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| layout | page_title | sidebar_current | description |
|---|---|---|---|
| intro | Registering Health Checks | gettingstarted-checks | We've now seen how simple it is to run Consul, add nodes and services, and query those nodes and services. In this step, we will continue our tour by adding health checks to both nodes and services. Health checks are a critical component of service discovery that prevent using services that are unhealthy. |
Health Checks
We've now seen how simple it is to run Consul, add nodes and services, and query those nodes and services. In this section, we will continue our tour by adding health checks to both nodes and services. Health checks are a critical component of service discovery that prevent using services that are unhealthy.
This step builds upon the Consul cluster created previously. At this point, you should have a two-node cluster running.
Defining Checks
Similar to a service, a check can be registered either by providing a check definition or by making the appropriate calls to the HTTP API.
We will use the check definition approach because, just like with services, definitions are the most common way to set up checks.
Create two definition files in the Consul configuration directory of the second node:
vagrant@n2:~$ echo '{"check": {"name": "ping", \
"script": "ping -c1 google.com >/dev/null", "interval": "30s"}}' \
>/etc/consul.d/ping.json
vagrant@n2:~$ echo '{"service": {"name": "web", "tags": ["rails"], "port": 80,\
"check": {"script": "curl localhost >/dev/null 2>&1", "interval": "10s"}}}' \
>/etc/consul.d/web.json
The first definition adds a host-level check named "ping". This check runs
on a 30 second interval, invoking ping -c1 google.com. On a script-based
health check, the check runs as the same user that started the Consul process.
If the command exits with a non-zero exit code, then the node will be flagged
unhealthy. This is the contract for any script-based health check.
The second command modifies the service named web, adding a check that sends a
request every 10 seconds via curl to verify that the web server is accessible.
As with the host-level health check, if the script exits with a non-zero exit code,
the service will be flagged unhealthy.
Now, restart the second agent or send it a SIGHUP signal. You should see the
following log lines:
==> Starting Consul agent...
...
[INFO] agent: Synced service 'web'
[INFO] agent: Synced check 'service:web'
[INFO] agent: Synced check 'ping'
[WARN] Check 'service:web' is now critical
The first few lines indicate that the agent has synced the new
definitions. The last line indicates that the check we added for
the web service is critical. This is because we're not actually running
a web server, so the curl test is failing!
Checking Health Status
Now that we've added some simple checks, we can use the HTTP API to inspect them. First, we can look for any failing checks using this command (note, this can be run on either node):
vagrant@n1:~$ curl http://localhost:8500/v1/health/state/critical
[{"Node":"agent-two","CheckID":"service:web","Name":"Service 'web' check","Status":"critical","Notes":"","ServiceID":"web","ServiceName":"web"}]
We can see that there is only a single check, our web service check, in the
critical state.
Additionally, we can attempt to query the web service using DNS. Consul will not return any results since the service is unhealthy:
dig @127.0.0.1 -p 8600 web.service.consul
...
;; QUESTION SECTION:
;web.service.consul. IN A
Next Steps
In this section, you learned how easy it is to add health checks. Check definitions
can be updated by changing configuration files and sending a SIGHUP to the agent.
Alternatively, the HTTP API can be used to add, remove, and modify checks dynamically.
The API also allows for a "dead man's switch", a
TTL-based check. TTL checks can be used to integrate an
application more tightly with Consul, enabling business logic to be evaluated as part
of assessing the state of the check.
Next, we will explore Consul's K/V store.