Why the volume folder is empty after a redis save? - docker

Question

Code
I'm trying to run a redis service defined inside a docker-compose.yml as follows:
version: '3'
services:
redis:
image: "redis:5-alpine"
volumes:
- ./redis-vol:/home/data
app:
build: .
ports:
- 8080:8080
volumes:
- .:/home/app/
This is the Dockerfile:
FROM python:2.7-alpine3.8
WORKDIR /home/app
COPY ./requirements.txt .
RUN apk add python2-dev build-base linux-headers pcre-dev && \
pip install -r requirements.txt
# Source files
COPY ./api.py .
COPY ./conf.ini .
CMD ["uwsgi", "--ini", "conf.ini"]
The app consists of this snippet running a uwsgi interface on port 8080
import uwsgi
import redis
r = redis.Redis('redis')
def application(env, start_response):
start_response('200 OK', [('Content-Type','text/html')])
r.append('hello', 'world!')
r.save()
return [b"Hello World"]
And this is the conf.ini file:
[uwsgi]
http = :8080
wsgi-file = api.py
master = true
process = 2
enable-threads = true
uid = 1001
gid = 1001
The app service is supposed to save a key:value pair through redis every time it receives a request to http://localhost:8080.
Upon a successful request, the docker-compose process returns the following log:
redis_1_bdf757fbb2bf | 1:M 26 Nov 2018 15:38:20.399 * DB saved on disk
app_1_5f729e6bcd36 | [pid: 17|app: 0|req: 1/1] 172.21.0.1 () {38 vars in 690 bytes} [Mon Nov 26 15:38:20 2018] GET / => generated 11 bytes in 8 msecs (HTTP/1.1 200) 1 headers in 44 bytes (1 switches on core 0)
redis_1_bdf757fbb2bf | 1:M 26 Nov 2018 15:38:20.998 * DB saved on disk
app_1_5f729e6bcd36 | [pid: 17|app: 0|req: 2/2] 172.21.0.1 () {40 vars in 691 bytes} [Mon Nov 26 15:38:20 2018] GET /favicon.ico => generated 11 bytes in 4 msecs (HTTP/1.1 200) 1 headers in 44 bytes (1 switches on core 0)
Problem
Despite the DB saved on disk log, the redis_vol folder is empty and the dump.rdb file doesn't seem to be saved anywhere else.
What I am doing wrong? I've also tried to use redis:alpine as image but I have the following error at startup:
redis_1_bdf757fbb2bf | 1:M 26 Nov 14:57:27.003 # Can't handle RDB format version 9
redis_1_bdf757fbb2bf | 1:M 26 Nov 14:57:27.003 # Fatal error loading the DB: Invalid argument. Exiting.
And I've also tried to map the dump.rdb in the redis service as follows:
redis:
image: "redis:5-alpine"
volumes:
- ./redis-vol/dump.rdb:/home/data/dump.rdb
but the docker creates a folder named dump.rdb/ instead of a readable file.

Answer 1

If you still face the problem even after changing the volume map to
<your-volume>:/data
Make sure to delete the previous container with
docker container prune
before restarting your container again

Answer 2

Accoring to the redis documentation on the DockerHub page
If persistence is enabled, data is stored in the VOLUME /data
So you are using the wrong volume path. Yout should use /data instead
volumes:
- ./redis-vol:/data

Answer 3

To be able to mount a single file for your container with docker-compose, use absolute path of the file you want to mount from your filesystem:
redis:
image: "redis:5-alpine"
volumes:
- /Users/username/dirname/redis-vol/dump.rdb:/home/data/dump.rdb

Answer 4

As codestation correctly pointed out, it is on the
official documenation but he is suggesting
mount point instead of volumes which has some additional cons.
In both cases in documentation there is also statement about "persistence
enabled":
$ docker run --name some-redis -d redis redis-server --appendonly yes
or in your docker-compose file:
redis:
image: "redis:alpine"
command: redis-server --appendonly yes
volumes:
- your-volume:/data