IEC 62541-11:2025
(Main)OPC Unified Architecture - Part 11: Historical Access
OPC Unified Architecture - Part 11: Historical Access
IEC 62541-11: 2025 defines the Information Model associated with Historical Access (HA). It particularly includes additional and complementary descriptions of the NodeClasses and Attributes needed for Historical Access, additional standard Properties, and other information and behaviour. The complete AddressSpace Model including all NodeClasses and Attributes is specified in IEC 62541‑3. The predefined Information Model is defined in IEC 62541‑5. The Services to detect and access historical data and events, and description of the ExtensibleParameter types are specified in IEC 62541‑4. This document includes functionality to compute and return Aggregates like minimum, maximum, average etc. The Information Model and the concrete working of Aggregates are defined in IEC 62541‑13. Conventions for Historical Access Clients are informatively provided in Annex A.
This fourth edition cancels and replaces the third edition published in 2020. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous edition:
a) a functionality has been added to support retrieving of modified events;
b) an Event has been added to indicate when a backfill occurred;
c) a new ReferenceType that can be used to indicate an external node has been defined;
d) the text has been improved to better explain the concept of annotation and remove conflicting explanations;
e) a default historian configuration (and where to find it) has been defined;
f) HistoricalEventConfigurationType, which provides general configuration information about the historical Event storage, has been added;
g) the text has been updated and optional fields have been added to HA configuration object to allow configuration to be defined for periodic data collection, not just for exception-based collection;
h) an ObjectType that can be used for external event collection has been provided as well as an example how historians can be configured.
Architecture unifiée OPC - Partie 11: Accès à l'historique
IEC 62541-11: 2025 définit le Modèle d'Information associé à l'Accès à l'historique (HA, Historical Access). Elle inclut en particulier des descriptions supplémentaires et complémentaires des NodeClasses et des Attributs nécessaires pour l'Accès à l'historique, des Propriétés normalisées supplémentaires et d'autres informations et comportements. Le Modèle complet de l'AddressSpace comprenant toutes les NodeClasses et tous les Attributs est spécifié dans l'IEC 62541‑3. Le Modèle d'information prédéfini est défini dans l'IEC 62541‑5. Les Services permettant de détecter et d'accéder aux données et événements historiques, ainsi qu'une description des types ExtensibleParameter, sont spécifiés dans l'IEC 62541‑4. Le présent document inclut une fonctionnalité permettant de calculer et de renvoyer des Agrégats (minimum, maximum, moyenne, etc.). Le Modèle d'information et la fonction concrète des Agrégats sont définis dans l'IEC 62541‑13. Les conventions pour les Clients d'Accès à l'historique sont données, à titre informatif, à l'Annexe A. Cette quatrième édition annule et remplace la troisième édition parue en 2020. Cette édition constitue une révision technique.
Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
a) une fonctionnalité qui permet de récupérer les événements modifiés a été ajoutée;
b) un Événement qui indique quand un remplissage a eu lieu a été ajouté;
c) un nouveau ReferenceType qui peut être utilisé pour indiquer un nœud externe a été défini;
d) le texte a été amélioré afin de mieux expliquer le concept d'annotation et de supprimer les explications contradictoires;
e) une configuration d'historique par défaut (et l'endroit où la trouver) a été définie;
f) l'HistoricalEventConfigurationType, qui fournit des informations de configuration générale sur le stockage des Événements historiques, a été ajouté;
g) le texte a été mis à jour et des champs facultatifs ont été ajoutés à l'objet de configuration HA afin de permettre la définition de la configuration pour la collecte de données périodique, et pas seulement pour la collecte fondée sur les exceptions;
h) un ObjectType qui peut être utilisé pour la collecte d'événements externes a été prévu, ainsi qu'un exemple de la façon dont les historiques peuvent être configurés.
