path: root/health/REFERENCE.md

# Configure alerts

Netdata's health watchdog is highly configurable, with support for dynamic thresholds, hysteresis, alert templates, and
more. You can tweak any of the existing alerts based on your infrastructure's topology or specific monitoring needs, or
create new entities.

You can use health alerts in conjunction with any of Netdata's [collectors](https://github.com/netdata/netdata/blob/master/collectors/README.md) (see
the [supported collector list](https://github.com/netdata/netdata/blob/master/collectors/COLLECTORS.md)) to monitor the health of your systems, containers, and
applications in real time.

While you can see active alerts both on the local dashboard and Netdata Cloud, all health alerts are configured _per
node_ via individual Netdata Agents. If you want to deploy a new alert across your
[infrastructure](https://github.com/netdata/netdata/blob/master/docs/quickstart/infrastructure.md), you must configure each node with the same health configuration
files.

## Edit health configuration files

You can configure the Agent's health watchdog service by editing files in two locations:

- The `[health]` section in `netdata.conf`. By editing the daemon's behavior, you can disable health monitoring
    altogether, run health checks more or less often, and more. See
    [daemon configuration](https://github.com/netdata/netdata/blob/master/daemon/config/README.md#health-section-options) for a table of
    all the available settings, their default values, and what they control.

- The individual `.conf` files in `health.d/`. These health entity files are organized by the type of metric they are
    performing calculations on or their associated collector. You should edit these files using the `edit-config`
    script. For example: `sudo ./edit-config health.d/cpu.conf`.

Navigate to your [Netdata config directory](https://github.com/netdata/netdata/blob/master/docs/configure/nodes.md) and
use `edit-config` to make changes to any of these files.

### Edit individual alerts

For example, to edit the `cpu.conf` health configuration file, run:

```bash
sudo ./edit-config health.d/cpu.conf
```

Each health configuration file contains one or more health _entities_, which always begin with `alarm:` or `template:`.
For example, here is the first health entity in `health.d/cpu.conf`:

```yaml
 template: 10min_cpu_usage
       on: system.cpu
    class: Utilization
     type: System
component: CPU
       os: linux
    hosts: *
   lookup: average -10m unaligned of user,system,softirq,irq,guest
    units: %
    every: 1m
     warn: $this > (($status >= $WARNING)  ? (75) : (85))
     crit: $this > (($status == $CRITICAL) ? (85) : (95))
    delay: down 15m multiplier 1.5 max 1h
  summary: CPU utilization
     info: Average cpu utilization for the last 10 minutes (excluding iowait, nice and steal)
       to: sysadmin
```

To tune this alert to trigger warning and critical alerts at a lower CPU utilization, change the `warn` and `crit` lines
to the values of your choosing. For example:

```yaml
    warn: $this > (($status >= $WARNING)  ? (60) : (75))
    crit: $this > (($status == $CRITICAL) ? (75) : (85))
```

Save the file and [reload Netdata's health configuration](#reload-health-configuration) to apply your changes.

## Disable or silence alerts

Alerts and notifications can be disabled permanently via configuration changes, or temporarily, via the
[health management API](https://github.com/netdata/netdata/blob/master/web/api/health/README.md). The
available options are described below.

### Disable all alerts

In the `netdata.conf` `[health]` section, set `enabled` to `no`, and restart the agent.

### Disable some alerts

In the `netdata.conf` `[health]` section, set `enabled alarms` to a
[simple pattern](https://github.com/netdata/netdata/edit/master/libnetdata/simple_pattern/README.md) that
excludes one or more alerts. e.g. `enabled alarms = !oom_kill *` will load all alerts except `oom_kill`.

You can also [edit the file where the alert is defined](#edit-individual-alerts), comment out its definition,
and [reload Netdata's health configuration](#reload-health-configuration).

### Silence an individual alert

You can stop receiving notification for an individual alert by [changing](#edit-individual-alerts) the `to:` line to `silent`.

```yaml
      to: silent
```

This action requires that you [reload Netdata's health configuration](#reload-health-configuration).

### Temporarily disable alerts at runtime

When you need to frequently disable all or some alerts from triggering during certain times (for instance
when running backups) you can use the
[health management API](https://github.com/netdata/netdata/blob/master/web/api/health/README.md).
The API allows you to issue commands to control the health engine's behavior without changing configuration,
or restarting the agent.

### Temporarily silence notifications at runtime

If you want health checks to keep running and alerts to keep getting triggered, but notifications to be
suppressed temporarily, you can use the
[health management API](https://github.com/netdata/netdata/blob/master/web/api/health/README.md).
The API allows you to issue commands to control the health engine's behavior without changing configuration,
or restarting the agent.

## Write a new health entity

While tuning existing alerts may work in some cases, you may need to write entirely new health entities based on how
your systems, containers, and applications work.

Read the [health entity reference](#health-entity-reference) for a full listing of the format,
syntax, and functionality of health entities.

To write a new health entity into a new file, navigate to your [Netdata config directory](https://github.com/netdata/netdata/blob/master/docs/configure/nodes.md),
then use `touch` to create a new file in the `health.d/` directory. Use `edit-config` to start editing the file.

As an example, let's create a `ram-usage.conf` file.

```bash
sudo touch health.d/ram-usage.conf
sudo ./edit-config health.d/ram-usage.conf
```

For example, here is a health entity that triggers a warning alert when a node's RAM usage rises above 80%, and a
critical alert above 90%:

```yaml
 alarm: ram_usage
    on: system.ram
lookup: average -1m percentage of used
 units: %
 every: 1m
  warn: $this > 80
  crit: $this > 90
  info: The percentage of RAM being used by the system.
```

Let's look into each of the lines to see how they create a working health entity.

- `alarm`: The name for your new entity. The name needs to follow these requirements:
  - Any alphabet letter or number.
  - The symbols `.` and `_`.
  - Cannot be `chart name`, `dimension name`, `family name`, or `chart variable names`.  

- `on`: Which chart the entity listens to.

- `lookup`: Which metrics the alert monitors, the duration of time to monitor, and how to process the metrics into a
    usable format.
  - `average`: Calculate the average of all the metrics collected.
  - `-1m`: Use metrics from 1 minute ago until now to calculate that average.
  - `percentage`: Clarify that we're calculating a percentage of RAM usage.
  - `of used`: Specify which dimension (`used`) on the `system.ram` chart you want to monitor with this entity.

- `units`: Use percentages rather than absolute units.

- `every`: How often to perform the `lookup` calculation to decide whether to trigger this alert.

- `warn`/`crit`: The value at which Netdata should trigger a warning or critical alert. This example uses simple
    syntax, but most pre-configured health entities use
    [hysteresis](#special-use-of-the-conditional-operator) to avoid superfluous notifications.

