How do we connect our docker app to a locally running postgres? - postgresql

Question

I'm looking for a way to connect a docker container app so it can access Postgres database via https://postgresapp.com/
Was wondering if there are certain ports to open and what the yml file would look like to enable docker-compose to work with locally running postgres.
Thanks!

Answer 1

You have to make changes in postgres config files in host machine, especially
1) pg_hba.conf
2) postgres.conf
You can find above configuration in /var/lib/postgresql/[version]/ path
First check your docker0 bridge interface it might be 172.17.0.0/16, if not change accordingly.
make changes in postgresql.conf path will be same as pg_hba.conf.
listenaddress to "*"
Then in pg_hba.conf add rule as
host all all 172.17.0.0/16 md5.
Then in docker application use host ip address to connect to postures running in host.

Answer 2

There is no difference between accessing a database from inside a container or outside a container.
Let's say the Postgres database is running on localhost. If you want to connect to it from a small Python script, you can do so as follows, as found in the Docs:
#!/usr/bin/python
import psycopg2
import sys
def main():
#Define our connection string
conn_string = "host='localhost' dbname='my_database' user='postgres' password='secret'"
# print the connection string we will use to connect
print "Connecting to database\n ->%s" % (conn_string)
# get a connection, if a connect cannot be made an exception will be raised here
conn = psycopg2.connect(conn_string)
# conn.cursor will return a cursor object, you can use this cursor to perform queries
cursor = conn.cursor()
print "Connected!\n"
if __name__ == "__main__":
main()
In order for this script to run you need to install Python and psycopg2 on your local machine, probably in a virtual environment. Alternatively, you could put it in a container. Instead of installing everything manually, you would define a Dockerfile with your installation instructions. It would probably look something like this:
FROM python:latest
ADD python-script.py /opt/www/python-script.py
RUN pip install psycopg2
CMD ["python", "/opt/www/python-script.py"]
And if we build and run...
dave-mbp:Desktop dave$ ls -la
-rw-r--r-- 1 dave staff 135 Dec 8 19:05 Dockerfile
-rw-r--r-- 1 dave staff 22 Dec 8 19:04 python-script.py
dave-mbp:Desktop dave$ docker build -t python-script .
Sending build context to Docker daemon 17.92kB
Step 1/4 : FROM python:latest
latest: Pulling from library/python
85b1f47fba49: Already exists
ba6bd283713a: Pull complete
817c8cd48a09: Pull complete
47cc0ed96dc3: Pull complete
4a36819a59dc: Pull complete
db9a0221399f: Pull complete
7a511a7689b6: Pull complete
1223757f6914: Pull complete
Digest: sha256:db9d8546f3ff74e96702abe0a78a0e0454df6ea898de8f124feba81deea416d7
Status: Downloaded newer image for python:latest
---> 79e1dc9af1c1
Step 2/4 : ADD python-script.py /opt/www/python-script.py
---> 21f31c8803f7
Step 3/4 : RUN pip install psycopg2
---> Running in f280c82d74e7
Collecting psycopg2
Downloading psycopg2-2.7.3.2-cp36-cp36m-manylinux1_x86_64.whl (2.7MB)
Installing collected packages: psycopg2
Successfully installed psycopg2-2.7.3.2
---> bd38f911bb6a
Removing intermediate container f280c82d74e7
Step 4/4 : CMD python /opt/www/python-script.py
---> Running in 159b70861893
---> 4aa783be5c90
Removing intermediate container 159b70861893
Successfully built 4aa783be5c90
Successfully tagged python-script:latest
dave-mbp:Desktop dave$ docker run python-script
>> Connected!
Containers are largely a packaging solution. We'll be able to connect to an external database or API endpoint as if we were running everything from outside of a container.
There is one thing to keep in mind though...localhost may resolve to the virtual machine if you're using docker toolbox/virtualbox. In which case, you wouldn't connect to localhost unless you were running a bridged network connection in the VM. Otherwise, just specify the host IP instead of localhost.

Answer 3

Postgres default port is 5432.
Docker offers different networking modes to use. I can tell you how to achieve your goal using bridge mode (used by default).
There is always a bridge network that allows you access host machine. It is created by default and called docker0.
Run the following command in terminal ip addr show docker0 to see your host ip available to all the containers you run.
Output on my machine:
developer#dlbra:~$ ip addr show docker0
3: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default
link/ether 02:42:b4:95:60:89 brd ff:ff:ff:ff:ff:ff
inet 172.17.0.1/16 scope global docker0
valid_lft forever preferred_lft forever
Therefore you don't need any additional configuration in docker-compose.yml
You just have to configure your db host to the ip address you saw. In my case it would be 172.17.0.1.
If your locally installed Postgres listens to a different port also specify that port in your application configuration.