Different data models and scalability
In this section we discuss scalability in the context of the different data models supported by ArangoDB.
The key/value store data model is the easiest to scale. In ArangoDB,
this is implemented in the sense that a document collection always has
a primary key
_key attribute and in the absence of further secondary
indexes the document collection behaves like a simple key/value store.
The only operations that are possible in this context are single key
lookups and key/value pair insertions and updates. If
_key is the
only sharding attribute then the sharding is done with respect to the
primary key and all these operations scale linearly. If the sharding is
done using different shard keys, then a lookup of a single key involves
asking all shards and thus does not scale linearly.
For the document store case even in the presence of secondary indexes essentially the same arguments apply, since an index for a sharded collection is simply the same as a local index for each shard. Therefore, single document operations still scale linearly with the size of the cluster, unless a special sharding configuration makes lookups or write operations more expensive.
Complex queries and joins
The AQL query language allows complex queries, using multiple collections, secondary indexes as well as joins. In particular with the latter, scaling can be a challenge, since if the data to be joined resides on different machines, a lot of communication has to happen. The AQL query execution engine organizes a data pipeline across the cluster to put together the results in the most efficient way. The query optimizer is aware of the cluster structure and knows what data is where and how it is indexed. Therefore, it can arrive at an informed decision about what parts of the query ought to run where in the cluster.
Nevertheless, for certain complicated joins, there are limits as to what can be achieved.
Graph databases are particularly good at queries on graphs that involve paths in the graph of an a priori unknown length. For example, finding the shortest path between two vertices in a graph, or finding all paths that match a certain pattern starting at a given vertex are such examples.
However, if the vertices and edges along the occurring paths are distributed across the cluster, then a lot of communication is necessary between nodes, and performance suffers. To achieve good performance at scale, it is therefore necessary to get the distribution of the graph data across the shards in the cluster right. Most of the time, the application developers and users of ArangoDB know best, how their graphs are structured. Therefore, ArangoDB allows users to specify, according to which attributes the graph data is sharded. A useful first step is usually to make sure that the edges originating at a vertex reside on the same cluster node as the vertex.