---
metadata:
  - name: generator
    content: Diplodoc Platform v5.50.4
alternate:
  - https://ydb-platform--ydb.viewer.diplodoc.com/en/dev/cdc.md
  - https://ydb-platform--ydb.viewer.diplodoc.com/ru/dev/cdc.md
sourcePath: en/core/dev/cdc.md
---
> **Documentation Index:** Fetch the complete configuration index at https://ydb-platform--ydb.viewer.diplodoc.com/en/llms.txt

# Change Data Capture

With [Change Data Capture](https://ydb-platform--ydb.viewer.diplodoc.com/en/concepts/cdc.md) (CDC), you can track changes in table data. YDB provides access to changefeeds so that data consumers can monitor changes in near real time.

## Enabling and disabling CDC {#add-drop}

CDC is represented as a data schema object: a changefeed that can be added to a table or deleted from them using the [ADD CHANGEFEED and DROP CHANGEFEED](https://ydb-platform--ydb.viewer.diplodoc.com/en/yql/reference/syntax/alter_table/changefeed.md) directives of the YQL `ALTER TABLE` statement.

## Reading data from a topic {#read}

Before reading data, add a [consumer](https://ydb-platform--ydb.viewer.diplodoc.com/en/concepts/datamodel/topic.md#consumer). Below is a sample command that adds a consumer named `my_consumer` to the `updates_feed` changefeed of the `table` table in the `my` directory:

```bash
ydb topic consumer add \
  my/table/updates_feed \
  --consumer=my_consumer
```

Next, you can use the created consumer to start tracking changes. Below is a sample command for tracking data changes in the CLI:

```bash
ydb topic read \
  my/table/updates_feed \
  --consumer=my_consumer \
  --format=newline-delimited \
  --wait
```

## Impact on table write performance {#performance-considerations}

When writing data to a table with CDC enabled, there are additional overheads for the following operations:

* Making records and saving them to a changefeed.
* Storing records in a changefeed.
* In some [modes](https://ydb-platform--ydb.viewer.diplodoc.com/en/yql/reference/syntax/alter_table/changefeed.md) (such as `OLD_IMAGE` and `NEW_AND_OLD_IMAGES`), data needs to be pre-fetched even if a user query doesn't require this.

As a result, queries may take longer to execute and size limits for stored data may be exceeded.

In real-world use cases, enabling CDC has virtually no impact on the query execution time (whatever the mode), since almost all data required for making records is stored in the cache , while the records themselves are sent to a topic asynchronously. However, record delivery background activity slightly (by 1% to 10%) increases CPU utilization.

When creating a changefeed for a table, the number of partitions of its storage (topic) is determined based on the current number of table partitions. If the number of source table partitions changes significantly (for example, after uploading a large amount of data or as a result of intensive accesses), an imbalance occurs between the table partitions and the topic partitions. This imbalance can also result in longer execution time for queries to modify data in the table or in unnecessary storage overheads for the changefeed. You can recreate the changefeed to correct the imbalance.

## Load testing {#workload}

As a load generator, you can use the feature of [emulating an online store](../reference/ydb-cli/commands/workload/stock) built into the YDB CLI:

1. [Initialize](../reference/ydb-cli/commands/workload/stock#init) a test.
1. Add a changefeed:

```yql
   ALTER TABLE `orders` ADD CHANGEFEED `updates` WITH (
       FORMAT = 'JSON',
       MODE = 'UPDATES'
   );
   ```

1. Create a consumer:

   ```bash
   ydb topic consumer add \
     orders/updates \
     --consumer=my_consumer
   ```

1. Start tracking changes:

   ```bash
   ydb topic read \
     orders/updates \
     --consumer=my_consumer \
     --format=newline-delimited \
     --wait
   ```

1. [Generate](../reference/ydb-cli/commands/workload/stock#run) a load.

   The following changefeed appears in the CLI:

   ```text
   ...
   {"update":{"created":"2022-06-24T11:35:00.000000Z","customer":"Name366"},"key":[13195699997286404932]}
   {"update":{"created":"2022-06-24T11:35:00.000000Z","customer":"Name3894"},"key":[452209497351143909]}
   {"update":{"created":"2022-06-24T11:35:00.000000Z","customer":"Name7773"},"key":[2377978894183850258]}
   ...
   ```