WSREP "Failed to open channel" solution

Error
ERROR WSREP "Failed to open channel 'my_mariadb_cluster'"

Detailed Error
WSREP: gcs/src/gcs.cpp:gcs_open():1458: Failed to open channel 'my_mariadb_cluster' at 'gcomm://10.10.10.10,10.10.10.11': -110 (Connection timed out)

Solution
On the first node: (e.g. node-1)
-> Stop mariadb service if it hangs
# systemctl stop mariadb.service

-> Make sure "safe_to_bootstrap" value is "1"
# cat /var/lib/mysql/grastate.dat
safe_to_bootstrap: 1

-> Run following command for starting cluster as new
# galera_new_cluster

-> mariadb service should be started automatically
# systemctl status mariadb.service

-> note wsrep_last_committed number by looking system table
# mysql -u root -p
> show status like 'wsrep_%';

-> note uuid number
# cat /var/lib/mysql/grastate.dat

On the second node: (e.g. node-2)
-> Stop mariadb service if it hangs
# systemctl stop mariadb.service

-> uuid and seqno (wsrep_last_committed) should be same with node-1
# vi /var/lib/mysql/grastate.dat
uuid: 0000
seqno: 100

-> start mariadb service
# systemctl start mariadb.service

How to find/follow postgresql server logs?

-> Check log level from config file
# cat /var/lib/pgsql/data/postgresql.conf
e.g.
log_min_messages= 'ERROR' (default value)
(options; DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, INFO, NOTICE, WARNING, ERROR, LOG, FATAL, and PANIC)

-> Determine log directory name (via postgres user)
$ psql
postgres=# show log_directory;
e.g. pg_log

-> Determine data directory name (via postgres user)
$ psql
postgres=# show data_directory;
e.g. /var/lib/pgsql/data

-> Determine log file name (via postgres user)
$ psql
postgres=# show log_filename;
e.g. postgresql-%a.log (%a will be "day" of the week)
e.g. postgresql-%a-%H.log (%H will be "hour" of the day)

-> Check log rotation age of the log files
postgres=# show log_rotation_age;
e.g. 1d (1d means one day)
* The database system generate log files daily
e.g. 1h (1h means one hour)
* The database system generate log files hourly

Eventually, the log path could be like this;
e.g. 
# ls /var/lib/pgsql/data/pg_log/postgresql-Mon.log
# ls /var/lib/pgsql/data/pg_log/postgresql-Mon-07.log

-> To follow the logs by tail
e.g.
# tail -100f /var/lib/pgsql/data/pg_log/postgresql-Mon.log
# tail -100f /var/lib/pgsql/data/pg_log/postgresql-Mon-07.log

Ref: https://www.postgresql.org/docs/9.5/static/runtime-config-logging.html

How to setup/configure/use mailx for Office365 account?

-> Update the OS
# yum -y update
or (y -> yes)
# yum update

-> Install mailx
# yum install -y mailx

-> Define soft link to use "email" the command
# ln -s /bin/mailx /bin/email

-> Prepare config file
# vi /etc/mail.rc
set smtp=outlook.office365.com
set smtp-auth-user=alper@alper.com
set smtp-auth-password=alper123
set ssl-verify=ignore
set nss-config-dir=/etc/pki/nssdb/
set smtp-use-starttls
set from="alper@alper.com(Alper Ozsoyler)"

-> Try to send an email to yourself
# echo "Your message is my message!" | email -v -s "Message Subject is Bla Bla" alper@alper.com

Ref: https://gist.github.com/ilkereroglu/aa6c868153d1c5d57cd8

How to solve "restarting/pending pod repeatedly" problem?

-> Check the pods status
e.g.
# kubectl get pods --all-namespaces
NAMESPACE     NAME                            READY     STATUS
kube-system   coredns-78fcdf6894-6ggkk        0/1       Pending
kube-system   coredns-78fcdf6894-tm52r        0/1       Pending 

-> Check the deployments
# kubectl get deployment --all-namespaces
NAMESPACE     NAME      DESIRED   CURRENT   UP-TO-DATE   AVAILABLE
kube-system   coredns   2         2         2            0

-> Needs to remove the deployments related to pod and service
e.g.
# kubectl delete deployment coredns -n kube-system
deployment.extensions "coredns" deleted

How to enable Hyper-V in Windows 10?

Press Windows key -> Type "Turn Windows features on or off" then click it -> Select "Hyper-V" check box then click "OK" -> Restart PC -> Done!

