memgraph/docs/user_technical/reference_guide/07_graph-streams.md

Graph Streams

Kafka

Memgraphs custom openCypher clause for creating a stream is:

CREATE STREAM stream_name AS
  LOAD DATA KAFKA 'URI'
  WITH TOPIC 'topic'
  WITH TRANSFORM 'URI'
  [BATCH_INTERVAL milliseconds]
  [BATCH_SIZE count]

The CREATE STREAM clause happens in a transaction.

WITH TOPIC parameter specifies the Kafka topic from which we'll stream data.

WITH TRANSFORM parameter should contain a URI of the transform script. We cover more about the transform script later, in the transform section.

BATCH_INTERVAL parameter defines the time interval in milliseconds which is the time between two successive stream importing operations.

BATCH_SIZE parameter defines the count of Kafka messages that will be batched together before import.

If both BATCH_INTERVAL and BATCH_SIZE parameters are given, the condition that is satisfied first will trigger the batched import.

Default value for BATCH_INTERVAL is 100 milliseconds, and the default value for BATCH_SIZE is 10.

The DROP clause deletes a stream:

DROP STREAM stream_name;

The SHOW clause enables you to see all configured streams:

SHOW STREAMS;

You can also start/stop streams with the START and STOP clauses:

START STREAM stream_name [LIMIT count BATCHES];
STOP STREAM stream_name;

A stream needs to be stopped in order to start it and it needs to be started in order to stop it. Starting a started or stopping a stopped stream will not affect that stream.

There are also convenience clauses to start and stop all streams:

START ALL STREAMS;
STOP ALL STREAMS;

Before the actual import, you can also test the stream with the TEST STREAM clause:

TEST STREAM stream_name [LIMIT count BATCHES];

When a stream is tested, data extraction and transformation occurs, but nothing is inserted into the graph.

A stream needs to be stopped in order to test it. When the batch limit is omitted, TEST STREAM will run for only one batch by default.

Transform

The transform script allows Memgraph users to have custom Kafka messages and still be able to import data in Memgraph by adding the logic to decode the messages in the transform script.

The entry point of the transform script from Memgraph is the stream function. Input for the stream function is a list of bytes that represent byte encoded Kafka messages, and the output of the stream function must be a list of tuples containing openCypher string queries and corresponding parameters stored in a dictionary.

To be more precise, the signature of the stream function looks like the following:

stream : [bytes] -> [(str, {str : type})]
type : none | bool | int | float | str | list | dict

An example of a simple transform script that creates vertices if the message contains one number (the vertex id) or it creates edges if the message contains two numbers (origin vertex id and destination vertex id) would look like the following:

def create_vertex(vertex_id):
  return ("CREATE (:Node {id: $id})", {"id": vertex_id})


def create_edge(from_id, to_id):
  return ("MATCH (n:Node {id: $from_id}), (m:Node {id: $to_id}) "\
          "CREATE (n)-[:Edge]->(m)", {"from_id": from_id, "to_id": to_id})


def stream(batch):
    result = []
    for item in batch:
        message = item.decode('utf-8').split()
        if len(message) == 1:
          result.append(create_vertex(message[0]))
        elif len(message) == 2:
          result.append(create_edge(message[0], message[1]))
    return result