitiatingHeapOccupancyPercent=40 -XX:G1NewSizePercent=25 -XX:ConcGCThreads=2 -XX:ParallelGCThreads=2 -Dfile.encoding=UTF8 -XX:+PrintGCTimeStamps -XX:+PrintGCDetails -Xloggc:tomcat_gc.log"
export JAVA_OPTS
Here is the result of command free, hardly any memory is in use (less than 80% for sure) when tomcat is NOT running
An explanation of this setting
vm.overcommit_memory=0 setting means that the allocated virtual memory is unlimited. Having unlimited virtual memory is important for database servers, which retain extra unused virtual memory allocations for dynamic memory management. Unreferenced allocated memory is not backed by RAM or paging space on Linux systems.
What worked for me is to set vm.overcommit_memory=1 and ensure Tomcat starts with memory size to use the free space. This is done by updating the variable vm.overcommit_memory to 1, then in tomcat configuration as mentioned earlier, set the CATALINA_OPTS="-server -Xms1024M -Xmx1024M". Once it was working, I gradually increase the memory allocation.
Restart the Tomcat application and watch Tomcat running smoothly. If all works, then this can be applied permanently by editing the file in /etc/sysctl.d
Now,
When Tomcat is running with 1024Mb memory assigned.
When Tomcat is running with 2048Mb memory assigned.
Creating a Vue component.
This is a continuation of Tutorial from Part 1.
Part 1:
Create the file src/components/Products.vue and add the contents
<template>
<div class="products">
<h1>{{ msg }}</h1>
</div>
</template>
<script>
export default {
name: 'Products',
props: {
msg: String
}
}
</script>
Update the App.vue template with data and methods to be used in the component.
Replace <HelloWorld msg="
with
<Products msg="
Replace import HelloWorld from './components/HelloWorld.vue'
with
import Products from './components/Products.vue'
Apply changes and run
Add data and for loop
Edit <SCRIPT> content in src/components/Products.vue
export default {
name: 'Products',
props: {
msg: String,
products: Array,
}
}
Edit <TEMPLATE> content in src/components/Products.vue
<div class="products">
<h1>{{ msg }}</h1>
<table border=1>
<thead>
<tr>
<th>Name</th>
<th>Description</th>
<th>Cost</th>
</tr>
</thead>
<tbody>
<tr v-for="product in products" :key="product.id">
<td>{{ product.name }}</td>
<td>{{ product.description }}</td>
<td>{{ product.cost }}</td>
</tr>
</tbody>
</table>
</div>
Edit conponents data in src/App.vue
data(){
return {
message: 'My Products',
products: [
{
id: 1,
name: "Leather shield",
description: "Tough leather",
cost: 120 },
{ id:2,
name: "Heavy Leather shield",
description: "Double leather layers",
cost: 150 },
],
}
Compile and run
npm install --save @popperjs/core bootstrap
Edit the file src/main.js to import the installed bootstrap.
import "bootstrap";
import "bootstrap/dist/css/bootstrap.min.css";
Lets use the example code from bootstrap. Create a new file src/components/Navigation.vue
Next, edit src/App.vue can replave the DIV with ID=app to the following
<div id="app" class="container-fluid">
<NavbarRow />
Within the tag <script>, add this new component
import NavbarRow from './components/Navigation.vue'
Add this to the components list
components: {
Products, NavbarRow,
},
Compile and run.
1. Create a new component called Footer.vue and like its name, setup within App.vue to display your name and email as the page footer.
2. Describe how each of the modules are loaded through a drawing.
3. Edit the menu item "Link" in the navigation bar to load a website of your preference.
Following is an introduction tutorial to building a Vue application. It works on MS Windows 10 and Centos Linux 7/8.
Part 1:
Nodejs version 12.x
NPM version 6.14.x
Most of the commands will be at the command line,
npm install -g @vue/cli
vue --version
@vue/cli 4.5.15
vue create helloworld
Select Vue 3, using arrow keys.
cd helloworld
npm run serve
Test the new site by opening a web browser and pointing to URL http://localhost:8080/
Press Ctrl-c to stop the server
Following are basic files and folders of interest;
src/main.js : root Vue instance is declared and configured.
src/App.vue : contains root Vue components. Notice the page title is defined here in the value "msg".
public/index.html : Rendering page of target "app" or should Vue instance fail.
src/components/HelloWorld.vue : Codes for the initial page that root Vue has declared.
This concludes creating a default Vue project. In each of the exercise below, view the results with
npm run serve
Replace the page welcome message with your own in the file src/App.vue
In Vue, there are short hand notations. One of this is ":msg" which is shorthand for our variable msg in "v-bind:msg".
