Category: Documentation

How to measure the sequential write/read speed of a Hard Disk or SSD?

Question: How to measure the sequential write speed of a hard disk or SSD?

Answer

We may use two ways. The most popular one is to use dd tools. For hard disks, we may not avoid using cache to measure the disk performance. As sequential writes can write the data to the cache, which is basically a DRAM and can perform excessively fast masking the original speed of the HDD. Let’s check how can we measure the HDD speed using dd bypassing HDD write cache:

dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct

/hddvz is my HDD mount, oflag=direct instruct the dd to confirm writes to the disk, not just the cache before saying the write is completed.

If it’s a good quality hard disk alone or soft raid 1, you can get speed up to 157-161MBps:

[root@bd3 ~]# dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 6.82675 s, 157 MB/s
[root@bd3 ~]#

With a good hardware raid controller and raid 10, you may be able to reach rates like 320MBps.

There is something to remember, software raid mdadm is able to read simultaneously from two disks. You may have the evidence, with two dd one by one, both in background as following:

[root@bd3 ~]# dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct &
[1] 12039
[root@bd3 ~]# dd if=/dev/zero of=/hddvz/testfile1.img bs=1G count=1 oflag=direct &
[2] 12040
[root@bd3 ~]# 1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 11.1064 s, 96.7 MB/s
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 10.7155 s, 100 MB/s

[1]-  Done                    dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct
[2]+  Done                    dd if=/dev/zero of=/hddvz/testfile1.img bs=1G count=1 oflag=direct

As you can see, both the request was able to reach 100MBps roughly while running in parallel.

Testing Read Speed Using hdparm

To check the read speed, you may use hdparm like the following:

[root@bd3 ~]# hdparm -Tt /dev/sda

/dev/sda:
 Timing cached reads:   40444 MB in  1.99 seconds = 20365.12 MB/sec
 Timing buffered disk reads: 396 MB in  3.06 seconds = 129.43 MB/sec
[root@bd3 ~]# hdparm -Tt /dev/sdb

/dev/sdb:
 Timing cached reads:   41006 MB in  1.99 seconds = 20649.43 MB/sec
 Timing buffered disk reads: 562 MB in  3.01 seconds = 186.81 MB/sec
[root@bd3 ~]#

How to connect CentOS Server to the Internet using USB Wifi Adapter?

I was recently working on an Asus RS300 server, installed with CentOS 7 minimal installation before placing it to the datacenter. I didn’t have any RJ45 cable to get this connected to the internet at home. I had a spare ‘Linksys wusb600n’ USB wifi adapter, that I wanted to try out. I connected it to the USB, and got the interface listed as wlp18s0b1 using ifconfig.

To connect this to wifi, we can use nmcli like the following:

Show the connection:

nmcli connection show

To connect to the wifi with the password, use the following:

nmcli dev wifi connect your-ssid password your-wifi-pass

Remember to replace ‘your-ssid’ with the wifi name of your and ‘your-wifi-pass’ with the password for your wifi.

Once you run the above command, this should get connected to the wifi. Now, you can see the connection details and up the device like the following:

nmcli connection show
nmcli connection up wlp18s0b1

Remember to replace ‘wlp18s0b1’ with the one you can see in ifconfig.

Good luck.

Server Boots to Grub – OVH Servers – How to Fix

Error Details

After you have completed updating your yum, you saw the kernel got updated, and hence restarted the server to take the new kernel. But you find out that the server has never come online. Once you visit the KVM or Serial Console (SOL) of the system, you could see, your system is booted to ‘grub>’ console instead of booting from disk. How can you fix the system now?

Solution Intro

This specific issue can appear for any linux server, along with many reasons. Although, if you are running an server from OVH and had faced a similar issue, the boat I am going to show you can navigate to destination. Please note, in many other case of similar situation, you might end up fixing the grub with the same solution.