- `info`: A description of the alert, which will appear in the dashboard and notifications.

In human-readable format:

> This health entity, named **ram_usage**, watches the **system.ram** chart. It looks up the last **1 minute** of
> metrics from the **used** dimension and calculates the **average** of all those metrics in a **percentage** format,
> using a **% unit**. The entity performs this lookup **every minute**.
>
> If the average RAM usage percentage over the last 1 minute is **more than 80%**, the entity triggers a warning alert.
> If the usage is **more than 90%**, the entity triggers a critical alert.

When you finish writing this new health entity, [reload Netdata's health configuration](#reload-health-configuration) to
see it live on the local dashboard or Netdata Cloud.

## Reload health configuration

To make any changes to your health configuration live, you must reload Netdata's health monitoring system. To do that
without restarting all of Netdata, run `netdatacli reload-health` or `killall -USR2 netdata`.

## Health entity reference

The following reference contains information about the syntax and options of _health entities_, which Netdata attaches
to charts in order to trigger alerts.

### Entity types

There are two entity types: **alarms** and **templates**. They have the same format and feature set—the only difference
is their label.

**Alerts** are attached to specific charts and use the `alarm` label.

**Templates** define rules that apply to all charts of a specific context, and use the `template` label. Templates help
you apply one entity to all disks, all network interfaces, all MySQL databases, and so on.

Alerts have higher precedence and will override templates.
If the `alert` and `template` entities have the same name and are attached to the same chart, Netdata will use `alarm`.

### Entity format

Netdata parses the following lines. Beneath the table is an in-depth explanation of each line's purpose and syntax.

- The `alarm` or `template` line must be the first line of any entity.
- The `on` line is **always required**.
- The `every` line is **required** if not using `lookup`.
- Each entity **must** have at least one of the following lines: `lookup`, `calc`, `warn`, or `crit`.
- A few lines use space-separated lists to define how the entity behaves. You can use `*` as a wildcard or prefix with
    `!` for a negative match. Order is important, too! See our [simple patterns docs](https://github.com/netdata/netdata/blob/master/libnetdata/simple_pattern/README.md) for
    more examples.
- Lines terminated by a `\` are spliced together with the next line. The backslash is removed and the following line is
    joined with the current one. No space is inserted, so you may split a line anywhere, even in the middle of a word.
    This comes in handy if your `info` line consists of several sentences.  

| line                                                | required        | functionality                                                                         |
|-----------------------------------------------------|-----------------|---------------------------------------------------------------------------------------|
| [`alarm`/`template`](#alert-line-alarm-or-template) | yes             | Name of the alert/template.                                                           |
| [`on`](#alert-line-on)                              | yes             | The chart this alert should attach to.                                                |
| [`class`](#alert-line-class)                        | no              | The general alert classification.                                                     |
| [`type`](#alert-line-type)                          | no              | What area of the system the alert monitors.                                           |
| [`component`](#alert-line-component)                | no              | Specific component of the type of the alert.                                          |
| [`os`](#alert-line-os)                              | no              | Which operating systems to run this chart.                                            |
| [`hosts`](#alert-line-hosts)                        | no              | Which hostnames will run this alert.                                                  |
| [`plugin`](#alert-line-plugin)                      | no              | Restrict an alert or template to only a certain plugin.                               |
| [`module`](#alert-line-module)                      | no              | Restrict an alert or template to only a certain module.                               |
| [`charts`](#alert-line-charts)                      | no              | Restrict an alert or template to only certain charts.                                 |
| [`families`](#alert-line-families)                  | no              | Restrict a template to only certain families.                                         |
| [`lookup`](#alert-line-lookup)                      | yes             | The database lookup to find and process metrics for the chart specified through `on`. |
| [`calc`](#alert-line-calc)                          | yes (see above) | A calculation to apply to the value found via `lookup` or another variable.           |
| [`every`](#alert-line-every)                        | no              | The frequency of the alert.                                                           |
| [`green`/`red`](#alert-lines-green-and-red)         | no              | Set the green and red thresholds of a chart.                                          |
| [`warn`/`crit`](#alert-lines-warn-and-crit)         | yes (see above) | Expressions evaluating to true or false, and when true, will trigger the alert.       |
| [`to`](#alert-line-to)                              | no              | A list of roles to send notifications to.                                             |
| [`exec`](#alert-line-exec)                          | no              | The script to execute when the alert changes status.                                  |
| [`delay`](#alert-line-delay)                        | no              | Optional hysteresis settings to prevent floods of notifications.                      |
| [`repeat`](#alert-line-repeat)                      | no              | The interval for sending notifications when an alert is in WARNING or CRITICAL mode.  |
| [`options`](#alert-line-options)                    | no              | Add an option to not clear alerts.                                                    |
| [`host labels`](#alert-line-host-labels)            | no              | Restrict an alert or template to a list of matching labels present on a host.         |
| [`chart labels`](#alert-line-chart-labels)          | no              | Restrict an alert or template to a list of matching labels present on a host.         |
| [`summary`](#alert-line-summary)                    | no              | A brief description of the alert.                                                     |
| [`info`](#alert-line-info)                          | no              | A longer text field that provides more information of this alert                      |

The `alarm` or `template` line must be the first line of any entity.

#### Alert line `alarm` or `template`

This line starts an alert or template based on the [entity type](#entity-types) you're interested in creating.

**Alert:**

```yaml
alarm: NAME
```

**Template:**

```yaml
template: NAME
```

`NAME` can be any alpha character, with `.` (period) and `_` (underscore) as the only allowed symbols, but the names
cannot be `chart name`, `dimension name`, `family name`, or `chart variables names`.

#### Alert line `on`

This line defines the chart this alert should attach to.

**Alerts:**

```yaml
on: CHART
```

The value `CHART` should be the unique ID or name of the chart you're interested in, as shown on the dashboard. In the
image below, the unique ID is `system.cpu`.

![Finding the unique ID of a
chart](https://user-images.githubusercontent.com/1153921/67443082-43b16e80-f5b8-11e9-8d33-d6ee052c6678.png)

**Template:**

```yaml
on: CONTEXT
```

The value `CONTEXT` should be the context you want this template to attach to.

Need to find the context? Hover over the date on any given chart and look at the tooltip. In the image below, which
shows a disk I/O chart, the tooltip reads: `proc:/proc/diskstats, disk.io`.

![Finding the context of a chart via the tooltip](https://user-images.githubusercontent.com/1153921/68882856-2b230880-06cd-11ea-923b-b28c4632d479.png)

You're interested in what comes after the comma: `disk.io`. That's the name of the chart's context.

If you create a template using the `disk.io` context, it will apply an alert to every disk available on your system.

