Linked Data Event Streams

1. Introduction

An LDES client is a piece of software used by a consumer that accepts the URL to an entry point, and returns a stream of members of the corresponding ldes:EventStream. The data stream emits the history that is available from this entry point, and once the consumer has caught up with the stream, it remains synchronized as new members are published.

The client does this by extending upon a subset of the W3C TREE hypermedia specification. This extension introduces specialized terms for dealing with append-only collections, referred to as event streams. For example, one can indicate what time-based property in the member is used for indicating the order of the event stream, indicate the retention policy as a promise from the producer to the consumer, or detail how to deal with version-based members.

More extensions should be specified w.r.t. HTTP status codes, or keeping state. This should be further detailled in a chapter after the overview.

2. Overview

An overview of the LDES specification.

A Linked Data Event Stream (LDES) (ldes:EventStream) is a collection (rdfs:subClassOf tree:Collection) of members that cannot be updated or removed once they are published, with each member being a set of RDF quads ([rdf-primer]). This way, the collection of members becomes an append-only log or event stream.

Following the TREE specification, this event stream is published using one or more HTTP resources. When more resources are used, these pages, or tree:Nodes, will be structured according to a search tree. Therefore we will use the terms root node for the first page, and subsequent node for every next page in the structure.

In the root node, the client will expect these properties to be described on the ldes:EventStream entity:

ldes:timestampPath: this is a SHACL property path (all further *Path properties are as well) that sets the chronological time with a xsd:dateTime literal within each member. This timestamp determines the chronological order in which members of the event stream are added. When ldes:timestampPath is set, no member can be added to the LDES with a timestamp earlier than the latest published member.
ldes:sequencePath: when the LDES producer wants to make clear what the ordering is within members with the same timestamp for the ldes:timestampPath, this property defines, based on the [xpath-functions-31] comparison operator, what xsd-literals define the order of processing. When no ldes:timestampPath has been set, the ldes:sequencePath defines the sequence for all members in the LDES.
ldes:versionOfPath: when your entities are versioned, this property points at the object that tells you what entity it is a version of (e.g., dcterms:isVersionOf).
ldes:versionTimestampPath: when those versions are not published chronologically, LDES providers have the possibility to indicate a different version timestamp to establish the latest version.
ldes:versionSequencePath: when the versions do not follow the order in ldes:timestampPath and ldes:sequencePath, and for when ldes:versionTimestampPath are the same for multiple members, or for when this property is not set. E.g., for out of order publishing of v0.1 → v0.2, in which v0.2 may have been published by the server before v0.1.
tree:shape: a [SHACL] shape that can be used to select a search tree in the discovery phase, as well as to validate the members in the event stream. The sh:NodeShape linked validates each target of the tree:member property. A validator in a consumer pipeline MUST ignore other targets.
tree:view: connects the collection to the current page, or points to one specific root node after dereferencing the ldes:EventStream identifier.

In the root node, the current node identified by the URL of the page (a provider can achieve this simply by using a relative IRI <>) will be further described using these properties:

ldes:retentionPolicy: indicates a retention policy (see next the dedicated chapter [](#retention)).
tree:viewDescription: can as well contain the retention policy, or other context data about this view of the LDES (e.g., the dcat:Distribution, the tree:SearchTree, or the ldes:EventSource) as a named entity. This is useful for example if a producer would like to disambiguate the IRI for the ldes:EventSource from the root tree:Node. By default, the tree:viewDescription points at the root node.

The client MUST implement the initialization of the TREE specification, and they MAY extract the tree:search form.

In any tree:Node – root node or subsequent node – the client expects to find 0 or more members of the ldes:EventStream using the tree:member property. The subject is the event stream instance and the object is the root focus node of a member. An LDES client MUST implement the Member Extraction Algorithm of the TREE specification to retrieve the full set of quads of the member. In an ldes:EventStream, the object of the tree:member triple can only be an IRI as this IRI will be used in the state to check whether the member has already been emitted or not.

