Publishing alerts from Prometheus

This page describes how you can configure Prometheus to monitor Liberator and publish alerts to Slack, raise alerts when Liberator is down, and publish those alerts to Slack via the Alertmanager alert message router.

Introduction to Prometheus in Caplin Platform

All Caplin Platform applications export a core set of Prometheus metrics supplemented by metrics specific to the application. See, for example, Liberator’s Prometheus metrics.

Enabling Prometheus endpoints

To enable the Prometheus endpoint in a Caplin application, set the application’s prometheus-port configuration item to a free port on the host.

Liberator 8, when deployed to the Caplin Deployment Framework, is configured by default to publish Prometheus metrics on port 8083:

prometheus-port 8083

The Prometheus endpoint is available at the URI /metrics:


Instrumenting your own Java DataSource adapters

If you write your own Java DataSource adapters, you can supplement the default Caplin Java DataSource metrics with metrics specific to your application.

Java DataSources use Micrometer for instrumentation. To add a custom metric to an adapter’s Prometheus endpoint, use Micrometer’s Metrics class:

Creating a Prometheus gauge metric
import io.micrometer.core.instrument.Metrics;

requestCountGauge = Metrics.globalRegistry.gauge(appName + "request.count", new AtomicInteger(0));

For more information on creating your own custom metrics, see the following resources:


In this walkthrough, we will deploy Liberator, Prometheus, and Alertmanager, and configure Prometheus and Alertmanager to publish alerts to Slack.

Deploy Liberator 8

Liberator 8 can be deployed in three different ways:

  • Deploy to the Caplin Deployment Framework

  • Deploy as a standalone server

  • Deploy as a Docker image

For the purposes of this walkthrough, we’ll keep things simple and run Liberator in the Platform Core Docker image, available from the registry (Caplin account required).

  1. Open a new terminal and run the command below to start Liberator in a Platform Core 8 container:

    docker run --rm --name liberator --interactive --tty --publish 8083:8083 --publish 18080:18080 /app/DeploymentFramework/dfw start-fg Liberator

    The command above runs Liberator in an interactive foreground container. When you want to stop Liberator, press Ctrl-C in the terminal window.

  2. To view the metrics exported by Liberator, navigate to http://localhost:8083/metrics. You will see output resembling the sample below:

    # HELP rttpd_session_connections_permitted gauge
    # TYPE rttpd_session_connections_permitted gauge
    rttpd_session_connections_permitted 1
    # HELP rttpd_process_fds gauge
    # TYPE rttpd_process_fds gauge
    rttpd_process_fds{type="non_socket_fds"} 27
    rttpd_process_fds{type="socket_fds"} 535
    # HELP rttpd_data_service_messages_read_total counter
    # TYPE rttpd_data_service_messages_read_total counter
    rttpd_data_service_messages_read_total{service_name="broadcast",message_type="nodata"} 0
    rttpd_data_service_messages_read_total{service_name="ContainerTestsService",message_type="map"} 0

Install Prometheus

In this section, we’ll install Prometheus and configure an alert when Liberator is down.

Follow the steps below:

  1. Download the latest version of Prometheus from the Prometheus Download page, and extract it to a local directory:

    Example directory structure following download and extraction
  2. In the Prometheus root directory, edit prometheus.yml and add a scrape job for Liberator in the scrape_configs sequence:

      - job_name: "liberator"
        scrape_timeout: 30s
        scrape_interval: 40s
          - targets: ["localhost:8083"]
  3. In the Prometheus root directory, create a rules.yml file containing the content below:

      - name: liberator
          - alert: InstanceDown
            expr: up == 0
            for: 1m

    For more information on writing alerting rules, see Alerting rules.

  4. In the Prometheus root directory, edit the prometheus.yml file and add rules.yml to the rule_files sequence:

      # - "first_rules.yml"
      # - "second_rules.yml"
      - "rules.yml"
  5. If Liberator is not already running, open a new terminal and start Liberator now:

    docker run --rm --name liberator --interactive --tty --publish 8083:8083 --publish 18080:18080 /app/DeploymentFramework/dfw start-fg Liberator
  6. In the Prometheus root directory, start Prometheus:

  7. In a web browser, navigate to http://localhost:9090 to confirm that Prometheus is running:

    prometheus home
  8. On the Prometheus home page, click Status > Targets to confirm that Prometheus has detected that the Liberator is 'up':

    liberator prometheus up
  9. In the Liberator terminal window, press Ctrl-C to stop the Liberator.

  10. In Prometheus, click Alerts. Notice that Prometheus has fired an alert for Liberator’s status change:

    liberator prometheus down

Install Alertmanager and route alerts to Slack

In the previous section we configured an alerting rule for a Liberator status change. At this point, the alert is visible only in the Prometheus GUI. To publish the alert to Slack, we use Prometheus’s sister application: Alertmanager.

Follow the steps below:

  1. Download Alertmanager from the Prometheus Download page and extract to a local directory:

    ~ alertmanager-0.26.0-linux-amd64 alertmanager alertmanager.ymlprometheus-2.49.1.linux-amd64prometheusprometheus.yml
    Example directory structure following download and extraction
  1. In Slack’s Manage Apps settings, create a webhook URL for the Slack channel you want to post alerts to.

  2. Add the following configuration to Alertmanager’s alertmanager.yml configuration file:

      resolve_timeout: 1m
      slack_api_url: '<your_webhook_url>'
      receiver: 'slack-notifications'
      - name: 'slack-notifications'
          - channel: '#<your_slack_channel>'
            send_resolved: true
  3. Add the configuration below to your prometheus.yml file to provide Prometheus with the location of your Alertmanager. By default, Alertmanager runs on port 9093.

    # Alertmanager configuration
        - static_configs:
          - targets:
            - localhost:9093
  4. If Liberator is not already running, start Liberator. Navigate to http://localhost:18080 to confirm Liberator has started.

    docker run --rm --name liberator --interactive --tty --publish 8083:8083 --publish 18080:18080 /app/DeploymentFramework/dfw start-fg Liberator
  5. Start Alertmanager. Navigate to localhost:9093 to confirm Alertmanager has started.

  6. Start Prometheus. Navigate to localhost:9090 to confirm Prometheus has started.

  7. In the Liberator terminal window, press Ctrl-C to stop the Liberator.

    Alertmanager sends a message to your configured Slack channel. For example:

    prometheus alertmanager basic alerts

More informative and interactive alerts can be created by using the Alertmanager’s templating syntax to produce alerts similar to the example below:

alertmanager styled down

For a detailed reference on configuring Alertmanager, including how to customise the presentation of alert notifications, see Alerting in the Prometheus documentation.