**Warning!**This page documents an earlier version of Kapacitor, which is no longer actively developed. Kapacitor v1.5 is the most recent stable version of Kapacitor.

Sample points or batches. One point will be emitted every count or duration specified.

Example:

```
stream
|sample(3)
```

Keep every third data point or batch.

Example:

```
stream
|sample(10s)
```

Keep only samples that land on the 10s boundary. See FromNode.Truncate, QueryNode.GroupBy time or WindowNode.Align for ensuring data is aligned with a boundary.

## Index

### Properties

### Chaining Methods

- Alert
- Bottom
- Combine
- Count
- Deadman
- Default
- Delete
- Derivative
- Difference
- Distinct
- Elapsed
- Eval
- First
- Flatten
- GroupBy
- HoltWinters
- HoltWintersWithFit
- HttpOut
- InfluxDBOut
- Join
- Last
- Log
- Max
- Mean
- Median
- Min
- Mode
- MovingAverage
- Percentile
- Sample
- Shift
- Spread
- Stats
- Stddev
- Sum
- Top
- Union
- Where
- Window

## Chaining Methods

Chaining methods create a new node in the pipeline as a child of the calling node.
They do not modify the calling node.
Chaining methods are marked using the `|`

operator.

### Alert

Create an alert node, which can trigger alerts.

```
node|alert()
```

Returns: AlertNode

### Bottom

Select the bottom `num`

points for `field`

and sort by any extra tags or fields.

```
node|bottom(num int64, field string, fieldsAndTags ...string)
```

Returns: InfluxQLNode

### Combine

Combine this node with itself. The data are combined on timestamp.

```
node|combine(expressions ...ast.LambdaNode)
```

Returns: CombineNode

### Count

Count the number of points.

```
node|count(field string)
```

Returns: InfluxQLNode

### Deadman

Helper function for creating an alert on low throughput, a.k.a. deadman's switch.

- Threshold – trigger alert if throughput drops below threshold in points/interval.
- Interval – how often to check the throughput.
- Expressions – optional list of expressions to also evaluate. Useful for time of day alerting.

Example:

```
var data = stream
|from()...
// Trigger critical alert if the throughput drops below 100 points per 10s and checked every 10s.
data
|deadman(100.0, 10s)
//Do normal processing of data
data...
```

The above is equivalent to this Example:

```
var data = stream
|from()...
// Trigger critical alert if the throughput drops below 100 points per 10s and checked every 10s.
data
|stats(10s)
.align()
|derivative('emitted')
.unit(10s)
.nonNegative()
|alert()
.id('node \'stream0\' in task \'{{ .TaskName }}\'')
.message('{{ .ID }} is {{ if eq .Level "OK" }}alive{{ else }}dead{{ end }}: {{ index .Fields "emitted" | printf "%0.3f" }} points/10s.')
.crit(lambda: "emitted" <= 100.0)
//Do normal processing of data
data...
```

The `id`

and `message`

alert properties can be configured globally via the 'deadman' configuration section.

Since the AlertNode is the last piece it can be further modified as usual. Example:

```
var data = stream
|from()...
// Trigger critical alert if the throughput drops below 100 points per 10s and checked every 10s.
data
|deadman(100.0, 10s)
.slack()
.channel('#dead_tasks')
//Do normal processing of data
data...
```

You can specify additional lambda expressions to further constrain when the deadman's switch is triggered. Example:

```
var data = stream
|from()...
// Trigger critical alert if the throughput drops below 100 points per 10s and checked every 10s.
// Only trigger the alert if the time of day is between 8am-5pm.
data
|deadman(100.0, 10s, lambda: hour("time") >= 8 AND hour("time") <= 17)
//Do normal processing of data
data...
```

```
node|deadman(threshold float64, interval time.Duration, expr ...ast.LambdaNode)
```

Returns: AlertNode

### Default

Create a node that can set defaults for missing tags or fields.

```
node|default()
```

Returns: DefaultNode

### Delete

Create a node that can delete tags or fields.

```
node|delete()
```

Returns: DeleteNode

### Derivative

Create a new node that computes the derivative of adjacent points.

```
node|derivative(field string)
```

Returns: DerivativeNode

### Difference

Compute the difference between points independent of elapsed time.

```
node|difference(field string)
```

Returns: InfluxQLNode

### Distinct

Produce batch of only the distinct points.

```
node|distinct(field string)
```

Returns: InfluxQLNode

### Elapsed

Compute the elapsed time between points

```
node|elapsed(field string, unit time.Duration)
```

Returns: InfluxQLNode

### Eval

Create an eval node that will evaluate the given transformation function to each data point. A list of expressions may be provided and will be evaluated in the order they are given. The results are available to later expressions.

```
node|eval(expressions ...ast.LambdaNode)
```

Returns: EvalNode

### First

Select the first point.

```
node|first(field string)
```

Returns: InfluxQLNode

### Flatten

Flatten points with similar times into a single point.

```
node|flatten()
```

Returns: FlattenNode

### GroupBy

Group the data by a set of tags.

