NGSIv2 Implementation Notes

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 attempt 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.

Note that NGSIv1 is deprecated. Thus, we don't recommend to use legacy notification format any longer.

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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 omitted, 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 ambiguity, Orion will always assume timezone Z when timezone designator is omitted.

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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User attributes or metadata matching builtin name

(The content of this section applies to all builtins except dateExpires attribute. Check the document on transient entities for specific information about dateExpires).

First of all: you are strongly encouraged to not use attributes or metadata with the same name as an NGSIv2 builtin. In fact, the NGSIv2 specification forbids that (check "Attribute names restrictions" and "Metadata names restrictions" sections in the specification).

However, if you are forced to have such attributes or metadata (maybe due to legacy reasons) take into account the following considerations:

  • You can create/update attributes and/or metadata which name is the same of a NGSIv2 builtin. Orion will let you do so.
  • User defined attributes and/or metadata are shown without need to explicit declare it in the GET request or subscription. For instance, if you created a dateModified attribute with value "2050-01-01" in entity E1, then GET /v2/entities/E1 will retrieve it. You don't need to use ?attrs=dateModified.
  • When rendered (in response to GET operations or in notifications) the user defined attribute/metadata will take preference over the builtin even when declared explicitly. For instance, if you created a dateModified attribute with value "2050-01-01" in entity E1 and you request GET /v2/entities?attrs=dateModified you will get "2050-01-01".
  • However, filtering (i.e. q or mq) is based on the value of the builtin. For instance, if you created a dateModified attribute with value "2050-01-01" in entity E1 and you request GET /v2/entities?q=dateModified>2049-12-31 you will get no entity. It happens that "2050-01-01" is greater than "2049-12-31" but the date you modified the entity (some date in 2018 or 2019 maybe) will not be greater than "2049-12-31". Note this is somehow inconsistent (i.e. user defined takes preference in rendering but not in filtering) and may change in the future.

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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 "Builtin 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, notifications 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 potentially 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 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.

Initial notification can be configurable using a new URI parameter option skipInitialNotification. For instance POST /v2/subscriptions?options=skipInitialNotification.

Check details in the document about initial notifications

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Oneshot subscriptions

Apart from the status values defined for subscription in the NGSIv2 specification, Orion also allows to use oneshot. Please find details in the oneshot subscription document

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lastFailureReason and lastSuccessCode subscriptions fields

Apart from the subscription fields described in NGSIv2 specification for GET /v2/subscriptions and GET /v2/subscriptions/subId requests, Orion supports this two extra fields within the notification field:

  • lastFailureReason: a text string describing the cause of the last failure (i.e. the failure occurred at lastFailure time).
  • lastSuccessCode: the HTTP code (200, 400, 404, 500, etc.) returned by receiving endpoint last time a successful notification was sent (i.e. the success occurred at lastSuccess time).

Both can be used to analyze possible problems with notifications. See section in the problem diagnosis procedures document for more details.

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forcedUpdate option

As extra URI param option to the ones included in the NGSIv2 specification, Orion implements forcedUpdate, than can be used to specify that an update operation have to trigger any matching subscription (and send corresponding notification) no matter if there is an actual attribute update or not. Remember that the default behaviour (i.e. without using the forcedUpdate URI param option) is to updated only if attribute is effectively updated.

The following requests can use the forcedUpdate URI param option:

  • POST /v2/entities/E/attrs?options=forcedUpdate
  • POST /v2/entities/E/attrs?options=append,forcedUpdate
  • POST /v2/op/update?options=forcedUpdate
  • PUT /v2/entities/E/attrs?options=forcedUpdate
  • PUT /v2/entities/E/attrs/A?options=forcedUpdate
  • PUT /v2/entities/E/attrs/A/value?options=forcedUpdate
  • PATCH /v2/entities/E/attrs?options=forcedUpdate

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Registrations

Orion implements registration management as described in the NGSIv2 specification, except for the following aspects:

  • PATCH /v2/registration/<id> is not implemented. Thus, registrations cannot be updated directly. I.e., updates must be done deleting and re-creating the registration. Please see this issue about this.
  • idPattern is supported but only for the exact regular expression .* And, right now (2019-03-21), only forwarding of queries is working for registrations with idPatterns. forwarded updates do not work for registrations with idPattern (the plan is to fix this asap).
  • typePattern is not implemented.
  • The only valid supportedForwardingMode is all. Trying to use any other value will end in a 501 Not Implemented error response. Please see this issue about this.
  • The expression field (within dataProvided) is not supported. The field is simply ignored. Please see this issue about it.
  • The inactive value for status is not supported. I.e., the field is stored/retrieved correctly, but the registration is always active, even when the value is inactive. Please see this issue about it.

According to NGSIv2 specification:

A NGSIv2 server implementation may implement query or update forwarding to context information sources.

The way in which Orion implements such forwarding is as follows:

Orion implements an additional field legacyForwarding (within provider) not included in NGSIv2 specification. If the value of legacyForwarding is true then NGSIv1-based query/update will be used for forwarding requests associated to that registration. Although NGSIv1 is deprecated, for the time being, NGSIv2-based forwarding has not been defined (see this issue about it) so the only valid option is to always use "legacyForwarding": true (otherwise a 501 Not Implemented error response will be the result).

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keyValues not supported in POST /v2/op/notify

The current Orion implementation doesn't support keyValues option in POST /v2/op/notify operation. If you attempt to use it you would get a 400 Bad Request error.

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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 compatibility.

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,*).

  • POST /v2/op/update accepts the same action types as NGSIv1, that is APPEND, APPEND_STRICT, UPDATE, DELETE and REPLACE. However, they shouldn't be used, preferring always the following counterparts: append, appendStrict, update, delete and replace.

  • attributes field in POST /v2/op/query is deprecated. It is a combination of attrs (to select which attributes to include in the response to the query) and unary attribute filter in q within expression (to return only entities which have these attributes). Use them instead.

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