What and How the Problem Happened

OVH has an interesting strategy of booting. They follow everything through network PXE, even if it is not ‘netboot’, but just the local drives. For this to work out, you need PXE to take the latest grub details pushed once a kernel is updated. This is one reason why, OVH also supplies a custom kernel from a cusstom repo. Although, if you are using the stock kernel, you might come up with a situation, where the latest grub hasn’t been pushed to PXE and your system fails to boot from drives. It then puts you in the ‘grub’ of network.

How to Fix the Problem

Now, one thing is clear, after you completed a kernel update, your grub is broken due to the latest machine code is not available to the booting system. You can go and follow a regular grub repair method for Grub 2, to fix the situation. A couple of things to remember, as your system’s grub is failing to load, you have to use an independent rescue kernel to fix this, this could either be from a personal network repository or a rescue disk available from your datacenter’s location, like ovh has one. Another thing to remember, is that, if you are using CentOS 7 or Ubuntu with UEFI system, using mdadm or linux software raid, it is highly likely, your boot efi is placed in a non raid partition. Preferably in the first drive’s first partition. You can always verify this from your fstab file.

So the first job, is to boot your system into the rescue disk/cd/kernel. I assume you have done that with no difficulty. Once done, first mount your partitions. In OVH cases, it loads the mdadm automatically. In my case, it was /dev/md2. 

mount /dev/md2 /mnt
# check what partition is used for /boot/efi
nano /mnt/etc/fstab
# in my case, it is /dev/nvme0n1p1 (It is a NVMe SSD, and the first partion is used for efi storage
mount /dev/nvme0n1p1 /mnt/boot/efi

Once we have mounted the partitions successfully, you may now chroot the system. Before chrooting, you want the dev, proc and sys to use the /mnt partitions respectively:

mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys

If these all goes well, now we can chroot the system:

chroot /mnt

Now you have successfully changed the root directory of the rescue kernel to the original drive’s root. All you need to do, is to remake the grub config, that will immediately generate the grub.cfg file and sync the machine code:

# we know grub.cfg is available in /boot/grub2/grub.cfg
grub2-mkconfig -o  /boot/grub2/grub.cfg
# once this is finished, we have to make sure, grub is also installed for both disks, for my case, these are /dev/nvme0n1 and /dev/nvme1n1
grub2-install /dev/nvme0n1
grub2-install /dev/nvme1n1

If you see the response is ‘No Error Reported’, then you are good go. You may now reboot your system back to hard disk, and can see your grub is able to load the latest kernel you installed from the original hard disk. Remember, for safety, you should umount all the partition, to avoid any data loss due to OS page cache:

# exit from chroot
exit
# unmount dev, proc, sys, /mnt/boot/efi, /mnt
umount /dev
umount /proc
umount /sys
umount /mnt/boot/efi
umount /mnt

Happy troubleshooting!

How to run composer with different PHP versions in Cpanel?

Question:

When we try to run a composer command, like update, we usually do the following:

composer update

Cpanel has multiple PHP binaries, but in this case, we are unable to select a specific PHP binary to use, instead we have to run it with the default one, how to run composer update with a different php binary in cpanel?

Solution

composer binary file, is a phar file. PHAR is necessarily a PHP Archive and usually automatically detect the running php. But as it is essentially written in php, you may explicitly run it with a different php binary, if you want. To run composer with different php binary, first, you need to find the location of composer. You may do so, using the following:

root@mirage [~]# which composer
/opt/cpanel/composer/bin/composer

Cpanel different php binaries are available under the following kind of directory:

/opt/cpanel/ea-phpXX/usr/bin/php

XX is the version number of PHP. So for example if you need to use PHP 7.4, you would need to run using the following:

/opt/cpanel/ea-php74/usr/bin/php

Now, to run composer update along with PHP 7.4 binary, you may do something like the following:

/opt/cpanel/ea-php74/usr/bin/php /opt/cpanel/composer/bin/composer update

First, make sure you are in the directory where you want to install laravel, for example, something like the following:

cd /home/username/public_html

Then, you may run the above command:

