ngsiv2_implementation_notes.md

NGSIv2 Implementation Notes

• Forbidden characters
• Custom payload decoding on notifications
• Option to disable custom notifications
• Non-modifiable headers in custom notifications
• Limit to attributes for entity location
• Legacy attribute format in notifications
• Datetime support
• Scope functionality
• Error responses
• Subscription payload validations
• actionType metadata
• noAttrDetail option
• Notification throttling
• Ordering between:$ different attribute value types
• Initial notifications
• Deprecated features

This document describes some considerations to take into account regarding the specific implementation done by Orion Context Broker of the NGSIv2 specification.

Forbidden characters

From "Field syntax restrictions" section at NGSIv2 specification:

In addition to the above rules, given NGSIv2 server implementations could add additional syntactical restrictions in those or other fields, e.g., to avoid cross script injection attacks.

The additional restrictions that apply to Orion are the ones describe in the forbidden characters section of the manual.

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Custom payload decoding on notifications

Due to forbidden characters restriction, Orion applies an extra decoding step to outgoing custom notifications. This is described in detail in this section of the manual.

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Option to disable custom notifications

Orion can be configured to disable custom notifications, using the -disableCustomNotifications CLI parameter.

In this case:

• httpCustom is interpreted as http, i.e. all sub-fields except url are ignored
• No ${...} macro substitution is performed.

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Non-modifiable headers in custom notifications

The following headers cannot be overwritten in custom notifications:

• Fiware-Correlator
• Ngsiv2-AttrsFormat

Any attemp of doing so (e.g. "httpCustom": { ... "headers": {"Fiware-Correlator": "foo"} ...} will be ignored.

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Limit to attributes for entity location

From "Geospatial properties of entities" section at NGSIv2 specification:

Client applications are responsible for defining which entity attributes convey geospatial properties (by providing an appropriate NGSI attribute type). Typically this is an entity attribute named location, but nothing prevents use cases where an entity contains more than one geospatial attribute. For instance, locations specified at different granularity levels or provided by different location methods with different accuracy. Nonetheless, it is noteworthy that spatial properties need special indexes which can be under resource constraints imposed by backend databases. Thus, implementations may raise errors when spatial index limits are exceeded. The recommended HTTP status code for those situations is 413, Request entity too large, and the reported error on the response payload must be NoResourcesAvailable.

In the case of Orion, that limit is one (1) attribute.

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Legacy attribute format in notifications

Apart from the values described for attrsFormat in the NGSIv2 specification, Orion also supports a legacy value, in order to send notifications in NGSIv1 format. This way, users can benefit from the enhancements of NGSIv2 subscriptions (e.g. filtering) with NGSIv1 legacy notification receivers.

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Datetime support

From "Special Attribute Types" section at NGSIv2 specification:

DateTime: identifies dates, in ISO8601 format. These attributes can be used with the query operators greater-than, less-than, greater-or-equal, less-or-equal and range.

The following considerations have to be taken into account at attribute creation/update time or when used in q and mq filters:

• Datetimes are composed of date, time and timezone designator, in one of the following patterns:
  • <date>
  • <date>T<time>
  • <date>T<time><timezone>
  • Note that the format <date><timezone> is not allowed. According to ISO8601: "If a time zone designator is required, it follows the combined date and time".
• Regarding <date> it must follow the pattern: YYYY-MM-DD
  • YYYY: year (four digits)
  • MM: month (two digits)
  • DD: day (two digits)
• Regarding <time> it must follow any of the patterns described in the ISO8601 specification:
  • hh:mm:ss.sss or hhmmss.sss. At the present moment, Orion is able to process times including microseconds (or even smaller resolutions) although internally they are stored as .00. However, this may change in the future (see related issue).
  • hh:mm:ss or hhmmss.
  • hh:mm or hhmm. Seconds are set to 00 in this case.
  • hh. Minutes and seconds are set to 00 in this case.
  • If <time> is ommited, then hours, minutes and seconds are set to 00.
• Regarding <timezones> it must follow any of the patterns described in the ISO8601 specification:
  • Z
  • ±hh:mm
  • ±hhmm
  • ±hh
• ISO8601 specifies that "if no UTC relation information is given with a time representation, the time is assumed to be in local time". However, this is ambiguous when client and server are in different zones. Thus, in order to solve this ambiguety, Orion will always assume timezone Z when timezone designator is ommited.

Orion always provides datetime attributes/metadata using the format YYYY-MM-DDThh:mm:ss.ssZ. Note it uses UTC/Zulu timezone (which is the best default option, as clients/receivers may be running in any timezone). This may change in the future (see related issue).

The string "ISO8601" as type for attributes and metadata is also supported. The effect is the same as when using "DateTime".

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Scope functionality

Orion implements a scope field in the POST /v2/op/update operation (you can see an example in the NGSIv2 walkthrough). However, note that this syntax is somewhat experimental and it hasn't been consolidated in the NGSIv2 specification.

