Why deploy two databases?

On my www.roo7690.me site, for example, user data is stored via postgresql and blog information (blog status, blog comments) is managed via mongodb.

Setting up the config

app_db/docker-compose.yml
version: 1.0.0

networks:  # setting up a network in our stack
  datanet:  # specifies the network name
    driver: bridge  # the network uses the bridge driver. This makes it a virtual network that links stack services.

volumes:  # additional storage space, for data persistence in the event of service restarts.
  postgres-data:
    driver: local
  mongo1-data:
    driver: local
  mongo2-data:
    driver: local
  mongo3-data:
    driver: local
  mongo4-data:
    driver: local

app_db/docker-compose.yml
version: 1.0.0

networks:
  datanet:
    driver: bridge

services:  # let's start defining our services.
  pg:  # the name assigned to the service in the network
    container_name: postgres.db
    image: postgres
    restart: always  # tells docker to always restart the container, should it stop due to any error, except in the case of an explicit shutdown.
    ports:
      - '5423:5432' # we map container port 5432 (the port on which the postresql server will start) to port 5423 on the host machine.
    networks:
      - datanet  # we connect it to the datanet virtual network
    volumes:
      - postgres-data:/var/lib/postgresql/data  # the directory /var/lib/postgresql/data will point to the postgres-data volume, defined locally as follows

volumes:
  postgres-data:
    driver: local
  mongo1-data:
    driver: local
  mongo2-data:
    driver: local
  mongo3-data:
    driver: local
  mongo4-data:
    driver: local

app_db/.env.local
POSTGRES_USER=username
POSTGRES_PASSWORD=password
POSTGRES_URL=postgres://username:password@pg:5432/

app_db/docker-compose.yml
version: 1.0.0

networks:
  datanet:
    driver: bridge

services:
  pg:
    container_name: postgres.db
    image: postgres
    restart: always
    ports:
      - '5423:5432'
    networks:
      - datanet
    volumes:
      - postgres-data:/var/lib/postgresql/data
    env_file:  # add environment variables
      - .env.local

  adminer:  # web platform for interaction with pg added
    container_name: adminer.db
    image: adminer
    restart: always
    networks:
      - datanet

volumes:
  postgres-data:
    driver: local
  mongo1-data:
    driver: local
  mongo2-data:
    driver: local
  mongo3-data:
    driver: local
  mongo4-data:
    driver: local

app_db/docker-compose.yml
version: 1.0.0

networks:
  datanet:
    driver: bridge

x-mongo: &mongo  # anchor the property block with the name mongo
  image: mongo
  restart: always
  env_file:
    - .env.local  # (TODO: environment variable to be defined)
  networks:
    - datanet
  command: --replSet rset --bind_ip_all --keyFile /data/db/keyfile  # command to be run at container startup with mongosh client.
  # start the server in replica set mode under the name rset (--replSet rset)
  # --bind_ip_all, listens on all networks
  # --keyFile, define the location of the file containing the replica set identification key (TODO: file to be imported)

services:
  mongo1:
    <<: *mongo  # by reference to mongo1, we give all the properties defined by mongo
    container_name: mongo1.db
    volumes:
      - mongo1-data:/data/db
      - ./key/keyfile:/data/db/keyfile  # copy ./key/keyfile to /data/db/keyfile (TODO: ./key/keyfile to be created)
    ports:
      - '27017:27017'

  mongo2:
    <<: *mongo
    container_name: mongo2.db
    volumes:
      - mongo2-data:/data/db
      - ./key/keyfile:/data/db/keyfile
    ports:
      - '27018:27017'

  mongo3:
    <<: *mongo
    container_name: mongo3.db
    volumes:
      - mongo3-data:/data/db
      - ./key/keyfile:/data/db/keyfile
    ports:
      - '27019:27017'

  mongo4:
    <<: *mongo
    container_name: mongo4.db
    volumes:
      - mongo4-data:/data/db
      - ./key/keyfile:/data/db/keyfile
    ports:
      - '27020:27017'

  mongo-express:  # added web platform for interaction with the cluster
    container_name: mongo-express.db
    image: mongo-express
    restart: always
    env_file:
      - .env.local
    networks:
      - database

  pg:
    container_name: postgres.db
    image: postgres
    restart: always
    ports:
      - '5423:5432'
    networks:
      - datanet
    volumes:
      - postgres-data:/var/lib/postgresql/data
    env_file:
      - .env.local

  adminer:
    container_name: adminer.db
    image: adminer
    restart: always
    networks:
      - datanet

volumes:
  postgres-data:
    driver: local
  mongo1-data:
    driver: local
  mongo2-data:
    driver: local
  mongo3-data:
    driver: local
  mongo4-data:
    driver: local

app_db/.env.local
#mongo
MONGO_INITDB_ROOT_USERNAME=username
MONGO_INITDB_ROOT_PASSWORD=password

#mongo-express
ME_CONFIG_MONGODB_ADMINUSERNAME=username
ME_CONFIG_MONGODB_ADMINPASSWORD=password
ME_CONFIG_MONGODB_URL="mongodb://username:password@mongo1:27017,mongo2:27017,mongo3:27017,mongo4:27017/?replicaSet=rs"
ME_CONFIG_BASICAUTH=true
ME_CONFIG_BASICAUTH_USERNAME=username
ME_CONFIG_BASICAUTH_PASSWORD=password

