---
domain: rfc.tango-controls.org
shortname: 5/Property
name: Property
status: stable
editor: Gwenaëlle Abeillé (gwenaelle.abeille@synchrotron-soleil.fr)
---

# 5/Property

This document describes the Tango Property model.

See also: [1/Tango](../1/Tango.md), [2/Device](../2/Device.md), [4/Attribute](../4/Attribute.md), [6/Database](../6/Database.md), [9/DataTypes](../9/DataTypes.md)

## Preamble

Copyright (c) 2019 Tango Controls Community.

This Specification is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version. This Specification is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, see <http://www.gnu.org/licenses>.

This Specification is a [free and open standard](https://web.archive.org/web/20161002091934/http://www.digistan.org/open-standard:definition) and is governed by the Digital Standards Organization's [Consensus-Oriented Specification System](https://web.archive.org/web/20161002092144/http://www.digistan.org/spec:1/coss).

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC-2119](https://tools.ietf.org/html/rfc2119).

## Tango Property Specification

This specification is intended to formally document the Tango Property model.
Runtime representation of Property in conforming Tango implementations MUST follow this specification.

### Goals

A Property represents a configuration parameter in Tango.

### Use Cases

Here are some Property use cases:

* To connect a real network equipment a Property can define the target IP address or hostname
* To change the representation of an information in order to be more user friendly a Property can define a new data format for the graphical user interface to convert the raw data
* To configure some alarms on Attribute Value (See [4/Attribute](../4/Attribute.md))
* To store the Attribute value of a Memorized Attribute (See [4/Attribute](../4/Attribute.md))
* To configure the labels associated to the possible values of a DevEnum Attribute

## Specification

A Property is a strict definition of a key/value pair:
  * The Property SHALL have one key, called Property Name.
  * The Property SHALL have one value, called Property Value.
    * It CAN be empty.

There are 4 kinds of Properties:

 *  Class Property: a Property related to a Device Class ([2/Device](../2/Device.md)).
 *  Device Property: a Property related to a Device ([2/Device](../2/Device.md)).
 *  Attribute Property: a Property related to an Attribute ([4/Attribute](../4/Attribute.md)).
 *  Free Property: a Property defined for the entire Control System ([1/Tango](../1/Tango.md)) i.e not related to any Class, Device or Attribute. A Free Property is attached to an entity called a Free Object.

A Property MAY be persisted in the Tango Database (See [1/Tango](../1/Tango.md) and [6/Database](../6/Database.md)) or into a file if no Tango Database is used.

Device Properties and Class Properties MAY have a _Default Value_ which is used if the Property Value has not been persisted.

A Tango Device Property MAY be defined as _Mandatory in Database_.
This metadata can be used when the device server programmer requires this Property to be persisted.
A Device Property defined as _Mandatory in Database_ SHALL not have a Default Value.

Device Property and Class Property MAY have a Type metadata.
It is RECOMMENDED to support the following Types for Device Properties and Class Properties (See [9/DataTypes](../9/DataTypes.md)):
  * DevBoolean
  * DevShort
  * DevUShort
  * DevLong
  * DevULong
  * DevLong64
  * DevULong64
  * DevFloat
  * DevDouble
  * DevString
  * DevVarShortArray
  * DevVarLongArray
  * DevVarLong64Array
  * DevVarFloatArray
  * DevVarDoubleArray
  * DevVarStringArray

Device and Class Properties MUST share the same scope. This means that a Device Property will always take precedence over a Class Property of the same name.

It is RECOMMENDED to provide a way to retrieve a persisted Property Value and to convert it to the above types.

It is RECOMMENDED to support _Not a Number (NaN)_ as well as _-infinity (-inf)_ and _+infinity (+inf)_ DevFloat and DevDouble values.

### Naming convention

* The Property's Name SHALL use the following convention:
``` ABNF
device_property_name = ALPHA *254acceptable-char
class_property_name = ALPHA *254acceptable-char
free_property_name = ALPHA *254acceptable-char
attribute_property_name = (ALPHA / underscore) *254acceptable-char
acceptable-char = ALPHA / DIGIT / underscore
underscore = %x5F
```

and Free Objects SHALL use:

```ABNF
free_object_name = ALPHA *254acceptable-char
acceptable-char = ALPHA / DIGIT / underscore
underscore = %x5F
```