Lightweight Edges

OrientDB supports Lightweight Edges as regular edges, but without an identity on database. Lightweight edges can be used only when no properties are defined on edge.

By avoiding the creation of the underlying Document, Lightweight Edges have the same impact on speed and space as with Document LINKs, but with the additional bonus to have bidirectional connections. This means you can use the MOVE VERTEX command to refactor your graph with no broken LINKs.

Regular Edge representation

Look at the figure below. With Regular Edges both vertices (#10:33 and #10:12) are connected through an Edge Document (#17:11). The outgoing out_Friend property in #10:33 document is a set of LINKs with #17:11 as item. Instead, in document #10:12 the relationship is as incoming, so the property in_Friend is used with the LINK to the same Edge #17:11.

When you cross this relationship, OrientDB loads the Edge document #17:11 to resolve the other part of the relationship.

  1. +---------------------+    +---------------------+    +---------------------+  
  2. |   Account Vertex    |    |     Friend Edge     |    |    Account Vertex   |
  3. |       #10:33        |    |       #17:11        |    |       #10:12        |
  4. +---------------------+    +---------------------+    +---------------------+
  5. |out_Friend: [#17:11] |<-->|out: [#10:33]        |    |                     |
  6. +---------------------+    |         in: [#10:12]|<-->|in_Friend: [#17:11]  |
  7.                            +---------------------+    +---------------------+

Lightweight Edge representation

With Lightweight Edge, instead, there is no Edge document, but both vertices (#10:33 and #10:12) are connected directly to each other. The outgoing out_Friend property in #10:33 document contains the direct LINK to the vertex #10:12. The same happens on Vertex document #10:12, where the relationship is as incoming and the property in_Friend contains the direct LINK to vertex #10:33.

When you cross this relationship, OrientDB doesn’t need to load any edge to resolve the other part of the relationship. Furthermore no edge document is created.

  1. +---------------------+    +---------------------+
  2. |   Account Vertex    |    |    Account Vertex   |
  3. |       #10:33        |    |       #10:12        |
  4. +---------------------+    +---------------------+
  5. |out_Friend: [#10:12] |<-->|in_Friend: [#10:33]  |
  6. +---------------------+    +---------------------+

Starting from OrientDB v2.0, Lightweight Edges are disabled by default with new databases. This is because having regular edges makes easier to act on edges from SQL. Many issues from beginner users were on Lightweight Edges. If you want to use Lightweight Edges, enable it via API:

  1. OrientGraph g = new OrientGraph("mygraph");
  2. g.setUseLightweightEdges(true);

Or via SQL:

  1. ALTER DATABASE custom useLightweightEdges=true

Changing useLightweightEdges setting to true, will not transform previous edges, but all new edges could be Lightweight Edges if they meet the requirements.

When use Lightweight Edges?

These are the PROS and CONS of Lightweight Edges vs Regular Edges:

PROS:

  • faster in creation and traversing, because don’t need an additional document to keep the relationships between 2 vertices

CONS:

  • cannot store properties
  • harder working with Lightweight edges from SQL, because there is no a regular document under the edge