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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 can have serious effects on the performance of your queries and the resource usage in ArangoDB. Especially in cluster setups they should not be used against much data, because this data will need to be sent over the network back and forth between DB-Servers and Coordinators, potentially adding a lot of latency. This can be mitigated by very selective FILTERs before calls to UDFs.
Since the optimizer doesn’t know anything about the nature of your function,
the optimizer can’t use indexes 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 a
Coordinator.
It is not possible to execute UDFs on DB-Servers, as no JavaScript execution
engine is available on DB-Servers. Queries that would push UDF execution to
DB-Servers are aborted with a parse error. This includes using UDFs in traversal
PRUNE conditions, as well as FILTER conditions that can be moved into the
traversal execution on a DB-Server. These limitations also apply to the
single server deployment mode to keep the differences to cluster deployments minimal.
As UDFs are written in JavaScript, each query that executes a UDF will acquire one V8 context to execute the UDFs in it. V8 contexts can be re-used across subsequent queries, but when UDF-invoking queries run in parallel, they will each require a dedicated V8 context.
Because UDFs use the V8 JavaScript engine, the engine’s default memory limit of 512 MB is applied.
Using UDFs in clusters may thus 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.
In addition, modification of global JavaScript variables from inside UDFs is unsupported, as is reading or changing the data of any collection or running queries from inside an AQL user function.
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.