#### Alert line `class`

This indicates the type of error (or general problem area) that the alert or template applies to. For example, `Latency` can be used for alerts that trigger on latency issues on network interfaces, web servers, or database systems. Example:

```yaml
class: Latency
```

<details>
<summary>Netdata's stock alerts use the following `class` attributes by default:</summary>

| Class       |
|-------------|
| Errors      |
| Latency     |
| Utilization |
| Workload    |

</details>

`class` will default to `Unknown` if the line is missing from the alert configuration.

#### Alert line `type`

Type can be used to indicate the broader area of the system that the alert applies to. For example, under the general `Database` type, you can group together alerts that operate on various database systems, like `MySQL`, `CockroachDB`, `CouchDB` etc. Example:

```yaml
type: Database
```

<details>
<summary>Netdata's stock alerts use the following `type` attributes by default, but feel free to adjust for your own requirements.</summary>

| Type            | Description                                                                                    |
|-----------------|------------------------------------------------------------------------------------------------|
| Ad Filtering    | Services related to Ad Filtering (like pi-hole)                                                |
| Certificates    | Certificates monitoring related                                                                |
| Cgroups         | Alerts for cpu and memory usage of control groups                                              |
| Computing       | Alerts for shared computing applications (e.g. boinc)                                          |
| Containers      | Container related alerts (e.g. docker instances)                                               |
| Database        | Database systems (e.g. MySQL, PostgreSQL, etc)                                                 |
| Data Sharing    | Used to group together alerts for data sharing applications                                    |
| DHCP            | Alerts for dhcp related services                                                               |
| DNS             | Alerts for dns related services                                                                |
| Kubernetes      | Alerts for kubernetes nodes monitoring                                                         |
| KV Storage      | Key-Value pairs services alerts (e.g. memcached)                                               |
| Linux           | Services specific to Linux (e.g. systemd)                                                      |
| Messaging       | Alerts for message passing services (e.g. vernemq)                                             |
| Netdata         | Internal Netdata components monitoring                                                         |
| Other           | When an alert doesn't fit in other types.                                                      |
| Power Supply    | Alerts from power supply related services (e.g. apcupsd)                                       |
| Search engine   | Alerts for search services (e.g. elasticsearch)                                                |
| Storage         | Class for alerts dealing with storage services (storage devices typically live under `System`) |
| System          | General system alerts (e.g. cpu, network, etc.)                                                |
| Virtual Machine | Virtual Machine software                                                                       |
| Web Proxy       | Web proxy software (e.g. squid)                                                                |
| Web Server      | Web server software (e.g. Apache, ngnix, etc.)                                                 |
| Windows         | Alerts for monitor of windows services                                                         |

</details>

If an alert configuration is missing the `type` line, its value will default to `Unknown`.

#### Alert line `component`

Component can be used to narrow down what the previous `type` value specifies for each alert or template. Continuing from the previous example, `component` might include `MySQL`, `CockroachDB`, `MongoDB`, all under the same `Database` type. Example:

```yaml
component: MySQL
```

As with the `class` and `type` line, if `component` is missing from the configuration, its value will default to `Unknown`.

#### Alert line `os`

The alert or template will be used only if the operating system of the host matches this list specified in `os`. The
value is a space-separated list.

The following example enables the entity on Linux, FreeBSD, and macOS, but no other operating systems.

```yaml
os: linux freebsd macos
```

#### Alert line `hosts`

The alert or template will be used only if the hostname of the host matches this space-separated list.

The following example will load on systems with the hostnames `server` and `server2`, and any system with hostnames that
begin with `database`. It _will not load_ on the host `redis3`, but will load on any _other_ systems with hostnames that
begin with `redis`.

```yaml
hosts: server1 server2 database* !redis3 redis*
```

#### Alert line `plugin`

The `plugin` line filters which plugin within the context this alert should apply to. The value is a space-separated
list of [simple patterns](https://github.com/netdata/netdata/blob/master/libnetdata/simple_pattern/README.md). For example,
you can create a filter for an alert that applies specifically to `python.d.plugin`:

```yaml
plugin: python.d.plugin
```

The `plugin` line is best used with other options like `module`. When used alone, the `plugin` line creates a very
inclusive filter that is unlikely to be of much use in production. See [`module`](#alert-line-module) for a
comprehensive example using both.

#### Alert line `module`

The `module` line filters which module within the context this alert should apply to. The value is a space-separated
list of [simple patterns](https://github.com/netdata/netdata/blob/master/libnetdata/simple_pattern/README.md). For
example, you can create an alert that applies only on the `isc_dhcpd` module started by `python.d.plugin`:

```yaml
plugin: python.d.plugin
module: isc_dhcpd
```

#### Alert line `charts`

The `charts` line filters which chart this alert should apply to. It is only available on entities using the
[`template`](#alert-line-alarm-or-template) line.
The value is a space-separated list of [simple patterns](https://github.com/netdata/netdata/blob/master/libnetdata/simple_pattern/README.md). For
example, a template that applies to `disk.svctm` (Average Service Time) context, but excludes the disk `sdb` from alerts:

```yaml
template: disk_svctm_alert
      on: disk.svctm
  charts: !*sdb* *
```

#### Alert line `families`

The `families` line, used only alongside templates, filters which families within the context this alert should apply
to. The value is a space-separated list.

The value is a space-separate list of simple patterns. See our [simple patterns docs](https://github.com/netdata/netdata/blob/master/libnetdata/simple_pattern/README.md) for
some examples.

For example, you can create a template on the `disk.io` context, but filter it to only the `sda` and `sdb` families:

```yaml
families: sda sdb
```

Please note that the use of the `families` filter is planned to be deprecated in upcoming Netdata releases. 
Please use [`chart labels`](#alert-line-chart-labels) instead.

#### Alert line `lookup`

This line makes a database lookup to find a value. This result of this lookup is available as `$this`.

The format is:

```yaml
lookup: METHOD AFTER [at BEFORE] [every DURATION] [OPTIONS] [of DIMENSIONS] [foreach DIMENSIONS]
```

The full [database query API](https://github.com/netdata/netdata/blob/master/web/api/queries/README.md) is supported. In short:

- `METHOD` is one of  the available [grouping methods](https://github.com/netdata/netdata/blob/master/web/api/queries/README.md#grouping-methods) such as `average`, `min`, `max` etc.
     This is required.

- `AFTER` is a relative number of seconds, but it also accepts a single letter for changing
     the units, like `-1s` = 1 second in the past, `-1m` = 1 minute in the past, `-1h` = 1 hour
     in the past, `-1d` = 1 day in the past. You need a negative number (i.e. how far in the past
     to look for the value). **This is required**.

