The open metadata ecosystem draws together metadata from many sources. Metadata provenance provides information about where each metadata element has come from and how it can be maintained (that is updated and deleted).
Inside the native open metadata repository found in a Metadata Access Store, metadata is organized into Metadata Collections. It has one local metadata collection which it uses to store the metadata elements that it homes. Each metadata element has one home metadata collection. Typically, this is the place where it was first created. The metadata element can only be updated in its home metadata collection.
The native open metadata repository can also store metadata elements from other metadata collections. These metadata elements are called reference copies, and they are read-only.
The metadata that a specific technology instance creates and maintains is referred to as a metadata collection. For example, in figure 1, there are (at least) three metadata collections. There is one in the database server on the left, (at least) one in the open metadata ecosystem and one in the downstream database server on the right.
Figure 1: Examples of metadata collections. In this example, metadata is originating from the database server in the left (labelled 1) and is being copied into the open metadata ecosystem where it is distributed to other metadata repositories, tools and systems as appropriate. This includes a downstream database server (labelled 3).
Inside the open metadata ecosystem are OMAG Servers that, depending on their type, perform a specific role in the management and distribution of open metadata. Specifically:
- The Metadata Access Stores include a native open metadata store that can manage any type of open metadata.
- The Repository Proxy acts as an open metadata adapter to a third party metadata repository.
The repositories associated with the Metadata Access Stores and Repository Proxies each contain a metadata collection that is inside the open metadata ecosystem. They are connected together via one or more cohorts.
Metadata inside the open metadata ecosystem is broken down into small elements called metadata instances - entities and relationships - and classification. Each metadata element can belong to a different metadata collection. The open metadata ecosystem is able to work with the metadata elements as if they were all stored together in a single repository.
Metadata Collection Identifiers¶
In open metadata, each metadata collection has a unique identifier, called the metadata collection id and an optional metadata collection name. When a metadata instance from the metadata collection is shared within the broader open metadata ecosystem, it includes the metadata collection id and metadata collection name in its header. This identifies the home metadata collection of the metadata instance.
The metadata instance header also includes a category name that describes how the metadata was introduced into the open metadata ecosystem. This category name is called the Instance Provenance Type by the Open Metadata Repository Services (OMRS) and Element Origin by the Open Connector Framework (OCF) and most of the Open Metadata Access Services (OMASs). We will use Element Origin for the rest of this description since it is the most commonly used name.
Figure 2 shows the different values of Element Origin and how they tie into the mechanism used to introduce the metadata to the open metadata ecosystem.
Figure 2: What the different values of Element Origin say about the source of metadata
Local Cohort means that the metadata instance is homed in a metadata collection inside the open metadata ecosystem. The home server will maintain the metadata and distribute any changes to the other cohort members. The metadata collection id and name is set up in the configuration of the cohort member's OMAG Server.
Deregistered Repository means that the metadata collection is owned by a server that used to be a member of one of the cohorts that the local server belongs too but is has deregistered from the cohort. This means it is no longer sending updates to the rest of the cohort. Its metadata is still visible because reference copies (read only copies) have been kept by the other members. There are two routes to making this metadata maintainable again:
- Connect the home server back into the cohort.
- If the home server has gone forever, re-home the metadata so that a current active member is its home.
External Source means the metadata originates in a third party technology and a copy is maintained through an Open Metadata Access Service (OMAS). Typically, the OMAS is called via an Open Metadata Integration Service (OMIS) running in an Integration Daemon. The metadata collection id and name is defined in Open Metadata as the unique identifier, or GUID, and unique name, or qualified name, of a Software Capability representing the source technology.
Configuration means the metadata comes from a Configuration Document. Configuration documents control the capabilities of an OMAG Server. This metadata is maintained through Egeria's Administration Services.
Content Pack means the metadata comes from an open metadata archive that contains a collection of standard definitions. Content packs could contain glossaries, reference data sets, definitions from regulations and other types of standards. The archive documents the metadata collection id and name for the collection. In addition, a content pack is used to define Open Metadata Types. The metadata from a content pack can be updated by loading a later version of the content pack. Updated instances and types from the archive are replaced by the newer versions.
Metadata Export is also content from an open metadata archive. However, it has been exported from a server that has never been a member of the connected open metadata repository cohort(s). It typically describes assets that are being imported from a third party (such as a business partner) that is also providing the accompanying metadata. Just as with content packs, this metadata is updated by loading a newer version of the metadata export archive. If the originating server joins one of the connected cohorts, updates it makes to its metadata are distributed directly across the cohort(s) and there is no longer any need for the open metadata archive.
Using provenance to manage metadata integrity¶
Egeria ensures that only the home of a metadata instance is permitted to update it. This enforcement makes use of the metadata provenance information in the metadata instance's header.
Typically, the home is the originator of the metadata instance, but it is possible to move a metadata instance's home from one metadata collection to another. This effectively changes the home to the new metadata collection.
When a metadata instance is saved into another repository, it is called a reference copy. Reference copies are read-only. The metadata provenance information from the originating repository is retained.
This means an open metadata repository may contain metadata from multiple metadata collections. It uses the metadata provenance information in the metadata instance header to determine if it is allowed to update the metadata.
Cohort events are used to broadcast changes to open metadata instances. They are sent by the home server.
Re-homing a metadata instance¶
The Open Metadata Repository Services supports commands to change the home metadata collection of a metadata instance. This is called re-homing. This should be done only if the instance needs to be edited and the technology supporting the home metadata collection is no longer available. For example, the instances belonging to a deregistered repository. The change needs to be made with care and planning ensuring that all members of the cohort are connected when the command is issued so that the change of home can be recorded consistently in all repositories.
- Find out more about membership of a cohort.
- Learn about how external sources can integrate with the open metadata ecosystem.
- Set up metadata solutions that integrate metadata from many sources.
- Create and load Open Metadata Archives into your open metadata ecosystem.
- Learn about Configuration Documents that control the behaviour of OMAG Servers and how to set them up.
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