V2/Concepts/Intro to Measurement

Measurement

Senfi deals with time seriesdata that can originate from a device or a complex system. Each record in this time series consists of:

• a timestamp indicating when the data was sampled (in milliseconds)
• a fixed set of tags
• a fixed set of metrics

All these records are kept in a dataset called a measurement. A measurement has a name (for display) and code (for integration).

A Measurement has the following components:

Measurement Code

A measurement code is a unique identifier of this measurement. You can use a combination of lowercase alphanumeric characters and underscore.

• The naming convention of a measurement code is as follows: <vendor>_<system>_<version>
• For example: nest_thermostat_v1. If you need to make changes to the measurement (eg. adding or removing metrics), you should create a new measurement with an incremented version, eg. nest_thermostat_v2.

A measurement code cannot be changed after creation

Timestamp

When sending a measurement to Senfi, timestamp is represented as tm_source, milliseconds since epoch.

Take note that timestamp refers to when the reading or measurement is taken, not when it is sent. There can be cases where readings are taken first, and sent in a batch later. In this case, the timestamp of the measurement will be older than the time of sending.

Tags

A tag is like an attribute of the measurement sample. There are 2 types of tags:

Required tags

You can think of required tags as the "composite key" to a row in a database table. That is, the combination of required tags will help you to identify the sensor that produced the measurement.

Non-required tags

Non-required tags act as a form of annotation, or label.

Tag names can be a combination of lowercase alphanumeric characters and underscore. Tag values should be sent as strings. Tag values can be empty ("") but not null.

Metric

These can be raw values from the sensing device (eg. rain_level), or derived values (eg. is_flooding). You can decide to compute the derived values before sending, or send the raw values to Senfi and make use of computed measurement to calculate it.

Similar to measurement, each metric has a name (for display), and a code (for integration). You should also specify a valid type for the metric.

Name

Any valid text (eg. Rain level)

Code

Combination of lowercase alphanumeric characters and underscore (eg. rain_level)

Type

One of the following: boolean, float, integer, string

Unit

Unit associated to metric (e.g. millimeters)

Readable

Decides if Metric can be read

Writable

Decides if Metric can be written into

There are also options to choose if a Measurement has lift or movable metrics. These are specialized metrics built for lift Measurement or Measurement with moving properties (e.g. robot, position tracking).

Metric type cannot be changed after creation

Access Group

Allows Admin Users to limit access of Measurement to certain groups of Users.

Data Retention

Decides how long the data will be retained in Senfi before it is purged.

What's Next

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• What is a Metric