DataWorks:ClickHouse Reader

Limits

• Only ApsaraDB for ClickHouse data sources of V20.8 or V21.8 are supported.
• ClickHouse Reader supports only exclusive resource groups for Data Integration, but not the shared resource group for Data Integration.
• ClickHouse Reader connects to a ClickHouse database by using JDBC and can read data from the database only by using JDBC Statement.
• ClickHouse Reader allows you to read data from the specified columns in an order different from that specified in the schema of the source table.
• You must make sure that the driver version is compatible with your ClickHouse database. ClickHouse Reader supports only the following version of the ClickHouse database driver:

  <dependency>
      <groupId>ru.yandex.clickhouse</groupId>
      <artifactId>clickhouse-jdbc</artifactId>
      <version>0.2.4.ali2-SNAPSHOT</version>
  </dependency>

Background information

ClickHouse Reader is designed for extract, transform, load (ETL) developers to read data from ClickHouse databases. ClickHouse Reader connects to a remote ClickHouse database by using JDBC, generates an SQL statement based on your configurations to read data from the database, and matches the protocol of the writer you use. Then, the writer writes data to the related engine by using the write API that is provided by the engine.

Supported data types

Parameters

Configure ClickHouse Reader by using the codeless UI

1. Configure data sources.
   Configure the source and destination for the synchronization node.

   Parameter	Description
   Connection	The name of the data source from which you want to read data. This parameter is equivalent to the datasource parameter that is specified in the preceding section.
   Table	The name of the table from which you want to read data. This parameter is equivalent to the table parameter that is specified in the preceding section.
   Filter	This parameter is equivalent to the where parameter that is specified in the preceding section. This parameter specifies the condition that is used to filter the data you want to read. Filtering based on the LIMIT keyword is not supported. The SQL syntax is determined based on the selected data source.
   Shard Key	The shard key. This parameter is equivalent to the splitPk parameter that is specified in the preceding section. You can use a column in the source table as the shard key. We recommend that you use the primary key column or an indexed column. Only integer columns are supported. If you configure this parameter, data sharding is performed based on the value of this parameter, and parallel threads can be used to read data. This improves data synchronization efficiency. Note The Shard Key parameter is displayed only after you select the data source for the synchronization node.

2. Configure field mappings. This operation is equivalent to setting the column parameter that is described in the preceding section.
   Fields in the source on the left have a one-to-one mapping with fields in the destination on the right. You can click Add to add a field. To remove an added field, move the pointer over the field and click the Remove icon.

   Operation	Description
   Map Fields with the Same Name	Click Map Fields with the Same Name to establish mappings between fields with the same name. The data types of the fields must match.
   Map Fields in the Same Line	Click Map Fields in the Same Line to establish mappings between fields in the same row. The data types of the fields must match.
   Delete All Mappings	Click Delete All Mappings to remove the mappings that are established.
   Auto Layout	Click Auto Layout. Then, the system automatically sorts the fields based on specific rules.
   Change Fields	Click the Change Fields icon. In the Change Fields dialog box, you can manually edit the fields in the source and destination tables. Each field occupies a row. The first and the last blank rows are included, whereas other blank rows are ignored.
   Add	Click Add to add a field. Take note of the following rules when you add a field: You can enter constants. Each constant must be enclosed in single quotation marks ('), such as 'abc' and '123'. You can use scheduling parameters, such as ${bizdate}. You can enter functions that are supported by relational databases, such as now() and count(1). If the field that you entered cannot be parsed, the value of Type for the field is Unidentified.

3. Configure channel control policies.

   Parameter	Description
   Expected Maximum Concurrency	The maximum number of parallel threads that the synchronization node uses to read data from the source or write data to the destination. You can configure the parallelism for the synchronization node on the codeless UI.
   Bandwidth Throttling	Specifies whether to enable throttling. You can enable throttling and specify a maximum transmission rate to prevent heavy read workloads on the source. We recommend that you enable throttling and set the maximum transmission rate to an appropriate value based on the configurations of the source.
   Dirty Data Records Allowed	The maximum number of dirty data records allowed.
   Distributed Execution	The distributed execution mode that allows you to split your node into pieces and distribute them to multiple Elastic Compute Service (ECS) instances for parallel execution. This speeds up synchronization. If you use a large number of parallel threads to run your synchronization node in distributed execution mode, excessive access requests are sent to the data sources. Therefore, before you use the distributed execution mode, you must evaluate the access load on the data sources. You can enable this mode only if you use an exclusive resource group for Data Integration. For more information about exclusive resource groups for Data Integration, see Exclusive resource groups for Data Integration and Create and use an exclusive resource group for Data Integration.

Configure ClickHouse Reader by using the code editor

{
    "type": "job",
    "version": "2.0",
    "steps": [
        {
            "stepType": "clickhouse", // The reader type. 
            "parameter": {
                "fetchSize":1024,// The number of data records to read at a time. 
                "datasource": "example",
                "column": [   // The names of the columns from which you want to read data. 
                    "id",
                    "name"
                ],
                "where": "",    // The WHERE clause. 
                "splitPk": "",  // The shard key. 
                "table": ""    // The name of the table from which you want to read data. 
            },
            "name": "Reader",
            "category": "reader"
        },
        {
            "stepType": "clickhouse",
            "parameter": {
                "postSql": [
                    "update @table set db_modify_time = now() where db_id = 1"
                ],
                "datasource": "example",    // The name of the data source. 
                "batchByteSize": "67108864",
                "column": [
                    "id",
                    "name"
                ],
                "writeMode": "insert",
                "encoding": "UTF-8",
                "batchSize": 1024,
                "table": "ClickHouse_table",
                "preSql": [
                    "delete from @table where db_id = -1"
                ]
            },
            "name": "Writer",
            "category": "writer"
        }
    ],
    "setting": {
        "executeMode": null,
        "errorLimit": {
            "record": "0"  // The maximum number of dirty data records allowed. 
        },
        "speed": {
         "throttle":true,// Specifies whether to enable throttling. The value false indicates that throttling is disabled, and the value true indicates that throttling is enabled. The mbps parameter takes effect only when the throttle parameter is set to true. 
            "concurrent":1 // The maximum number of parallel threads. 
            "mbps":"12",// The maximum transmission rate.
        }
    },
    "order": {
        "hops": [
            {
                "from": "Reader",
                "to": "Writer"
            }
        ]
    }
}