A Cohort Member is an OMAG Server that is capable of joining an open metadata repository cohort.
The open metadata repository cohort (or cohort for short) is a group of OMAG servers that are exchanging metadata using a peer-to-peer replication protocol and federated queries. This is shown in Figure 1.
Figure 1: OMAG Servers connected via a cohort
The cohort is self-configuring. At the heart of it is a shared topic. Each member puts a registration request on the topic when they want to join. This is picked up by the existing members who add this new server to their registry of members and re-send their registration information to allow the new member to build up its own registry.
When an OMAG server leaves the cohort, it sends an unregistration request. This enables the other members to remove the parting member from their registries.
The purpose of the registry in each member is to configure its federated query capability. The registration information includes the URL Root and server name of the member. The federation capability in each OMAG server allows it to issue metadata create, update, delete and search requests to each and every member of the cohort. This is the primary mechanism for accessing metadata.
In addition, any change to metadata made by a member is replicated to the other members of the cohort through the cohort topic. This gives the other members to opportunity to maintain cached copies of the metadata for performance / availability reasons. A member may also request that metadata is “refreshed” across the cohort. The originator of the requested metadata then sends the latest version of this metadata to the rest of the cohort through the cohort topic. This mechanism is useful to seed the cache in a new member of the cohort and is invoked as a result of a federated query issued from the new member. (A federated query occurs whenever an access service make a request ofr metadata.)
The exchange of metadata is using the Open Metadata Repository Services (OMRS) interfaces which gives fine-grained control of metadata notifications and updates. (See the OMRS metamodel for more details). This level of control is necessary for metadata repositories that are managing specific subsets of large-grained objects such as Assets.
Figure 2 shows the different types of cohort members. Follow the links below the diagram to find out more about each one’s purpose.
Figure 2: Different types of OMAG Servers that can be connected via a cohort
There is more detailed information about the operation of an open metadata repository cohort in the repository services documentation.
The administration hands on lab called “Understanding Cohort Configuration Lab” provides an opportunities to query the cohort registries of cohort members as they exchange metadata for Coco Pharmaceuticals. Instructions for running the labs can be found here.
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