- `at BEFORE` is by default 0 and is not required. Using this you can define the end of the
     lookup. So data will be evaluated between `AFTER` and `BEFORE`.

- `every DURATION` sets the updated frequency of the lookup (supports single letter units as
     above too).

- `OPTIONS` is a space separated list of `percentage`, `absolute`, `min2max`, `unaligned`,
     `match-ids`, `match-names`. Check the [badges](https://github.com/netdata/netdata/blob/master/web/api/badges/README.md) documentation for more info.

- `of DIMENSIONS` is optional and has to be the last parameter. Dimensions have to be separated
     by `,` or `|`. The space characters found in dimensions will be kept as-is (a few dimensions
     have spaces in their names). This accepts Netdata simple patterns _(with `words` separated by
     `,` or `|` instead of spaces)_ and the `match-ids` and `match-names` options affect the searches
     for dimensions.

- `foreach DIMENSIONS` is optional and works only with [templates](#alert-line-alarm-or-template), will always be the last parameter, and uses the same `,`/`|`
     rules as the `of` parameter. Each dimension you specify in `foreach` will use the same rule
     to trigger an alert. If you set both `of` and `foreach`, Netdata will ignore the `of` parameter
     and replace it with one of the dimensions you gave to `foreach`. This option allows you to
     [use dimension templates to create dynamic alerts](#use-dimension-templates-to-create-dynamic-alerts).

The result of the lookup will be available as `$this` and `$NAME` in expressions.
The timestamps of the timeframe evaluated by the database lookup is available as variables
`$after` and `$before` (both are unix timestamps).

#### Alert line `calc`

A `calc` is designed to apply some calculation to the values or variables available to the entity. The result of the
calculation will be made available at the `$this` variable, overwriting the value from your `lookup`, to use in warning
and critical expressions.

When paired with `lookup`, `calc` will perform the calculation just after `lookup` has retrieved a value from Netdata's
database.

You can use `calc` without `lookup` if you are using [other available variables](#variables).

The `calc` line uses [expressions](#expressions) for its syntax.

```yaml
calc: EXPRESSION
```

#### Alert line `every`

Sets the update frequency of this alert.  This is the same to the `every DURATION` given
in the `lookup` lines.

Format:

```yaml
every: DURATION
```

`DURATION` accepts `s` for seconds, `m` is minutes, `h` for hours, `d` for days.

#### Alert lines `green` and `red`

Set the green and red thresholds of a chart. Both are available as `$green` and `$red` in expressions. If multiple
alerts define different thresholds, the ones defined by the first alert will be used. Eventually it will be visualized
on the dashboard, so only one set of them is allowed If you need multiple sets of them in different alerts, use
absolute numbers instead of `$red` and `$green`.

Format:

```yaml
green: NUMBER
red: NUMBER
```

#### Alert lines `warn` and `crit`

Define the expression that triggers either a warning or critical alert. These are optional, and should evaluate to
either true or false (or zero/non-zero).

The format uses Netdata's [expressions syntax](#expressions).

```yaml
warn: EXPRESSION
crit: EXPRESSION
```

#### Alert line `to`

This will be the first script parameter that will be executed when the alert changes its status. Its meaning is left up to
the `exec` script.

The default `exec` script, `alarm-notify.sh`, uses this field as a space separated list of roles, which are then
consulted to find the exact recipients per notification method.

Format:

```yaml
to: ROLE1 ROLE2 ROLE3 ...
```

#### Alert line `exec`

Script to be executed when the alert status changes.

Format:

```yaml
exec: SCRIPT
```

The default `SCRIPT` is Netdata's `alarm-notify.sh`, which supports all the notifications methods Netdata supports,
including custom hooks.

#### Alert line `delay`

This is used to provide optional hysteresis settings for the notifications, to defend against notification floods. These
settings do not affect the actual alert - only the time the `exec` script is executed.

Format:

```yaml
delay: [[[up U] [down D] multiplier M] max X]
```

- `up U` defines the delay to be applied to a notification for an alert that raised its status
     (i.e. CLEAR to WARNING, CLEAR to CRITICAL, WARNING to CRITICAL). For example, `up 10s`, the
     notification for this event will be sent 10 seconds after the actual event. This is used in
     hope the alert will get back to its previous state within the duration given. The default `U`
     is zero.

- `down D` defines the delay to be applied to a notification for an alert that moves to lower
     state (i.e. CRITICAL to WARNING, CRITICAL to CLEAR, WARNING to CLEAR). For example, `down 1m`
     will delay the notification by 1 minute. This is used to prevent notifications for flapping
     alerts. The default `D` is zero.

- `multiplier M` multiplies `U` and `D` when an alert changes state, while a notification is
     delayed. The default multiplier is `1.0`.

- `max X`  defines the maximum absolute notification delay an alert may get. The default `X`
     is `max(U * M, D * M)` (i.e. the max duration of `U` or `D` multiplied once with `M`).

    Example:

    `delay: up 10s down 15m multiplier 2 max 1h`

    The time is `00:00:00` and the status of the alert is CLEAR.

    | time of event | new status | delay               | notification will be sent | why                                                                           |
    |---------------|------------|---------------------|---------------------------|-------------------------------------------------------------------------------|
    | 00:00:01      | WARNING    | `up 10s`            | 00:00:11                  | first state switch                                                            |
    | 00:00:05      | CLEAR      | `down 15m x2`       | 00:30:05                  | the alert changes state while a notification is delayed, so it was multiplied |
    | 00:00:06      | WARNING    | `up 10s x2 x2`      | 00:00:26                  | multiplied twice                                                              |
    | 00:00:07      | CLEAR      | `down 15m x2 x2 x2` | 00:45:07                  | multiplied 3 times.                                                           |

    So:

  - `U` and `D` are multiplied by `M` every time the alert changes state (any state, not just
        their matching one) and a delay is in place.
  - All are reset to their defaults when the alert switches state without a delay in place.

#### Alert line `repeat`

Defines the interval between repeating notifications for the alerts in CRITICAL or WARNING mode. This will override the
default interval settings inherited from health settings in `netdata.conf`. The default settings for repeating
notifications are `default repeat warning = DURATION` and `default repeat critical = DURATION` which can be found in
health stock configuration, when one of these interval is bigger than 0, Netdata will activate the repeat notification
for `CRITICAL`, `CLEAR` and `WARNING` messages.

Format:

```yaml
repeat: [off] [warning DURATION] [critical DURATION]
```

- `off`: Turns off the repeating feature for the current alert. This is effective when the default repeat settings has
    been enabled in health configuration.
- `warning DURATION`: Defines the interval when the alert is in WARNING state. Use `0s` to turn off the repeating
    notification for WARNING mode.
- `critical DURATION`: Defines the interval when the alert is in CRITICAL state. Use `0s` to turn off the repeating
    notification for CRITICAL mode.