An example record from a sensor observation dataset in the [turtle] format:

ex:Observations a ldes:EventStream ;
                ldes:timestampPath sosa:resultTime ;
                tree:shape ex:shape1.shacl ;
                tree:view <> ;
                tree:member ex:Observation1 .

ex:Observation1 a sosa:Observation ;
                sosa:resultTime "2026-01-01T00:00:00Z"^^xsd:dateTime ;
                sosa:hasSimpleResult "..." .

An example record from a base registry of addresses in the [trig] format:

ex:AddressRecords a ldes:EventStream ;
                  ldes:timestampPath dcterms:created ;
                  ldes:versionOfPath dcterms:isVersionOf ;
                  tree:shape ex:shape2.shacl ;
                  tree:view <> ;
                  tree:member ex:AddressRecord1-activity1 .

ex:AddressRecord1-activity1 dcterms:created "2026-01-01T00:00:00Z"^^xsd:dateTime ;
                            adms:versionNotes "First version of this address" ;
                            dcterms:isVersionOf ex:AddressRecord1 .
                           
ex:AddressRecord1-activity1 {
    ex:AddressRecord1 dcterms:title "Streetname X, ZIP Municipality, Country" .
}

In any tree:Node, root node or subsequent node, the client expects to find zero or more tree:relation properties, containing a description of the tree:Relations from this node to subsequent nodes. A client MUST traverse the relations cfr. the TREE chapters on traversing the search tree. A client MUST keep its own state to know when to refetch certain tree:Nodes.

We should refer here to a new next chapter on how to gracefully iterate over the pages and how to keep the state in more detail cfr. the extensions in Issue 1. We can then also indicate that a client MAY implement the text on pruning branches related to interpreting comparators for xsd:dateTime literals if it wants to detect immutable pages via the timestampPath.

More specific server documentation should be found in a Server Primer (to do), such as containing a link to the JSON-LD context, official SHACL shapes for LDES to validate your pages, best practices for publishing an LDES for reaching an optimal performance, best practices for enveloping your data using named graphs, how to build a status log for the use case of an aggregator or harvester, etc.

3. Context information

A client MUST have a way to make available context information to processors further down the pipeline set up by a consumer. A consumer will be able to make decisions on how to further process the event stream using this context information.

3.1. Versions and transactions

Consumers can use the LDES version properties to decide what action to take. E.g., when the consumer understands the members are versioned, it can upsert the members on each update. If it understands something was created instead of updated, it can just add it into the store without removing statements first, and when a deletion comes in, it knows it can remove the statements associated with the previous insert or upsert. To that extent, on the ldes:EventStream entity, these properties can be used, which are further explained in the vocabulary.

ldes:versionOfPath - such as dcterms:isVersionOf or as:object
ldes:versionDeleteObject - such as as:Delete
ldes:versionCreateObject - such as as:Create
ldes:versionUpdateObject - such as as:Update
ldes:versionDeletePath - defaults to rdf:type
ldes:versionCreatePath - defaults to rdf:type
ldes:versionUpdatePath - defaults to rdf:type

A consumer can also understand how to process the event stream in a way that the resulting knowledge graph will be consistent, by interpreting transactions using these properties:

ldes:transactionPath - points at an identifier for the transaction. The result of evaluating the path can be a literal or a IRI.
ldes:transactionFinalizedPath - points at the object
ldes:transactionFinalizedObject - the value that the object needs to be in order to be finalized. Defaults to the "true"^^xsd:boolean.

If one of the processors in a pipeline relies on transactions, the LDES client MUST force to emit the members in chronological order according to the ldes:timestampPath and ldes:sequencePath. When processing multiple members of the transaction with the same ldes:timestampPath, the client MUST ensure the member that finalizes the transaction is emitted last.

When the IRI in the object of the tree:member triple is also used as a named graph, an LDES consumer MAY assume the payload of the upsert is in the named graph. A consumer MUST implement a way to find back this group of triples in case an update or deletion comes in.