General Information
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Standards Content (Sample)
IEC 62541-11 ®
Edition 3.0 2025-12
INTERNATIONAL
STANDARD
REDLINE VERSION
OPC unified architecture -
Part 11: Historical Access
ICS 25.040.40; 35.100.05 ISBN 978-2-8327-0974-0
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CONTENTS
FOREWORD . 5
1 Scope . 8
2 Normative references . 8
3 Terms, definitions and abbreviations . 8
3.1 Terms and definitions. 8
3.2 Abbreviated terms . 10
4 Concepts . 11
4.1 General . 11
4.2 Data architecture . 11
4.3 Historians and interruption of data collection . 12
4.4 Modification of Historical Data/Events . 12
4.5 Timestamps . 13
4.6 Bounding Values and time domain . 13
4.7 Changes in AddressSpace over time . 15
5 Historical Information Model . 16
5.1 HistoricalNodes . 16
5.1.1 General . 16
5.1.2 Annotations Property . 16
5.2 HistoricalDataNodes . 17
5.2.1 General . 17
5.2.2 HistoricalDataConfigurationType . 17
5.2.3 Attributes . 19
5.2.4 Historical Data Configuration Object . 19
5.3 References . 20
5.3.1 Overview . 20
5.3.2 HasHistoricalConfiguration ReferenceType . 21
5.3.3 HasCurrentData ReferenceType . 21
5.3.4 HasCurrentEvent ReferenceType . 22
5.4 HistoricalEventNodes . 22
5.4.1 General . 22
5.4.2 HistoricalEventFilter Property . 22
5.4.3 HistoricalEventConfigurationType . 23
5.4.4 HistoricalEventNode Attributes . 23
5.5 External History sources . 24
5.5.1 General . 24
5.5.2 External Historical Event Node . 24
5.6 Example Object Models in Historian Servers (informative) . 25
5.6.1 Overview . 25
5.6.2 HistoricalDataNodes Address Space Model . 25
5.6.3 Historical data . 27
5.6.4 HistoricalEventNodes Address Space Model . 27
5.6.5 Historical Events . 30
5.7 Exposing supported functions and capabilities . 30
5.7.1 General . 30
5.7.2 HistoryServerCapabilitiesType . 32
5.7.3 Default configuration. 35
5.8 Historical Audit Events . 36
5.8.1 General . 36
5.8.2 AuditHistoryEventUpdateEventType . 36
5.8.3 AuditHistoryValueUpdateEventType . 37
5.8.4 AuditHistoryAnnotationUpdateEventType . 37
5.8.5 AuditHistoryDeleteEventType . 38
5.8.6 AuditHistoryRawModifyDeleteEventType . 39
5.8.7 AuditHistoryAtTimeDeleteEventType . 40
5.8.8 AuditHistoryEventDeleteEventType . 40
5.8.9 AuditHistoryConfigurationChangeEventType . 41
5.8.10 AuditHistoryBulkInsertEventType . 41
6 Historical Access specific usage of Services. 42
6.1 General . 42
6.2 Historical Nodes StatusCodes . 43
6.2.1 Overview . 43
6.2.2 Operation level result codes . 43
6.2.3 Semantics changed . 45
6.3 Continuation Points . 45
6.4 Arrays, index ranges and substrings . 45
6.5 HistoryReadDetails parameters . 47
6.5.1 Overview . 47
6.5.2 ReadEventDetails structure . 49
6.5.3 ReadRawModifiedDetails structure . 52
6.5.4 ReadProcessedDetails structure . 55
6.5.5 ReadAtTimeDetails structure . 57
6.5.6 ReadAnnotationDataDetails structure . 58
6.6 HistoryData parameters returned . 59
6.6.1 Overview . 59
6.6.2 HistoryData type . 59
6.6.3 HistoryModifiedData type . 59
6.6.4 HistoryEvent type DataType . 60
6.5.5 HistoryAnnotationData type .
6.6.5 HistoryModifiedEvent DataType . 61
6.6.6 Annotation DataType . 62
6.7 HistoryUpdateType Enumeration . 62
6.8 PerformUpdateType Enumeration . 63
6.9 HistoryUpdateDetails parameter . 63
6.9.1 Overview . 63
6.9.2 UpdateDataDetails structure . 65
6.9.3 UpdateStructureDataDetails structure . 67
6.9.4 UpdateEventDetails structure . 69
6.9.5 DeleteRawModifiedDetails structure . 71
6.9.6 DeleteAtTimeDetails structure . 72
6.9.7 DeleteEventDetails structure . 73
Annex A (informative) Client conventions . 75
A.1 How clients can request timestamps . 75
A.2 Determining the first or last historical data point. 76
Bibliography . 78
Figure 1 – Possible OPC UA Server supporting Historical Access . 11
Figure 2 – ReferenceType hierarchy . 20
Figure 3 – Historical Variable with Historical Data Configuration and Annotations . 26
Figure 4 – Historical Variable with Historical Data Configuration and Annotations . 27
Figure 5 – Representation of an Event with History in the AddressSpace . 29
Figure 6 – Event History configuration example . 30
