Managing Consumers

import Tabs from "@theme/Tabs"; import TabItem from "@theme/TabItem";

TDengine provides data subscription and consumption interfaces similar to those of message queue products. In many scenarios, by adopting TDengine's time-series big data platform, there is no need to integrate additional message queue products, thus simplifying application design and reducing maintenance costs. This chapter introduces the related APIs and usage methods for data subscription with various language connectors. For basic information on data subscription, please refer to Data Subscription

Creating Topics

Please use TDengine CLI or refer to the Execute SQL section to execute the SQL for creating topics: CREATE TOPIC IF NOT EXISTS topic_meters AS SELECT ts, current, voltage, phase, groupid, location FROM meters

The above SQL will create a subscription named topic_meters. Each record in the messages obtained using this subscription is composed of the columns selected by this query statement SELECT ts, current, voltage, phase, groupid, location FROM meters.

Note

In the implementation of TDengine connectors, there are the following limitations for subscription queries.

• Only data subscription is supported, and subscription with with meta is not supported.
  • Java(WebSocket connection), Go, and Rust connectors support subscribing to databases, super tables, and SELECT queries.
  • Java(Native connection), C#, Python, and Node.js connectors only support subscribing to SELECT statements and do not support other types of SQL, such as subscribing to databases or super tables.
• Raw data query: Subscription queries can only query raw data, not aggregated or calculated results.
• Time order limitation: Subscription queries can only query data in chronological order.

Creating Consumers

The concept of TDengine consumers is similar to Kafka, where consumers receive data streams by subscribing to topics. Consumers can be configured with various parameters, such as connection methods, server addresses, automatic Offset submission, etc., to suit different data processing needs. Some language connectors' consumers also support advanced features such as automatic reconnection and data transmission compression to ensure efficient and stable data reception.

Creation Parameters

There are many parameters for creating consumers, which flexibly support various connection types, Offset submission methods, compression, reconnection, deserialization, and other features. The common basic configuration items applicable to all language connectors are shown in the following table:

• 0: TLS encryption disabled. If the server enables TLS, the client will automatically upgrade the connection.
• 1: TLS encryption enabled, but the server certificate is not verified.
• 2: TLS encryption enabled, the server certificate is verified, but the hostname is not verified.
• 3: TLS encryption enabled, both the server certificate and hostname are verified (the server certificate must include SAN; CN will be ignored) (supported from version 3.3.8.12) | Default is 0 | | ws.tls.version | string | List of TLS protocol versions supported by the C WebSocket connections, separated by commas. Optional values: TLSv1.2, TLSv1.3 (supported from version 3.3.8.12) | Default is TLSv1.3 | | ws.tls.ca | string | The C WebSocket connection is used to verify the CA certificate file path or PEM format certificate content of the server certificate. This certificate should be a CA certificate that issued the server certificate (supported from version 3.3.8.12) | |

Below are the connection parameters for connectors in various languages: <Tabs defaultValue="java" groupId="lang"> <TabItem value="java" label="Java"> The parameters for creating a consumer with the Java connector are Properties. For a list of parameters you can set, please refer to Consumer Parameters
For other parameters, refer to the common basic configuration items mentioned above.

Supported properties list for creating consumers:

• ws.url: WebSocket connection address.
• ws.message.channelLen: WebSocket message channel buffer length, default 0.
• ws.message.timeout: WebSocket message timeout, default 5m.
• ws.message.writeWait: WebSocket message write timeout, default 10s.
• ws.message.enableCompression: Whether to enable compression for WebSocket, default false.
• ws.autoReconnect: Whether WebSocket should automatically reconnect, default false.
• ws.reconnectIntervalMs: WebSocket reconnect interval in milliseconds, default 2000.
• ws.reconnectRetryCount: WebSocket reconnect retry count, default 3.
• timezone: The timezone used for parsing time-type data in subscription results, using the IANA timezone format, e.g., Asia/Shanghai(supported in v3.7.4 and above).

See the table above for other parameters.

• useSSL: Whether to use SSL connection, default false.
• token: Token for connecting to TDengine cloud.
• ws.message.enableCompression: Whether to enable WebSocket compression, default false.
• ws.autoReconnect: Whether to automatically reconnect, default false.
• ws.reconnect.retry.count: Reconnect attempts, default 3.
• ws.reconnect.interval.ms: Reconnect interval in milliseconds, default 2000.