Can pass literal * to group by all dimensions. Example:

```
|groupBy(*)
```

```
node|groupBy(tag ...interface{})
```

Returns: GroupByNode

### HoltWinters

Compute the holt-winters forecast of a data set.

```
node|holtWinters(field string, h int64, m int64, interval time.Duration)
```

Returns: InfluxQLNode

### HoltWintersWithFit

Compute the holt-winters forecast of a data set. This method also outputs all the points used to fit the data in addition to the forecasted data.

```
node|holtWintersWithFit(field string, h int64, m int64, interval time.Duration)
```

Returns: InfluxQLNode

### HttpOut

Create an HTTP output node that caches the most recent data it has received.
The cached data are available at the given endpoint.
The endpoint is the relative path from the API endpoint of the running task.
For example, if the task endpoint is at `/kapacitor/v1/tasks/<task_id>`

and endpoint is
`top10`

, then the data can be requested from `/kapacitor/v1/tasks/<task_id>/top10`

.

```
node|httpOut(endpoint string)
```

Returns: HTTPOutNode

### InfluxDBOut

Create an influxdb output node that will store the incoming data into InfluxDB.

```
node|influxDBOut()
```

Returns: InfluxDBOutNode

### Join

Join this node with other nodes. The data are joined on timestamp.

```
node|join(others ...Node)
```

Returns: JoinNode

### Last

Select the last point.

```
node|last(field string)
```

Returns: InfluxQLNode

### Log

Create a node that logs all data it receives.

```
node|log()
```

Returns: LogNode

### Max

Select the maximum point.

```
node|max(field string)
```

Returns: InfluxQLNode

### Mean

Compute the mean of the data.

```
node|mean(field string)
```

Returns: InfluxQLNode

### Median

Compute the median of the data. Note, this method is not a selector,
if you want the median point use `.percentile(field, 50.0)`

.

```
node|median(field string)
```

Returns: InfluxQLNode

### Min

Select the minimum point.

```
node|min(field string)
```

Returns: InfluxQLNode

### Mode

Compute the mode of the data.

```
node|mode(field string)
```

Returns: InfluxQLNode

### MovingAverage

Compute a moving average of the last window points. No points are emitted until the window is full.

```
node|movingAverage(field string, window int64)
```

Returns: InfluxQLNode

### Percentile

Select a point at the given percentile. This is a selector function, no interpolation between points is performed.

```
node|percentile(field string, percentile float64)
```

Returns: InfluxQLNode

### Sample

Create a new node that samples the incoming points or batches.

One point will be emitted every count or duration specified.

```
node|sample(rate interface{})
```

Returns: SampleNode

### Shift

Create a new node that shifts the incoming points or batches in time.

```
node|shift(shift time.Duration)
```

Returns: ShiftNode

### Spread

Compute the difference between `min`

and `max`

points.

```
node|spread(field string)
```

Returns: InfluxQLNode

### Stats

Create a new stream of data that contains the internal statistics of the node. The interval represents how often to emit the statistics based on real time. This means the interval time is independent of the times of the data points the source node is receiving.

```
node|stats(interval time.Duration)
```

Returns: StatsNode

### Stddev

Compute the standard deviation.

```
node|stddev(field string)
```

Returns: InfluxQLNode

### Sum

Compute the sum of all values.

```
node|sum(field string)
```

Returns: InfluxQLNode

### Top

Select the top `num`

points for `field`

and sort by any extra tags or fields.

```
node|top(num int64, field string, fieldsAndTags ...string)
```

Returns: InfluxQLNode

### Union

Perform the union of this node and all other given nodes.

```
node|union(node ...Node)
```

Returns: UnionNode

### Where

Create a new node that filters the data stream by a given expression.

```
node|where(expression ast.LambdaNode)
```

Returns: WhereNode

### Window

Create a new node that windows the stream by time.

NOTE: Window can only be applied to stream edges.

```
node|window()
```

Returns: WindowNode