#### Alert line `options`

The only possible value for the `options` line is

```yaml
options: no-clear-notification
```

For some alerts we need compare two time-frames, to detect anomalies. For example, `health.d/httpcheck.conf` has an
alert template called `web_service_slow` that compares the average http call response time over the last 3 minutes,
compared to the average over the last hour. It triggers a warning alert when the average of the last 3 minutes is twice
the average of the last hour. In such cases, it is easy to trigger the alert, but difficult to tell when the alert is
cleared. As time passes, the newest window moves into the older, so the average response time of the last hour will keep
increasing. Eventually, the comparison will find the averages in the two time-frames close enough to clear the alert.
However, the issue was not resolved, it's just a matter of the newer data "polluting" the old. For such alerts, it's a
good idea to tell Netdata to not clear the notification, by using the `no-clear-notification` option.

#### Alert line `host labels`

Defines the list of labels present on a host. See our [host labels guide](https://github.com/netdata/netdata/blob/master/docs/guides/using-host-labels.md) for
an explanation of host labels and how to implement them.

For example, let's suppose that `netdata.conf` is configured with the following labels:

```yaml
[host labels]
    installed = 20191211
    room = server
```

And more labels in `netdata.conf` for workstations:

```yaml
[host labels]
    installed = 201705
    room = workstation
```

By defining labels inside of `netdata.conf`, you can now apply labels to alerts. For example, you can add the following
line to any alerts you'd like to apply to hosts that have the label `room = server`.

```yaml
host labels: room = server
```

The `host labels` is a space-separated list that accepts simple patterns. For example, you can create an alert
that will be applied to all hosts installed in the last decade with the following line:

```yaml
host labels: installed = 201*
```

See our [simple patterns docs](https://github.com/netdata/netdata/blob/master/libnetdata/simple_pattern/README.md) for more examples.

#### Alert line `chart labels`

Similar to host labels, the `chart labels` key can be used to filter if an alert will load or not for a specific chart, based on
whether these chart labels match or not.

The list of chart labels present on each chart can be obtained from http://localhost:19999/api/v1/charts?all

For example, each `disk_space` chart defines a chart label called `mount_point` with each instance of this chart having
a value there of which mount point it monitors.

If you have an e.g. external disk mounted on `/mnt/disk1` and you don't wish any related disk space alerts running for
it (but you do for all other mount points), you can add the following to the alert's configuration:

```yaml
chart labels: mount_point=!/mnt/disk1 *`
```

The `chart labels` is a space-separated list that accepts simple patterns. If you use multiple different chart labels,
then the result is an OR between them. i.e. the following:

```yaml
chart labels: mount_point=/mnt/disk1 device=sda`
```

Will create the alert if the `mount_point` is `/mnt/disk1` or the `device` is `sda`. Furthermore, if a chart label name
is specified that does not exist in the chart, the chart won't be matched.

See our [simple patterns docs](https://github.com/netdata/netdata/blob/master/libnetdata/simple_pattern/README.md) for more examples.

#### Alert line `summary`

The summary field contains a brief title of the alert. It is used as the subject for the notifications, and in
dashboard list of alerts. An example for the `ram_available` alert is:

```yaml
summary: Available Ram
```

summary fields can contain special variables in their text that will be replaced during run-time to provide more specific
alert information. Current variables supported are:

| variable            | description                                                       |
|---------------------|-------------------------------------------------------------------|
| ${family}           | Will be replaced by the family instance for the alert (e.g. eth0) |
| ${label:LABEL_NAME} | The variable will be replaced with the value of the chart label   |

For example, a summry field like the following:

```yaml
summary: 1 minute received traffic overflow for ${label:device}
```

Will be rendered on the alert acting on interface `eth0` as:

```yaml
info: 1 minute received traffic overflow for ${label:device}
```

> Please note that variable names are case-sensitive.

#### Alert line `info`

The info field can contain a small piece of text describing the alert or template. This will be rendered in
notifications and UI elements whenever the specific alert is in focus. An example for the `ram_available` alert is:

```yaml
info: Percentage of estimated amount of RAM available for userspace processes, without causing swapping
```

info fields can contain special variables in their text that will be replaced during run-time to provide more specific
alert information. Current variables supported are:

| variable            | description                                                       |
|---------------------|-------------------------------------------------------------------|
| ${family}           | Will be replaced by the family instance for the alert (e.g. eth0) |
| ${label:LABEL_NAME} | The variable will be replaced with the value of the chart label   |

For example, an info field like the following:

```yaml
info: average inbound utilization for the network interface ${family} over the last minute
```

Will be rendered on the alert acting on interface `eth0` as:

```yaml
info: average inbound utilization for the network interface eth0 over the last minute
```

An alert acting on a chart that has a chart label named e.g. `target`, with a value of `https://netdata.cloud/`,
can be enriched as follows:

```yaml
info: average ratio of HTTP responses with unexpected status over the last 5 minutes for the site ${label:target}
```

Will become:

```yaml
info: average ratio of HTTP responses with unexpected status over the last 5 minutes for the site https://netdata.cloud/
```

> Please note that variable names are case-sensitive.

## Expressions

Netdata has an internal infix expression parser under `libnetdata/eval`. This parses expressions and creates an internal
structure that allows fast execution of them.

These operators are supported `+`, `-`, `*`, `/`, `<`, `==`, `<=`, `<>`, `!=`, `>`, `>=`, `&&`, `||`, `!`, `AND`, `OR`, `NOT`.
Boolean operators result in either `1` (true) or `0` (false).

The conditional evaluation operator `?` is supported too. Using this operator IF-THEN-ELSE conditional statements can be
specified. The format is: `(condition) ? (true expression) : (false expression)`. So, Netdata will first evaluate the
`condition` and based on the result will either evaluate `true expression` or `false expression`.

Example: `($this > 0) ? ($avail * 2) : ($used / 2)`.

Nested such expressions are also supported (i.e. `true expression` and `false expression` can contain conditional
evaluations).

Expressions also support the `abs()` function.

Expressions can have variables. Variables start with `$`. Check below for more information.

There are two special values you can use:

- `nan`, for example `$this != nan` will check if the variable `this` is available. A variable can be `nan` if the
    database lookup failed. All calculations (i.e. addition, multiplication, etc.) with a `nan` result in a `nan`.

- `inf`, for example `$this != inf` will check if `this` is not infinite. A value or variable can be set to infinite
    if divided by zero. All calculations (i.e. addition, multiplication, etc.) with a `inf` result in a `inf`.

