Extending AQL with User Functions

AQL comes with a built-in set of functions, but it is not a fully-featured programming language.

To add missing functionality or to simplify queries, users may add their own functions to AQL in the selected database. These functions are written in JavaScript, and are deployed via an API; see Registering Functions.

In order to avoid conflicts with existing or future built-in function names, all user defined functions (UDF) have to be put into separate namespaces. Invoking a UDF is then possible by referring to the fully-qualified function name, which includes the namespace, too; see Conventions.

Technical Details

Known Limitations

UDFs have some implications you should be aware of. Otherwise they can introduce serious effects on the performance of your queries and the resource usage in ArangoDB.

Since the optimizer doesn’t know anything about the nature of your function, the optimizer can’t use indices for UDFs. So you should never lean on a UDF as the primary criterion for a FILTER statement to reduce your query result set. Instead, put a another FILTER statement in front of it. You should make sure that this FILTER statement is effective to reduce the query result before passing it to your UDF.

Rule of thumb is, the closer the UDF is to your final RETURN statement (or maybe even inside it), the better.

When used in clusters, UDFs are always executed on the coordinator.

Using UDFs in clusters may result in a higher resource allocation in terms of used V8 contexts and server threads. If you run out of these resources, your query may abort with a cluster backend unavailable error.

To overcome these mentioned limitations, you may want to increase the number of available V8 contexts (at the expense of increased memory usage), and the number of available server threads.

Deployment Details

Internally, UDFs are stored in a system collection named _aqlfunctions of the selected database. When an AQL statement refers to such a UDF, it is loaded from that collection. The UDFs will be exclusively available for queries in that particular database.

Since the coordinator doesn’t have own local collections, the _aqlfunctions collection is sharded across the cluster. Therefore (as usual), it has to be accessed through a coordinator - you mustn’t talk to the shards directly. Once it is in the _aqlfunctions collection, it is available on all coordinators without additional effort.

Keep in mind that system collections are excluded from dumps created with arangodump by default. To include AQL UDF in a dump, the dump needs to be started with the option --include-system-collections true.