--- title: EvalNode (Kapacitor TICKscript node) description: EvalNode evaluates expressions on each data point it receives. A list of expressions may be provided and will be evaluated in the order they are given. The results of expressions are available to later expressions in the list. note: Auto generated by tickdoc menu: kapacitor_1_4: name: EvalNode identifier: eval_node weight: 90 parent: TICKscript nodes --- ### Constructor | Chaining Method | Description | |:---------|:---------| | **[eval](#descr) ( `expressions` `...ast.LambdaNode`)** | 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. | ### Property Methods | Setters | Description | |:---|:---| | **[as](#as) ( `names` `...string`)** | List of names for each expression. The expressions are evaluated in order. The result of an expression may be referenced by later expressions via the name provided. | | **[keep](#keep) ( `fields` `...string`)** | If called the existing fields will be preserved in addition to the new fields being set. If not called then only new fields are preserved. (Tags are always preserved regardless how `keep` is used.) | | **[quiet](#quiet) ( )** | Suppress errors during evaluation. | | **[tags](#tags) ( `names` `...string`)** | Convert the result of an expression into a tag. The result must be a string. Use the `string()` expression function to convert types. | ### Chaining Methods [Alert](/kapacitor/v1.4/nodes/eval_node/#alert), [Barrier](/kapacitor/v1.4/nodes/eval_node/#barrier), [Bottom](/kapacitor/v1.4/nodes/eval_node/#bottom), [Combine](/kapacitor/v1.4/nodes/eval_node/#combine), [Count](/kapacitor/v1.4/nodes/eval_node/#count), [CumulativeSum](/kapacitor/v1.4/nodes/eval_node/#cumulativesum), [Deadman](/kapacitor/v1.4/nodes/eval_node/#deadman), [Default](/kapacitor/v1.4/nodes/eval_node/#default), [Delete](/kapacitor/v1.4/nodes/eval_node/#delete), [Derivative](/kapacitor/v1.4/nodes/eval_node/#derivative), [Difference](/kapacitor/v1.4/nodes/eval_node/#difference), [Distinct](/kapacitor/v1.4/nodes/eval_node/#distinct), [Ec2Autoscale](/kapacitor/v1.4/nodes/eval_node/#ec2autoscale), [Elapsed](/kapacitor/v1.4/nodes/eval_node/#elapsed), [Eval](/kapacitor/v1.4/nodes/eval_node/#eval), [First](/kapacitor/v1.4/nodes/eval_node/#first), [Flatten](/kapacitor/v1.4/nodes/eval_node/#flatten), [GroupBy](/kapacitor/v1.4/nodes/eval_node/#groupby), [HoltWinters](/kapacitor/v1.4/nodes/eval_node/#holtwinters), [HoltWintersWithFit](/kapacitor/v1.4/nodes/eval_node/#holtwinterswithfit), [HttpOut](/kapacitor/v1.4/nodes/eval_node/#httpout), [HttpPost](/kapacitor/v1.4/nodes/eval_node/#httppost), [InfluxDBOut](/kapacitor/v1.4/nodes/eval_node/#influxdbout), [Join](/kapacitor/v1.4/nodes/eval_node/#join), [K8sAutoscale](/kapacitor/v1.4/nodes/eval_node/#k8sautoscale), [KapacitorLoopback](/kapacitor/v1.4/nodes/eval_node/#kapacitorloopback), [Last](/kapacitor/v1.4/nodes/eval_node/#last), [Log](/kapacitor/v1.4/nodes/eval_node/#log), [Max](/kapacitor/v1.4/nodes/eval_node/#max), [Mean](/kapacitor/v1.4/nodes/eval_node/#mean), [Median](/kapacitor/v1.4/nodes/eval_node/#median), [Min](/kapacitor/v1.4/nodes/eval_node/#min), [Mode](/kapacitor/v1.4/nodes/eval_node/#mode), [MovingAverage](/kapacitor/v1.4/nodes/eval_node/#movingaverage), [Percentile](/kapacitor/v1.4/nodes/eval_node/#percentile), [Sample](/kapacitor/v1.4/nodes/eval_node/#sample), [Shift](/kapacitor/v1.4/nodes/eval_node/#shift), [Sideload](/kapacitor/v1.4/nodes/eval_node/#sideload), [Spread](/kapacitor/v1.4/nodes/eval_node/#spread), [StateCount](/kapacitor/v1.4/nodes/eval_node/#statecount), [StateDuration](/kapacitor/v1.4/nodes/eval_node/#stateduration), [Stats](/kapacitor/v1.4/nodes/eval_node/#stats), [Stddev](/kapacitor/v1.4/nodes/eval_node/#stddev), [Sum](/kapacitor/v1.4/nodes/eval_node/#sum), [SwarmAutoscale](/kapacitor/v1.4/nodes/eval_node/#swarmautoscale), [Top](/kapacitor/v1.4/nodes/eval_node/#top), [Union](/kapacitor/v1.4/nodes/eval_node/#union), [Where](/kapacitor/v1.4/nodes/eval_node/#where), [Window](/kapacitor/v1.4/nodes/eval_node/#window)

### Description Evaluates expressions on each data point it receives. A list of expressions may be provided and will be evaluated in the order they are given. The results of expressions are available to later expressions in the list. See the property [EvalNode.As](/kapacitor/v1.4/nodes/eval_node/#as) for details on how to reference the results. Example: ```javascript stream |eval(lambda: "error_count" / "total_count") .as('error_percent') ``` The above example will add a new field `error_percent` to each data point with the result of `error_count / total_count` where `error_count` and `total_count` are existing fields on the data point. Available Statistics: * eval_errors -- number of errors evaluating any expressions. 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. Property methods are marked using the `.` operator. ### As List of names for each expression. The expressions are evaluated in order. The result of an expression may be referenced by later expressions via the name provided. Example: ```javascript stream |eval(lambda: "value" * "value", lambda: 1.0 / "value2") .as('value2', 'inv_value2') ``` The above example calculates two fields from the value and names them `value2` and `inv_value2` respectively. ```javascript eval.as(names ...string) ``` ### Keep If called the existing fields will be preserved in addition to the new fields being set. If not called then only new fields are preserved. (Tags are always preserved regardless how `keep` is used.) Optionally, intermediate values can be discarded by passing a list of field names to be kept. Only fields in the list will be retained, the rest will be discarded. If no list is given then all fields are retained. Example: ```javascript stream |eval(lambda: "value" * "value", lambda: 1.0 / "value2") .as('value2', 'inv_value2') .keep('value', 'inv_value2') ``` In the above example the original field `value` is preserved. The new field `value2` is calculated and used in evaluating `inv_value2` but is discarded before the point is sent on to child nodes. The resulting point has only two fields: `value` and `inv_value2`. ```javascript eval.keep(fields ...string) ``` ### Quiet Suppress errors during evaluation. ```javascript eval.quiet() ``` ### Tags Convert the result of an expression into a tag. The result must be a string. Use the `string()` expression function to convert types. Example: ```javascript stream |eval(lambda: string(floor("value" / 10.0))) .as('value_bucket') .tags('value_bucket') ``` The above example calculates an expression from the field `value`, casts it as a string, and names it `value_bucket`. The `value_bucket` expression is then converted from a field on the point to a tag `value_bucket` on the point. Example: ```javascript stream |eval(lambda: string(floor("value" / 10.0))) .as('value_bucket') .tags('value_bucket') .keep('value') // keep the original field `value` as well ``` The above example calculates an expression from the field `value`, casts it as a string, and names it `value_bucket`. The `value_bucket` expression is then converted from a field on the point to a tag `value_bucket` on the point. The `keep` property preserves the original field `value`. Tags are always kept since creating a tag implies you want to keep it. ```javascript eval.tags(names ...string) ``` 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. ```javascript eval|alert() ``` Returns: [AlertNode](/kapacitor/v1.4/nodes/alert_node/) ### Barrier Create a new Barrier node that emits a BarrierMessage periodically One BarrierMessage will be emitted every period duration ```javascript eval|barrier() ``` Returns: [BarrierNode](/kapacitor/v1.4/nodes/barrier_node/) ### Bottom Select the bottom `num` points for `field` and sort by any extra tags or fields. ```javascript eval|bottom(num int64, field string, fieldsAndTags ...string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Combine Combine this node with itself. The data is combined on timestamp. ```javascript eval|combine(expressions ...ast.LambdaNode) ``` Returns: [CombineNode](/kapacitor/v1.4/nodes/combine_node/) ### Count Count the number of points. ```javascript eval|count(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### CumulativeSum Compute a cumulative sum of each point that is received. A point is emitted for every point collected. ```javascript eval|cumulativeSum(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### 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: ```javascript 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: ```javascript 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](/kapacitor/v1.4/nodes/alert_node/) is the last piece it can be further modified as usual. Example: ```javascript 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: ```javascript 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... ``` ```javascript eval|deadman(threshold float64, interval time.Duration, expr ...ast.LambdaNode) ``` Returns: [AlertNode](/kapacitor/v1.4/nodes/alert_node/) ### Default Create a node that can set defaults for missing tags or fields. ```javascript eval|default() ``` Returns: [DefaultNode](/kapacitor/v1.4/nodes/default_node/) ### Delete Create a node that can delete tags or fields. ```javascript eval|delete() ``` Returns: [DeleteNode](/kapacitor/v1.4/nodes/delete_node/) ### Derivative Create a new node that computes the derivative of adjacent points. ```javascript eval|derivative(field string) ``` Returns: [DerivativeNode](/kapacitor/v1.4/nodes/derivative_node/) ### Difference Compute the difference between points independent of elapsed time. ```javascript eval|difference(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Distinct Produce batch of only the distinct points. ```javascript eval|distinct(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Ec2Autoscale Create a node that can trigger autoscale events for a ec2 autoscalegroup. ```javascript eval|ec2Autoscale() ``` Returns: [Ec2AutoscaleNode](/kapacitor/v1.4/nodes/ec2_autoscale_node/) ### Elapsed Compute the elapsed time between points ```javascript eval|elapsed(field string, unit time.Duration) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### 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. ```javascript eval|eval(expressions ...ast.LambdaNode) ``` Returns: [EvalNode](/kapacitor/v1.4/nodes/eval_node/) ### First Select the first point. ```javascript eval|first(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Flatten Flatten points with similar times into a single point. ```javascript eval|flatten() ``` Returns: [FlattenNode](/kapacitor/v1.4/nodes/flatten_node/) ### GroupBy Group the data by a set of tags. Can pass literal * to group by all dimensions. Example: ```javascript |groupBy(*) ``` ```javascript eval|groupBy(tag ...interface{}) ``` Returns: [GroupByNode](/kapacitor/v1.4/nodes/group_by_node/) ### HoltWinters Compute the holt-winters (/{{< latest "influxdb" "v1" >}}/query_language/functions/#holt-winters) forecast of a data set. ```javascript eval|holtWinters(field string, h int64, m int64, interval time.Duration) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### HoltWintersWithFit Compute the holt-winters (/{{< latest "influxdb" "v1" >}}/query_language/functions/#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. ```javascript eval|holtWintersWithFit(field string, h int64, m int64, interval time.Duration) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### 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 `/kapacitor/v1/tasks/` and endpoint is `top10`, then the data can be requested from `/kapacitor/v1/tasks//top10`. ```javascript eval|httpOut(endpoint string) ``` Returns: [HTTPOutNode](/kapacitor/v1.4/nodes/http_out_node/) ### HttpPost Creates an HTTP Post node that POSTS received data to the provided HTTP endpoint. HttpPost expects 0 or 1 arguments. If 0 arguments are provided, you must specify an endpoint property method. ```javascript eval|httpPost(url ...string) ``` Returns: [HTTPPostNode](/kapacitor/v1.4/nodes/http_post_node/) ### InfluxDBOut Create an influxdb output node that will store the incoming data into InfluxDB. ```javascript eval|influxDBOut() ``` Returns: [InfluxDBOutNode](/kapacitor/v1.4/nodes/influx_d_b_out_node/) ### Join Join this node with other nodes. The data is joined on timestamp. ```javascript eval|join(others ...Node) ``` Returns: [JoinNode](/kapacitor/v1.4/nodes/join_node/) ### K8sAutoscale Create a node that can trigger autoscale events for a kubernetes cluster. ```javascript eval|k8sAutoscale() ``` Returns: [K8sAutoscaleNode](/kapacitor/v1.4/nodes/k8s_autoscale_node/) ### KapacitorLoopback Create an kapacitor loopback node that will send data back into Kapacitor as a stream. ```javascript eval|kapacitorLoopback() ``` Returns: [KapacitorLoopbackNode](/kapacitor/v1.4/nodes/kapacitor_loopback_node/) ### Last Select the last point. ```javascript eval|last(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Log Create a node that logs all data it receives. ```javascript eval|log() ``` Returns: [LogNode](/kapacitor/v1.4/nodes/log_node/) ### Max Select the maximum point. ```javascript eval|max(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Mean Compute the mean of the data. ```javascript eval|mean(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### 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)`. ```javascript eval|median(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Min Select the minimum point. ```javascript eval|min(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Mode Compute the mode of the data. ```javascript eval|mode(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### MovingAverage Compute a moving average of the last window points. No points are emitted until the window is full. ```javascript eval|movingAverage(field string, window int64) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Percentile Select a point at the given percentile. This is a selector function, no interpolation between points is performed. ```javascript eval|percentile(field string, percentile float64) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Sample Create a new node that samples the incoming points or batches. One point will be emitted every count or duration specified. ```javascript eval|sample(rate interface{}) ``` Returns: [SampleNode](/kapacitor/v1.4/nodes/sample_node/) ### Shift Create a new node that shifts the incoming points or batches in time. ```javascript eval|shift(shift time.Duration) ``` Returns: [ShiftNode](/kapacitor/v1.4/nodes/shift_node/) ### Sideload Create a node that can load data from external sources ```javascript eval|sideload() ``` Returns: [SideloadNode](/kapacitor/v1.4/nodes/sideload_node/) ### Spread Compute the difference between `min` and `max` points. ```javascript eval|spread(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### StateCount Create a node that tracks number of consecutive points in a given state. ```javascript eval|stateCount(expression ast.LambdaNode) ``` Returns: [StateCountNode](/kapacitor/v1.4/nodes/state_count_node/) ### StateDuration Create a node that tracks duration in a given state. ```javascript eval|stateDuration(expression ast.LambdaNode) ``` Returns: [StateDurationNode](/kapacitor/v1.4/nodes/state_duration_node/) ### 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. ```javascript eval|stats(interval time.Duration) ``` Returns: [StatsNode](/kapacitor/v1.4/nodes/stats_node/) ### Stddev Compute the standard deviation. ```javascript eval|stddev(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Sum Compute the sum of all values. ```javascript eval|sum(field string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### SwarmAutoscale Create a node that can trigger autoscale events for a docker swarm cluster. ```javascript eval|swarmAutoscale() ``` Returns: [SwarmAutoscaleNode](/kapacitor/v1.4/nodes/swarm_autoscale_node/) ### Top Select the top `num` points for `field` and sort by any extra tags or fields. ```javascript eval|top(num int64, field string, fieldsAndTags ...string) ``` Returns: [InfluxQLNode](/kapacitor/v1.4/nodes/influx_q_l_node/) ### Union Perform the union of this node and all other given nodes. ```javascript eval|union(node ...Node) ``` Returns: [UnionNode](/kapacitor/v1.4/nodes/union_node/) ### Where Create a new node that filters the data stream by a given expression. ```javascript eval|where(expression ast.LambdaNode) ``` Returns: [WhereNode](/kapacitor/v1.4/nodes/where_node/) ### Window Create a new node that windows the stream by time. NOTE: Window can only be applied to stream edges. ```javascript eval|window() ``` Returns: [WindowNode](/kapacitor/v1.4/nodes/window_node/)