Figure 7 – Server and HistoryServer Capabilities . 32
Figure 8 – History Array example . 46
Figure 9 – Historian Array Illustration 2 . 47
Table 1 – Bounding Value examples . 14
Table 2 – Annotations Property . 16
Table 3 – HistoricalDataConfigurationType definition . 17
Table 4 – ExceptionDeviationFormat Values Items . 18
Table 5 – ExceptionDeviationFormat definition . 19
Table 6 – Historical Access configuration definition . 20
Table 7 – HasHistoricalConfiguration ReferenceType . 21
Table 8 – HasCurrentData ReferenceType . 21
Table 9 – HasCurrentEvent ReferenceType . 22
Table 10 – Standard Historical Events Properties . 23
Table 11 – HistoricalEventConfigurationType definition . 23
Table 12 – HistoricalExternalEventSourceType definition . 24
Table 13 – HistoryServerCapabilitiesType Definition . 33
Table 14 – DefaultHAConfiguration definition . 35
Table 15 – DefaultHEConfiguration definition . 35
Table 16 – AuditHistoryEventUpdateEventType definition . 36
Table 17 – AuditHistoryValueUpdateEventType definition . 37
Table 18 – AuditHistoryAnnotationUpdateEventType definition . 38
Table 19 – AuditHistoryDeleteEventType definition . 39
Table 20 – AuditHistoryRawModifyDeleteEventType definition . 39
Table 21 – AuditHistoryAtTimeDeleteEventType definition . 40
Table 22 – AuditHistoryEventDeleteEventType definition . 41
Table 23 – AuditHistoryConfigurationChangeEventType definition . 41
Table 24 – AuditHistoryBulkInsertEventType definition . 42
Table 25 – Bad operation level result codes . 43
Table 26 – Good operation level result codes . 44
Table 27 – HistoryReadDetails parameterTypeIds parameter Symbolic Names . 48
Table 28 – HistoryReadDetails definition . 49
Table 29 – ReadEventDetails Structure . 49
Table 30 – ReadEventDetails definition. 49
Table 31 – ReadEventDetails2 Structure . 51
Table 32 – ReadEventDetails2 definition . 51
Table 33 – ReadRawModifiedDetails structure . 52
Table 34 – ReadRawModifiedDetails definition . 52
Table 35 – ReadProcessedDetails structure. 55
Table 36 – ReadProcessedDetails definition . 55
Table 37 – ReadAtTimeDetails structure . 57
Table 38 – ReadAtTimeDetails definition . 57
Table 39 – ReadAnnotationDataDetails Structure . 58
Table 40 – ReadAnnotationDataDetails definition . 58
Table 41 – HistoryData Details structure . 59
Table 42 – HistoryData definition . 59
Table 43 – HistoryModifiedData Details structure . 60
Table 44 – HistoryModifiedData definition . 60
Table 45 – HistoryEvent Details structure . 60
Table 46 – HistoryEvent definition . 61
Table 47 – HistoryModifiedEvent structure . 61
Table 48 – HistoryModifiedEvent definition . 61
Table 49 – Annotation Structure . 62
Table 50 – Annotation definition . 62
Table 51 – HistoryUpdateType Enumeration Items . 62
Table 52 – HistoryUpdateType definition . 63
Table 53 – PerformUpdateType Enumeration Items . 63
Table 54 – PerformUpdateType definition . 63
Table 55 – HistoryUpdateDetails parameter TypeIds Symbolic Names . 64
Table 56 – HistoryUpdateDetails Structure. 65
Table 57 – HistoryUpdateDetails definition . 65
Table 58 – UpdateDataDetails Structure . 66
Table 59 – UpdateDataDetails definition . 66
Table 60 – UpdateStructureDataDetails Structure . 67
Table 61 – UpdateStructureDataDetails definition . 68
Table 62 – UpdateEventDetails Structure. 69
Table 63 – UpdateEventDetails definition . 70
Table 64 – DeleteRawModifiedDetails Structure . 72
Table 65 – DeleteRawModifiedDetails definition. 72
Table 66 – DeleteAtTimeDetails Structure. 73
Table 67 – DeleteAtTimeDetails definition . 73
Table 68 – DeleteEventDetails Structure . 73
Table 69 – DeleteEventDetails definition . 74
Table A.1 – Time keyword definitions . 76
Table A.2 – Time offset definitions . 76
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPC unified architecture -
Part 11: Historical Access
FOREWORD
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This redline version of the official IEC Standard allows the user to identify the changes made
to the previous edition IEC 62541-11:2020. A vertical bar appears in the margin wherever a
change has been made. Additions are in green text, deletions are in strikethrough red text.
