AtomSpace RocksDB Backend

Save and restore AtomSpace contents to a RocksDB database. The RocksDB database is a single-user, local-host-only file-backed database. That means that only one AtomSpace can connect to it at any given moment.

In ASCII-art:

 +---------------------+
 |                     |
 |      AtomSpace      |
 |                     |
 +-- StorageNode API --+
 |                     |
 |  RocksStorageNode   |
 |                     |
 +---------------------+
 |       RocksDB       |
 +---------------------+
 |     filesystem      |
 +---------------------+

Each box is a shared library. Library calls go downwards. The StorageNode API is the same for all StorageNodes; the RocksStorageNode is just one of them.

RocksDB (see https://rocksdb.org/) is an "embeddable persistent key-value store for fast storage." The goal of layering the AtomSpace on top of it is to provide fast persistent storage for the AtomSpace. There are several advantages to doing this:

• RocksDB is file-based, and so it is straight-forward to make backup copies of datasets, as well as to share these copies with others. (You don't need to be a DB Admin to do this!)
• RocksDB runs locally, and so the overhead of pushing bytes through the network is eliminated. The remaining inefficiencies/bottlenecks have to do with converting between the AtomSpace's natural in-RAM format, and the position-independent format that all databases need. (Here, we say "position-independent" in that the DB format does not contain any C/C++ pointers; all references are managed with local unique ID's.)
• RocksDB is a "real" database, and so enables the storage of datasets that might not otherwise fit into RAM. This back-end does not try to guess what your working set is; it is up to you to load, work with and save those Atoms that are important for you. The examples demonstrate exactly how that can be done.

This backend, together with the CogServer-based network AtomSpace backend provides a building-block out of which more complex distributed and/or decentralized AtomSpaces can be built.

Status

This is Version 1.5.1. All unit tests pass. It has been used in at least one major project, to process tens of millions of Atoms.

This code is 2x or 3x faster than Postgres on synthetic benchmarks, and has been observed to run 12x faster in a real-world application.

Building and Installing

The build and install of atomspace-rocks follows the same pattern as other AtomSpace projects.

RocksDB is a prerequisite. On Debian/Ubuntu, apt install librocks-dev Then build, install and test:

    cd to project dir atomspace-rocks
    mkdir build
    cd build
    cmake ..
    make -j4
    sudo make install
    make check

Example Usage

See the examples directory for details. In brief:

$ guile
scheme@(guile-user)> (use-modules (opencog))
scheme@(guile-user)> (use-modules (opencog persist))
scheme@(guile-user)> (use-modules (opencog persist-rocks))
scheme@(guile-user)> (define sto (RocksStorageNode "rocks:///tmp/foo.rdb/"))
scheme@(guile-user)> (cog-open sto)
scheme@(guile-user)> (load-atomspace)
scheme@(guile-user)> (cog-close sto)

That's it! You've loaded the entire contents of foo.rdb into the AtomSpace! Of course, loading everything is not generally desirable, especially when the file is huge and RAM space is tight. More granular load and store is possible; see the examples directory for details.

Contents

There are two implementations in this repo: a simple one, suitable for users who use only a single AtomSpace, and a sophisticated one, intended for sophisticated users who need to work with complex DAG's of AtomSpaces. These two are accessed by using either MonoStorageNode or by using RocksStorageNode. Both use the standard StorageNode API.

The implementation of MonoStorageNode is smaller and simpler, and is the easier of the two to understand.

The implementation of RocksStorageNode provides full support for deep stacks (DAG's) of AtomSpaces, layered one on top another (called "Frames", a name meant to suggest "Kripke Frames" and "stackframes"). An individual "frame" can be thought of as a change-set, a collection of deltas to the next frame further down in the DAG. A frame inheriting from multiple AtomSpaces contains the set-union of Atoms in the contributing AtomSpaces. Atoms and Values can added, changed and removed in each changeset, without affecting Atoms and Values in deeper frames.

Design

This is a minimalistic implementation. There has been no performance tuning. There's only just enough code to make everything work; that's it. This does nothing at all fancy/sophisticated with RocksDB, and it might be possible to improve performance and squeeze out some air. However, the code is not sloppy, so it might be hard to make it go faster.

If you are creating a new StorageNode to some other kind of database, using the code here as a starting point would make an excellent design choice. All the hard problems have been solved, and yet the overall design remains fairly simple. All you'd need to do is to replace all of the references to RocksDB to your favorite, desired DB.