> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-fix-nav-issues.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.
> This engine allows integrating ClickHouse with RabbitMQ.
# RabbitMQ table engine
This engine allows integrating ClickHouse with [RabbitMQ](https://www.rabbitmq.com).
`RabbitMQ` lets you:
* Publish or subscribe to data flows.
* Process streams as they become available.
Creating a table
```sql theme={null}
CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
name1 [type1],
name2 [type2],
...
) ENGINE = RabbitMQ SETTINGS
rabbitmq_host_port = 'host:port' [or rabbitmq_address = 'amqp(s)://guest:guest@localhost/vhost'],
rabbitmq_exchange_name = 'exchange_name',
rabbitmq_format = 'data_format'[,]
[rabbitmq_exchange_type = 'exchange_type',]
[rabbitmq_routing_key_list = 'key1,key2,...',]
[rabbitmq_secure = 0,]
[rabbitmq_schema = '',]
[rabbitmq_num_consumers = N,]
[rabbitmq_num_queues = N,]
[rabbitmq_queue_base = 'queue',]
[rabbitmq_persistent = 0,]
[rabbitmq_skip_broken_messages = N,]
[rabbitmq_max_block_size = N,]
[rabbitmq_flush_interval_ms = N,]
[rabbitmq_queue_settings_list = 'x-dead-letter-exchange=my-dlx,x-max-length=10,x-overflow=reject-publish',]
[rabbitmq_queue_consume = false,]
[rabbitmq_address = '',]
[rabbitmq_vhost = '/',]
[rabbitmq_username = '',]
[rabbitmq_password = '',]
[rabbitmq_commit_on_select = false,]
[rabbitmq_max_rows_per_message = 1,]
[rabbitmq_handle_error_mode = 'default']
```
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](/reference/formats/index) 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_schema` – Parameter that must be used if the format requires a schema definition. For example, [Cap'n Proto](https://capnproto.org/) requires the path to the schema file and the name of the root `schema.capnp:Message` object.
* `rabbitmq_num_consumers` – The number of consumers per table. Specify more consumers if the throughput of one consumer is insufficient. Default: `1`
* `rabbitmq_num_queues` – Total number of queues. Increasing this number can significantly improve performance. Default: `1`.
* `rabbitmq_queue_base` - Specify a hint for queue names. Use cases of this setting are described below.
* `rabbitmq_persistent` - If set to 1 (true), in insert query delivery mode will be set to 2 (marks messages as 'persistent'). Default: `0`.
* `rabbitmq_skip_broken_messages` – RabbitMQ message parser tolerance to schema-incompatible messages per block. If `rabbitmq_skip_broken_messages = N` then the engine skips *N* RabbitMQ messages that cannot be parsed (a message equals a row of data). Default: `0`.
* `rabbitmq_max_block_size` - Number of row collected before flushing data from RabbitMQ. Default: [max\_insert\_block\_size](/reference/settings/session-settings#max_insert_block_size).
* `rabbitmq_flush_interval_ms` - Timeout for flushing data from RabbitMQ. Default: [stream\_flush\_interval\_ms](/reference/settings/session-settings#stream_flush_interval_ms).
* `rabbitmq_queue_settings_list` - allows to set RabbitMQ settings when creating a queue. Available settings: `x-max-length`, `x-max-length-bytes`, `x-message-ttl`, `x-expires`, `x-priority`, `x-max-priority`, `x-overflow`, `x-dead-letter-exchange`, `x-queue-type`. The `durable` setting is enabled automatically for the queue.
* `rabbitmq_address` - Address for connection. Use ether this setting or `rabbitmq_host_port`.
* `rabbitmq_vhost` - RabbitMQ vhost. Default: `'/'`.
* `rabbitmq_queue_consume` - Use user-defined queues and do not make any RabbitMQ setup: declaring exchanges, queues, bindings. Default: `false`.
* `rabbitmq_username` - RabbitMQ username.
* `rabbitmq_password` - RabbitMQ password.
* `reject_unhandled_messages` - Reject messages (send RabbitMQ negative acknowledgement) in case of errors. This setting is automatically enabled if there is a `x-dead-letter-exchange` defined in `rabbitmq_queue_settings_list`.
* `rabbitmq_commit_on_select` - Commit messages when select query is made. Default: `false`.
* `rabbitmq_max_rows_per_message` — The maximum number of rows written in one RabbitMQ message for row-based formats. Default : `1`.
* `rabbitmq_empty_queue_backoff_start_ms` — A start backoff point to reschedule read if the rabbitmq queue is empty.
* `rabbitmq_empty_queue_backoff_end_ms` — An end backoff point to reschedule read if the rabbitmq queue is empty.
* `rabbitmq_empty_queue_backoff_step_ms` — A backoff step to reschedule read if the rabbitmq queue is empty.