/opt/cpanel/ea-php74/usr/bin/php /opt/cpanel/composer/bin/composer update
or in case, you want to to install
/opt/cpanel/ea-php74/usr/bin/php /opt/cpanel/composer/bin/composer install
or may be, you wan to run update with no-scripts
/opt/cpanel/ea-php74/usr/bin/php /opt/cpanel/composer/bin/composer update --no-scripts

How to Update PATH Variable in Linux

A PATH variable is a system variable that stores the information about the binary files location that you may run for commands. When you log in as an user, or use a custom control panel like Plesk/Cyberpanel/Cpanel, you might want to add some custom paths as a user to take binary commands. One of the example, could be to change the default php path, or a laravel command location from vendor folder. To do this, you need to extend/update the PATH variable for a specific user.

PATH variable extends with the “:”. If you type the following, in your shell, you may see the existing paths in the PATH variable:

[elastic-keldysh@pl1 ~]$ echo $PATH
/usr/share/Modules/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin

Now, if I want to extend this to take the php binary available in /opt/plesk/php/7.2/bin/php, then we can extend the PATH variable using the following:

PATH=$PATH:/opt/plesk/php/7.2/bin/

Now, if you check, the PATH variable again, you can see it is added:

[elastic-keldysh@pl1 ~]$ echo $PATH
/usr/share/Modules/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/opt/plesk/php/7.2/bin/
[elastic-keldysh@pl1 ~]$

We have successfully modified the PATH variable, but only for the existing session. If you want to persist the changes, then, you need to add the command in .bashrc/.profile/.bash_profile file depending on your shell type and OS. You can add to either of the file and test with the following command:

[elastic-keldysh@pl1 ~]$ echo "PATH=$PATH:/opt/plesk/php/7.2/bin/" >> .profile

Replace .profile with .bashrc or .bash_profile depending on the file that works for you. You may logout and relogin, and then run the echo command again to see if the $PATH is persisting or not.

How to Install Let’s Encrypt in Cpanel

Let’s Encrypt is a popular tool to use free SSL for your website. Cpanel comes with Sectigo free ssl service through requesting and pooling system. Although, you might feel interested in getting the SSL released immediately without a queue based approach, and would prefer to use Let’s Encrypt that’s why.

There are two ways, you may install Let’s Encrypt in Cpanel.

  1. Using Cpanel Plugin

First one would be using the plugin created by Cpanel. Login to your server as root:

ssh root@server_ip

Then, run the following to install Let’s Encrypt in your cpanel system

/usr/local/cpanel/scripts/install_lets_encrypt_autossl_provider

It might take a couple of minutes, then it should install Let’s Encrypt as a provider in AutoSSL.

Now, go to WHM >> Manage AutoSSL and select Let’s Encrypt as the provider instead of Sectigo Cpanel default. You need to check the Agreement rules under the Let’s Encrypt selection and you may create the account in Let’s Encrypt using the same tool.

Once done, your new SSLs would be issued using the Let’s Encrypt tool through Cpanel AutoSSL plugin.

2. Using FleetSSL

There is a 3rd party tool, existed before Cpanel provided a plugin for Let’s Encrypt. It’s FleetSSL. One key benefit of using FleetSSL is that, it allows the Cpanel end users to control issuing and renewing the SSL from Cpanel. One key cons of using FleetSSL is that, it is not free of charge, it comes with 30$ one time fees. But mainly hosting provider would not mind to use this as it is a nice addition for the end user feature set in a hosting provider’s point of view.

You may check for details here:

https://letsencrypt-for-cpanel.com/

Now, once you complete installing Let’s Encrypt SSL, you may now use Let’s Encrypt for different cpanel services like webmail/cpanel/whm/calenders/MTA services. You may check the following to know how to:

How To: Install AutoSSL for Server Hostname – Cpanel Server

How to Install Odoo 13 in CentOS 7

Odoo is currently one of the most popular tool for business purposes. It has a community edition, that allows managing ERP at very low cost. Odoo was previously known as OpenERP. Odoo requires to be installed on a dedicated server or VPS. Odoo 13 had come out on October, 2019. Odoo 14 hasn’t been released yet for production. I will have a straight forward how to on installing the latest Odoo 13 in CentOS 7.

