Streams

Memgraph can connect to existing stream sources. To use streams, a user must:

1. Create a transformation module
2. Load the transformation module into Memgraph
3. Create the stream with a CREATE <streaming platform> STREAM query and optionally set its offset with CALL mg.kafka_set_stream_offset(stream_name, offset)
4. Start the stream with a START STREAM query

tip

Check out the example-streaming-app on GitHub to see a sample Memgraph-Kafka application.

Create a stream

The syntax for creating a stream depends on the type of the source because each specific type supports a different set of configuration options.

There is no strict order for specifying the configuration options.

Kafka

CREATE KAFKA STREAM <stream name>
  TOPICS <topic1> [, <topic2>, ...]
  TRANSFORM <transform procedure>
  [CONSUMER_GROUP <consumer group>]
  [BATCH_INTERVAL <batch interval duration>]
  [BATCH_SIZE <batch size>]
  [BOOTSTRAP_SERVERS <bootstrap servers>]
  [CONFIGS { <key1>: <value1> [, <key2>: <value2>, ...]}]
  [CREDENTIALS { <key1>: <value1> [, <key2>: <value2>, ...]}];

Option	Description	Type	Example	Default
stream name	Name of the stream in Memgraph	plain text	my_stream	/
topic	Name of the topic in Kafka	plain text	my_topic	/
transform procedure	Name of the transformation file followed by a procedure name	function	my_transformation.my_procedure	/
consumer group	Name of the consumer group in Memgraph	plain text	my_group	mg_consumer
batch interval duration	Maximum waiting time in milliseconds for consuming messages before calling the transform procedure	int	9999	100
batch size	Maximum number of messages to wait for before calling the transform procedure	int	99	1000
bootstrap servers	Comma-separated list of bootstrap servers	string	"localhost:9092"	/
configs	String key-value pairs of configuration options for the Kafka consumer	map with string key-value pairs	{"sasl.username": "michael.scott"}	/
credentials	String key-value pairs of configuration options for the Kafka consumer, but their value aren't shown in the Kafka specific stream information	map with string key-value pairs	{"sasl.password": "password"}	/

danger

The credentials are stored on the disk without any encryption, which means everybody who has access to the data directory of Memgraph is able to get the credentials.

To check the list of possible configuration options and their values, please check the documentation of librdkafka library which is used in Memgraph. At the time of writing this documentation Memgraph uses version 1.7.0 of librdkafka.

Pulsar

CREATE PULSAR STREAM <stream name>
  TOPICS <topic1> [, <topic2>, ...]
  TRANSFORM <transform procedure>
  [BATCH_INTERVAL <batch interval duration>]
  [BATCH_SIZE <batch size>]
  [SERVICE_URL <service url>];

Option	Description	Type	Example	Default
stream name	Name of the stream in Memgraph	plain text	my_stream	/
topic	Name of the topic in Pulsar	plain text	my_topic	/
transform procedure	Name of the transformation file followed by a procedure name	function	my_transformation.my_procedure	/
batch interval duration	Maximum waiting time in milliseconds for consuming messages before calling the transform procedure	int	9999	100
batch size	Maximum number of messages to wait for before calling the transform procedure	int	99	1000
service url	URL to the running Pulsar cluster	string	"127.0.0.1:6650"	/

The transformation procedure is called if either the BATCH_INTERVAL or the BATCH_SIZE is reached, and there is at least one received message.

The BATCH_INTERVAL starts when the:

• the stream is started
• the processing of the previous batch is completed
• the previous batch interval ended without receiving any messages

The user who executes the CREATE query is the owner of the stream.

Memgraph Community doesn't support authentication and authorization, so the owner is always Null, and the privileges are not checked.

In Memgraph Enterprise, owner privileges are checked upon executing the queries returned from the transformation procedures. If the owner doesn't have the required privileges, the execution of the queries will fail. Find more information about how the owner affects the stream in the reference guide.

Delete a stream

DROP STREAM <stream name>;

Drops a stream with the name <stream name>.

Start a stream

START STREAM <stream name>;

START ALL STREAMS;

Starts a specific stream or all streams.

When a stream is started, it resumes ingesting data from the last committed offset. If there is no committed offset for the consumer group, then the largest offset will be used. Therefore, only the new messages will be consumed.

Stop a stream

STOP STREAM <stream name>;

STOP ALL STREAMS;

Stops a specific stream or all streams.

Show streams

SHOW STREAMS;

Shows a list of existing streams with the following information:

• stream name
• stream type
• batch interval
• batch size
• transformation procedure name
• the owner of the streams
• whether the stream is running or not

Check stream

CHECK STREAM <stream name> [BATCH_LIMIT <count>] [TIMEOUT <milliseconds>];

The CHECK STREAM clause does a dry-run on the stream with name <stream name> with <count> number of batches and returns the result of the transformation, that is, the queries and parameters that would be executed in a normal run. If <count> number of batches are not processed within the specified TIMEOUT, probably because not enough messages were received, an exception is thrown.

The default value of <count> is 1. TIMEOUT is measured in milliseconds, and its default value is 30000.

Kafka producer delivery semantics

In stream processing, it is important to consider how failures are handled. When connecting an external application such as Memgraph to a Kafka stream, there are two possible ways to handle failures during message processing:

1. Every message is processed at least once: the message offsets are committed to the Kafka cluster after the processing is done. This means if the committing fails, the messages can get processed multiple times.
2. Every message is processed at most once: the message offsets are committed to the Kafka cluster right after they are received before the processing is started. This means if the processing fails, the same messages won't be processed again.

Missing a message can result in missing an edge that would connect two independent components of a graph. Therefore, we think that missing some information is a bigger problem in graphs databases than having duplicated information, so we implemented our streams using the at least once semantics, i.e. the queries returned by the transformations are first executed and committed to the database for every batch of messages and only then is the message offset committed to the Kafka cluster.

However, even though we cannot guarantee exactly once semantics, we tried to minimize the possibility of processing messages multiple times. This means committing the message offsets to the Kafka cluster happens right after the transaction is committed to the database.

Configuring stream transactions

A stream can fail for various reasons. One important type of failure is when a transaction (in which the returned queries of the transformation are executed) fails to commit because of another conflicting transaction. This is a side effect of isolation levels and can be remedied by the following Memgraph flag:

--stream-transaction-conflict-retries=TIMES_TO_RETRY

By default, Memgraph will always try to execute a transaction once. However, for streams, if Memgraph fails because of transaction conflicts it will retry to execute the transaction again for up to TIMES_TO_RETRY times (default value is 30).

Moreover, the interval of retries is also important and can be configured with the following Memgraph flag:

--stream-transaction-retry-interval=INTERVAL_TIME

The INTERVAL_TIME is measured in milliseconds and its default value is 500ms.

Setting a stream offset

When using a Kafka stream, you can manually set the offset of the next consumed message with a call to the query procedure mg.kafka_set_stream_offset:

CALL mg.kafka_set_stream_offset(stream_name, offset)

Option	Description	Type	Example	Default
stream_name	Name of the stream to set the offset for	string	"my_stream"	/
offset	Offset number	int	0	/

• An offset of -1 denotes the start of the stream, i.e., the beginning offset available for the given topic/partition.
• An offset of -2 denotes the end of the stream, i.e., for each topic/partition, its logical end such that only the next produced message will be consumed.

Keep in mind that a stream can consume from multiple topics with multiple partitions. Therefore, when setting the offsets to an arbitrary number be aware that setting the offset of a stream internally sets all of the associated offsets of that stream (topics/partitions) to that value.