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Error responses

The error response rules defined in telefonicaid#1286 takes precedence over the ones described in "Error Responses" section in the NGSIv2 specification. In particular, Orion Context Broker never responds with "InvalidModification (422)", using "Unprocessable (422)" instead.

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Subscription payload validations

The particular validations that Orion implements on NGSIv2 subscription payloads are the following ones:

• description: optional (max length 1024)
• subject: mandatory
  • entities: mandatory
    • id or idPattern: one of them is mandatory (but both at the same time is not allowed). id must follow NGSIv2 restrictions for IDs. idPattern must be not empty and a valid regex.
    • type or typePattern: optional (but both at the same time is not allowed). type must follow NGSIv2 restrictions for IDs. type must not be empty. typePattern must be a valid regex, and non-empty.
  • condition: optional (but if present it must have a content, i.e. {} is not allowed)
    • attrs: optional (but if present it must be a list; empty list is allowed)
    • expression: optional (but if present it must have a content, i.e. {} is not allowed)
      • q: optional (but if present it must be not empty, i.e. "" is not allowed)
      • mq: optional (but if present it must be not empty, i.e. "" is not allowed)
      • georel: optional (but if present it must be not empty, i.e. "" is not allowed)
      • geometry: optional (but if present it must be not empty, i.e. "" is not allowed)
      • coords: optional (but if present it must be not empty, i.e. "" is not allowed)
• notification:
  • http: must be present if httpCustom is omitted, forbidden otherwise
    • url: mandatory (must be a valid URL)
  • httpCustom: must be present if http is omitted, forbidden otherwise
    • url: mandatory (must be not empty)
    • headers: optional (but if present it must have a content, i.e. {} is not allowed)
    • qs: optional (but if present it must have a content, i.e. {} is not allowed)
    • method: optional (but if present it must be a valid HTTP method)
    • payload: optional (empty string is allowed)
  • attrs: optional (but if present it must be a list; empty list is allowed)
  • metadata: optional (but if present it must be a list; empty list is allowed)
  • exceptAttrs: optional (but it cannot be present if attrs is also used; if present it must be a non-empty list)
  • attrsFormat: optional (but if present it must be a valid attrs format keyword)
• throttling: optional (must be an integer)
• expires: optional (must be a date or empty string "")
• status: optional (must be a valid status keyword)

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actionType metadata

From NGSIv2 specification section ""System/builtin in metadata"", regarding actionType metadata:

Its value depend on the request operation type: update for updates, append for creation and delete for deletion. Its type is always Text.

Current Orion implementation supports "update" and "append". The "delete" case will be supported upon completion of this issue.

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noAttrDetail option

The value noAttrDetail of the URI param options may be used in order to avoid NGSIv2 type browsing queries (GET /v2/types and GET /v2/types/<type>) to provide attribute type details. When used, the types list associated to each attribute name is set to [].

Using this option, Orion solves these queries much faster, especially in the case of a large number of attributes, each one with a different type. This can be very useful if your use case doesn't need the attribute type detail. In some cases savings from 30 seconds to 0.5 seconds with the noAttrDetails option have been detected.

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Notification throttling

From NGSIv2 specification regarding subscription throttling:

throttling: Minimal period of time in seconds which must elapse between two consecutive notifications. It is optional.

The way in which Orion implements this is discarding notifications during the throttling guard period. Thus, nofications may be lost if they arrive too close in time. If your use case doesn't support losing notifications this way, then you should not use throttling.

In addition, Orion implements throttling in a local way. In multi-CB configurations, take into account that the last-notification measure is local to each Orion node. Although each node periodically synchronizes with the DB in order to get potencially newer values (more on this here) it may happen that a particular node has an old value, so throttling is not 100% accurate.

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Ordering between different attribute value types

From NGISv2 specification "Ordering Results" section:

Operations that retrieve lists of entities permit the orderBy URI parameter to specify the attributes or properties to be be used as criteria when ordering results

It is an implementation aspect how each type is ordered with regard to other types. In the case of Orion, we use the same criteria as the one used by the underlying implementation (MongoDB). See the following link for details.

From lowest to highest:

1. Null
2. Number
3. String
4. Object
5. Array
6. Boolean

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Initial notifications

The NGSIv2 specification describes in section "Subscriptions" the rules that trigger notifications corresponding to a given subscription, based on updates to the entities covered by the subscription. Apart from that kind of regular notifications, Orion may send also an initial notification at subscription creation/update time. Check details in the document about initial notifications

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Deprecated features

Although we try to minimize the changes in the stable version of the NGSIv2 specification, a few changes have been needed in the end. Thus, there is changed functionality that doesn't appear in the current NGSIv2 stable specification document but that Orion still supports (as deprecated functionality) in order to keep backward compability.

In particular, the usage of dateCreated and dateModified in the options parameter (introduced in stable RC-2016.05 and removed in RC-2016.10.) is still supported, e.g. options=dateModified. However, you are highly encouraged to use attrs instead (i.e. attrs=dateModified,*).

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