3.2. Retention policies

The goal of a retention policy is to indicate in what way a specific view will not be able to provide a complete history of the event stream to the consumer. This can help a consumer in the discovery phase to pick an specific LDES view, or help the consumer to detect non-viable synchronization set-ups.

When no retention policy is provided in the root node, the consumer MUST assume that all members, that have once been added to the ldes:EventStream, are still available from this root node. When a retention policy is provided however, a consumer MUST assume it will not be able to find members outside of the retention policy.

An overview of the existing retention policies in LDES

An example retention policy combining different features from the overview.

ex:LDES a ldes:EventStream ;
        ldes:timestampPath as:updated ;
        ldes:transactionPath ex:transactionId ;
        ldes:transactionFinalizedPath ex:transactionEnded ;
        ldes:transactionFinalizedObject true ;
        ldes:versionOfPath as:object ;
        ldes:versionDeleteObject as:Delete ;
        ldes:versionCreateObject as:Create ;
        ldes:versionUpdateObject as:Update ;
        tree:view <> .

<> a ldes:EventSource ;
     ldes:retentionPolicy [
         ldes:fullLogDuration "P1Y"^^xsd:duration ;
         ldes:versionAmount 1 ;
         ldes:versionDeleteDuration "P1Y"^^xsd:duration ;
     ] .

A retention policy will be described on the root node. The root node itself can contain this information using the property ldes:retentionPolicy, or the root node can refer through the property tree:viewDescription to an entity on which the retention policy is described using the property ldes:retentionPolicy. When the client is processing the root node, it MUST look for a retention policy in both ways.

In the example above, the retention policy has been set on the root node (double typed as the ldes:EventSource). When the ldes:retentionPolicy would refer to an entity without further statements in the current page, the client MUST assume this view keeps no members at all. Multiple properties can then be added to make the scope of members that are kept larger:

ldes:startingFrom: this view only retaines members starting from this xsd:dateTime with timezone. In combination with other retention policies, this property only enforces the period before the timestamp for which the view will not retain any member.
ldes:fullLogDuration: the duration from current time from which all members are retained. Only in combination with ldes:startingFrom, and when the ldes:startingFrom timestamp is within this window, not all members within the member are retained. No other properties can influence this property.
ldes:versionAmount: the amount of versions to keep.
ldes:versionDuration: the duration from current time from which an amount of version are kept, to be used together with ldes:versionAmount. Defaults the duration of the full event stream.
ldes:versionDeleteDuration: the period of time from current time the deletions in the event stream are retained. Before this period, deletions are not retained, regardless of ldes:versionAmount or ldes:versionDuration.

When using the current time in calculations, the consumer MUST take into account a safe buffer to mitigate clock inaccuracies. The ldes:timestampPath points at the timestamp in the member that can be compared with the current time minus the durations. When the ldes:versionTimestampPath has been set, the two version durations are to be compared with this timestamp.

Historically, there are more specific type of retention policies that MUST remain supported although their use is discouraged in favor of the just introduced retention policy design. These retention policies types are:

ldes:DurationAgoPolicy: a time-based retention policy in which data generated before a specified duration is not retained.
ldes:LatestVersionSubset: a version subset based on the latest versions of an entity in the stream.
ldes:PointInTimePolicy: a point-in-time retention policy in which data generated before a specific time is not retained.

A ldes:LatestVersionSubset uses the property ldes:amount with as range an xsd:integer datatype, indicating the number of versions to keep. By default, this value is set to 1. A ldes:PointInTimePolicy uses a ldes:pointInTime with an xsd:dateTime-typed literal to indicate the point in time on or after which data is kept when compared to a member’s timestamp.

4. Vocabulary

Next to re-using terms from the tree: vocabulary, the ldes: namespace introduced in this document provides a couple of new terms. The base IRI for LDES is https://w3id.org/ldes#, and the preferred prefix is ldes:. There is a Turtle version available at https://w3id.org/ldes#Vocabulary.

4.1. ldes:EventStream

The class ldes:EventStream is a subclass of tree:Collection. The specialization being that all members are immutable, and thus that this tree:Collection is append-only.