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CMDBf Interface Modes: Push and Pull

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The draft CMDBf spec identifies two modes of operation: push and pull. Here's a quick overview.

In a CMDBf implementation, push mode means that a Management Data Repository registers its items and relationships into the Federating CMDB. Pull mode means that the Federating CMDB can access items and relationships by issuing queries to the MDR. A Federating CMDB can support both modes of operation.

In order to support push mode, the Federating CMDB must provide a "Registration Service". The Registration service allows the MDR to "push" or register items and relationships. If the MDR uses push mode, it is not required to provide a "Query Service", but one is strongly recommended. The reason for this is that the Registration Service may only cache the identifying properties of registered items. The Federating CMDB and clients would still need a standard way to access the additional properties and records that are not cached by the Federating CMDB.

If the Federating CMDB only supports pull mode, each MDR is required to provide a Query Service. In this case, the Federating CMDB must know the location of the MDR Query Service. The draft spec does not define how the Federating CMDB will determine the location of the MDR or the record types it supports - this must still be defined by the technical committee.

Assuming this is done, pull mode could be implemented in these ways:

1) The Federating CMDB can pull the items and relationships from the MDR using the MDR Query Service, and cache any required information to support its own Query Service on these items. This behavior would mirror push mode registration.

2) Clients can access the MDR Query Service directly to get additional properties and records.

3) The Federating CMDB can pull information from the MDR on demand and pass the information through to the client via its own Query Service.

Implementations may differ on which modes they support, and particularly when and how information is reconciled or normalized by the Federating CMDB. MDR implementations should support the Query Service in any case - this is the key to an interoperable federation.