### Special use of the conditional operator

A common (but not necessarily obvious) use of the conditional evaluation operator is to provide
[hysteresis](https://en.wikipedia.org/wiki/Hysteresis) around the critical or warning thresholds.  This usage helps to
avoid bogus messages resulting from small variations in the value when it is varying regularly but staying close to the
threshold value, without needing to delay sending messages at all.

An example of such usage from the default CPU usage alerts bundled with Netdata is:

```yaml
warn: $this > (($status >= $WARNING)  ? (75) : (85))
crit: $this > (($status == $CRITICAL) ? (85) : (95))
```

The above say:

- If the alert is currently a warning, then the threshold for being considered a warning is 75, otherwise it's 85.

- If the alert is currently critical, then the threshold for being considered critical is 85, otherwise it's 95.

Which in turn, results in the following behavior:

- While the value is rising, it will trigger a warning when it exceeds 85, and a critical alert when it exceeds 95.

- While the value is falling, it will return to a warning state when it goes below 85, and a normal state when it goes
    below 75.

- If the value is constantly varying between 80 and 90, then it will trigger a warning the first time it goes above
    85, but will remain a warning until it goes below 75 (or goes above 85).

- If the value is constantly varying between 90 and 100, then it will trigger a critical alert the first time it goes
    above 95, but will remain a critical alert goes below 85 (at which point it will return to being a warning).

## Variables

You can find all the variables that can be used for a given chart, using
`http://NODE:19999/api/v1/alarm_variables?chart=CHART_NAME`, replacing `NODE` with the IP address or hostname for your
Agent dashboard. For example, [variables for the `system.cpu` chart of the
registry](https://registry.my-netdata.io/api/v1/alarm_variables?chart=system.cpu).

> If you don't know how to find the CHART_NAME, you can read about it [here](https://github.com/netdata/netdata/blob/master/web/README.md#charts).

Netdata supports 3 internal indexes for variables that will be used in health monitoring.

<details markdown="1"><summary>The variables below can be used in both chart alerts and context templates.</summary>

Although the `alarm_variables` link shows you variables for a particular chart, the same variables can also be used in
templates for charts belonging to a given [context](https://github.com/netdata/netdata/blob/master/web/README.md#contexts). The reason is that all charts of a given
context are essentially identical, with the only difference being the [family](https://github.com/netdata/netdata/blob/master/web/README.md#families) that
identifies a particular hardware or software instance. Charts and templates do not apply to specific families anyway,
unless if you explicitly limit an alert with the [alert line `families`](#alert-line-families).

</details>

- **chart local variables**. All the dimensions of the chart are exposed as local variables. The value of `$this` for
    the other configured alerts of the chart also appears, under the name of each configured alert.

     Charts also define a few special variables:

  - `$last_collected_t` is the unix timestamp of the last data collection
  - `$collected_total_raw` is the sum of all the dimensions (their last collected values)
  - `$update_every` is the update frequency of the chart
  - `$green` and `$red` the threshold defined in alerts (these are per chart - the charts inherits them from the first alert that defined them)

        Chart dimensions define their last calculated (i.e. interpolated) value, exactly as
        shown on the charts, but also a variable with their name and suffix `_raw` that resolves
        to the last collected value - as collected and another with suffix `_last_collected_t`
        that resolves to unix timestamp the dimension was last collected (there may be dimensions
        that fail to be collected while others continue normally).

- **family variables**. Families are used to group charts together. For example all `eth0`
     charts, have `family = eth0`. This index includes all local variables, but if there are
     overlapping variables, only the first are exposed.

- **host variables**. All the dimensions of all charts, including all alerts, in fullname.
     Fullname is `CHART.VARIABLE`, where `CHART` is either the chart id or the chart name (both
     are supported).

- **special variables\*** are:

  - `$this`, which is resolved to the value of the current alert.

  - `$status`, which is resolved to the current status of the alert (the current = the last
         status, i.e. before the current database lookup and the evaluation of the `calc` line).
         This values can be compared with `$REMOVED`, `$UNINITIALIZED`, `$UNDEFINED`, `$CLEAR`,
         `$WARNING`, `$CRITICAL`. These values are incremental, e.g. `$status > $CLEAR` works as
         expected.

  - `$now`, which is resolved to current unix timestamp.

## Alert statuses

Alerts can have the following statuses:

- `REMOVED` - the alert has been deleted (this happens when a SIGUSR2 is sent to Netdata
     to reload health configuration)

- `UNINITIALIZED` - the alert is not initialized yet

- `UNDEFINED` - the alert failed to be calculated (i.e. the database lookup failed,
     a division by zero occurred, etc.)

- `CLEAR` - the alert is not armed / raised (i.e. is OK)

- `WARNING` - the warning expression resulted in true or non-zero

- `CRITICAL` - the critical expression resulted in true or non-zero

The external script will be called for all status changes.

## Example alerts

Check the `health/health.d/` directory for all alerts shipped with Netdata.

Here are a few examples:

### Example 1 - check server alive

A simple check if an apache server is alive:

```yaml
template: apache_last_collected_secs
      on: apache.requests
    calc: $now - $last_collected_t
   every: 10s
    warn: $this > ( 5 * $update_every)
    crit: $this > (10 * $update_every)
```

The above checks that Netdata is able to collect data from apache. In detail:

```yaml
template: apache_last_collected_secs
```

The above defines a **template** named `apache_last_collected_secs`.
The name is important since `$apache_last_collected_secs` resolves to the `calc` line.
So, try to give something descriptive.

```yaml
      on: apache.requests
```

The above applies the **template** to all charts that have `context = apache.requests`
(i.e. all your apache servers).

```yaml
    calc: $now - $last_collected_t
```

- `$now` is a standard variable that resolves to the current timestamp.

- `$last_collected_t` is the last data collection timestamp of the chart.
     So this calculation gives the number of seconds passed since the last data collection.

```yaml
   every: 10s
```

The alert will be evaluated every 10 seconds.

```yaml
    warn: $this > ( 5 * $update_every)
    crit: $this > (10 * $update_every)
```

If these result in non-zero or true, they trigger the alert.

- `$this` refers to the value of this alert (e.g. the result of the `calc` line).
     We could also use `$apache_last_collected_secs`.

`$update_every` is the update frequency of the chart, in seconds.

So, the warning condition checks if we have not collected data from apache for 5
iterations and the critical condition checks for 10 iterations.

### Example 2 - disk space

Check if any of the disks is critically low on disk space:

```yaml
template: disk_full_percent
      on: disk.space
    calc: $used * 100 / ($avail + $used)
   every: 1m
    warn: $this > 80
    crit: $this > 95
  repeat: warning 120s critical 10s
```

`$used` and `$avail`  are the `used` and `avail` chart dimensions as shown on the dashboard.

So, the `calc` line finds the percentage of used space. `$this` resolves to this percentage.