Log in to your system and update

First step would be to login to your system and then update the system using yum.

ssh root@server_ip

You may check the CentOS version from the redhat release file using the following:

cat /etc/redhat-release

It should show you something like the following if you 

CentOS Linux release 7.8.2003 (Core)

Now, you may try updating the system with yum

yum update -y

Once done, now install the EPEL repository as we need it to satisfy a couple of dependecies:

yum install epel-release

Install Python 3.6 packages and Odoo dependencies

We need Python 3.6 at least to run Odoo 13. Odoo 12 had support for Python 3.5, unfortunately, Odoo 13 doesn’t. We will use ‘Software Collection (scl)’ repository to install and use Python 3.6. To find the available Python versions in SCL, you may check the following:

SCL Repository for Python

Now, to install Python 3.6 using SCL, we first need to install the SCL repository for Centos:

yum install centos-release-scl

Once the SCL is loaded, now, you may install the python 3.6 using the following command:

yum install rh-python36

Once the Python is installed, now we will install several tools and packages for Odoo dependencies with the following command:

yum install git gcc nano wget nodejs-less libxslt-devel bzip2-devel openldap-devel libjpeg-devel freetype-devel

Create Odoo User

We now need to create a system user and group for Odoo and define a home directory to /opt/odoo

useradd -m -U -r -d  /opt/odoo -s /bin/bash odoo

You may use any username here, but remember to create the same username for the PostgreSQL as well.

Install PostgreSQL

CentOS base repository unfortunately, comes with Postgresql 9.2. But we want to use PostgreSQL 9.6 for our Odoo installation. You may check the available PostgreSQL for CentOS 7 using the following command:

yum list postgresql*

As by default CentOS 7 does not provide the PostgreSQL 9.6, we would use PostgreSQL official repository to download and install the 9.6 version.

First, we install the Postgres Yum Repository using the following command:

yum install https://download.postgresql.org/pub/repos/yum/9.6/redhat/rhel-7-x86_64/pgdg-redhat-repo-latest.noarch.rpm

Now, you may install PostgreSQL 9.6 and related required packages using the following command:

yum install postgresql96 postgresql96-server postgresql96-contrib postgresql96-libs

Now, we need to initialize the postgres database and start it. You may do that using the following:

# Initialize the DB
/usr/pgsql-9.6/bin/postgresql96-setup initdb

# Start the database
systemctl start postgresql-9.6.service

Now you may enable Postgres to start when booting up using the systemctl enable command:

systemctl enable postgresql-9.6.service

Now, we need to create a database user for our Odoo installation. You may do that using the following:

su - postgres -c "createuser -s odoo"

Note: If you have created a different user for Odoo installation other than ‘odoo’ than you should change the username here as well.

Install Wkhtmltopdf

Wkhtmltopdf is a open source tool to make html in pdf format so that you may print pdf reports. This tool is used by Odoo and requires to be installed as dependency. CentOS 7 repository does not provide the latest version of this tool, and Odoo requires you to use the latest version. Hence, we require to download the latest version from the Wkhtmltopdf website and install it. To do that, you may first visit the page:

https://wkhtmltopdf.org/downloads.html

The page gives you the direct rpm download link for each version of CentOS/Ubuntu/Mac etc. Download the stable version for CentOS 7. At the time of writing, the URL for CentOS 7 x86_64 bit is the following:

https://github.com/wkhtmltopdf/packaging/releases/download/0.12.6-1/wkhtmltox-0.12.6-1.centos7.x86_64.rpm

You may install this using the following:

cd /opt/
wget https://github.com/wkhtmltopdf/packaging/releases/download/0.12.6-1/wkhtmltox-0.12.6-1.centos7.x86_64.rpm
yum localinstall wkhtmltox-0.12.6-1.centos7.x86_64.rpm

Install and Configure Odoo 13

If you have come all through here, that means you are done with the all dependency installations before starting to download Odoo 13 source code. We will download Odoo 13 from it’s Github repo and use virtualenv to create an isolated python environment to install this python software.

