I'm working on a side-project in my spare time since the end of 2012 (before at the university) with some gaps, which is an append-only DBS with time travel capabilities using a custom storage engine based on COW tries and a (still I think) novel page versioning strategy called Sliding Snapshot. Recently I began work on pooling pages for reuse and a custom allocator for variable page sizes. Before, I had created new instances whenever a page was read from disk instead of reusing instances, so the allocation rate was really high for parallel transactions.
I'm working on an evolutionary, accumulate-only database system called SirixDB written in Java (and a module in Kotlin) in my spare time and in my day to day job I'm also working as a software engineer (embedded; besides I'm dancing a lot -- swing dances, mainly Lindy Hop and travel to festivals in Europe often times in my holidays) :-)
Furthermore, I'm working on Brackit (http://brackit.io), a query engine for JSON and XML using a JSONiq like query language. It can be used as ad-hoc in-memory query processor as the query processor of a data store. We envision, that it encapsulates common proven optimizations whereas the individual data stores can add further stages to rewrite the queries for index matching and physical optimizations.
SirixDB (https://sirix.io or https://github.com/sirixdb/sirix) is all about efficient versioning of your data. That is on the one hand it reduces the storage cost of storing a new revision during each transaction-commit while balancing read- and write-performance through a novel sliding snapshot algorithm and dynamic page compression. On the other hand Sirix supports easy query capabilities for instance to open a specific revision by a timestamp or several revisions by a given timespan, to navigate to future or past versions of nodes in the tree-structure and so on. It basically never overwrites data and is heavily inspired by ZFS and Git and borrows some ideas and puts these to test on the sub-file level.
In stark contrast to other approaches SirixDB combines copy-on-write semantics with node-level versioning and does not require a write-ahead-log for consistency.
It all started around 2006 as a university / Ph.D. project of Marc Kramis and I worked on the project since 2007 and already did my Bachelor's Thesis, Master's Thesis as well as several HiWi-Jobs on topics regarding the project and I'm still more eager than ever to put forth the idea of a versioned, analytics plattform to perform analytical tasks based on current as well as the history of the data.
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https://sirix.io | https://github.com/sirixdb/sirix | http://brackit.io