This is a repeating alert and if the alert becomes CRITICAL it repeats the notifications every 10 seconds. It also
repeats notifications every 2 minutes if the alert goes into WARNING mode.

### Example 3 - disk fill rate

Predict if any disk will run out of space in the near future.

We do this in 2 steps:

Calculate the disk fill rate:

```yaml
    template: disk_fill_rate
          on: disk.space
      lookup: max -1s at -30m unaligned of avail
        calc: ($this - $avail) / (30 * 60)
       every: 15s
```

In the `calc` line: `$this` is the result of the `lookup` line (i.e. the free space 30 minutes
ago) and `$avail` is the current disk free space. So the `calc` line will either have a positive
number of GB/second if the disk is filling up, or a negative number of GB/second if the disk is
freeing up space.

There is no `warn` or `crit` lines here. So, this template will just do the calculation and
nothing more.

Predict the hours after which the disk will run out of space:

```yaml
    template: disk_full_after_hours
          on: disk.space
        calc: $avail / $disk_fill_rate / 3600
       every: 10s
        warn: $this > 0 and $this < 48
        crit: $this > 0 and $this < 24
```

The `calc` line estimates the time in hours, we will run out of disk space. Of course, only
positive values are interesting for this check, so the warning and critical conditions check
for positive values and that we have enough free space for 48 and 24 hours respectively.

Once this alert triggers we will receive an email like this:

![image](https://cloud.githubusercontent.com/assets/2662304/17839993/87872b32-6802-11e6-8e08-b2e4afef93bb.png)

### Example 4 - dropped packets

Check if any network interface is dropping packets:

```yaml
template: 30min_packet_drops
      on: net.drops
  lookup: sum -30m unaligned absolute
   every: 10s
    crit: $this > 0
```

The `lookup` line will calculate the sum of the all dropped packets in the last 30 minutes.

The `crit` line will issue a critical alert if even a single packet has been dropped.

Note that the drops chart does not exist if a network interface has never dropped a single packet.
When Netdata detects a dropped packet, it will add the chart, and it will automatically attach this
alert to it.

### Example 5 - CPU usage

Check if user or system dimension is using more than 50% of cpu:

```yaml
template: cpu_template
      on: system.cpu
      os: linux
  lookup: average -1m foreach system,user
   units: %
   every: 10s
    warn: $this > 50
    crit: $this > 80
```

The `lookup` line will calculate the average CPU usage from system and user over the last minute. Because we have
the foreach in the `lookup` line, Netdata will create two independent alerts called `cpu_template_system`
and `dim_template_user` that will have all the other parameters shared among them.

### Example 6 - CPU usage

Check if all dimensions are using more than 50% of cpu:

```yaml
template: cpu_template
      on: system.cpu
      os: linux
  lookup: average -1m foreach *
   units: %
   every: 10s
    warn: $this > 50
    crit: $this > 80
```

The `lookup` line will calculate the average of CPU usage from system and user over the last minute. In this case
Netdata will create alerts for all dimensions of the chart.

### Example 7 - Z-Score based alert

Derive a "[Z Score](https://en.wikipedia.org/wiki/Standard_score)" based alert on `user` dimension of the `system.cpu` chart:

```yaml
 alarm: cpu_user_mean
    on: system.cpu
lookup: mean -60s of user
 every: 10s

 alarm: cpu_user_stddev
    on: system.cpu
lookup: stddev -60s of user
 every: 10s

 alarm: cpu_user_zscore
    on: system.cpu
lookup: mean -10s of user
  calc: ($this - $cpu_user_mean) / $cpu_user_stddev
 every: 10s
  warn: $this < -2 or $this > 2
  crit: $this < -3 or $this > 3
```

Since [`z = (x - mean) / stddev`](https://en.wikipedia.org/wiki/Standard_score) we create two input alerts, one for `mean` and one for `stddev` and then use them both as inputs in our final `cpu_user_zscore` alert.

### Example 8 - [Anomaly rate](https://github.com/netdata/netdata/blob/master/ml/README.md#anomaly-rate) based CPU dimensions alert

Warning if 5 minute rolling [anomaly rate](https://github.com/netdata/netdata/blob/master/ml/README.md#anomaly-rate) for any CPU dimension is above 5%, critical if it goes above 20%:

```yaml
template: ml_5min_cpu_dims
      on: system.cpu
      os: linux
   hosts: *
  lookup: average -5m anomaly-bit foreach *
    calc: $this
   units: %
   every: 30s
    warn: $this > (($status >= $WARNING)  ? (5) : (20))
    crit: $this > (($status == $CRITICAL) ? (20) : (100))
    info: rolling 5min anomaly rate for each system.cpu dimension
```

The `lookup` line will calculate the average anomaly rate of each `system.cpu` dimension over the last 5 minues. In this case
Netdata will create alerts for all dimensions of the chart.

### Example 9 - [Anomaly rate](https://github.com/netdata/netdata/blob/master/ml/README.md#anomaly-rate) based CPU chart alert

Warning if 5 minute rolling [anomaly rate](https://github.com/netdata/netdata/blob/master/ml/README.md#anomaly-rate) averaged across all CPU dimensions is above 5%, critical if it goes above 20%:

```yaml
template: ml_5min_cpu_chart
      on: system.cpu
      os: linux
   hosts: *
  lookup: average -5m anomaly-bit of *
    calc: $this
   units: %
   every: 30s
    warn: $this > (($status >= $WARNING)  ? (5) : (20))
    crit: $this > (($status == $CRITICAL) ? (20) : (100))
    info: rolling 5min anomaly rate for system.cpu chart
```

The `lookup` line will calculate the average anomaly rate across all `system.cpu` dimensions over the last 5 minues. In this case
Netdata will create one alert for the chart.

### Example 10 - [Anomaly rate](https://github.com/netdata/netdata/blob/master/ml/README.md#anomaly-rate) based node level alert

Warning if 5 minute rolling [anomaly rate](https://github.com/netdata/netdata/blob/master/ml/README.md#anomaly-rate) averaged across all ML enabled dimensions is above 5%, critical if it goes above 20%:

```yaml
template: ml_5min_node
      on: anomaly_detection.anomaly_rate
      os: linux
   hosts: *
  lookup: average -5m of anomaly_rate
    calc: $this
   units: %
   every: 30s
    warn: $this > (($status >= $WARNING)  ? (5) : (20))
    crit: $this > (($status == $CRITICAL) ? (20) : (100))
    info: rolling 5min anomaly rate for all ML enabled dims
```

The `lookup` line will use the `anomaly_rate` dimension of the `anomaly_detection.anomaly_rate` ML chart to calculate the average [node level anomaly rate](https://github.com/netdata/netdata/blob/master/ml/README.md#node-anomaly-rate) over the last 5 minues.

