default Function

Summary

default([<timeWindow>,] [<delayTime>,] <defaultValue>, <tsExpression> [.orElse<defaultIfNoData>])

Fills in gaps in the time series described by tsExpression, by inserting data points with the value defaultValue.

• Specify timeWindow to fill in data for just a limited period of time after each existing point.
• Specify delayTime to allow a gap before the inserted data.

Parameters

Description

The default() function allows you to specify the value that you would like to assign to gaps of missing data on a chart. This is the only missing data function that allows you to specify the value you’d like to assign to gaps of missing data. The default() function only fills the gaps after a data point, not after a given timestamp.

Basic Usage

For the simplest case, you can use default() to set the default value of a query to 0 if the specified metric does not exist:

default(0, ts(my.metric))

ts(my.metric).orElse(0)

Using default() With .orElse

While the default() function allows you to specify a value for missing points on a chart, the function shows NO DATA if the time series reported no data at all in the specified time window. In that situation, you can use default() with the .orElse operator to specify a value to return if no data are found.

Examples

Examples for Basic Usage of default()

The first screenshot shows two time series. The lines are dashed when there are no data:

If we wrap default() and specify 0 as the default, missing data are replaced with 0 in the display.

Examples for default() with .orElse

• If the time series exists in the time window, default() fills in the values and .orElse is not needed (does nothing).

  default(100, ts('my.metric').orElse(25))
  

• If the time series does not exist in the time window, i.e. is not reporting in the past four weeks, NO DATA is shown by default. However, .orElse specifies a value of 25, which is shown for the time series.

  default(100, ts('metric_not_there').orElse(25))
  

Caveats

Use default() with care:

• Sometimes using default() is just what you need - but sometimes it does not behave the way you might expect.
• In many cases default() does not add value when used with alerts.
• default() can affect performance - and in some cases prevent alerts from firing.

Here are some things to watch out for – and suggestions how you can rewrite the query without using default() in many cases:

Time Series Churn with default()

Use of default() leads to slower queries if there’s time series churn, that is, old time series stop reporting and new time series start reporting all the time. This can happen easily if sources are dynamically provisioned, for example, in case of an EC2 instance.

For example, consider the following query:

align(1m, default(0, ts("filehandles.used"))) / align(1m, default(0, ts("filehandles.total"))) * 100 > 60.

Assume your environment has about 350 active time series at any moment, but within the last 2 weeks (the metrics obsolescence period if the default configuration is not changed), ~7200 unique time series were active.

In this case, default() is not needed at all - filehandles.used and filehandles.total always report together. The following query is more than 20x faster:

ts("filehandles.used") / ts("filehandles.total") * 100 > 60

Alerts Don’t Fire with default()

When a metric arrives with a delay of more than 1 minute, the use of default() can prevent an associated alert from firing because the value for the last minute evaluates to false.

Instead of accounting for sparse metrics – success.count is reporting all the time, but failure.count is reporting a value only when there’s a problem – approach the query from a different angle.

Instead of:

ts(success.count) * 100 / (default(0, ts(failure.count)) + ts(success.count)) < 95

Use

ts(failure.count) * 100 / (ts(failure.count) + ts(success.count)) > 5

Unexpected Behavior with highpass() and default()

Using highpass() after default() with a higher highpass value than default reverts the effects of default().

Instead of

highpass(..., default(0, ts(...)))

Use

highpass(..., ts(...))

Using msum() Makes default() Redundant

Using msum() after default(0, ) is redundant because msum() always returns a value for all active series where default(0, ) backfills a value.

Instead of

msum(..., default(0, ts(...)))

Use

msum(..., ts(...))

Using rawsum() Makes default() Redundant

Using rawsum() after default(0, ) is usually redundant. If you are sure that default() is necessary:

Instead of

rawsum(default(0, ts(...)))

Use

default(0, rawsum(ts(...)))

Time Windows and default()

To improve performance, limit the time window.

• Use the timeWindow parameter to limit to the current time window.
• Use the delayTime parameter introduce a delay time.

Histogram and default()

You cannot apply default() to a histogram. Even if you convert the histogram to a tsExpression, an error results if you then apply default().

See Also

Using Moving and Tumbling Windows to Highlight trends

Other missing data functions include:

• interpolate
• next
• last
• orElse Operator