How to install/configure/enable NTP service?

-> Install ntp service
# yum install ntp

-> Once NTP server pool zones are taking
e.g. check from https://www.pool.ntp.org/zone/europe

-> Add specific pool zones into config file if they are not existed
# vi /etc/ntp.conf
server 0.europe.pool.ntp.org

-> Enable service
# systemctl enable ntpd

-> Run service
# systemctl start ntpd 

-> Lastly, check the service and the settings

# timedatectl 

How to check linux operating system info?

-> show kernel version and other info
# uname -a

-> show machine info
# hostnamectl

-> show specific release version
# rpm --query centos-release 

How to check available yum packages?

e.g. install/update ansible by yum

-> Go to repo path
# cd /etc/yum.repos.d/

-> Add source paths into repo file to be able to install ansible
# vi epel.repo
[epel]
name=Extra Packages for Enterprise Linux 7 - $basearch
#baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch
metalink=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch
failovermethod=priority
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7

[epel-debuginfo]
name=Extra Packages for Enterprise Linux 7 - $basearch - Debug
#baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch/debug
metalink=https://mirrors.fedoraproject.org/metalink?repo=epel-debug-7&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1

[epel-source]
name=Extra Packages for Enterprise Linux 7 - $basearch - Source
#baseurl=http://download.fedoraproject.org/pub/epel/7/SRPMS
metalink=https://mirrors.fedoraproject.org/metalink?repo=epel-source-7&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1

-> Search yum package
# yum search ansible

-> Check available yum package
# yum list available | grep ansible

-> Check available all yum packages with its duplicates
# yum list available --showduplicates | grep ansible

-> Install ansible 
# yum install ansible

-> Download only yum package
# yum install ansible --downloadonly --downloaddir=/root

-> Check available yum packages for upgrade
# yum check-updates | grep ansible

-> Update ansible
# yum update ansible

-> Check yum history
# yum history list

How to install/configure/run ansible?

-> Set hostname
# hostnamectl set-hostname ansible_host

-> Add remote hosts into /etc/hosts file
# vi /etc/hosts
10.10.10.2 remote_host_2
10.10.10.3 remote_host_3

-> Generate ssh key without passphase
# ssh-keygen

-> Install ssh key into remote servers
# ssh-copy-id remote_host_2
# ssh-copy-id remote_host_3

-> Install ansible rpm packages
# yum install ansible

-> Add remote hosts as nodes into /etc/ansible/hosts file
# vi /etc/ansible/hosts
[remote_hosts]
remote_host_2
remote_host_3

-> Prepare an ansible playbook in /etc/ansible/
# vi my_very_first_playbook.yml
# example playbook
- name: Run the role for me 
  hosts: remote_hosts
  roles:
  - example-role

-> Prepare a role for the playbook in /etc/ansible/roles/example-role/
# vi main.yml
- name: checking Command Scheduler service
  service: 
   name=crond.service
   state=started

-> Run the playbook in /etc/ansible/
# ansible-playbook my_very_first_playbook.yml
or
# ansible-playbook -i hosts my_very_first_playbook.yml
(i -> inventory, its default location is /etc/ansible/hosts)

-> To check the ansible logs
# tail -f /var/log/ansible.log

usage of "run_once: true" option

If you want to run only once a specific task during all playbook execution, you may add "run_once: true" option into your task to do this.

e.g. example task
- name: prompt for directory name
  run_once: true
  pause:
   prompt: "Please enter directory name: "
  register: input_dir_name

- name: show input
  debug: 
   msg="{{ input_dir_name.user_input }}"

How to know last login info on the system?

-> show all records
$ last

-> show all records page by page
$ last | more

-> show first 10 records
$ last | head

-> show first 100 records
$ last | head -100

-> show last 10 records
$ last | tail -10

-> show last 100 records
$ last | tail -100

How to change service name?

e.g.
current service name: postgresql.service
desired service name: postgresql-9.1.service

Run following commands to change service name;

Step 1: Stop current service
# systemctl stop postgresql.service
# systemctl daemon-reload

Step 2: Change service name
# cd /usr/lib/systemd/system
# mv postgresql.service postgresql-9.1.service

Step 3: Start and enable new service
# systemctl start postgresql-9.1.service
# systemctl enable postgresql-9.1.service

Step 4: Check service status:
# systemctl status postgresql-9.1.service

Step 5: Make sure that service runs properly
# journalctl -u postgresql-9.1.service

How to solve remote connection error cause of iptables/firewalld service?

