qpc-github / akutan

A distributed knowledge graph store

Akutan

There's a blog post that's a good introduction to Akutan.

Akutan is a distributed knowledge graph store, sometimes called an RDF store or a triple store. Knowledge graphs are suitable for modeling data that is highly interconnected by many types of relationships, like encyclopedic information about the world. A knowledge graph store enables rich queries on its data, which can be used to power real-time interfaces, to complement machine learning applications, and to make sense of new, unstructured information in the context of the existing knowledge.

How to model your data as a knowledge graph and how to query it will feel a bit different for people coming from SQL, NoSQL, and property graph stores. In a knowledge graph, data is represented as a single table of facts, where each fact has a subject, predicate, and object. This representation enables the store to sift through the data for complex queries and to apply inference rules that raise the level of abstraction. Here's an example of a tiny graph:

subject	predicate	object
<John_Scalzi>	<born>	<Fairfield>
<John_Scalzi>	<lives>	<Bradford>
<John_Scalzi>	<wrote>	<Old_Mans_War>

To learn about how to represent and query data in Akutan, see docs/query.md.

Akutan is designed to store large graphs that cannot fit on a single server. It's scalable in how much data it can store and the rate of queries it can execute. However, Akutan serializes all changes to the graph through a central log, which fundamentally limits the total rate of change. The rate of change won't improve with a larger number of servers, but a typical deployment should be able to handle tens of thousands of changes per second. In exchange for this limitation, Akutan's architecture is a relatively simple one that enables many features. For example, Akutan supports transactional updates and historical global snapshots. We believe this trade-off is suitable for most knowledge graph use cases, which accumulate large amounts of data but do so at a modest pace. To learn more about Akutan's architecture and this trade-off, see docs/central_log_arch.md.

Akutan isn't ready for production-critical deployments, but it's useful today for some use cases. We've run a 20-server deployment of Akutan for development purposes and off-line use cases for about a year, which we've most commonly loaded with a dataset of about 2.5 billion facts. We believe Akutan's current capabilities exceed this capacity and scale; we haven't yet pushed Akutan to its limits. The project has a good architectural foundation on which additional features can be built and higher performance could be achieved.

Akutan needs more love before it can be used for production-critical deployments. Much of Akutan's code consists of high-quality, documented, unit-tested modules, but some areas of the code base are inherited from Akutan's earlier prototype days and still need attention. In other places, some functionality is lacking before Akutan could be used as a critical production data store, including deletion of facts, backup/restore, and automated cluster management. We have filed GitHub issues for these and a few other things. There are also areas where Akutan could be improved that wouldn't necessarily block production usage. For example, Akutan's query language is not quite compatible with Sparql, and its inference engine is limited.

So, Akutan has a nice foundation and may be useful to some people, but it also needs additional love. If that's not for you, here are a few alternative open-source knowledge and property graph stores that you may want to consider (we have no affiliation with these projects):

• Blazegraph: an RDF store. Supports several query languages, including SPARQL and Gremlin. Disk-based, single-master, scales out for reads only. Seems unmaintained. Powers https://query.wikidata.org/.
• Dgraph: a triple-oriented property graph store. GraphQL-like query language, no support for SPARQL. Disk-based, scales out.
• Neo4j: a property graph store. Cypher query language, no support for SPARQL. Single-master, scales out for reads only.
• See also Wikipedia's Comparison of Triplestores page.

The remainder of this README describes how to get Akutan up and running. Several documents under the docs/ directory describe aspects of Akutan in more detail; see docs/README.md for an overview.

Installing dependencies and building Akutan

Akutan has the following system dependencies:

• It's written in Go. You'll need v1.11.5 or newer.
• Akutan uses Protocol Buffers extensively to encode messages for gRPC, the log of data changes, and storage on disk. You'll need protobuf version 3. We reccomend 3.5.2 or later. Note that 3.0.x is the default in many Linux distributions, but doesn't work with the Akutan build.
• Akutan's Disk Views store their facts in RocksDB.

On Mac OS X, these can all be installed via Homebrew:

$ brew install golang protobuf rocksdb zstd

On Ubuntu, refer to the files within the docker/ directory for package names to use with apt-get.

After cloning the Akutan repository, pull down several Go libraries and additional Go tools:

$ make get

Finally, build the project:

$ make build

Running Akutan locally

The fastest way to run Akutan locally is to launch the in-memory log store:

$ bin/plank

Then open another terminal and run:

$ make run

This will bring up several Akutan servers locally. It starts an API server that listens on localhost for gRPC requests on port 9987 and for HTTP requests on port 9988, such as http://localhost:9988/stats.txt.

The easiest way to interact with the API server is using bin/akutan-client. See docs/query.md for examples. The API server exposes the FactStore gRPC service defined in proto/api/akutan_api.proto.

Deployment concerns

The log

Earlier, we used bin/plank as a log store, but this is unsuitable for real usage! Plank is in-memory only, isn't replicated, and by default, it only keeps 1000 entries at a time. It's only meant for development.

Akutan also supports using Apache Kafka as its log store. This is recommended over Plank for any deployment. To use Kafka, follow the Kafka quick start guide to install Kafka, start ZooKeeper, and start Kafka. Then create a topic called "akutan" (not "test" as in the Kafka guide) with partitions set to 1. You'll want to configure Kafka to synchronously write entries to disk.

To use Kafka with Akutan, set the akutanLog's type to kafka in your Akutan configuration (default: local/config.json), and update the locator's addresses accordingly (Kafka uses port 9092 by default). You'll need to clear out Akutan's Disk Views' data before restarting the cluster. The Disk Views by default store their data in $TMPDIR/rocksdb-akutan-diskview-{space}-{partition} so you can delete them all with rm -rf $TMPDIR/rocksdb-akutan-diskview*

Docker and Kubernetes

This repository includes support for running Akutan inside Docker and Minikube. These environments can be tedious for development purposes, but they're useful as a step towards a modern and robust production deployment.

See cluster/k8s/Minikube.md file for the steps to build and deploy Akutan services in Minikube. It also includes the steps to build the Docker images.

Distributed tracing

Akutan generates distributed OpenTracing traces for use with Jaeger. To try it, follow the Jaeger Getting Started Guide for running the all-in-one Docker image. The default make run is configured to send traces there, which you can query at http://localhost:16686. The Minikube cluster also includes a Jaeger all-in-one instance.

Development

VS Code

You can use whichever editor you'd like, but this repository contains some configuration for VS Code. We suggest the following extensions:

• Go
• Code Spell Checker
• Rewrap
• vscode-proto3
• Docker

Override the default settings in .vscode/settings.json with ./vscode-settings.json5.

Test targets

The Makefile contains various targets related to running tests:

Target	Description
make test	run all the akutan unit tests
make cover	run all the akutan unit tests and open the web-based coverage viewer
make lint	run basic code linting
make vet	run all static analysis tests including linting and formatting

License Information

Copyright 2019 eBay Inc.

Primary authors: Simon Fell, Diego Ongaro, Raymond Kroeker, Sathish Kandasamy

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at https://www.apache.org/licenses/LICENSE-2.0.

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

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Note the project was renamed to Akutan in July 2019.