Table of Contents
There are various limitations and "gotchas" that one should be aware of when using JanusGraph. Some of these limitations are necessary design choices and others are issues that will be rectified as JanusGraph development continues. Finally, the last section provides solutions to common issues.
These limitations reflect long-term tradeoffs design tradeoffs which are either difficult or impractical to change. These limitations are unlikely to be removed in the near future.
JanusGraph can store up to a quintillion edges (2^60) and half as many vertices. That limitation is imposed by JanusGraph’s id scheme.
When declaring the data type of a property key using dataType(Class) JanusGraph will enforce that all properties for that key have the declared type, unless that type is Object.class. This is an equality type check, meaning that sub-classes will not be allowed. For instance, one cannot declare the data type to be Number.class and use Integer or Long. For efficiency reasons, the type needs to match exactly. Hence, use Object.class as the data type for type flexibility. In all other cases, declare the actual data type to benefit from increased performance and type safety.
Retrieving an edge by id, e.g tx.getEdge(edge.getId()), is not a constant time operation because it requires an index call on one of its adjacent vertices. Hence, the cost of retrieving an individual edge by its id is O(log(k)) where k is the number of incident edges on the adjacent vertex. JanusGraph will attempt to pick the adjacent vertex with the smaller degree.
This also applies to index retrievals for edges via a standard or external index.
The definition of an edge label, property key, or vertex label cannot be changed once it has been committed to the graph. However, a type can be renamed and new types can be created at runtime to accommodate an evolving schema.
There are certain keywords that JanusGraph uses internally for types that cannot be used otherwise. These types include vertex labels, edge labels, and property keys. The following are keywords that cannot be used:
- vertex
- element
- edge
- property
- label
- key
For example, if you attempt to create a vertex with the label of property, you will receive an exception regarding protected system types.
These are limitations in JanusGraph’s current implementation. These limitations could reasonably be removed in upcoming versions of JanusGraph.
Mixed indexes only support a subset of the data types that JanusGraph supports. See Mixed Index Data Types for a current listing. Also, mixed indexes do not currently support property keys with SET or LIST cardinality.
JanusGraph provides a batch loading mode that can be enabled through the graph configuration. However, this batch mode only facilitates faster loading into the storage backend, it does not use storage backend specific batch loading techniques that prepare the data in memory for disk storage. As such, batch loading in JanusGraph is currently slower than batch loading modes provided by single machine databases. Chapter 30, Bulk Loading contains information on speeding up batch loading in JanusGraph.
Another limitation related to batch loading is the failure to load millions of edges into a single vertex at once or in a short time of period. Such supernode loading can fail for some storage backends. This limitation also applies to dense index entries.