First, login as odoo from root:

su - odoo

Clone the Odoo source code from Github repository:

git clone https://www.github.com/odoo/odoo --depth 1 --branch 13.0 /opt/odoo/odoo13

This will bring the Odoo 13 branch from the Odoo repository and put it inside the folder /opt/odoo/odoo13

Now, we need to enable software collections in order to access python binaries:

scl enable rh-python36 bash

Then we need to create a virtual environment to complete the installation:

cd /opt/odoo
python3 -m venv odoo13-venv

Now, you may activate the virtual environment you have just created:

source odoo13-venv/bin/activate

Now, we upgrade the pip and install the wheel library:

pip install --upgrade pip
pip3 install wheel

Once done, now we can using pip3 to install all the required Python modules from the requirements.txt file:

pip3 install -r odoo13/requirements.txt

Once the installation is complete, now we can deactivate the virtual environment and get back to the root user

deactivate && exit ; exit

If you think, you will create custom modules, you may now create it and give odoo the permission accordingly:

mkdir /opt/odoo/odoo13-custom-addons
chown odoo: /opt/odoo/odoo13-custom-addons

Now, we can fill up the odoo configuration file. First open the odoo.conf file:

nano /etc/odoo.conf

You may paste the following inside:

[options]
; This is the password that allows database operations:
admin_passwd = set_the_password_to_create_odoo_database
db_host = False
db_port = False
db_user = odoo
db_password = False
addons_path = /opt/odoo/odoo13/addons,/opt/odoo/odoo13-custom-addons
; You can enable log file with uncommenting the next line
; logfile = /var/log/odoo13/odoo.log

Please do not forget to change the password ‘set_the_password_to_create_odoo_database’ with a new strong password. This would be used to create Odoo databases from the login screen.

Create the systemd service file and start Odoo 13

Now, we will create a service file, to be able to start, stop and restart Odoo daemon. To do that, first create a service file using the following:

nano /etc/systemd/system/odoo13.service

and paste the following:

[Unit]
Description=Odoo13
Requires=postgresql-9.6.service
After=network.target postgresql-9.6.service

[Service]
Type=simple
SyslogIdentifier=odoo13
PermissionsStartOnly=true
User=odoo
Group=odoo
ExecStart=/usr/bin/scl enable rh-python35 -- /opt/odoo/odoo13-venv/bin/python3 /opt/odoo/odoo13/odoo-bin -c /etc/odoo.conf
StandardOutput=journal+console

[Install]
WantedBy=multi-user.target

Now, save the file and exit.

Now, you need to reload the systemd daemon to be able to read the latest changes you have made to services. To do that, run:

systemctl daemon-reload

Finally, now we can start Odoo 13 instance using the following command:

systemctl start odoo13

If you are interested to check the status of the instance, you may do this:

systemctl status odoo13
[root@hr ~]# systemctl status odoo13
● odoo13.service - Odoo13
   Loaded: loaded (/etc/systemd/system/odoo13.service; enabled; vendor preset: disabled)
   Active: active (running) since Sun 2020-09-13 08:26:46 EDT; 23h ago
 Main PID: 24502 (scl)
   CGroup: /system.slice/odoo13.service
           ├─24502 /usr/bin/scl enable rh-python36 -- /opt/odoo/odoo13-venv/bin/python3 /opt/odoo/odoo13/odoo-bin -c /etc/odoo.conf
           ├─24503 /bin/bash /var/tmp/sclSWH04z
           └─24507 /opt/odoo/odoo13-venv/bin/python3 /opt/odoo/odoo13/odoo-bin -c /etc/odoo.conf

It show green active running, if everything worked out. If you see no error, you may now enable Odoo to start during the boot:

systemctl enable odoo13

If you would like to see the logs, you may either use the journal tools like the following:

journalctl -u odoo13

or uncomment the following line to log the debugs in /etc/odoo.conf

logfile = /var/log/odoo13/odoo.log

After making any change to /etc/odoo.conf, do not forget the restart the Odoo13 instance using systemctl.

