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Target Scrape Failures

Introduction​

In Prometheus, a monitoring system that collects metrics from configured targets, one of the most common issues you might encounter is target scrape failures. These occur when Prometheus cannot successfully collect metrics from a specified target. Understanding why these failures happen and how to resolve them is crucial for maintaining a reliable monitoring system.

A scrape failure can happen for various reasons - network issues, misconfiguration, authentication problems, or target service unavailability. This guide will help you identify, debug, and fix scrape failures in your Prometheus setup.

Understanding Target Scrape Failures​

What is a Scrape?​

Before diving into failures, let's understand what a scrape operation is:

Prometheus works on a pull model where it periodically sends HTTP requests (typically to /metrics endpoints) to collect data from targets. This data collection process is called a "scrape."

What Constitutes a Scrape Failure?​

A scrape failure occurs when:

  1. Prometheus cannot reach the target
  2. The target returns a non-2xx HTTP status code
  3. The connection times out
  4. The response format is invalid
  5. Authentication fails

Identifying Scrape Failures​

Using the Prometheus UI​

The simplest way to identify scrape failures is through the Prometheus UI:

  1. Navigate to your Prometheus web interface (typically at http://your-prometheus-server:9090)
  2. Go to the "Status" dropdown menu
  3. Select "Targets"
  4. Look for targets with state "DOWN" or any error messages

![Target status page showing failures]

Using Prometheus Metrics​

Prometheus exposes metrics about its own scraping performance:

# HELP up Whether the target is up (1) or not (0)
# TYPE up gauge
up{job="<job_name>", instance="<instance>"} 0

You can query failed targets with:

promql
up == 0

Common Error Messages​

Here are some typical error messages and what they indicate:

  • context deadline exceeded: Timeout connecting to the target
  • connection refused: Target service is not running or not accepting connections
  • 401 Unauthorized: Authentication issue
  • 404 Not Found: Incorrect endpoint path
  • 500 Internal Server Error: Target is experiencing issues

Troubleshooting Scrape Failures​

1. Network Connectivity Issues​

Symptoms: context deadline exceeded, connection refused, or dial tcp: i/o timeout

Debugging Steps:

bash
# Check if you can reach the target from the Prometheus server
curl -v http://target-host:port/metrics

# Test using telnet
telnet target-host port

# Check for firewall rules
sudo iptables -L

Resolution:

  • Ensure network connectivity between Prometheus and target
  • Verify firewall rules allow traffic
  • Check if the target service is running

2. Configuration Issues​

Symptoms: Wrong endpoint, incorrect port, or invalid parameters

Debugging:

Check your Prometheus configuration file (prometheus.yml):

yaml
scrape_configs:
  - job_name: 'example-job'
    scrape_interval: 15s
    metrics_path: /metrics
    scheme: http
    static_configs:
      - targets: ['localhost:9090', 'localhost:9100']

Resolution:

  • Verify the correct port, metrics path, and scheme
  • Ensure target IP/hostname is correct
  • Apply configuration changes and reload Prometheus:
bash
curl -X POST http://prometheus-host:9090/-/reload

3. Authentication Problems​

Symptoms: 401 Unauthorized or 403 Forbidden

Resolution:

  • Configure the correct authentication in your scrape config:
yaml
scrape_configs:
  - job_name: 'secure-endpoint'
    scheme: https
    basic_auth:
      username: 'prometheus'
      password: 'password'
    static_configs:
      - targets: ['secure-host:8443']

4. TLS/SSL Issues​

Symptoms: x509: certificate signed by unknown authority

Resolution:

  • Configure TLS settings correctly:
yaml
scrape_configs:
  - job_name: 'https-endpoint'
    scheme: https
    tls_config:
      cert_file: /path/to/certificate.cert
      key_file: /path/to/key.key
      insecure_skip_verify: false  # Set to true to skip verification (not recommended for production)
    static_configs:
      - targets: ['secure-host:8443']

5. Target Service Issues​

Symptoms: 500 Internal Server Error or metrics endpoint not working

Debugging:

  • Check the logs of the target service
  • Manually query the metrics endpoint:
bash
curl -v http://target-host:port/metrics

Resolution:

  • Restart the target service if needed
  • Fix any issues with the target's metrics exporter

Real-World Example: Troubleshooting Node Exporter​

Let's walk through a complete example of troubleshooting a scrape failure with Node Exporter:

Scenario​

You've configured Prometheus to scrape a Node Exporter instance, but it's showing as "DOWN" in the targets page.

Step 1: Check the error message​

In the Prometheus UI under Status > Targets, you see:

Error: connection refused

Step 2: Verify the Node Exporter is running​

bash
# Check if process is running
ps aux | grep node_exporter

# If not running, start it
node_exporter

Step 3: Verify connectivity​

bash
# Test from Prometheus server
curl http://node-exporter-host:9100/metrics

Step 4: Check your Prometheus configuration​

yaml
scrape_configs:
  - job_name: 'node'
    static_configs:
      - targets: ['node-exporter-host:9100']

Step 5: Solve the issue​

In this case, the Node Exporter wasn't running. After starting it:

bash
# Start Node Exporter with proper permissions
sudo systemctl start node_exporter

# Or run manually
./node_exporter

After restarting the service, the target shows as "UP" in Prometheus.

Advanced Troubleshooting: Custom Relabel Configurations​

Sometimes targets fail because of relabeling issues. Check your relabel configurations:

yaml
scrape_configs:
  - job_name: 'relabeled-job'
    static_configs:
      - targets: ['host:port']
    relabel_configs:
      - source_labels: [__address__]
        regex: '(.*):(.*)'
        target_label: instance
        replacement: '$1'

Summary​

Target scrape failures are a common issue in Prometheus deployments. By systematically checking for network, configuration, authentication, and target service issues, you can quickly identify and resolve these problems.

Remember these key points:

  • Use the Prometheus UI to quickly identify failed targets
  • Query up == 0 to get a list of failed targets
  • Check connectivity from the Prometheus server to the target
  • Verify your configuration for correct endpoints and parameters
  • Examine target service logs for any issues
  • Restart Prometheus after configuration changes with prometheus --config.file=prometheus.yml

Additional Resources​

Exercises​

  1. Deliberately misconfigure a target in your Prometheus setup and practice troubleshooting it.
  2. Set up a secure endpoint requiring TLS and basic authentication, then configure Prometheus to scrape it.
  3. Create a dashboard in Grafana that shows all your failed targets based on the up metric.
  4. Use the Prometheus API to fetch the status of all targets programmatically.

If you spot any mistakes on this website, please let me know at [email protected]. I’d greatly appreciate your feedback! :)