IEC 62541-11 has been prepared by subcommittee 65E: Devices and integration in enterprise
systems, of IEC technical committee 65: Industrial-process measurement, control and
automation. It is an International Standard.
This fourth edition cancels and replaces the third edition published in 2020. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) a functionality has been added to support retrieving of modified events;
b) an Event has been added to indicate when a backfill occurred;
c) a new ReferenceType that can be used to indicate an external node has been defined;
d) the text has been improved to better explain the concept of annotation and remove
conflicting explanations;
e) a default historian configuration (and where to find it) has been defined;
f) HistoricalEventConfigurationType, which provides general configuration information about
the historical Event storage, has been added;
g) the text has been updated and optional fields have been added to HA configuration object
to allow configuration to be defined for periodic data collection, not just for exception-based
collection;
h) an ObjectType that can be used for external event collection has been provided as well as
an example how historians can be configured.
The text of this International Standard is based on the following documents:
Draft Report on voting
65E/1058/CDV 65E/1096/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
Throughout this document and the other parts of the IEC 62541 series, certain document
conventions are used:
Italics are used to denote a defined term or definition that appears in the "Terms and definitions"
clause in one of the parts of the IEC 62541 series.
Italics are also used to denote the name of a service input or output parameter or the name of
a structure or element of a structure that are usually defined in tables.
The italicized terms and names are, with a few exceptions, written in camel-case (the practice
of writing compound words or phrases in which the elements are joined without spaces, with
each element's initial letter capitalized within the compound). For example, the defined term is
AddressSpace instead of Address Space. This makes it easier to understand that there is a
single definition for AddressSpace, not separate definitions for Address and Space.
A list of all parts in the IEC 62541 series, published under the general title OPC Unified
Architecture, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
1 Scope
This part of IEC 62541 belongs to the OPC Unified Architecture standards series and defines
the Information Model associated with Historical Access (HA). It particularly includes additional
and complementary descriptions of the NodeClasses and Attributes needed for Historical
Access, additional standard Properties, and other information and behaviour.
The complete AddressSpace Model including all NodeClasses and Attributes is specified in
IEC 62541-3. The predefined Information Model is defined in IEC 62541-5. The Services to
detect and access historical data and events, and description of the ExtensibleParameter types
are specified in IEC 62541-4.
This document includes functionality to compute and return Aggregates like minimum,
maximum, average etc. The Information Model and the concrete working of Aggregates are
defined in IEC 62541-13.
Conventions for Historical Access Clients are informatively provided in Annex A.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC TR 62541-1, OPC Unified Architecture - Part 1: Overview and Concepts
IEC 62541-3, OPC Unified Architecture - Part 3: Address Space Model
IEC 62541-4, OPC Unified Architecture - Part 4: Services
IEC 62541-5, OPC Unified Architecture - Part 5: Information Model
IEC 62541-7, OPC Unified Architecture - Part 7: Profiles
IEC 62541-8, OPC Unified Architecture - Part 8: Data Access
IEC 62541-13, OPC Unified Architecture - Part 13: Aggregates
3 Terms, definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TR 62541-1,
IEC 62541-3, IEC 62541-4, IEC 62541-13 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1.1
Annotation
metadata associated with an item at a given instance in time
Note 1 to entry: An Annotation is metadata that is associated with an item at a given instance in time.