POSTGRES_USER=username
POSTGRES_PASSWORD=password
POSTGRES_URL=postgres://username:password@pg:5432/

app_db/nginx.conf
user www-data;
worker_processes auto;
pid /run/nginx.pid;

events {
  worker_connections 1024;
}

http {  # http protocol configuration
  include /etc/nginx/mime.types;
  default_type application/octet-stream;

  sendfile on;
  tcp_nopush on;
  tcp_nodelay on;
  keepalive_timeout 65;
  types_hash_max_size 2048;

  include /etc/nginx/conf.d/*.conf;
}

app_db/webapp.conf
# define a server to be included in our protocol
server {
  listen 8082;
  server_name nom_de_domaine;

  # we define the headers
  proxy_set_header Host $host;
  proxy_http_version 1.1;
  proxy_set_header X-Real-IP $remote_addr;
  proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
  proxy_set_header X-Forwarded-Proto $scheme;
  proxy_set_header X-NginX-Proxy true;
  real_ip_header X-Real-IP;
  proxy_connect_timeout 300;
  proxy_set_header Upgrade $http_upgrade;
  proxy_cache_bypass $http_upgrade;
  proxy_set_header Connection "upgrade";
  chunked_transfer_encoding off;
  add_header X-Frame-Options "DENY";
  add_header X-Content-Type-Options "nosniff";
  add_header X-XSS-Protection "1; mode=block";

  # we define the proxies
  location /mongo/ {
    rewrite ^/mongo/(.*) /$1 break;
    # you'll have to rewrite the html code received in the mongo-express response
    sub_filter 'href="/' 'href="/mongo/';
    sub_filter 'src="/' 'src="/mongo/';
    sub_filter 'action="/' 'action="/mongo/';
    sub_filter 'url(/' 'url(/mongo/';
    sub_filter_once off;
    proxy_redirect <http://mongo-express:8082/> https://nom_de_domaine/mongo/;

    proxy_pass <http://mongo-express:8081/>;
  }

  location /checkValid {
    # you'll have to redirect mongo-express's /checkValid route
    proxy_pass <http://mongo-express:8081/checkValid>;
  }

  location /db {
    # rediriger la route /db de mongo-express
    return 301 https://nom_de_domaine/mongo$request_uri;
  }

  location /postgres/ {
    rewrite ^/postgres/(.*) /$1 break;
    # rewriting the html code received in adminer's reply
    sub_filter 'href="/' 'href="/postgres/';
    sub_filter 'src="/' 'src="/postgres/';
    sub_filter 'action="/' 'action="/postgres/';
    sub_filter 'url(/' 'url(/postgres/';
    sub_filter_once off;
    proxy_redirect / /postgres/;

    proxy_pass <http://adminer:8080/>;
  }
}

app_db/docker-compose.yml
version: 1.0.0

networks:
  datanet:
    driver: bridge

x-mongo: &mongo
  image: mongo
  restart: always
  env_file:
    - .env.local
  networks:
    - datanet
  command: --replSet rset --bind_ip_all --keyFile /data/db/keyfile

services:
  db:
    container_name: nginx.db
    image: nginx
    volumes:  # copy configuration files
      - ./webapp.conf:/etc/nginx/conf.d/webapp.conf
      - ./nginx.conf:/etc/nginx/nginx.conf
      - ./.env.local:/etc/nginx/.env.local
    ports:
      - '8082:8082'  # Instead, we set up a server which, once started, will listen on port 8082.
      # For my part, I map it to port 8082 on the host machine. My port 443 (the default https port is occupied by a nginx server that handles proxying to 8082 and all other servers).
    networks:
      - datanet

  mongo1:
    <<: *mongo
    container_name: mongo1.db
    volumes:
      - mongo1-data:/data/db
      - ./key/keyfile:/data/db/keyfile
    ports:
      - '27017:27017'

  mongo2:
    <<: *mongo
    container_name: mongo2.db
    volumes:
      - mongo2-data:/data/db
      - ./key/keyfile:/data/db/keyfile
    ports:
      - '27018:27017'

  mongo3:
    <<: *mongo
    container_name: mongo3.db
    volumes:
      - mongo3-data:/data/db
      - ./key/keyfile:/data/db/keyfile
    ports:
      - '27019:27017'

  mongo4:
    <<: *mongo
    container_name: mongo4.db
    volumes:
      - mongo4-data:/data/db
      - ./key/keyfile:/data/db/keyfile
    ports:
      - '27020:27017'

  mongo-express:
    container_name: mongo-express.db
    image: mongo-express
    restart: always
    env_file:
      - .env.local
    networks:
      - database

  pg:
    container_name: postgres.db
    image: postgres
    restart: always
    ports:
      - '5423:5432'
    networks:
      - datanet
    volumes:
      - postgres-data:/var/lib/postgresql/data
    env_file:
      - .env.local

  adminer:
    container_name: adminer.db
    image: adminer
    restart: always
    networks:
      - datanet

volumes:
  postgres-data:
    driver: local
  mongo1-data:
    driver: local
  mongo2-data:
    driver: local
  mongo3-data:
    driver: local
  mongo4-data:
    driver: local

app_db/key
openssl rand -base64 756 > keyfile
chmod 400 keyfile
chown mongodb:mongodb keyfile

app_db
docker-compose up -d

app_db
docker exec -it mongo1 mongosh --username username --password password --authenticationDatabase admin

mongo1
cf=rs.conf();
cf._id='rset';
cf.members=[
  { _id:0,host:'mongo1',priority:1},
  { _id:1,host:'mongo2',priority:0.5},
  { _id:2,host:'mongo3',priority:0.5},
  { _id:3,host:'mongo4',priority:0.5}
];
rs.reconfig(cf,{force:true});