How to monitor external resources with proxy requests and entities

Proxy entities allow Sensu to monitor external resources on systems or devices where a Sensu agent cannot be installed, like a network switch or a website. You can create proxy entities using sensuctl, the Sensu API, or the proxy_entity_name check attribute. When executing checks that include a proxy_entity_name, Sensu agents report the resulting event under the proxy entity instead of the agent entity.

This guide requires a running Sensu backend, a running Sensu agent, and a sensuctl instance configured to connect to the backend as a user with read and create permissions for entities, checks, and events.

Using a proxy entity to monitor a website

In this section, we’ll monitor the status of sensu.io by configuring a check with a proxy entity name so that Sensu creates an entity representing the site and reports the status of the site under this entity.

Installing an HTTP check script

First, we’ll install a bash script, named http_check.sh, to perform an HTTP check using curl.

sudo curl https://raw.githubusercontent.com/sensu/sensu-go/5.1.0/examples/checks/http_check.sh \
-o /usr/local/bin/http_check.sh && \
sudo chmod +x /usr/local/bin/http_check.sh

PRO TIP: While this command may be appropriate when running a few agents, you should consider using Sensu assets or a configuration management tool to provide runtime dependencies.

Creating the check

Now that our script is installed, we’ll create a check named check-http, which runs the command http_check.sh https://sensu.io, at an interval of 60 seconds, for all entities subscribed to the proxy subscription, using the sensu-site proxy entity name.

Create a file called check.json and add the following check definition.

{
  "type": "CheckConfig",
  "api_version": "core/v2",
  "metadata": {
    "name": "check-http",
    "namespace": "default"
  },
  "spec": {
    "command": "http_check.sh https://sensu.io",
    "interval": 60,
    "proxy_entity_name": "sensu-site",
    "publish": true,
    "subscriptions": [
      "proxy"
    ]
  }
}

Now we can use sensuctl to add this check to Sensu.

sensuctl create --file check.json

sensuctl check list
    Name                 Command               Interval   Cron   Timeout   TTL   Subscriptions   Handlers   Assets   Hooks   Publish?   Stdin?   Metric Format   Metric Handlers  
──────────── ──────────────────────────────── ────────── ────── ───────── ───── ─────────────── ────────── ──────── ─────── ────────── ──────── ─────────────── ───────────────── 
 check-http   http_check.sh https://sensu.io         60                0     0   proxy                                       true       false

Adding the subscription

To run the check, we’ll need a Sensu agent with the subscription proxy. After installing an agent, open /etc/sensu/agent.yml and add the proxy subscription so the subscription configuration looks like:

subscriptions:
  - "proxy"

Then restart the agent.

sudo service sensu-agent restart

Validating the check

Now we can use sensuctl to see that Sensu has created the proxy entity sensu-site.

sensuctl entity list
      ID        Class    OS           Subscriptions                   Last Seen            
────────────── ─────── ─────── ─────────────────────────── ─────────────────────────────── 
sensu-centos   agent   linux   proxy,entity:sensu-centos   2019-01-16 21:50:03 +0000 UTC  
sensu-site     proxy           entity:sensu-site           N/A

And that Sensu is now monitoring sensu-site using the check-http check.

sensuctl event info sensu-site check-http
=== sensu-site - check-http
Entity:    sensu-site
Check:     check-http
Output:    
Status:    0
History:   0,0
Silenced:  false
Timestamp: 2019-01-16 21:51:53 +0000 UTC

NOTE: It might take a few moments for Sensu to execute the check and create the proxy entity.

We can also see our new proxy entity in the Sensu dashboard.

Using proxy requests to monitor a group of websites

Now let’s say that, instead of monitoring just sensu.io, we want to monitor multiple sites, for example: docs.sensu.io, packagecloud.io, and github.com. In this section of the guide, we’ll use the proxy_requests check attribute, along with entity labels and token substitution, to monitor three sites using the same check. Before we get started, go ahead and install the HTTP check script if you haven’t already.

Installing an HTTP check script

If you haven’t already, install a bash script, named http_check.sh, to perform an HTTP check using curl.

sudo curl https://raw.githubusercontent.com/sensu/sensu-go/5.1.0/examples/checks/http_check.sh \
-o /usr/local/bin/http_check.sh && \
sudo chmod +x /usr/local/bin/http_check.sh

PRO TIP: While this command may be appropriate when running a few agents, you should consider using Sensu assets or a configuration management tool to provide runtime dependencies.

Creating proxy entities

Instead of creating a proxy entity using the proxy_entity_name check attribute, we’ll be using sensuctl to create proxy entities to represent the three sites we want to monitor. Our proxy entities need the entity_class attribute set to proxy to mark them as proxy entities as well as a few custom labels that we’ll use to identify them as a group and pass in individual URLs.

