RabbitMQ Engine 

This engine allows integrating ClickHouse with RabbitMQ.

RabbitMQ lets you:

  • Publish or subscribe to data flows.
  • Process streams as they become available.

Creating a Table 

CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1],
    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2],
    ...
) ENGINE = RabbitMQ SETTINGS
    rabbitmq_host_port = 'host:port',
    rabbitmq_exchange_name = 'exchange_name',
    rabbitmq_format = 'data_format'[,]
    [rabbitmq_exchange_type = 'exchange_type',]
    [rabbitmq_routing_key_list = 'key1,key2,...',]
    [rabbitmq_row_delimiter = 'delimiter_symbol',]
    [rabbitmq_num_consumers = N,]
    [rabbitmq_num_queues = N,]
    [rabbitmq_transactional_channel = 0]

Required parameters:

  • rabbitmq_host_port – host:port (for example, localhost:5672).
  • rabbitmq_exchange_name – RabbitMQ exchange name.
  • rabbitmq_format – Message format. Uses the same notation as the SQL FORMAT function, such as JSONEachRow. For more information, see the Formats section.

Optional parameters:

  • rabbitmq_exchange_type – The type of RabbitMQ exchange: direct, fanout, topic, headers, consistent-hash. Default: fanout.
  • rabbitmq_routing_key_list – A comma-separated list of routing keys.
  • rabbitmq_row_delimiter – Delimiter character, which ends the message.
  • rabbitmq_num_consumers – The number of consumers per table. Default: 1. Specify more consumers if the throughput of one consumer is insufficient.
  • rabbitmq_num_queues – The number of queues per consumer. Default: 1. Specify more queues if the capacity of one queue per consumer is insufficient. Single queue can contain up to 50K messages at the same time.
  • rabbitmq_transactional_channel – Wrap insert queries in transactions. Default: 0.

Required configuration:

The RabbitMQ server configuration should be added using the ClickHouse config file.

 <rabbitmq>
    <username>root</username>
    <password>clickhouse</password>
 </rabbitmq>

Example:

  CREATE TABLE queue (
    key UInt64,
    value UInt64
  ) ENGINE = RabbitMQ SETTINGS rabbitmq_host_port = 'localhost:5672',
                            rabbitmq_exchange_name = 'exchange1',
                            rabbitmq_format = 'JSONEachRow',
                            rabbitmq_num_consumers = 5;

Description 

SELECT is not particularly useful for reading messages (except for debugging), because each message can be read only once. It is more practical to create real-time threads using materialized views. To do this:

  1. Use the engine to create a RabbitMQ consumer and consider it a data stream.
  2. Create a table with the desired structure.
  3. Create a materialized view that converts data from the engine and puts it into a previously created table.

When the MATERIALIZED VIEW joins the engine, it starts collecting data in the background. This allows you to continually receive messages from RabbitMQ and convert them to the required format using SELECT.
One RabbitMQ table can have as many materialized views as you like.

Data can be channeled based on rabbitmq_exchange_type and the specified rabbitmq_routing_key_list.
There can be no more than one exchange per table. One exchange can be shared between multiple tables - it enables routing into multiple tables at the same time.

Exchange type options:

  • direct - Routing is based on exact matching of keys. Example table key list: key1,key2,key3,key4,key5, message key can eqaul any of them.
  • fanout - Routing to all tables (where exchange name is the same) regardless of the keys.
  • topic - Routing is based on patterns with dot-separated keys. Examples: *.logs, records.*.*.2020, *.2018,*.2019,*.2020.
  • headers - Routing is based on key=value matches with a setting x-match=all or x-match=any. Example table key list: x-match=all,format=logs,type=report,year=2020.
  • consistent-hash - Data is evenly distributed between all bound tables (where exchange name is the same). Note that this exchange type must be enabled with RabbitMQ plugin: rabbitmq-plugins enable rabbitmq_consistent_hash_exchange.

If exchange type is not specified, then default is fanout and routing keys for data publishing must be randomized in range [1, num_consumers] for every message/batch (or in range [1, num_consumers * num_queues] if rabbitmq_num_queues is set). This table configuration works quicker then any other, especially when rabbitmq_num_consumers and/or rabbitmq_num_queues parameters are set.

If rabbitmq_num_consumers and/or rabbitmq_num_queues parameters are specified along with rabbitmq_exchange_type, then:

  • rabbitmq-consistent-hash-exchange plugin must be enabled.
  • message_id property of the published messages must be specified (unique for each message/batch).

Example:

  CREATE TABLE queue (
    key UInt64,
    value UInt64
  ) ENGINE = RabbitMQ SETTINGS rabbitmq_host_port = 'localhost:5672',
                            rabbitmq_exchange_name = 'exchange1',
                            rabbitmq_exchange_type = 'headers',
                            rabbitmq_routing_key_list = 'format=logs,type=report,year=2020',
                            rabbitmq_format = 'JSONEachRow',
                            rabbitmq_num_consumers = 5;

  CREATE TABLE daily (key UInt64, value UInt64)
    ENGINE = MergeTree();

  CREATE MATERIALIZED VIEW consumer TO daily
    AS SELECT key, value FROM queue;

  SELECT key, value FROM daily ORDER BY key;

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