Exploring Expression Browser

In this lesson we’ll walk through the Prometheus expression browser — the built-in web UI for running PromQL queries directly against your Prometheus server. You’ll learn how to run ad-hoc queries, inspect raw time series, visualize results, and explore useful server pages for configuration, targets, rules, and alerts. To open the expression browser, point your web browser to the IP or hostname of your Prometheus server and the Prometheus port (default: 9090). For a local install, use: http://localhost:9090 Key UI elements:

Expression input: type PromQL queries here.
Execute button: run the query at the chosen evaluation time.
Graph / Console tabs: visualize results or view raw series.
Autocomplete / Highlighting / History: help craft and re-run queries.

Below is the expression browser with metric autocomplete suggesting metrics related to “up”.

The image shows the Prometheus time series database interface on a web browser, with a search bar suggesting metric names related to "up".

Basic queries Start with a simple built-in metric, up, which returns one time series per monitored target with labels that identify that target. Example Console output:

up{instance="192.168.1.168:9100", job="node"}
up{instance="192.168.21.43:80", job="ec2"}
up{instance="192.168.40.248:80", job="ec2"}
up{instance="localhost:9090", job="prometheus"}

In Prometheus, a value of 1 means the target is reachable (UP).
A value of 0 means the target is unreachable (DOWN).
Press Execute and switch to the Console tab to see the raw series and their current values.

Drilling into metrics More detailed metrics produce many series — for example, CPU metrics are split by CPU/core, mode, instance, and job. Example:

node_cpu_seconds_total{cpu="0", instance="192.168.1.168:9100", job="node", mode="idle"} 1434.34
node_cpu_seconds_total{cpu="0", instance="192.168.1.168:9100", job="node", mode="iowait"} 7.67
node_cpu_seconds_total{cpu="0", instance="192.168.1.168:9100", job="node", mode="irq"} 0
node_cpu_seconds_total{cpu="0", instance="192.168.1.168:9100", job="node", mode="nice"} 24.7
node_cpu_seconds_total{cpu="0", instance="192.168.1.168:9100", job="node", mode="softirq"} 0.51
node_cpu_seconds_total{cpu="0", instance="192.168.1.168:9100", job="node", mode="steal"} 0
node_cpu_seconds_total{cpu="0", instance="192.168.1.168:9100", job="node", mode="system"} 21.6
node_cpu_seconds_total{cpu="0", instance="192.168.1.168:9100", job="node", mode="user"} 60.38

Time-travel evaluation The expression browser supports historical evaluation. Use the calendar/time selector to set a past evaluation time and re-run the query to see values at that timestamp.

The image shows a Prometheus web interface on Firefox running on Ubuntu, displaying a data query with a calendar and time selection tool. The interface includes options like "Enable autocomplete" and displays CPU seconds for different nodes.

Graphing queries Switch to the Graph tab to visualize query results over a time range. You can:

Adjust the time window (e.g., 1h, 30m, custom range).
Choose chart styles and display options.
Filter or aggregate series if a metric returns many lines (otherwise all series will be plotted).

The image shows a Prometheus graph displaying CPU usage metrics over time with various data series, while running on a Linux operating system within a Firefox web browser.

Query authoring helpers The expression input includes autocomplete, syntax highlighting, and a query history — toggle these features via the checkboxes in the UI. Use autocomplete to discover metric names and label keys quickly. Useful server pages The expression browser exposes several server pages under the Status and main navigation menus. These pages are essential for debugging Prometheus itself.

Page	Purpose	What you’ll find
`Status > Configuration`	Inspect the active Prometheus config	The YAML file Prometheus loaded at startup (scrape intervals, scrape configs, alerting settings)
`Status > Targets`	Troubleshoot scraping issues	All scrape targets, their UP/DOWN status, last scrape time, and scrape errors
`Status > Rules`	View recording & alerting rules	Loaded recording rules and alerting rules with evaluation state
`Alerts`	Review alerts	Active and pending alerts generated from alerting rules

Example (truncated) of a running prometheus.yml shown on the Configuration page:

global:
  scrape_interval: 15s
  evaluation_interval: 15s
alerting:
  alertmanagers:
  - follow_redirects: true
    enable_http2: true
    scheme: http
    timeout: 10s
    api_version: v2
    static_configs:
    - targets: []
scrape_configs:
- job_name: prometheus
  honor_timestamps: true
  scrape_interval: 15s
  scrape_timeout: 10s
  metrics_path: /metrics
  scheme: http
  follow_redirects: true
  enable_http2: true
  static_configs:
  - targets:
    - localhost:9090
- job_name: node
  honor_timestamps: true
  scrape_interval: 15s
  scrape_timeout: 10s
  metrics_path: /metrics
  scheme: http
  follow_redirects: true
  enable_http2: true
  static_configs:
  - targets:
    - 192.168.1.168:9100
- job_name: ec2
  honor_timestamps: true
  scrape_interval: 15s
  scrape_timeout: 10s
  metrics_path: /metrics
  scheme: http
  follow_redirects: true
  enable_http2: true
  static_configs:
  - targets: []

The Targets page helps you spot unreachable endpoints and scrape errors. The example below shows some endpoints marked DOWN with scrape errors and others UP:

The image shows a Prometheus web interface displaying the status of various endpoints, with some marked as "DOWN" and others as "UP". It highlights scrape errors for two "ec2" endpoints and a proper connection for "node" and "prometheus".

Best practices and next steps

Use the expression browser for exploratory, ad-hoc queries and troubleshooting. For long-term dashboards and richer visualizations, connect Prometheus to Grafana and build persistent dashboards there.

Do not expose the Prometheus expression browser directly to the public internet. Use network controls, authentication proxies, or restricted access to avoid leaking metrics and configuration.

Links and references

Prometheus: https://prometheus.io/
PromQL basics: https://prometheus.io/docs/prometheus/latest/querying/basics/
Grafana: https://grafana.com/

Summary This covers the core usage of the Prometheus expression browser:

Run PromQL queries and use autocomplete to author queries faster.
View raw time series in the Console and plot metrics in the Graph tab.
Use time-travel evaluation to inspect historical metric values.
Inspect Prometheus configuration, targets, rules, and alerts via the server pages for effective troubleshooting and monitoring.

Introduction

Observability Fundamentals

Prometheus Fundamentals

PromQL

Dashboarding & Visualization

Application Instrumentation

Service Discovery

Push Gateway

Alerting

Monitoring Kubernetes

Conclusion

Scaling & Long Term Storage

Exploring Expression Browser

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