In the file src/App.vue edit the components
components: {
HelloWorld
},
data(){
return {
message: 'This is my custom title!'
}
}
Replace HelloWorld place holder with
<HelloWorld :msg="message"/>
Edit file src/App.vue, add after the line <HelloWorld .....
<input type="text" v-model="message" />
<button @click="alertMessage">Ok</button>
Add another component methods, for the function alertMessage to be called
data(){
return {
message: 'Hello World!'
}
},
methods: {
alertMessage(){
alert(this.message)
}
}
Basics on Kerberos on Centos Linux 8.
A user or service is defined as a principal. E.g. a user dev, a service postgres and postgres/TBOX.SITE
An instance is used to manage principals.
E.g. root/admin, where
root = principal
/admin = instance
A Key Distribution Center (KDC) contains database of all principals and
sudo kadmin.local
listprincs
ImageMagick provide tools on Linux to manipulate images such as those of the JPEG, TIFF, PNM, GIF and Photo CD image formats. Default installation includes special effects that can be applied to the images.
Using other Linux scripts and commands, there are many uses to the tool provided by ImageMagick.
sudo yum install ImageMagick
Or
sudo dnf install ImageMagick
convert original_apple.jpg -resize 1024 new_apple.jpg
find directory_name -type f -size +2M -exec convert {} -resize 2048 {} \;
Change image compression (value between 1 and 100)
convert original_apple.jpg -quality 95 new_apple.jpg
convert original_apple.jpg -resize x200 new_apple.jpg
convert original_apple.jpg -rotate 180 new_apple.jpg
convert original_apple.png original_apple.jpg
This will remove any transparency in PNG.
Apply an image effect.
convert original_apple.png -swirl 45 original_apple.jpg
Following are effects available:
-annotate geometry text
-charcoal radius
-enhance
-implode amount
-monochrome
-sketch geometry
-swirl degrees
-transpose
-transverse
The benefits of GIT is being able to work in different branches from the master codebase. Commonly a new feature being developed would be published to a new branch for review before being merged to the master branch. Once it is merged to the master branch, the new branch would be removed from the remote site.
Before merge to the master branch, it is possible to compare between the 2 branches. This will allow a check on files changes and where are the code changes. Here, lets see how to retrieve the new branch from remote, then compare changes made. The git command format in general would be;
git option <arguments>
Assumptions:
git log
git log --oneline
This might not be part of the open source, but its just a thing to blog.
Recently a windows 10 Home, login screen after successfully entering the PIN popup the message "your organization requires you to change your password" and forced users to change the pin before proceeding. Those who were not aware, changed it, but did not remember this new PIN the next time they needed to login.
I am trying out this method, but can't say it works as I have not done more test. Its just for my record
In Windows 10, press Win+r and type regedit
Scroll through Registry Editor, explorer like navigation as follow
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Policies\PassportForWork
Look for the item within that sub-folder named Policies\PINComplexity where it contain several value. Change the value named "Expiration". Double click and choose decimal. In Value data put in the number of days before the PIN expires.
I can't say whats the correct value to disable, but for now I am using the Value data=600 (in decimal). Expecting the PIN to last 600 days. Maybe Windows 11 would be out by then.
Virtualisation provide a more efficient use of memory, disk and all CPU on a server. Separate servers can be managed within a single server. Centos 8 provide virtual machine (VM) management through KVM (kernel virtual machine).
Each VM is given a domain name. Common practice before upgrading a server operating system, is to create a backup the current VM, or as its known here, a snapshot. Default format in use is qcow2, where tools for snapshot will work. Through Virtual Machine Manager and Cockpit, virtual machines can be created and managed.
Let's work with an existing VM named "Centos7_php". This virtual machine is installed with the operating system Centos Linux 8 and all the other packages required as a PHP development server. This requires the VM to be shutdown (and there are no snapshots)
Syntax:
virsh list
In Centos 7 and earlier, SysV manage Linux services. To list such services,
config --list
Centos 7 and 8 uses Systemd to manage Linux services. The syntax for this command is
systemctl [OPTIONS...] COMMAND [UNIT...]
Centos 7 was a transition phase towards the newer Systemd, and it maintained a mix of service types.
Here are a list of common command to check Linux services.
List active systemd services
systemctl
List all systemd services
systemctl -a
List only SysV services (centos 7 and older)
systemctl list-unit-files
List by service types
systemctl list-units --type service
systemctl list-units --type mount
Where
LOAD = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB = The low-level unit activation state, values depend on unit type.