Test the Installation

You may now test the installation using http://your_server_ip:8069. If everything worked, it should come up. If it doesn’t, you may try stopping your ‘firewalld’ to see if firewall is blocking the port or not:

systemctl stop firewalld

At Mellowhost, we provide Odoo installation and configuration assistance for absolute free of charge. If you are willing to try out any of our VPS for Odoo, you may do so and talk with us through the Live chat or the ticket for Odoo assistance.

Furthermore, Good luck.

How to Fix zmconfigd failed in Zimbra – Starting zmconfigd…failed.

Sometimes, if you restart Zimbra, you see zmconfigd is not starting or saying it’s failed. You may also see the zmconfigd service is not running in the Zimbra admin panel. There are couple of common reasons why zmconfigd fails to start.

Disable IPv6

One reason of zmconfigd fails to start is IPv6, for some reason, it fails to route the IPv6 and fails to start. A quick solution to this problem is to disable ipv6 and restart zmconfigd. You may do this like the following:

#Edit your sysctl.conf file
nano /etc/sysctl.conf

# paste the following inside the file
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

# Save the file, and update sysctl in realtime
sysctl -p

# now try to restart zmconfigd
su - zimbra
zmconfigdctl restart

Now you can check the zmconfigd status with the following, to know if it’s running or not:

[root@mailapp ~]# cat /opt/zimbra/log/zmconfigd.pid
19722

If it returns an ID, it means the zmconfigd is running.

Netcat is not installed

Another reason of the error could be because nc is not installed in your system. Zimbra zmconfigd has a dependency on netcat package. Netcat is available through nmap-ncat in centos systems. You may run the following to install netcat:

yum install nc
# or 
yum install nmap-netcat

How to Recover Innodb Table when ib_logfile / ibdata is/are crashed/deleted/lost without backup

If you are here, that means, you probably have panicked the same way, I did around 12 years back. I lost my ib_logfile0/ib_logfile1/ibdata1 all at once for a server that excessively utilizes Innodb tables. I had to recover vital data from the same situation today on a random request who does not have backups, and thought it is better to keep this as a document for future.

One key purpose of utilizing Innodb tables instead of MyISAM is that, the benefit on writes. It always outperform MyISAM in writes due to the use of extra efficient buffers. But, this also causes Innodb to vulnerable from crashing. As Innodb stores some sensitive data to 3 specific files, loosing them, also looses some serious mapping instruments for the database engines to recognizes Innodb table structure and data.

Who can follow this technique?

If you have lost any of ib_logfile0, ib_logfile1, ibdata1 or all of them, but still manages to keep the database folder intact with the .frm and .ibd files (which you would, if you have accidentally deleted the log file or the data only) and also have the following option NOT DISABLED in your mysql configuration ‘innodb_file_per_table’. This option is enabled by default, until you are explicitly disabling this to increase performance. A suggestion: only do this, if you keeping real time backups of your databases. Otherwise, it is better to have this enabled

What is ‘innodb_file_per_table’?

Primarily the tablespace stores and uses data from system tablespace for Innodb. But, as this creates a single point of failure from ibdata and log files, Innodb by defaults also stores the tablespace in table’s own data file, which is .ibd file. That means, if I lose the ibdata/logfile mappings, I can still use the .ibd file to restore my tablespace and do the schema to data mapping only if I allowed innodb to store these information to the database’s own .ibd file. You may read more about the parameters from MySQL documentation:

File-Per-Table Tablespaces at dev.mysql.com

How to Recover an Innodb Table from database files only?

There are two steps to this process. One is to identify and recognize the database schema from the frm file and then basically find a way to import the tablespace from .ibd file and introduce it to innodb engine system tablespace.

First Step First: How to get the schema from .frm files?

