Promtail and Grafana - json log file from docker container not displayed - docker

Question

my application's services are deployed via docker-compose. Currently, I also deployed Grafana, Loki and Promtail within the same docker-compose network.
Following the getting-started guide, collecting and displaying the log files from /var/log with the config
- job_name: system
entry_parser: raw
static_configs:
- targets:
- localhost
labels:
job: varlogs
__path__: /var/log/*log
works fine.
However, my backend logs (NestJS) into a log file which is stored in a docker volume.
Example log entry:
{"message":"Mapped {/api/drink, POST} route","context":"RouterExplorer","level":"info","timestamp":"2021-03-23T17:08:16.334Z"}
The path to the log is
/var/lib/docker/volumes/my_volume/_data/general.log
When I add the following config to Promtail
- job_name: backend
pipeline_stages:
- json:
expressions:
level: level
message: message
timestamp: timestamp
context: context
static_configs:
- targets:
- localhost
labels:
job: backend
__path__: /var/lib/docker/volumes/my_volume/_data/general.log
and use the query {job="backend"} in Grafana, nothing is displayed.
Furthermore, the log of the promtail container doesn't give any information.
What am I missing?
Thank you in advance!

Answer 1

In your pipeline stages you need to store the extracted values:
pipeline_stages:
- json:
expressions:
level: level
message: message
timestamp: timestamp
context: context
- timestamp:
source: timestamp
- labels:
level:
context:
- output:
source: message
This will set the timestamp, the labels context, level and the message will be the log line.
Documentation can be found here.