See the table above for other parameters.

WebSocket Connection

Introduces how connectors in various languages use WebSocket connection method to create consumers. Specify the server address to connect, set auto-commit, start consuming from the latest message, specify group.id and client.id, etc. Some language connectors also support deserialization parameters.

{{#include docs/examples/JDBC/JDBCDemo/src/main/java/com/taos/example/WsConsumerLoopFull.java:create_consumer}}

{{#include docs/examples/python/tmq_websocket_example.py:create_consumer}}

{{#include docs/examples/rust/restexample/examples/tmq.rs:create_consumer_dsn}}

{{#include docs/examples/rust/restexample/examples/tmq.rs:create_consumer_ac}}

    {{#include docs/examples/node/websocketexample/tmq_example.js:create_consumer}}

{{#include docs/examples/c-ws-new/tmq_demo.c:create_consumer_2}}

Call the build_consumer function to attempt to obtain the consumer instance tmq. Print a success log if successful, and a failure log if not. </TabItem> <TabItem label="REST API" value="rest"> Not supported </TabItem> </Tabs>

Native Connection

Introduce how connectors in various languages use native connections to create consumers. Specify the server address for the connection, set auto-commit, start consuming from the latest message, and specify information such as group.id and client.id. Some language connectors also support deserialization parameters.

{{#include docs/examples/JDBC/JDBCDemo/src/main/java/com/taos/example/ConsumerLoopFull.java:create_consumer}}

{{#include docs/examples/python/tmq_native.py:create_consumer}}

{{#include docs/examples/rust/nativeexample/examples/tmq.rs:create_consumer_ac}}

{{#include docs/examples/c/tmq_demo.c:create_consumer_1}}

{{#include docs/examples/c/tmq_demo.c:create_consumer_2}}

Call the build_consumer function to attempt to obtain the consumer instance tmq. Print a success log if successful, and a failure log if not. </TabItem> <TabItem label="REST API" value="rest"> Not supported </TabItem> </Tabs>

Subscribe to Consume Data

After subscribing to a topic, consumers can start receiving and processing messages from these topics. The example code for subscribing to consume data is as follows:

WebSocket Connection

{{#include docs/examples/JDBC/JDBCDemo/src/main/java/com/taos/example/WsConsumerLoopFull.java:poll_data_code_piece}}

• The parameters of the subscribe method mean: the list of topics subscribed to (i.e., names), supporting subscription to multiple topics simultaneously.
• poll is called each time to fetch a message, which may contain multiple records.
• ResultBean is a custom internal class, whose field names and data types correspond one-to-one with the column names and data types, allowing objects of type ResultBean to be deserialized using the value.deserializer property's corresponding deserialization class.
• When subscribing to a database, you need to set value.deserializer to com.taosdata.jdbc.tmq.MapEnhanceDeserializer when creating a consumer, and then create a consumer of type TaosConsumer<TMQEnhMap>. This way, each row of data can be deserialized into a table name and a Map.

{{#include docs/examples/python/tmq_websocket_example.py:subscribe}}

• The parameters of the subscribe method mean: the list of topics subscribed to (i.e., names), supporting subscription to multiple topics simultaneously.
• poll is called each time to fetch a message, which may contain multiple records.
• records contains multiple block segments, each of which may contain multiple records. </TabItem>

{{#include docs/examples/rust/restexample/examples/tmq.rs:consume}}

• Consumers can subscribe to one or more TOPIC, generally it is recommended that a consumer subscribes to only one TOPIC.
• TMQ message queue is a futures::Stream type, which can be used with the corresponding API to consume each message and mark it as consumed through .commit.
• Record is a custom structure, whose field names and data types correspond one-to-one with the column names and data types, allowing objects of type Record to be deserialized using serde.

{{#include docs/examples/node/websocketexample/tmq_seek_example.js}}

{{#include docs/examples/c-ws-new/tmq_demo.c:basic_consume_loop}}

{{#include docs/examples/c-ws-new/tmq_demo.c:msg_process}}

{{#include docs/examples/c-ws-new/tmq_demo.c:subscribe_3}}

Steps for subscribing and consuming data:

1. Call the build_topic_list function to create a topic list topic_list.
2. If topic_list is NULL, it means creation failed, and the function returns -1.
3. Use the tmq_subscribe function to subscribe to the topic list specified by tmq. If the subscription fails, print an error message.
4. Destroy the topic list topic_list to free resources.
5. Call the basic_consume_loop function to start the basic consumption loop, processing the subscribed messages.