First, you must install mysql-utilities tools to get access mysqlfrm tool, you may get the instructions to install this here:

How to Install Mysqlfrm / Mysql-utilities in CentOS 7

Once this is done, now you have two options to read mysqlfrm files. My favorite way is to use the ‘diagnostic’ attribute. To achieve this, run the following:

mysqlfrm --diagnostic /var/lib/mysql/your_database/assets.frm

I assumed, your database name is ‘your_database’ and the table you are trying to recover is ‘assets’. The above command will return you the schema of ‘CREATE TABLE’ you need to use. First, create a new database, and run this on the SQL console to generate the table first on the new database.

Second Step: Get your data and mapping back from .ibd to system tablespace

Once the database has the table, it will also create a .frm and .ibd file for you. What we need to do, is to first, make it forget the existing .ibd file it created, sync the .ibd file from our collapsed database, make the mysql innodb engine to recognize tablespace from the backup tablespace of this .ibd file and store & use it from system tablespace. These lines are complex, and might sound a bit difficult. No worry, let’s do it.

Run the following command first to let it forget the .ibd it has created now:

alter table assets discard tablespace;

Remember the following, our table name is ‘assets’. If you have a different table name, make sure to replace this accordingly. What this has done, is removed the assets.ibd file it created in /var/lib/mysql/new_database/ folder as we asked him to forget the existing .ibd file. Now we first need to copy the backup/old .ibd file to this location with the correct permission. I would use rsync to make sure permissions remains intact here:

rsync -vrplogDtH /var/lib/mysql/your_database/assets.ibd /var/lib/mysql/new_database/

Once this is done, we know, .ibd contains a backup of our original tablespace. We only need to make mysql & innodb recognize this. To achieve this, you may do the following from the Sql console:

alter table assets import tablespace;

If it throws a warning on not being able to file the .cfg file, you may forget it, because it is not essential to have a .cfg to recognize permissions/configurations.

If everything runs well, you should see your rows are back. It’s because innodb has now fetched your tablespace data from .ibd file to system tablespace and it can now recognizes the mapping to your data, viola! All you now need is to repeat the process for all of your innodb tables, and recover the whole database.

Can You Cancel/Abort/Rollback a HTTP Request?

There are cases, where Frontend developers wants to cancel the REST Requests that they submit using Ajax. It is one of the popular question among the Frontend developers community. Let’s try to focus on this case today.

To answer the question in short, No, you can not cancel a HTTP Request once it is submitted. There are catches require shear understanding here. Let’s elaborate a bit.

HTTP is Stateless by Design

HTTP is stateless by design, so what happens when there is no state for a request? It means we do not know the destination, what is happening with that specific requests at the back, but only can acknowledge the response. For example, if you have a REST API behind a load balancer. If you put a POST request to the API, you will not know which server going to serve your requests out of many HTTP servers behind the load balancer. For such cases, we create ‘cookies’ or ‘sessions’ on our application end for our full fledged web applications and extend the functionality of HTTP, to store state of a request and following it. For each form request for example, we submit a state along with the app, to let load balancer understand the state and follow me to the destination. Isn’t it? But we have to remember, this is not a HTTP feature, this is done at application level to add statefullness in HTTP requests.

What does this mean? It means, as there is no state for REST, we are not aware of the destination here, but only the response. So how can we cancel a requests, whose actual destination we are not aware of? Now you might think that, how about adding a cookies and cancel it? Yes, we do that on web applications layer, but can REST HTTP Requests be stateful? Not really, that is not by design of HTTP or REST unfortunately.

So, Does That Mean We Can Never Cancel an API Call?

If you are using REST, then it means you can never cancel the API call. If it gets submitted, it will continue processing in the backend even if you stop it in the frontend. But if ‘Cancelling’ is much important for your application through the API call, then you must consider other alternatives, like SOAP or RPC. RPC is stateful API architecture, and it is possible to design a cancel request for this. Please note, RPC doesn’t implement ‘cancel’ by default, but as this is stateful, you are able to design a ‘cancel’ request with the RPC call. Google has a RPC called ‘gRPC’, which is a stateful API architecture. That means, it is possible for you to implement cancel/abort or event rollback/restore a state with gRPC.

A Google application called ‘Firestore’ database has support for gRPC which is basically a stateful version of stateless REST API of Firestore.