Create a file called entities.json and add the following entity definitions.

{
  "type": "Entity",
  "api_version": "core/v2",
  "metadata": {
    "name": "sensu-docs",
    "namespace": "default",
    "labels": {
      "proxy_type": "website",
      "url": "https://docs.sensu.io"
    }
  },
  "spec": {
    "entity_class": "proxy"
  }
}
{
  "type": "Entity",
  "api_version": "core/v2",
  "metadata": {
    "name": "packagecloud-site",
    "namespace": "default",
    "labels": {
      "proxy_type": "website",
      "url": "https://packagecloud.io"
    }
  },
  "spec": {
    "entity_class": "proxy"
  }
}
{
  "type": "Entity",
  "api_version": "core/v2",
  "metadata": {
    "name": "github-site",
    "namespace": "default",
    "labels": {
      "proxy_type": "website",
      "url": "https://github.com"
    }
  },
  "spec": {
    "entity_class": "proxy"
  }
}

PRO TIP: When creating proxy entities, you can add whatever custom labels make sense for your environment. For example, when monitoring a group of routers, you may want to add ip_address labels.

Now we can use sensuctl to add these proxy entities to Sensu.

sensuctl create --file entities.json

sensuctl entity list
         ID           Class    OS           Subscriptions                   Last Seen            
 ─────────────────── ─────── ─────── ─────────────────────────── ─────────────────────────────── 
  github-site         proxy                                       N/A                            
  packagecloud-site   proxy                                       N/A                            
  sensu-centos        agent   linux   proxy,entity:sensu-centos   2019-01-16 23:05:03 +0000 UTC  
  sensu-docs          proxy                                       N/A

Creating a reusable HTTP check

Now that we have our three proxy entities set up, each with a proxy_type and url label, we can use proxy requests and token substitution to create a single check that monitors all three sites.

Create a file called check-proxy-requests.json and add the following check definition.

{
  "type": "CheckConfig",
  "api_version": "core/v2",
  "metadata": {
    "name": "check-http-proxy-requests",
    "namespace": "default"
  },
  "spec": {
    "command": "http_check.sh {{ .labels.url }}",
    "interval": 60,
    "subscriptions": [
      "proxy"
    ],
    "publish": true,
    "proxy_requests": {
      "entity_attributes": [
        "entity.entity_class == 'proxy'",
        "entity.labels.proxy_type == 'website'"
      ],
      "splay": true,
      "splay_coverage": 90
    }
  }
}

Our check-http-proxy-requests check uses the proxy_requests attribute to specify the applicable entities. In our case, we want to run the check-http-proxy-requests check on all entities of entity class proxy and proxy type website. To make sure that Sensu runs the check for all applicable entities, we need to set the splay attribute to true with a splay coverage percentage value of 90. This gives Sensu 90% of the check interval, 60 seconds in this case, to execute the check for all applicable entities. Since we’re using this check to monitor multiple sites, we can use token substitution to apply the correct url in the check command.

Now we can use sensuctl to add this check to Sensu.

sensuctl create --file check-proxy-requests.json

sensuctl check list
          Name                          Command               Interval   Cron   Timeout   TTL   Subscriptions   Handlers   Assets   Hooks   Publish?   Stdin?   Metric Format   Metric Handlers  
─────────────────────────── ───────────────────────────────── ────────── ────── ───────── ───── ─────────────── ────────── ──────── ─────── ────────── ──────── ─────────────── ───────────────── 
  check-http                  http_check.sh https://sensu.io          60                0     0   proxy                                       true       false                                     
  check-http-proxy-requests   http_check.sh {{ .labels.url }}         60                0     0   proxy                                       true       false                                                            true       false

Validating the check

Before validating the check, make sure that you’ve added the proxy subscription to a Sensu agent if you haven’t already.

Now we can use sensuctl to see that Sensu is monitoring docs.sensu.io, packagecloud.io, and github.com using the check-http-proxy-requests.

sensuctl event list
      Entity                   Check             Output   Status   Silenced             Timestamp            
─────────────────── ─────────────────────────── ──────── ──────── ────────── ─────────────────────────────── 
github-site         check-http-proxy-requests                 0   false      2019-01-17 17:10:31 +0000 UTC  
packagecloud-site   check-http-proxy-requests                 0   false      2019-01-17 17:10:34 +0000 UTC  
sensu-centos        keepalive                                 0   false      2019-01-17 17:10:34 +0000 UTC  
sensu-docs          check-http-proxy-requests                 0   false      2019-01-17 17:06:59 +0000 UTC

Next steps

You now know how to run a proxy check to verify the status of a website, as well as using proxy requests to run a check on two different proxy entities based on label evaluation. From this point, here are some recommended resources: