Query using conditional logic
Flux provides if, then, and else conditional expressions that allow for powerful and flexible Flux queries.
Conditional expression syntax
// Pattern
if <condition> then <action> else <alternative-action>
// Example
if color == "green" then "008000" else "ffffff"
Conditional expressions are most useful in the following contexts:
- When defining variables.
- When using functions that operate on a single row at a time (
filter(),map(),reduce()).
Evaluating conditional expressions
Flux evaluates statements in order and stops evaluating once a condition matches.
For example, given the following statement:
if r._value > 95.0000001 and r._value <= 100.0 then "critical"
else if r._value > 85.0000001 and r._value <= 95.0 then "warning"
else if r._value > 70.0000001 and r._value <= 85.0 then "high"
else "normal"
When r._value is 96, the output is “critical” and the remaining conditions are not evaluated.
Examples
- Conditionally set the value of a variable
- Create conditional filters
- Conditionally transform column values with map()
- Conditionally increment a count with reduce()
Conditionally set the value of a variable
The following example sets the overdue variable based on the
dueDate variable’s relation to now().
dueDate = 2019-05-01
overdue = if dueDate < now() then true else false
Create conditional filters
The following example uses an example metric variable to change how the query filters data.
metric has three possible values:
- Memory
- CPU
- Disk
metric = "Memory"
from(bucket: "telegraf/autogen")
|> range(start: -1h)
|> filter(fn: (r) =>
if v.metric == "Memory"
then r._measurement == "mem" and r._field == "used_percent"
else if v.metric == "CPU"
then r._measurement == "cpu" and r._field == "usage_user"
else if v.metric == "Disk"
then r._measurement == "disk" and r._field == "used_percent"
else r._measurement != ""
)
Conditionally transform column values with map()
The following example uses the map() function
to conditionally transform column values.
It sets the level column to a specific string based on _value column.
from(bucket: "telegraf/autogen")
|> range(start: -5m)
|> filter(fn: (r) => r._measurement == "mem" and r._field == "used_percent" )
|> map(fn: (r) => ({
r with
level:
if r._value >= 95.0000001 and r._value <= 100.0 then "critical"
else if r._value >= 85.0000001 and r._value <= 95.0 then "warning"
else if r._value >= 70.0000001 and r._value <= 85.0 then "high"
else "normal"
})
)
from(bucket: "telegraf/autogen")
|> range(start: -5m)
|> filter(fn: (r) => r._measurement == "mem" and r._field == "used_percent" )
|> map(fn: (r) => ({
// Retain all existing columns in the mapped row
r with
// Set the level column value based on the _value column
level:
if r._value >= 95.0000001 and r._value <= 100.0 then "critical"
else if r._value >= 85.0000001 and r._value <= 95.0 then "warning"
else if r._value >= 70.0000001 and r._value <= 85.0 then "high"
else "normal"
})
)
Conditionally increment a count with reduce()
The following example uses the aggregateWindow()
and reduce()
functions to count the number of records in every five minute window that exceed a defined threshold.
threshold = 65.0
from(bucket: "telegraf/autogen")
|> range(start: -1h)
|> filter(fn: (r) => r._measurement == "mem" and r._field == "used_percent" )
|> aggregateWindow(
every: 5m,
fn: (column, tables=<-) => tables |> reduce(
identity: {above_threshold_count: 0.0},
fn: (r, accumulator) => ({
above_threshold_count:
if r._value >= threshold then accumulator.above_threshold_count + 1.0
else accumulator.above_threshold_count + 0.0
})
)
)
threshold = 65.0
from(bucket: "telegraf/autogen")
|> range(start: -1h)
|> filter(fn: (r) => r._measurement == "mem" and r._field == "used_percent" )
// Aggregate data into 5 minute windows using a custom reduce() function
|> aggregateWindow(
every: 5m,
// Use a custom function in the fn parameter.
// The aggregateWindow fn parameter requires 'column' and 'tables' parameters.
fn: (column, tables=<-) => tables |> reduce(
identity: {above_threshold_count: 0.0},
fn: (r, accumulator) => ({
// Conditionally increment above_threshold_count if
// r.value exceeds the threshold
above_threshold_count:
if r._value >= threshold then accumulator.above_threshold_count + 1.0
else accumulator.above_threshold_count + 0.0
})
)
)