About the Dgraph cache

The MDEX Engine cache (or the Dgraph cache) is a storage area in memory that the Dgraph uses to dynamically save potentially useful data structures, such as partial and complete results of processing queries.

Since the Dgraph has direct access to the structures it needs, it does not need to repeat the computational work previously done. The structures that are chosen for storing enable the Dgraph to answer queries faster by using fewer server resources.

The Dgraph cache is unified and adaptive:

The cache is unified in that all sorts of data structures go into the same cache. The whole Dgraph has one cache, and all the threads share it.
The cache is adaptive in that it uses a dynamic mechanism for evicting those data structures that it finds no longer useful. Its eviction algorithm attaches value to each cache object based on the empirical evidence of how useful the object actually is to the Engine, based on your current data, and responding to your visitors’ current queries. When this information changes, the Dgraph cache detects the change and adjusts, but you do not have to retune it.

The default Dgraph cache size (specified by the --cmem flag) is 1024MB (1GB).

The Dgraph cache improves both throughput and latency by taking advantage of similarities between processed queries. When a query is processed, the Dgraph checks to see whether processing time can be saved by looking up the results of some or all of the query computation from an earlier query.

The Dgraph cache is used to dynamically cache query results as well as partial or intermediate results. For example, if you perform a text search query the result will be stored, if it was not already, in the cache. If you then refine the results by selecting a dimension value, your original text search query is augmented with a refinement. It is likely that the Dgraph can take advantage of the cached text search result from your original query and avoid recomputing that result. If the navigation refinement result is also in the cache, the Engine does not need to do that work either.

To a large extent, the contents of the Dgraph cache are self-adjusting: what information is saved there and how long it is kept is decided automatically.

However, when deploying a Dgraph you need to decide how much memory to allocate for the Dgraph cache.

Allocating more memory to the cache improves performance by increasing the amount of information that can be stored in it. Thus, this information does not have to be recomputed.

Your MDEX Engine is well-tuned only when the Dgraph cache and the file system cache are well-balanced; therefore you need to understand them both.