## Use dimension templates to create dynamic alerts

In v1.18 of Netdata, we introduced **dimension templates** for alerts, which simplifies the process of
writing [alert entities](#health-entity-reference) for
charts with many dimensions.

Dimension templates can condense many individual entities into one—no more copy-pasting one entity and changing the
`alarm`/`template` and `lookup` lines for each dimension you'd like to monitor.

### The fundamentals of `foreach`

> **Note**: works only with [templates](#alert-line-alarm-or-template).

Our dimension templates update creates a new `foreach` parameter to the
existing [`lookup` line](#alert-line-lookup). This
is where the magic happens.

You use the `foreach` parameter to specify which dimensions you want to monitor with this single alert. You can separate
them with a comma (`,`) or a pipe (`|`). You can also use
a [Netdata simple pattern](https://github.com/netdata/netdata/blob/master/libnetdata/simple_pattern/README.md) to create
many alerts with a regex-like syntax.

The `foreach` parameter _has_ to be the last parameter in your `lookup` line, and if you have both `of` and `foreach` in
the same `lookup` line, Netdata will ignore the `of` parameter and use `foreach` instead.

Let's get into some examples, so you can see how the new parameter works.

> ⚠️ The following entities are examples to showcase the functionality and syntax of dimension templates. They are not
> meant to be run as-is on production systems.

### Condensing entities with `foreach`

Let's say you want to monitor the `system`, `user`, and `nice` dimensions in your system's overall CPU utilization.
Before dimension templates, you would need the following three entities:

```yaml
 alarm: cpu_system
    on: system.cpu
lookup: average -10m of system
 every: 1m
  warn: $this > 50
  crit: $this > 80

 alarm: cpu_user
    on: system.cpu
lookup: average -10m of user
 every: 1m
  warn: $this > 50
  crit: $this > 80

 alarm: cpu_nice
    on: system.cpu
lookup: average -10m of nice
 every: 1m
  warn: $this > 50
  crit: $this > 80
```

With dimension templates, you can condense these into a single template. Take note of the `lookup` line.

```yaml
template: cpu_template
      on: system.cpu
  lookup: average -10m foreach system,user,nice
   every: 1m
    warn: $this > 50
    crit: $this > 80
```

The `template` line specifies the naming scheme Netdata will use. You can use whatever naming scheme you'd like, with `.`
and `_` being the only allowed symbols.

The `lookup` line has changed from `of` to `foreach`, and we're now passing three dimensions.

In this example, Netdata will create three alerts with the names `cpu_template_system`, `cpu_template_user`, and
`cpu_template_nice`. Every minute, each alert will use the same database query to calculate the average CPU usage for
the `system`, `user`, and `nice` dimensions over the last 10 minutes and send out alerts if necessary.

You can find these three alerts active by clicking on the **Alerts** button in the top navigation, and then clicking on
the **All** tab and scrolling to the **system - cpu** collapsible section.

![Three new alerts created from the dimension template](https://user-images.githubusercontent.com/1153921/66218994-29523800-e67f-11e9-9bcb-9bca23e2c554.png)

Let's look at some other examples of how `foreach` works, so you can best apply it in your configurations.

### Using a Netdata simple pattern in `foreach`

In the last example, we used `foreach system,user,nice` to create three distinct alerts using dimension templates. But
what if you want to quickly create alerts for _all_ the dimensions of a given chart?

Use a [simple pattern](https://github.com/netdata/netdata/blob/master/libnetdata/simple_pattern/README.md)! One example of a simple pattern is a single wildcard
(`*`).

Instead of monitoring system CPU usage, let's monitor per-application CPU usage using the `apps.cpu` chart. Passing a
wildcard as the simple pattern tells Netdata to create a separate alert for _every_ process on your system:

```yaml
 alarm: app_cpu
    on: apps.cpu
lookup: average -10m percentage foreach *
 every: 1m
  warn: $this > 50
  crit: $this > 80
```

This entity will now create alerts for every dimension in the `apps.cpu` chart. Given that most `apps.cpu` charts have
10 or more dimensions, using the wildcard ensures you catch every CPU-hogging process.

To learn more about how to use simple patterns with dimension templates, see
our [simple patterns documentation](https://github.com/netdata/netdata/blob/master/libnetdata/simple_pattern/README.md).

### Using `foreach` with alert templates

Dimension templates also work
with [alert templates](#alert-line-alarm-or-template).
Alert templates help you create alerts for all the charts with a given context—for example, all the cores of your
system's CPU.

By combining the two, you can create dozens of individual alerts with a single template entity. Here's how you would
create alerts for the `system`, `user`, and `nice` dimensions for every chart in the `cpu.cpu` context—or, in other
words, every CPU core.

```yaml
template: cpu_template
      on: cpu.cpu
  lookup: average -10m percentage foreach system,user,nice
   every: 1m
    warn: $this > 50
    crit: $this > 80
```

On a system with a 6-core, 12-thread Ryzen 5 1600 CPU, this one entity creates alerts on the following charts and
dimensions:

- `cpu.cpu0`
  - `cpu_template_user`
  - `cpu_template_system`
  - `cpu_template_nice`

- `cpu.cpu1`
  - `cpu_template_user`
  - `cpu_template_system`
  - `cpu_template_nice`

- `cpu.cpu2`
  - `cpu_template_user`
  - `cpu_template_system`
  - `cpu_template_nice`

- ...

- `cpu.cpu11`
  - `cpu_template_user`
  - `cpu_template_system`
  - `cpu_template_nice`

And how just a few of those dimension template-generated alerts look like in the Netdata dashboard.

![A few of the created alerts in the Netdata dashboard](https://user-images.githubusercontent.com/1153921/66219669-708cf880-e680-11e9-8b3a-7bfe178fa28b.png)

All in all, this single entity creates 36 individual alerts. Much easier than writing 36 separate entities in your
health configuration files!

## Troubleshooting

You can compile Netdata with [debugging](https://github.com/netdata/netdata/blob/master/daemon/README.md#debugging) and then set in `netdata.conf`:

```yaml
[global]
   debug flags = 0x0000000000800000
```

Then check your `/var/log/netdata/debug.log`. It will show you how it works. Important: this will generate a lot of
output in debug.log.

You can find the context of charts by looking up the chart in either `http://NODE:19999/netdata.conf` or
`http://NODE:19999/api/v1/charts`, replacing `NODE` with the IP address or hostname for your Agent dashboard.

You can find how Netdata interpreted the expressions by examining the alert at
`http://NODE:19999/api/v1/alarms?all`. For each expression, Netdata will return the expression as given in its
config file, and the same expression with additional parentheses added to indicate the evaluation flow of the
expression.