Workshop – Accessing log data with Loki

Log Study

© Lead Image © Kheng Ho Toh,

© Lead Image © Kheng Ho Toh,

Article from Issue 249/2021

Loki is a powerful, scalable, and easy-to-use solution for aggregating log data.

One day, during one of my company's cloud project meetings, a developer colleague said, "I need to find a way to quickly access logs for debugging and troubleshooting." I already had some experience with the Grafana-Prometheus, so I said I would help find a solution.

It turns out, the solution we settled on was Loki [1], from Grafana Labs. The Grafana Labs website describes Loki as "…a horizontally scalable, highly available, multi-tenant log aggregation system inspired by Prometheus." Loki is designed to aggregate logs efficiently, extracting metrics and alerts – all without requiring a massive indexing configuration. Once you have extracted the information you need, you can then use Grafana to visualize the data.

This workshop offers a quick look at how to access log data using Loki. In this scenario, I will push logs generated by an Apache web server hosting a sample Nextcloud deployment, then evaluate the data using Loki's own query language, LogQL.

In addition to a Loki server, I'll install the companion application Promtail [2], which Grafana Labs maintains as an agent to push data to Loki.

Log Factory

The first step is to start logging. If you already have a log-filled folder, just skip this step. In this case, I'll run a Nextcloud Docker container. I'll "mount" (bind) the Apache web server log folder inside the container to a local folder on the workstation. This step will allow Promtail, which will run locally, to access the files.

Of course you need to have the Docker engine installed. If you don't, see the box entitled "Get Docker Ready."

Get Docker Ready

This tutorial uses Docker as a way to spin up services quickly without installing unnecessary packages. In order to use Docker, you'll need the Docker engine, which you can install with a one-liner:

# curl -sSL | sudo bash

This command will fetch the latest official installation script. The script will detect which Linux distribution you're running, add the proper package manager repositories, and install.

Warning: Make sure you check the contents of a script every time you plan to pipe directly to sudo bash.

docker run --name nextcloud -d -p 8080:80 -v /somelogsdir:/var/log/apache2 nextcloud

Once the container is running, you can sail a browser to:


and perform some actions on the Nextcloud instance. It doesn't really matter what you do as long as it generates entries in the access.log and error.log Apache default log files.


Download the binary release of the most recent version of both Loki and Promtail. Regarding Loki, fetch both the binary distribution and a sample config file:

wget -O loki_config.yaml

The Loki sample config is already good enough for this workshop, so I'll execute it and keep it running:

./loki-linux-amd64 -config.file=loki_config.yaml

If you see a bunch of creating table messages, that is a good sign – it means Loki is creating the structure to host the log entries.

The next step is to set up and run Promtail:

grafana/loki/master/cmd/promtail/promtail-local-config.yaml-O promtail_config.yaml

Before running Promtail, you'll need to tweak the configuration to set up a Loki URL and log source folder (lines 9 and 18 in Listing 1).

Listing 1

Promtail Configuration

01 server:
02  http_listen_port: 9080
03  grpc_listen_port: 0
05 positions:
06  filename: /tmp/positions.yaml
08 clients:
09   - url: "http://localhost:3100/loki/api/v1/push"
10 scrape_configs:
11 - job_name: apache
12   static_configs:
13   - targets:
14       - localhost
15     labels:
16       job: "apache"
17       instance: "localserver"
18       __path__: /somelogsdir/*.log

Once you have successfully configured Promtail, run it with:

./promtail-linux-amd64 -config.file=promtail_config.yaml

If everything is working as expected, you won't get any output yet from the logs being pushed to Loki.


Loki has no built-in UI, so the only way to query logs at this point is to make use of the excellent Loki RESTful API (see the box entitled "Structure").


Once the logs are stored in Loki, they will be organized into streams. Each stream is identified by labels. Some labels are automatically generated (for example, "filename") and some are custom-made (See Listing 1, rows 16 and 17.)

In this case, I'll use {job="apache"} and {instance="localserver"}.

Loki will eventually store the log entries as pairs composed by a timestamp and the actual content.

The following query asks Loki to provide the most recent log entries, limiting the result to three entries:

curl -G -s  "http://localhost:3100/loki/api/v1/query_range?limit=3" --data-urlencode 'query={job="apache"}' | jq

The results of the query appear in Listing 2.

Listing 2

Sample Log Query Result Object

01 {
02   "status": "success",
03   "data": {
04     "resultType": "streams",
05     "result": [
06       {
07         "stream": {
08           "filename": "/somelogsdir/access.log",
09           "instance": "localserver",
10           "job": "apache"
11         },
12         "values": [
13           [
14             "1620748522681322318",
15             " - - [11/May/2021:15:55:22 +0000] \"GET /csrftoken HTTP/1.1\" 200 928 \"-\"\"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:88.0) Gecko/20100101 Firefox/88.0\""
16           ],
17           [
18             "1620748504911781382",
19             " - - [11/May/2021:15:55:04 +0000] \"GET /csrftoken HTTP/1.1\" 200 929 \"-\"\"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:88.0) Gecko/20100101 Firefox/88.0\""
20           ],
21           [
22             "1620748336477761747",
23             " - - [11/May/2021:15:50:54 +0000] \"GET /cron.php HTTP/1.1\" 200 931 \"-\"\"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:88.0) Gecko/20100101 Firefox/88.0\""
24           ]
25         ]
26       }
27     ],
28    "stats": {}
29 }

Buy this article as PDF

Express-Checkout as PDF
Price $2.95
(incl. VAT)

Buy Linux Magazine

Get it on Google Play

US / Canada

Get it on Google Play

UK / Australia

Related content

  • Ubuntu 22.04 LTS

    Ubuntu 22.04 LTS features an updated Linux kernel, numerous programming language updates, and improved virtualization and container tools, making it useful for developers and admins.

  • Skydive

    If you don't speak fluent Ethernet, it sometimes helps to get a graphical view of what your network is doing. Skydive offers visual insights that could reveal complex error patterns.

  • Security Lessons

    Moving data to and from Linux systems under the radar.

  • Virtual Test Network

    If you don't have room on your desk for a whole laboratory of servers, simply hitch up a virtual playground on your own workstation.

  • Perl – Elasticsearch

    Websites often offer readers links to articles about similar topics. Using Elasticsearch, the free search engine, is one way to find related documents instantly and automatically.

comments powered by Disqus
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters

Support Our Work

Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.

Learn More