When we need to use swap partition in Linux?

Recently i have an issue on my LightSail small instance which have 512MB of RAM,

I can not run react-script build on it

npm run build

Because of the issue: https://github.com/kitze/custom-react-scripts/blob/master/packages/react-scripts/template/README.md#npm-run-build-exits-too-early

So create a Linux swap is life saver.

Firstly, check if swap available :

sudo swapon --show

if it show nothing, then you don't have any swap yet.

Now, create a swap file, it should be 2x or 4x of your RAM capacity

sudo fallocate -l 1G /swapfile

To verify the swap is created

ls -lh /swapfile

To enable swap, you may need to change the permission

sudo chmod 600 /swapfile

make swap file

          sudo mkswap /swapfile        

then

sudo swapon /swapfile

check if it is ready

sudo swapon --show

 To enable swap permanent

Backup file

sudo cp /etc/fstab /etc/fstab.bak

Add the line at the end of file

/swapfile swap swap defaults 0 0

Benchmarking web application tools

1. Apache benchmarking 

Home page https://httpd.apache.org/docs/2.4/programs/ab.html

• c: ("Concurrency"). Indicates how many clients (people/users) will be hitting the site at the same time. While ab runs, there will be -c clients hitting the site. This is what actually decides the amount of stress your site will suffer during the benchmark.
• n: Indicates how many requests are going to be made. This just decides the length of the benchmark. A high -n value with a -c value that your server can support is a good idea to ensure that things don't break under sustained stress: it's not the same to support stress for 5 seconds than for 5 hours.
• k: This does the "KeepAlive" functionality browsers do by nature. You don't need to pass a value for -k as it it "boolean" (meaning: it indicates that you desire for your test to use the Keep Alive header from HTTP and sustain the connection). Since browsers do this and you're likely to want to simulate the stress and flow that your site will have from browsers, it is recommended you do a benchmark with this.
  The final argument is simply the host. By default it will hit http:// protocol if you don't specify it.

ab -k -c 350 -n 20000 example.com/ 

By issuing the command above, you will be hitting http://example.com/ with 350 simultaneous connections until 20 thousand requests are met. It will be done using the keep alive header.

E.g: Get user profile

ab -n2000 -c10 -k -m GET -T "application/json" -H "${accessToken}" http://localhost:8080/api/v1/users/profile

Limitation : ab does not support dynamic body per request -  https://stackoverflow.com/questions/2602841/changing-post-data-used-by-apache-bench-per-iteration

2. Apache Jmeter

Home Page: http://jmeter.apache.org/

Create many users using __UUID() random method example:

References:

https://stackoverflow.com/questions/12732182/ab-load-testing

https://stackoverflow.com/questions/44800557/apache-jmeter-add-random-data-in-body-for-request/54059660#54059660

https://www.roseindia.net/jmeter/using-jmeter.shtml

MacOS basic commands

1. Copy a folder into another folder

cp -r /path/folder/a /path/folder/b

result:

/path/folder/b
/path/folder/b/a

2. Move a folder into another folder

mv /path/folder/a /path/folder/b

result:

/path/folder/b
/path/folder/b/a

3. Remove files

rm -rf /path/to/file
rm -rf /path/to/file*

4. Process and ports

lsof -iTCP -sTCP:LISTEN -n -P
lsof -iTCP -sTCP:LISTEN -n -P | grep {PORT}

5. Kill a process

kill -9 ${PID}

6. Change owner of a folder

# for current user
sudo chown -R `id -un` /path/to/folder
# a user
sudo chown -R ${username} /path/to/folder

MongoDB basic commands

1. Setup MongoDB

• Download: https://www.mongodb.com/download-center/community


• Extract folder
  

  tar -zxvf mongodb-osx-x86_64-xx.tgz -C /path/to/container/folder



• Add to $PATH
  Find out which current shell you are working on.
  zsh open ~/.zshrc
  bash open ~/.bashrc
  add lines at the end of the file.
  

  export MONGODB_HOME=~/Workspace/mongodb-osx-x86_64-xx/
  export PATH=$MONGODB_HOME/bin:$PATH

  
  Reload the environment variables for current shell
  

  # e.g
  source ~/.zshrc



• Create the directory where MongoDB stores data
  

  sudo mkdir /data/db
  sudo chown -R `id -un` /data/db



• Start MongoDB
  

  mongod

  
  For now , you have a mongodb server is running on default port 2017 without authentication, the data is stored on /data/db

2. Basic client commands

# connect to server
mongo

# list databases;
show databases;

# use a database
use ${db_name}

# list collections;
show collections;

# form now we can perform collection methods
db.${collectionName}.${command}

find({})
findOne()
find({"_id": new IdObject("${docId}")})
find({ <field>: { $regex: /pattern/, $options: '<options>' }})
deleteMany({})
deleteOne({})
remove({})
drop()

Checkout collection methods

2.Delete database

# from commandline
mongo ${dbName} --eval "db.dropDatabase()"

# inside mongoClient
use ${dbName}
db.dropDatabase()