Airflow, Kubernetes and Docker Documentation : October 2021

How to run PostgreSQL on Docker

Postgres on Docker

Postgres is most advanced object relational database management system(ORDBMS). Postgres implements majority of SQL:2011 standard. It's ACID compliant and It avoids locking issues using multiversion concurrency control. So today we are going to run Postgres on Docker.

To start with Postgres we first need to pull the image from DockerHub. DockerHub is image repository for all images. Let's run the below command and pull the image:

docker pull postgres

Using default tag: latest

latest: Pulling from library/postgres

a9eb63951c1c: Pull complete

b192c7f382df: Pull complete

e7ce3f587986: Pull complete

4098744a1414: Pull complete

4c98d6f3399d: Pull complete

65e57fefc38a: Pull complete

d61d9528cfd5: Pull complete

de6b20f44659: Pull complete

25db13ff0bef: Pull complete

7f74f4b0e936: Pull complete

144c847b11fb: Pull complete

cf0afd1be009: Pull complete

fe0c14991327: Pull complete

Now let's check that we have downloaded the image.

docker images

REPOSITORY TAG IMAGE ID CREATED SIZE

postgres latest 83ce63c594ee 5 days ago 355MB

Let's run the image and start a container.

docker run --name test -e POSTGRES_PASSWORD=Test@123 -d postgres

Just run the docker ps command to check if container is running

docker ps

CONTAINER ID IMAGE COMMAND CREATED STATUS

83ec4a222 postgres "docker-entrypoint.s…" 2 minutes ago Up

Let's enter in bash shell of container by running below command

docker exec -it 83ec4a222 bash

root@83ec4a222:/#

Connect to Postgres now:

psql -h localhost -p 5432 -U postgres -w

psql (14.0 (Debian 14.0-1.pgdg110+1))

Type "help" for help.

You are connected to Postgres now. Lets' create some tables and execute some queries.

postgres=# \l

List of databases

-----------+----------+----------+------------+------------+-----------------------

(3 rows)

postgres=#

Let's check the current database name by running below command.

postgres=# select current_database();

current_database

------------------

postgres

(1 row)

So current database is Postgres. We'll check now how many databases are there on the system.

postgres=# select datname from pg_catalog.pg_database;

datname

-----------

postgres

template1

template0

(3 rows)

There are total 3 databases on system.

You can check all tables on a database by querying information schema.

postgres=# select table_name from information_schema.tables limit 10;

table_name

-----------------------

pg_statistic

pg_type

pg_foreign_table

pg_authid

pg_shadow

pg_statistic_ext_data

pg_roles

pg_settings

pg_file_settings

pg_hba_file_rules

(10 rows)

We can do a lot more than this on Postgres this was just a small part about Postgres. We can get all information about all tables and databases just by using information schema. Docker can be very useful in this case when we don't want to install it on system and want to run Postgres inside container and can leverage the power of Docker.

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How to dockerize your python application in docker

Dockerize your python application:

Docker is a technology which lets you build, deploy and run your applications. Docker enables you separate your infrastructure from your application. With Docker all you need to do is just write your code,.

dockerize it and distribute it in form of image. That way any one can use your application who is running the Docker.

What do you mean by Dockerize application?

Dockerize mean you write your code on your system then you prepare the image and distribute it over the internet or on DockerHub. You don't have to worry about the underlying infrastructure and dependencies.

Let's write a python program which will count the occurrence of words from a given string.

#Input :  string = "Docker is a technology which 
# lets you build, deploy and run your applications.";
#Count occurence of words from a given string example 


def findFreq(s):
    dictt = {}
    strng = s.split(" ")
    strr1 = set(strng)
    for word in strr1:
        dictt[word] = s.count(word)
    return dictt  
if __name__ == "__main__":
    x = input("Enter your string:")
    #raw_input in python 2.x and input() in python 3.x
    print(findFreq(x))
       

#Output: {'a': 4, 'and': 1, 'run': 1, '': 80, 
# 'deploy': 1, 'technology': 1, 'is': 1, 
# 'you': 2, 'lets': 1, 'applications.': 1, 
# 'which': 1, 'build,': 1, 'Docker': 1, 'your': 1}

Save this file with findfrequency.py in same directory. I am saving it in current directory for my convenience but you can save it anywhere and pass the absolute path.

Now lets create a Dockerfile.

FROM python:3

We need to use python in docker so we are using FROM keyword so this will create layer from python image. Means your image is based on python image.

Now we need to run our python file so we need to add this file to Dockerfile.

ADD findfrequency.py /

Use CMD to execute commands when image loads

CMD ["python", "./findfrequency.py"]

Combine all above lines and create a Dockerfile.

FROM python:3
ADD findfrequency.py /
CMD ["python", "./findfrequency.py"]

So we have created a Dockerfile now. I saved it with the name "Dockerfile" in current directory. When you run docker build . command then docker looks for Dockerfile if ithis file doesn't exist or file name is wrong or extension is wrong you'll get file not exists error.

Now we are ready to build image from the dockerfile.

Open the terminal and run the below command and make sure you are in the same directory where you saved your Dockerfile as well as python file.

docker build -t myapp .

-t : This is tagging a name to your image. In this case I gave my image a name "myapp"

.(dot) : Is current directory

Ok so you have successfully build your image. Now Let's check what's inside the image by inspecting it.

docker inspect myapp

[

{

"Id": "sha256:c4595feabbd0b9aba4ae67037ea3c43a8c0aaf2abe6f6fd28d25b22a7cf9",

"RepoTags": [

"myapp:latest"

"RepoDigests": [],

"Parent": "",

"Comment": "buildkit.dockerfile.v0",

"Created": "2021-10-01T08:42:53.450488763Z",

"Container": "",

"ContainerConfig": {

"Hostname": "",

"Domainname": "",

"User": "",

"AttachStdin": false,

"AttachStdout": false,

"AttachStderr": false,

"Tty": false,

"OpenStdin": false,

"StdinOnce": false,

"Env": null,

"Cmd": null,

"Image": "",

"Volumes": null,

"WorkingDir": "",

"Entrypoint": null,

"OnBuild": null,

"Labels": null

"DockerVersion": "",

"Author": "",

"Config": {

"Hostname": "",

"Domainname": "",

"User": "",

"AttachStdin": false,

"AttachStdout": false,

"AttachStderr": false,

"Tty": false,

"OpenStdin": false,

"StdinOnce": false,

"Env": [

"LANG=C.UTF-8",

"PYTHON_VERSION=3.9.7",

"PYTHON_PIP_VERSION=21.2.4",

"PYTHON_SETUPTOOLS_VERSION=57.5.0",

"PYTHON_GET_PIP_SHA256=fa6f3fb93cce234cd4e8dd2be9c247653b52855a48dd44e6b21ff28b"

"Cmd": [

"python",

"./findfrequency.py"

You'll see output something like above. Our python function is there inside the output under CMD tag.

Let's run the image.

docker run -it myapp

Enter your string: This is my test to test dockerfile.

{'': 37, 'is': 2, 'dockerfile.': 1, 'to': 1, 'my': 1, 'test': 2, 'This': 1}

See the output above and pass the desired string to count the words.

So we have successfully dockerized our application. You can send this image to others so that they can use your program and they don't have to worry about installing any dependencies which can cause your program to crash.