Native Connection

{{#include docs/examples/JDBC/JDBCDemo/src/main/java/com/taos/example/WsConsumerLoopFull.java:poll_data_code_piece}}

• The parameters of the subscribe method mean: the list of topics (i.e., names) to subscribe to, supporting subscription to multiple topics simultaneously.
• poll is called each time to get a message, which may contain multiple records.
• ResultBean is a custom internal class, whose field names and data types correspond one-to-one with the column names and data types, allowing objects of type ResultBean to be deserialized based on the value.deserializer property's corresponding deserialization class.

{{#include docs/examples/python/tmq_native.py:subscribe}}

• The parameters of the subscribe method mean: the list of topics (i.e., names) to subscribe to, supporting subscription to multiple topics simultaneously.
• poll is called each time to get a message, which may contain multiple records.
• records contains multiple blocks, each of which may contain multiple records. </TabItem>

{{#include docs/examples/rust/restexample/examples/tmq.rs:consume}}

• Consumers can subscribe to one or more TOPIC, generally it is recommended that a consumer subscribes to only one TOPIC.
• The TMQ message queue is a futures::Stream type, which can be used with the corresponding API to consume each message and mark it as consumed with .commit.
• Record is a custom structure, whose field names and data types correspond one-to-one with the column names and data types, allowing objects of type Record to be deserialized through serde.

{{#include docs/examples/c/tmq_demo.c:basic_consume_loop}}

{{#include docs/examples/c/tmq_demo.c:msg_process}}

{{#include docs/examples/c/tmq_demo.c:subscribe_3}}

Subscription and consumption data steps:

1. Call the build_topic_list function to create a topic list topic_list.
2. If topic_list is NULL, it means creation failed, and the function returns -1.
3. Use the tmq_subscribe function to subscribe to the topic list specified by tmq. If the subscription fails, print an error message.
4. Destroy the topic list topic_list to free resources.
5. Call the basic_consume_loop function to start the basic consumption loop, processing the subscribed messages.

Specifying the Subscription Offset

Consumers can specify to start reading messages from a specific Offset in the partition, allowing them to reread messages or skip processed messages. Below is how connectors in various languages specify the subscription Offset.

WebSocket Connection

{{#include docs/examples/JDBC/JDBCDemo/src/main/java/com/taos/example/WsConsumerLoopFull.java:consumer_seek}}

1. Use the consumer.poll method to poll data until data is obtained.
2. For the first batch of polled data, print the content of the first message and obtain the current consumer's partition assignment information.
3. Use the consumer.seekToBeginning method to reset the offset of all partitions to the starting position and print the successful reset message.
4. Poll data again using the consumer.poll method and print the content of the first message.

{{#include docs/examples/python/tmq_websocket_example.py:assignment}}

{{#include docs/examples/rust/nativeexample/examples/tmq.rs:seek_offset}}

1. By calling the consumer.assignments() method, obtain the consumer's current partition assignment information and record the initial assignment status.
2. Traverse each partition assignment information, for each partition: extract the topic, consumer group ID (vgroup_id), current offset (current), starting offset (begin), and ending offset (end). Record this information.
3. Call the consumer.offset_seek method to set the offset to the starting position. If the operation fails, record the error information and current assignment status.
4. After adjusting the offset for all partitions, obtain and record the consumer's partition assignment information again to confirm the status after the offset adjustment.

    {{#include docs/examples/node/websocketexample/tmq_seek_example.js:offset}}

1. Use the tmq_get_topic_assignment function to obtain the assignment information for a specific topic, including the number of assignments and the details of each assignment.
2. If fetching the assignment information fails, print an error message and return.
3. For each assignment, use the tmq_offset_seek function to set the consumer's offset to the earliest offset.
4. If setting the offset fails, print an error message.
5. Release the assignment information array to free resources.
6. Call the basic_consume_loop function to start a new consumption loop and process messages.

Native Connection

{{#include docs/examples/JDBC/JDBCDemo/src/main/java/com/taos/example/WsConsumerLoopFull.java:consumer_seek}}

1. Use the consumer.poll method to poll data until data is obtained.
2. For the first batch of polled data, print the content of the first data item and obtain the current consumer's partition assignment information.
3. Use the consumer.seekToBeginning method to reset the offset of all partitions to the beginning position and print a message of successful reset.
4. Poll data again using the consumer.poll method and print the content of the first data item.

{{#include docs/examples/python/tmq_native.py:assignment}}

{{#include docs/examples/rust/nativeexample/examples/tmq.rs:seek_offset}}

1. Obtain the consumer's current partition assignment information by calling the consumer.assignments() method and record the initial assignment status.
2. For each partition assignment, extract the topic, consumer group ID (vgroup_id), current offset, beginning offset, and ending offset. Record this information.
3. Use the consumer.offset_seek method to set the offset to the beginning position. If the operation fails, record the error information and the current assignment status.
4. After adjusting the offset for all partitions, obtain and record the consumer's partition assignment information again to confirm the status after the offset adjustment.

1. Use the tmq_get_topic_assignment function to obtain the assignment information for a specific topic, including the number of assignments and the details of each assignment.
2. If fetching the assignment information fails, print an error message and return.
3. For each assignment, use the tmq_offset_seek function to set the consumer's offset to the earliest offset.
4. If setting the offset fails, print an error message.
5. Release the assignment information array to free resources.
6. Call the basic_consume_loop function to start a new consumption loop and process messages.

Commit Offset

After a consumer has read and processed messages, it can commit the Offset, indicating that the consumer has successfully processed messages up to this Offset. Offset commits can be automatic (committed periodically based on configuration) or manual (controlled by the application when to commit). When creating a consumer, if the property enable.auto.commit is set to false, the offset can be manually committed.

Note: Before manually submitting the consumption progress, ensure that the message has been processed correctly; otherwise, the incorrectly processed message will not be consumed again. Automatic submission may commit the consumption progress of the previous message during the current poll, so please ensure that the message processing is completed before the next poll or message retrieval.

WebSocket Connection

{{#include docs/examples/JDBC/JDBCDemo/src/main/java/com/taos/example/WsConsumerLoopFull.java:commit_code_piece}}

{{#include docs/examples/python/tmq_websocket_example.py:commit_offset}}

You can manually submit the consumption progress using the consumer.commit method. </TabItem>

    {{#include docs/examples/node/websocketexample/tmq_example.js:commit}}

You can manually submit the consumption progress using the tmq_commit_sync function.

Native Connection

{{#include docs/examples/JDBC/JDBCDemo/src/main/java/com/taos/example/WsConsumerLoopFull.java:commit_code_piece}}

{{#include docs/examples/python/tmq_native.py:commit_offset}}

You can manually submit the consumption progress using the consumer.commit method. </TabItem> <TabItem label="Node.js" value="node"> Not supported </TabItem> <TabItem label="C#" value="csharp">

{{#include docs/examples/csharp/subscribe/Program.cs:commit_offset}}

You can manually commit the consumption progress using the tmq_commit_sync function.

Unsubscribe and Close Consumption

Consumers can unsubscribe from topics and stop receiving messages. When a consumer is no longer needed, the consumer instance should be closed to release resources and disconnect from the TDengine server.

WebSocket Connection

{{#include docs/examples/JDBC/JDBCDemo/src/main/java/com/taos/example/WsConsumerLoopFull.java:unsubscribe_data_code_piece}}

{{#include docs/examples/python/tmq_websocket_example.py:unsubscribe}}

Note: Once the consumer unsubscribes and is closed, it cannot be reused. If you want to subscribe to a new topic, please recreate the consumer. </TabItem>

    {{#include docs/examples/node/websocketexample/tmq_example.js:unsubscribe}}

Native Connection

{{#include docs/examples/JDBC/JDBCDemo/src/main/java/com/taos/example/WsConsumerLoopFull.java:unsubscribe_data_code_piece}}

{{#include docs/examples/python/tmq_native.py:unsubscribe}}

Note: After the consumer unsubscribes, it is closed and cannot be reused. If you want to subscribe to a new topic, please create a new consumer. </TabItem> <TabItem label="Node.js" value="node"> Not supported </TabItem> <TabItem label="C#" value="csharp">

{{#include docs/examples/csharp/subscribe/Program.cs:close}}

Complete Examples

WebSocket Connection

Note: The value of the value.deserializer configuration parameter should be adjusted according to the package path of the test environment.

Native Connection

Note: The value of the value.deserializer configuration parameter should be adjusted according to the package path in the test environment.