Support Nebula Graph

rfcs/048-support-nebula-graph-as-backend-storage.md

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+- Feature Name: Support Nebula Graph
+- Start Date: 2022-04-16
+- RFC PR: [amundsen-io/rfcs#48](https://github.com/amundsen-io/rfcs/pull/48)
+- Amundsen Issue: [amundsen-io/amundsen#1816](https://github.com/amundsen-io/amundsen/issues/1816)
+
+# Nebula Graph Support
+
+## Summary
+
+The support includes the Nebula Graph data builder and a new proxy for Nebula Graph in metadata service.
+
+## Motivation
+
+Metadata can be published into Nebula Graph from Amundsen data builder now and this RFC is going to make metadata retrieval API work for Nebula Graph in metadata service.  
+
+## Guide-level Explanation (aka Product Details)
+
+The goal of this RFC is to add additional loaders, publishers, and serializers to the library suite so that Nebula Graph is supported.
+
+## UI/UX-level Explanation
+
+N/A
+
+## Reference-level Explanation (aka Technical Details)
+
+A new proxy for Nebula Graph is added to support Nebula Graph in the metadata service.
+
+To support Nebula Graph in the data builder, several new components are added:
+
+- Nebula Extractor
+- Nebula Search Data Extractor
+- Nebula CSV Loader
+- Nebula CSV Publisher
+- Nebula Serializer
+- Nebula Sample Data Loader
+
+### Nebula Graph Schema handling
+
+They worked quite similarly to those components for Neo4j(it even speaks a dialect of OpenCypher) but differentiated in one thing: Nebula is not schemaless like Neo4j, that is, a label(named tag in Nebula Graph) or an edge type should be created before it's being referred in a query.
+
+Instead of maintaining a versioned schema by the user(with extra interfaces introduced), the proposed design is to parse the schema from data being published(Nebula CSV Publisher), which will do a schema check and DDL change when needed automatically, where the schema information will be in a single source of truth: the data model of the data builder.
+
+This requires the user to run the Nebula data builder sample script to initialize the Nebula Graph Schema, which could be discussed/revisited.
+
+### Nebula Graph Index
+
+Another difference in Nebula towards Neo4j is, when it comes to "starting point seeking" of an OpenCypher `MATCH` Query, if non of the `key`(it's called Vertex ID in Nebula Graph) is provided, an index on the LABEL(tag)  should be created(or a `LIMIT` clause is added), that is either no conditions at all: `MATCH (n: Table) return n` or conditions for starting point is only under a property: `MATCH (n: Table{category: "foo"}) return n` in the query. [Here](https://siwei.io/en/nebula-index-explained/) is a post where I explained why it's designed so.
+
+All those data models with queries in meta service API required indexes were handled by Nebula Publisher now, included in `NEBULA_INDEX_TAG_FIELDS`.
+
+## Drawbacks
+
+The RFC adds support for another datastore which brings in additional components and increases the code size of the repo.
+
+## Alternatives
+
+For the Nebula Schema auto adaptation in Nebula Publisher, one alternative is adding an extra interface/utility to manage the Nebula Graph schema.
+
+## Prior art
+
+N/A
+
+## Unresolved questions
+
+N/A
+
+## Future possibilities
+
+None so far.