Despite the maturity of commercial graph databases, little consensus has been reached so far on the standardization of data definition languages (DDLs) for property graphs (PG). The discussion on the characteristics of PG schemas is ongoing in many standardization and community groups. Although some basic aspects of a schema are already present in Neo4j 3.5, like in most commercial graph databases, full support is missing allowing to constraint property graphs with more or less flexibility.In this paper, we focus on two different perspectives from which a PG schema should be considered, as being descriptive or prescriptive, and we show how it would be possible to switch from one to another as the application under development gains more stability. Apart from proposing concise schema DDL inspired by Cypher syntax, we show how schema validation can be enforced through homomorphisms between PG schemas and PG instances; and how schema evolution can be described through the use of graph rewriting operations. Our prototypical implementation demonstrates feasibility and shows the need of offering high-level query primitives to accommodate flexible graph schema requirements as showcased in our work. * Currently on leave at the University of Waterloo, Canada.

Three documents from 2018 by Neo4j Inc. authors describing property graph data model, schema and mapping to SQL schema

This paper briefly summarizes the work towards the final version of 'Distributed Transaction Processing' (OSI TP). Several well-known optimizations of the presumed abort protocol are introduced: dynamic flow of READY-messages, a one-phase protocol, a read-only extension. Moreover, some useful extensions such as containment of heuristic decisions and reporting of the completion status of a transaction are presented. The requirements and the functionality are discussed especially from the user's point of view. Finally, the evolving requirements for distributed transaction processing are discussed given the increasing number of applications on the Internet (Electronic Commerce).

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