3.1.2
BoundingValues
values associated with the starting and ending time
Note 1 to entry: BoundingValues are the values that are associated with the starting and ending time of a
ProcessingInterval specified when reading from the historian. BoundingValues may can be required by Clients to
determine the starting and ending values when requesting raw data RawData over a time range. If a raw data
RawData value exists at the start or end point, it is considered the bounding value even though it is part of the data
request. If no raw data RawData value exists at the start or end point, then the Server will determine the boundary
value, which may possibly requires data from a data point outside of the requested range. See 4.6 for details on
using BoundingValues.
3.1.3
Historian
application storing time series data and/or time series events
3.1.4
HistoricalNode
Object, Variable, Property or View in the AddressSpace where a Client can access historical
data or Events
Note 1 to entry: A HistoricalNode is a term used in this document to represent any Object, Variable, Property or
View in the AddressSpace for which a Client may can read and/or update historical data or Events. The terms
"HistoricalNodes's history" or "history of a HistoricalNodes" will refer to the time series data or Events stored for this
HistoricalNode. The term HistoricalNode refers to both HistoricalDataNodes and HistoricalEventNodes.
3.1.5
HistoricalDataNode
Variable or Property in the AddressSpace where a Client can access historical data
Note 1 to entry: A HistoricalDataNode represents any Variable or Property in the AddressSpace for which a Client
may can read and/or update historical data. "HistoricalDataNode history" or "history of a HistoricalDataNode" refers
to the time series data stored for this HistoricalNode. Examples of such data are:
• device data (like temperature sensors)
• calculated data
• status information (open/closed, moving)
• dynamically changing system data (like stock quotes)
• diagnostic data
The term HistoricalDataNodes is used when referencing aspects of the standard that apply to accessing historical
data only.
3.1.6
HistoricalEventNode
Object or View in the AddressSpace for which a Client can access historical Events
Note 1 to entry: "HistoricalEventNode's history" or "history of a HistoricalEventNode" refers to the time series
Events stored in some historical system. Examples of such data are:
• Notifications
• system Alarms
• operator action Events
• system triggers (such as new orders to be processed)
The term HistoricalEventNode is used when referencing aspects of the standard that apply to accessing historical
Events only.
3.1.7
modified values
ModifiedValues
HistoricalDataNode's value that has been changed (or manually inserted or deleted) after it was
stored in the historian
Note 1 to entry: For some Servers, a lab data entry value is not a modified value, but if a user corrects a lab value,
the original value would be considered a modified value, and would be returned during a request for modified values
ModifiedValues. Also manually inserting a value that was missed by a standard collection system can be considered
a modified value. Unless specified otherwise, all historical Services operate on the current, or most recent, value for
the specified HistoricalDataNode at the specified timestamp. Requests for modified values ModifiedValues are used
to access values that have been superseded, deleted or inserted. It is up to a system to determine what is considered
a modified value. Whenever a Server has modified data available for an entry in the historical collection it shall set
the ExtraData bit in the StatusCode.
3.1.8
raw data
RawData
data that is stored within the historian for a HistoricalDataNode
Note 1 to entry: The data can be all data collected for the DataValue or it can be some subset of the data depending
on the historian and the storage rules invoked when the item's values were saved.
3.1.9
StartTime/EndTime
bounds of a history request which define the time domain
Note 1 to entry: For all requests, a value falling at the end time of the time domain is not included in the domain,
so that requests made for successive, contiguous time domains will include every value in the historical collection
exactly once.
3.1.10
TimeDomain
interval of time covered by a particular request, or response
Note 1 to entry: In general, if the start time is earlier than or the same as the end time, the time domain is considered
to begin at the start time and end just before the end time; if the end time is earlier than the start time, the time
domain still begins at the start time and ends just before the end time, with time "running backward" for the particular
request and response. In both cases, any value which falls exactly at the end time of the TimeDomain is not included
in the TimeDomain. See the examples in 4.6. BoundingValues effect the time domain as described in 4.6.
