**Warning!**This page documents an old version of Kapacitor, which is no longer actively developed. Kapacitor v1.2 is the most recent stable version of Kapacitor.

Compute the derivative of a stream or batch. The derivative is computed on a single field and behaves similarly to the InfluxQL derivative function. Deriviative is not a MapReduce function and as a result is not part of the normal influxql functions.

Example:

```
stream
.from().measurement('net_rx_packets')
.derivative('value')
.unit(1s) // default
.nonNegative()
...
```

Computes the derivative via: (current - previous ) / ( time_difference / unit)

For batch edges the derivative is computed for each point in the batch and because of boundary conditions the number of points is reduced by one.

## Properties

Property methods modify state on the calling node. They do not add another node to the pipeline, and always return a reference to the calling node.

### As

The new name of the derivative field. Default is the name of the field used when calculating the derivative.

```
node.as(value string)
```

### NonNegative

If called the derivative will skip negative results.

```
node.nonNegative()
```

### Unit

The time unit of the resulting derivative value. Default: 1s

```
node.unit(value time.Duration)
```

## 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.

### Alert

Create an alert node, which can trigger alerts.

```
node.alert()
```

Returns: AlertNode

### Deadman

Helper function for creating an alert on low throughput, aka 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)
.derivative('collected')
.unit(10s)
.nonNegative()
.alert()
.id('node \'stream0\' in task \'{{ .TaskName }}\'')
.message('{{ .ID }} is {{ if eq .Level "OK" }}alive{{ else }}dead{{ end }}: {{ index .Fields "collected" | printf "%0.3f" }} points/10s.')
.crit(lamdba: "collected" <= 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 normal. Example:

```
var data = stream.from()...
// Trigger critical alert if the throughput drops below 100 points per 1s 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 ...tick.Node)
```

Returns: AlertNode

### Derivative

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

```
node.derivative(field string)
```

Returns: DerivativeNode

### 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 and results of previous expressions are made available to later expressions.

```
node.eval(expressions ...tick.Node)
```

Returns: EvalNode

### 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

### HttpOut

Create an http output node that caches the most recent data it has received. The cached data is 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 "/api/v1/task/<task_name>" and endpoint is "top10", then the data can be requested from "/api/v1/task/<task_name>/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 is joined on timestamp.

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

Returns: JoinNode

### MapReduce

Perform a map-reduce operation on the data.
The built-in functions under `influxql`

provide the
selection,aggregation, and transformation functions
from the InfluxQL language.

MapReduce may be applied to either a batch or a stream edge. In the case of a batch each batch is passed to the mapper independently. In the case of a stream all incoming data points that have the exact same time are combined into a batch and sent to the mapper.

```
node.mapReduce(mr MapReduceInfo)
```

Returns: ReduceNode

### 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

### 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

### 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 tick.Node)
```

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