!SECTION Different data models and scalability

In this section we discuss scalability in the context of the different
data models supported by ArangoDB.

!SUBSECTION Key/value pairs

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.

!SUBSECTION document store

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.

!SUBSECTION complex queries and joins

TODO

!SUBSECTION graph database

TODO