All timestamps which can legally be represented in a UtcTime DataType are valid timestamps, and the Server may
has the choice to, or not to return an invalid argument result code due to the timestamp being outside of the range
for which the Server has data. See IEC 62541-3 for a description of the range and granularity of this DataType.
Servers are expected to handle out-of-bounds timestamps gracefully and return the proper StatusCodes to the Client.
3.1.11
StructuredHistoryData
structured data stored in a history collection where parts of the structure are used to uniquely
identify the data within the data collection
Note 1 to entry: Most historical data applications assume only one current value per timestamp. Therefore, the
timestamp of the data is considered the unique identifier for that value. Some data or meta data such as Annotations
may can permit multiple values to exist at a single timestamp. In such cases, the Server would use one or more
parameters of the StructuredHistoryData entry to uniquely identify each element within the history collection.
Annotations are examples of StructuredHistoryData.
3.2 Abbreviated terms
DA Data Access
HA Historical Access
HDA Historical Data Access
UA Unified Architecture
4 Concepts
4.1 General
This document defines the handling of historical time series data and historical Event data in
the OPC Unified Architecture (in a Historian). Included is the specification of the representation
of historical data and Events in the AddressSpace.
Annex A defines some useful, but not normative, conventions for OPC UA Clients.
4.2 Data architecture
A Server supporting Historical Access provides Clients with transparent access to different
historical data and/or historical Event sources (e.g., process Historians, event Historians, etc.).
The historical data or Events may can be located in a proprietary data collection, database or
a short-term buffer within memory. A Server supporting Historical Access will provide historical
data and Events for all or a subset of the available Variables, Objects, Properties or Views
within the Server AddressSpace.
Figure 1 illustrates how the AddressSpace of a UA Server might can consist of a broad range
of different historical data and/or historical Event sources.
Figure 1 – Possible OPC UA Server supporting Historical Access
The Server may Historian can be implemented as a standalone OPC UA Server that collects
data from another OPC UA Server or another data source. The Historian can also just aggregate
historical data from underlying Historians. The Client that references the OPC UA Server
supporting Historical Access for historical data may can be simple trending packages that desire
values over a given time frame or they may can be complex reports that require data in multiple
formats.
There are general requirements for Historians, but Historians can vary in functionality. A
consistent requirement for all Historians is that they store Historical data including a timestamp.
All historical data should include status information for each value, but a Historian can compress
this to only storing status information that indicates a problem (Bad status) and/or status
change, instead of storing a status for every time series data item. The status of historical data
can be complex. What is required is that the values returned as part of the timeseries raw data
match the data that would have been observed if the Value was subscribed to at that point in
time.
Historical Events are more complicated. In a stream of Events each Event can have a different
list of fields. EventTypes are defined in a hierarchical manner, where each EventType inherits
fields from its parent type and can add additional fields. Some of these additional fields can be
mandatory and are required to understand or process the given EventType. A Historian that
stores Events, shall be configurable to store all mandatory fields for any EventTypes that it
historizes. If it receives for storage an EventType that it does not support all mandatory fields
for, it can store it as one of its supertype EventTypes (one that it does support all mandatory
fields for), but then it shall not claim that it supports historizing of that EventType (see 5.4.3).
The Historian shall also provide information about the fields that are currently being historized
(see 5.4.3).
4.3 Historians and interruption of data collection
When an Historian is collecting and storing data, the data collection can be interrupted. The
interruption can have been for collecting the current values of data or for an event stream. The
interruption can have been due to an interruption in the source of a value or an interruption of
the forwarding of historical data from an underlying Historian. The interruption can also have
been due to an action that stopped the collection of HistoricalData or historical Events. Some
of these interruptions can recover with no loss of data, others can result in data gaps. The
Historian shall report any gaps when a client is accessing the stored historical data with an
error code of Bad_DataLost.
For example, if a subscription for data breaks, and the historian recovers the subscription after
several minutes, it can check the SourceTimestamp of the initial values in the subscription. If
the initial value SourceTimestamp matches the last stored value SourceTimestamp, then no
data was lost and nothing needs to be stored indicating the given HistoricalDataNode's data
collection was interrupted. But if the SourceTimestamp of the initial value is later than the
SourceTimestamp of the last stored value, then the historian has no way of knowing
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