Case
Machine A(192.168.1.2) wants to connect on Machine B(192.168.1.5) by database port.

Action
In Machine A;
# su - postgres
$ psql -h 192.168.1.5 -d alper -U alper -p 5433

Error
psql: could not connect to server: No route to host
        Is the server running on host "192.168.1.5" and accepting
        TCP/IP connections on port 5433?

Solution
In Machine B;
For iptables service:
# iptables -I INPUT -p tcp --dport 5433 -j ACCEPT
# iptables-save

For firewalld service:
# firewall-cmd --zone=public --add-port=5433/tcp --permanent

# firewall-cmd --reload

Check
In Machine A;

# nc -vz 192.168.1.5 5433

Ncat: Connected to 192.168.1.5:5433

How to solve database module error?

Action
# ansible-playbook -i myhosts example.yml -v

Error
TASK [example role : example task] *******************
fatal: [local]: FAILED! => {"changed": false, "msg": "The MySQL-python module is required."} 

Solution
# yum install MySQL-python

The other solutions related to database tasks:
# yum install python-redis (Redisdb)
# yum install python-psycopg2 (PostgreSQL)

Check it for up to date list of database modules:
https://docs.ansible.com/ansible/latest/modules/list_of_database_modules.html

How to prevent sleep mode on laptop when lid closes?

Press Window Key then type "Power Options" and click on it -> Click "Choose what closing the lid does" -> Set "When I close the lid" to "Do nothing" -> Click "Save changes" -> Done!

How to run a SQL query/statement in a task?

You are able to run a SQL query/statement with "echo" command by shell module for the ansible task.

# an example task for sql query/statement
- name: set default schema for user
  shell: echo "ALTER ROLE alper SET SEARCH_PATH TO myschema" | psql
  become: yes
  become_user: postgres

benefit of "gather_facts: no" setting

If you do not need any facts for your variables, you can run your playbook more fast via gather_facts setting.

For this, just add following setting in your playbook:

# example playbook
- name: Run the role for me 
  hosts: my_nodes
  gather_facts: no
  roles:
  - example-role

"database "example" is being accessed by other users" solution

Action
postgres=# drop database example;

Error
ERROR:  database "example" is being accessed by other users
DETAIL:  There is 1 other session using the database.

Solution
Run following sql code;

postgres=# 
SELECT pg_terminate_backend(pg_stat_activity.pid) FROM pg_stat_activity WHERE pg_stat_activity.datname = 'example' AND pid <> pg_backend_pid();

All sessions should be terminated where belong to "example" database except backend sessions.

Lastly, try to drop it again.

How to backup database in XE?

It is very simple to backup your database in Oracle XE with already prepared .bat file quickly.

Steps;
Press Windows Key -> Type "Backup Database" -> Click "Backup Database" link -> .bat file "Backup.bat" runs -> Follow the steps -> Done

Finally, check the backup path;
...\oracle\fast_recovery_area\XE

e.g. truncate table cascade

Preparation of Test Data
test=# 
create table employees (id integer, name varchar, department_id integer);

test=# 
create table departments (id int, department_name varchar);

test=# 
ALTER TABLE DEPARTMENTS
ADD CONSTRAINT DEPT_PK 
PRIMARY KEY (ID);

test=# 
ALTER TABLE EMPLOYEES
ADD CONSTRAINT EMP_DEPT_FK 
FOREIGN KEY (DEPARTMENT_ID)
REFERENCES DEPARTMENTS(ID);

test=# 
insert into departments
values (1,'IT');

test=# 
insert into departments
values (2,'Sales');

test=# 
insert into departments
values (3,'Marketing');

test=# 
insert into employees
values (1,'John',1);

test=# 
insert into employees
values (2,'Alex',1);

test=# 
insert into employees
values (3,'Guru',2);

test=# 
insert into employees
values (4,'Maximus',3);

test=# 
commit;

Data Check
-- select all rows of parent table 
test=# 
select * from departments;

-- select all rows of child table
test=# 
select * from employees;

Action
-- by using truncate table cascade, parent table and child table data will be removed. 
test=# 
truncate table departments cascade;

Last Data Check
-- select all rows of parent table 
test=# 
select * from departments;

-- select all rows of child table
test=# 
select * from employees;

Info: In Oracle 12c, you are able to use "cascade" clause with "truncate table" statement as well.