How to use query modules

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

note

For detailed technical information on query modules, check out the reference guide.

Loading query modules

Upon startup, Memgraph will attempt to load the query modules form all *.so and *.py files it finds in the default (/usr/lib/memgraph/query_modules) directory.

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).

Avilable query modules

MAGE, also known as Memgraph Advanced Graph Extensions, is an open-source repository that contains graph algorithms in the form of query modules written by the team behind Memgraph and its users. You can find and contribute implementations of various algorithms in multiple programming languages, all runnable inside Memgraph.

MAGE Documentation

Check out the MAGE documentation for a detailed overview of all the available modules and installation instructions.

MAGE Source code

Check out the MAGE source code on GitHub if you are interested in the underlying implementation.

Syntax for calling procedures

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

CALL module.procedure(arg1, arg2, ...) YIELD res1, res2, ...;

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 the procedure doesn't return any fields, then the YIELD part can be omitted.

Procedures may be called standalone as the above, 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 module.procedure(node) YIELD result RETURN *;

For writeable procedures we have some other limitations also:

• the rest of the query has to be read-only
• the writeable procedure call has to be the last cause in the query apart from the RETURN clause. The last example also works with writeable procedures because it satisfies all of these requirements.

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 or the called procedure is a writeable procedure. 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 (node) CALL module.procedure(42) YIELD node AS 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 module.procedure(arg1, arg2, ...) PROCEDURE MEMORY LIMIT 100 KB YIELD res1, res2, ...;
CALL module.procedure(arg1, arg2, ...) PROCEDURE MEMORY LIMIT 100 MB YIELD res1, res2, ...;
CALL module.procedure(arg1, arg2, ...) PROCEDURE MEMORY UNLIMITED YIELD res1, res2, ...;

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

Implementing custom query modules

Query modules can be implemented by either using the C API or Python API provided by Memgraph.

Modules implemented using the C API need to be compiled to a shared library (.so file), so they can be loaded when Memgraph starts. The C API is well documented in the /usr/include/memgraph/mg_procedure.h header and in the reference guide.

Modules implemented using the Python API need to be written in Python version 3.5.0 and above. The Python API is well documented in the /usr/lib/memgraph/python_support/mgp.py file and in the reference guide.

danger

If your programming language of choice throws exceptions, these exceptions must never leave the scope of your module! You should have a top level exception handler which returns with an error value and potentially logs the error message. Exceptions which cross the module boundary will cause all sorts of unexpected issues.

For a more detailed example on how to implement your own query modules, we suggest you take a look at the Implement custom query modules guide.