* `rabbitmq_handle_error_mode` — How to handle errors for RabbitMQ engine. Possible values: default (the exception will be thrown if we fail to parse a message), stream (the exception message and raw message will be saved in virtual columns `_error` and `_raw_message`), dead\_letter\_queue (error related data will be saved in system.dead\_letter\_queue).
SSL connection
Use either `rabbitmq_secure = 1` or `amqps` in connection address: `rabbitmq_address = 'amqps://guest:guest@localhost/vhost'`.
The default behaviour of the used library is not to check if the created TLS connection is sufficiently secure. Whether the certificate is expired, self-signed, missing or invalid: the connection is simply permitted. More strict checking of certificates can possibly be implemented in the future.
Also format settings can be added along with rabbitmq-related settings.
Example:
```sql theme={null}
CREATE TABLE queue (
key UInt64,
value UInt64,
date DateTime
) ENGINE = RabbitMQ SETTINGS rabbitmq_host_port = 'localhost:5672',
rabbitmq_exchange_name = 'exchange1',
rabbitmq_format = 'JSONEachRow',
rabbitmq_num_consumers = 5,
date_time_input_format = 'best_effort';
```
The RabbitMQ server configuration should be added using the ClickHouse config file.
Required configuration:
```xml theme={null}
root
clickhouse
```
Additional configuration:
```xml theme={null}
clickhouse
```
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](/reference/statements/create/view). 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 the exact matching of keys. Example table key list: `key1,key2,key3,key4,key5`, message key can equal 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 the exchange name is the same). Note that this exchange type must be enabled with RabbitMQ plugin: `rabbitmq-plugins enable rabbitmq_consistent_hash_exchange`.
Setting `rabbitmq_queue_base` may be used for the following cases:
* to let different tables share queues, so that multiple consumers could be registered for the same queues, which makes a better performance. If using `rabbitmq_num_consumers` and/or `rabbitmq_num_queues` settings, the exact match of queues is achieved in case these parameters are the same.
* to be able to restore reading from certain durable queues when not all messages were successfully consumed. To resume consumption from one specific queue - set its name in `rabbitmq_queue_base` setting and do not specify `rabbitmq_num_consumers` and `rabbitmq_num_queues` (defaults to 1). To resume consumption from all queues, which were declared for a specific table - just specify the same settings: `rabbitmq_queue_base`, `rabbitmq_num_consumers`, `rabbitmq_num_queues`. By default, queue names will be unique to tables.
* to reuse queues as they are declared durable and not auto-deleted. (Can be deleted via any of RabbitMQ CLI tools.)
To improve performance, received messages are grouped into blocks the size of [max\_insert\_block\_size](/reference/settings/session-settings#max_insert_block_size). If the block wasn't formed within [stream\_flush\_interval\_ms](/reference/settings/server-settings/settings) milliseconds, the data will be flushed to the table regardless of the completeness of the block.
If `rabbitmq_num_consumers` and/or `rabbitmq_num_queues` settings 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).
For insert query there is message metadata, which is added for each published message: `messageID` and `republished` flag (true, if published more than once) - can be accessed via message headers.
Do not use the same table for inserts and materialized views.
Example:
```sql theme={null}
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() ORDER BY key;
CREATE MATERIALIZED VIEW consumer TO daily
AS SELECT key, value FROM queue;
SELECT key, value FROM daily ORDER BY key;
```
Virtual columns
* `_exchange_name` - RabbitMQ exchange name. Data type: `String`.
* `_channel_id` - ChannelID, on which consumer, who received the message, was declared. Data type: `String`.
* `_delivery_tag` - DeliveryTag of the received message. Scoped per channel. Data type: `UInt64`.
* `_redelivered` - `redelivered` flag of the message. Data type: `UInt8`.
* `_message_id` - messageID of the received message; non-empty if was set, when message was published. Data type: `String`.
* `_timestamp` - timestamp of the received message; non-empty if was set, when message was published. Data type: `UInt64`.
Additional virtual columns when `rabbitmq_handle_error_mode='stream'`:
* `_raw_message` - Raw message that couldn't be parsed successfully. Data type: `Nullable(String)`.
* `_error` - Exception message happened during failed parsing. Data type: `Nullable(String)`.
Note: `_raw_message` and `_error` virtual columns are filled only in case of exception during parsing, they are always `NULL` when message was parsed successfully.
Caveats
Even though you may specify [default column expressions](/reference/statements/create/table#default_values) (such as `DEFAULT`, `MATERIALIZED`, `ALIAS`) in the table definition, these will be ignored. Instead, the columns will be filled with their respective default values for their types.
RabbitMQ engine supports all [formats](/reference/formats/index) supported in ClickHouse.
The number of rows in one RabbitMQ message depends on whether the format is row-based or block-based:
* For row-based formats the number of rows in one RabbitMQ message can be controlled by setting `rabbitmq_max_rows_per_message`.
* For block-based formats we cannot divide block into smaller parts, but the number of rows in one block can be controlled by general setting [max\_block\_size](/reference/settings/session-settings#max_block_size).