Check status of a service
systemctl status sshd.service
sudo systemctl status nfs-server.service
To stop or start or restart a service
sudo systemctl stop nfs-server.service
sudo systemctl start nfs-server.service
sudo systemctl restart nfs-server.service
To determine service is active or enabled the following will return 0 (active) and non zero (disabled)
systemctl is-active sshd
systemctl is-enabled sshd
systemctl is-active nfs-server
systemctl is-enabled nfs-server
To list a service and its dependencies;
systemctl list-dependencies nfs-server
List services and their number of running task, CPU percentage, memory usage and I/O continuously
systemd-cgtop
List services as above once
systemd-cgtop -n 1
To sort, use the following based on the column titles
systemd-cgtop -t
systemd-cgtop -c
systemd-cgtop -m
systemd-cgtop -i
List services and their open ports
netstat -a
List services that use TCP and UDP and their open ports
netstat -tulpn
Display which user is used for a filename
fuser . -v
Display running service
top
Display running services in pretty formatting and enable F1-F10 commands.
htop
One of the most common operations in Laravel involves the index.blade.php file. It displays a list of all the contents that an object retrieves from the database. In situations where there are thousand of rows of data, the view would best display it across many pages.
In Laravel, Pagination by default is compatible with Tailwind CSS framework and Bootstrap 4. When the number of rows are huge, its essential that the database column where the "whereby" clause is used, is index.
Lets see 2 methods to apply pagination in a Laravel project using Bootstrap 4.
Edit file App\Providers\AppServiceProvider.php
Lets consider ArticleController that retrieves Article objects from the database through the function index(). This will display all the articles through a View called index.blade.php. For simplicity, only a total of 3 items will be displayed per page.
Users will be able to view pagination with page numbers.
Edit app\Http\Controllers\ArticleController.php
Edit resources\views\articles\index.blade.php and below the displayed date listing add the pagination.
Users will be able to view pagination with only the previous, and next navigation.
Edit app\Http\Controllers\ArticleController.php
Edit resources\views\articles\index.blade.php and below the displayed date listing add the pagination.
Additional functions available to use with Cursor
Here are the basic steps to backup and restore Postgresql database. At this point of writing, I am using Postgresql 10 and 11 on Centos Linux 8.
In this article, backup database is inventory_system and restore of the backup is to another database that I will call cupboard.
Step 1: Login as postgres user
su - postgres
or
sudo su postgres -
Step 2: Dump the database content to a file using the tool pg_dump
pg_dump inventory_system > inventory_system-20210312.bak
or when database is located at IP 10.1.1.123 with port 5432
pg_dump -h 10.1.1.123 -p 5432 inventory_system > inventory_system-20210312.bak
Step 3: Create the new database
createdb cupboard
Step 4: Restore the database file with psql
psql cupboard < inventory_system-20210312.bak
On MS Windows 10 prompt that is provided through Laragon, the postgres user login is required
pg_dump -U postgres cm2020_1 > test.bak
Cron job is suitable to automate the backup process mentioned above. To schedule a cron job, use the command
crontab -e
Then save the following instruction to run at midnight of every Saturday
0 0 * * 6 pg_dump -U postgres inventory_system > ~/postgres/backups/inventory_system-20210312.bak
Save and exit from the text editor.
Refer to Cron activities in the file /var/log/cron
For automated backup, there is a good note with the scripts at Postgresql wiki.
Having successfully installed a new Raspberry Pi with Raspbian OS, it is common that the next reboot the screen display just leaves a blank screen. You just know its the display setting run when at the boot screen the system messages can be seen.
Firstly, the most important file you must know is the /boot/config.txt
This contains many configuration, that require the file to be save and RPi to be restarted.
If your display is working, but the resolution is wrong, then access the start menu And look for Configuration, or directly access the resolution setting, type
raspi-config
When the display is a touch screen, it would be a good idea to install a virtual keyboard. Do this by
sudo apt install florence
Back to troubleshooting a display that's not showing. Always check your display is powered, and that system messages appear at boot before proceeding here.
Ensure RPi is connected to your local network, then SSH to your RPi. On MS Windows there is a software named Putty.
Determine your device supported resolutions. Use either of following commands;
tvservice -m CEA
tvservice -m DMT
Depending on whether you are connected to a TV (CEA) or computer monitor (DMT). Generally, a TV with VGA cable will use CEA.
Edit the file /boot/config.txt
Use the settings identified in Step 2. If you are using CEA then the value of hdmi_group=1,
if its DMT then hdmi_group=2.
Eg 1. when connected to a monitor with resolution 1280×720 and frequency 60Hz, these are the settings;
hdmi_safe=1
hdmi_group=2
hdmi_mode=85
Eg 2. when connected to a TV through HDMI with resolution 640x480 and frequency 60Hz. Since the TV have a standard HDMI connector, hdmi_safe will be commented out. These are the settings;
#hdmi_safe=1
hdmi_group=2
hdmi_mode=4
Reference: Raspberry site
