Merge branch 'health_check_docs' into 'master'

Add health check feature documentation

for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3888

@virtuacreative @axil 

See merge request !4199

doc/README.md

@@ -41,6 +41,7 @@
 - [Git LFS configuration](workflow/lfs/lfs_administration.md)
 - [Housekeeping](administration/housekeeping.md) Keep your Git repository tidy and fast.
 - [GitLab Performance Monitoring](monitoring/performance/introduction.md) Configure GitLab and InfluxDB for measuring performance metrics
+- [Monitoring uptime](monitoring/health_check.md) Check the server status using the health check endpoint
 - [Sidekiq Troubleshooting](administration/troubleshooting/sidekiq.md) Debug when Sidekiq appears hung and is not processing jobs
 - [High Availability](administration/high_availability/README.md) Configure multiple servers for scaling or high availability
 

doc/monitoring/health_check.md

+# Health Check
+
+>**Note:** This feature was [introduced][ce-3888] in GitLab 8.8.
+
+GitLab provides a health check endpoint for uptime monitoring on the `health_check` web
+endpoint. The health check reports on the overall system status based on the status of
+the database connection, the state of the database migrations, and the ability to write
+and access the cache. This endpoint can be provided to uptime monitoring services like
+[Pingdom][pingdom], [Nagios][nagios-health], and [NewRelic][newrelic-health].
+
+## Access Token
+
+An access token needs to be provided while accessing the health check endpoint. The current
+accepted token can be found on the `admin/heath_check` page of your GitLab instance.
+
+![access token](img/health_check_token.png)
+
+The access token can be passed as a URL parameter:
+
+```
+https://gitlab.example.com/health_check.json?token=ACCESS_TOKEN
+```
+
+or as an HTTP header:
+
+```bash
+curl -H "TOKEN: ACCESS_TOKEN" https://gitlab.example.com/health_check.json
+```
+
+## Using the Endpoint
+
+Once you have the access token, health information can be retrieved as plain text, JSON,
+or XML using the `health_check` endpoint:
+
+- `https://gitlab.example.com/health_check?token=ACCESS_TOKEN`
+- `https://gitlab.example.com/health_check.json?token=ACCESS_TOKEN`
+- `https://gitlab.example.com/health_check.xml?token=ACCESS_TOKEN`
+
+You can also ask for the status of specific services:
+
+- `https://gitlab.example.com/health_check/cache.json?token=ACCESS_TOKEN`
+- `https://gitlab.example.com/health_check/database.json?token=ACCESS_TOKEN`
+- `https://gitlab.example.com/health_check/migrations.json?token=ACCESS_TOKEN`
+
+For example, the JSON output of the following health check:
+
+```bash
+curl -H "TOKEN: ACCESS_TOKEN" https://gitlab.example.com/health_check.json
+```
+
+would be like:
+
+```
+{"healthy":true,"message":"success"}
+```
+
+## Status
+
+On failure, the endpoint will return a `500` HTTP status code. On success, the endpoint
+will return a valid successful HTTP status code, and a `success` message. Ideally your
+uptime monitoring should look for the success message.
+
+[ce-3888]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3888
+[pingdom]: https://www.pingdom.com
+[nagios-health]: https://nagios-plugins.org/doc/man/check_http.html
+[newrelic-health]: https://docs.newrelic.com/docs/alerts/alert-policies/downtime-alerts/availability-monitoring