Memgraph supports extending the query language with user-written procedures. These procedures are grouped into modules, which can then be loaded on startup.

Loading query modules on startup

The Memgraph installation comes with the example.so and py_example.py query modules which are located in /usr/lib/memgraph/query_modules directory. Assuming the standard installation on Debian, you would run Memgraph with the following command:

systemctl start memgraph

When using Docker, the equivalent would be the following:

docker run -p 7687:7687 \
  -v mg_lib:/var/lib/memgraph -v mg_log:/var/log/memgraph -v mg_etc:/etc/memgraph \
  memgraph

Memgraph will now attempt to load the query modules from all *.so and *.py files it finds in the default (/usr/lib/memgraph/query_modules) directory. The *.so modules are written using the C API and the *.py modules are written using the Python API. Each file corresponds to one query module. Names of these files will be mapped to query module names. So in our case, we have an example.so which will be mapped to example module and a py_example.py which will be mapped to py_example module in the query language.

Each query module can define multiple procedures. Both of our examples define a single procedure creatively named procedure.

If you want to change the directory in which Memgraph searches for query modules, just change the --query-modules-directory flag in the main configuration file (/etc/memgraph/memgraph.conf) or supply it as a command-line parameter (e.g. when using Docker).

Utility query module

Query procedures that allow the users to gain more insight into other query modules and their procedures are written under our utility mg query module. This module offers three procedures with the following signatures:

• mg.procedures() :: (name :: STRING, signature :: STRING): lists loaded

  procedures and their signatures

• mg.load(module_name :: STRING) :: (): loads or reloads the given module

• mg.load_all() :: (): loads or reloads all modules

For example, invoking mg.procedures() from openCypher like so:

CALL mg.procedures() YIELD *;

might yield the following result:

+---------------------+-------------------------------------------------------------------+
| name                | signature                                                         |
+---------------------+-------------------------------------------------------------------+
| ...                 | ...                                                               |
| graph_analyzer.help | graph_analyzer.help() :: (name :: STRING, value :: STRING)        |
| mg.load             | mg.load(module_name :: STRING) :: ()                              |
| mg.load_all         | mg.load_all() :: ()                                               |
| mg.procedures       | mg.procedures() :: (name :: STRING, signature :: STRING)          |
| ...                 | ...                                                               |
+---------------------+-------------------------------------------------------------------+

In this case, we can see that Memgraph has successfully loaded all the procedures implemented in query modules including the utility query procedures.

To load a module (named e.g. hello) that wasn't loaded on startup (perhaps because it was added to Memgraph's query modules directory afterwards), we can simply invoke:

CALL mg.load("hello");

If we wish to reload an existing module, say the graph_analyzer module above, we again use the same procedure:

CALL mg.load("graph_analyzer");

Lastly, if we wish to reload all existing modules and load any newly added ones we can use:

CALL mg.load_all();

Syntax for calling procedures

OpenCypher has a special syntax for calling procedures in loaded query modules. For example:

CALL example.procedure("string-argument") YIELD args, result;

Each procedure returns zero or more records, where each record contains named fields. The YIELD part is used to select fields we are interested in. If we are not interested in any fields, the YIELD part can be omitted. The procedure will still run in such a case, but the record fields will not be stored in variables. In the above example, we assume that example.procedure will produce a record with 2 fields, args and result. Trying to YIELD fields that are not part of the produced record will result in an error.

Procedures may be called standalone as in the above example, or as a part of a larger query. This is useful if we want the procedure to work on data the query is producing. For example:

MATCH (node) CALL example.procedure(node) YIELD result RETURN *;

Unfortunately, when we use CALL in a larger query, we have to explicitly RETURN from the query to get the results. Naturally, the RETURN is not needed if we perform updates after CALL. This follows the openCypher convention that read-only queries need to end with a RETURN, while queries which update something don't need to RETURN anything.

If a procedure returns a record with a field name that may clash with some variable we already have in a query, that field name can be aliased into some other name. For example:

MATCH (result) CALL example.procedure(42) YIELD result AS procedure_result RETURN *;

Controlling procedure memory usage

When running a procedure, Memgraph controls the maximum memory usage that the procedure may consume during its execution. By default, the upper memory limit when running a procedure is 100 MB. If your query procedure requires more memory to be able to yield its results, you can increase the memory limit using the following syntax:

CALL example.procedure(arg1, arg2, ...) MEMORY LIMIT 100 KB YIELD result;
CALL example.procedure(arg1, arg2, ...) MEMORY LIMIT 100 MB YIELD result;
CALL example.procedure(arg1, arg2, ...) MEMORY UNLIMITED YIELD result;

The limit can either be specified to a specific value (either in KB or in MB), or it can be set to unlimited.