Our purpose in building Devii was, first and foremost, to support our own application development; however, we had always envisioned that Devii could itself be a product, in addition to supporting our apps as products. Thus, whenever possible, we aim for generality, automation, flexibility, and configurability. That said, there were a few ways in which we had to restrict our focus, in order to create a usable system.
We limited our supported data storage backends to SQL databases, specifically those supported out of the box by Python’s SQLAlchemy library: MySQL, PostgreSQL, Oracle, Microsoft SQL Server, Amazon Aurora, and SQLite. These constitute the “big four” of the SQL RDBMS market, plus the most popular embedded SQL database format. SQL databases not supported by default, but for which SQLAlchemy has drivers, are available on request, but we do not package drivers for them by default.
We have no plans to support “NoSQL” databases such as MongoDB, CouchDB, Redis, etc. This is for three reasons.
First, SQL’s relational character, with its primary and foreign keys, allows for easy generation of relationships in GraphQL schemas. Those relationships would have to be hand-coded in a NoSQL schema, as most NoSQL databases do not have a means of representing relationships, and those that do, do so in their own idiosyncratic way.
Second, SQL databases have a regularity, based on their use of a standardized query language, that allows creation of a single backend library to communicate with them. SQLAlchemy is such a backend library, and is able to leverage the commonalities in SQL systems and handle the relatively minor differences between dialects. To support a variety of SQL database backends, all we have to do is use SQLAlchemy and install the appropriate drivers.
By contrast, each NoSQL database has its own query language or API; efforts such as JSONiq to create a standardized query language for NoSQL have not been very successful. To support each NoSQL database, we would have to add new code with a new library, which would require a great deal more coding time and effort.
The final reason is that most NoSQL systems do not have a defined schema for their data. The lack of schema makes for a dynamic data storage backend, but is unsuitable for automatically generating a GraphQL schema. If any record could store any set of values, how can types be introspected and generated? A lack of defined schema makes any stored data uninterpretable, beyond what its original application intended; this is very similar, ironically, to the problems inherent in old COBOL database files, where the data was unreadable without having the original COBOL source’s “data dictionary” that defined the record fields.