# Limitations of TiCDC

__*TiCDC limits*__

+ TiCDC can only synchronize tables that have at least one valid index
  + The primary key (PRIMARY KEY) is a valid index
  + A unique index (UNIQUE INDEX) that also satisfies the following conditions is a valid index
    + Each column in the index is explicitly defined as not null (NOT NULL) in the table schema
    + VIRTUAL GENERated COLUMNS does not exist in the index

+ scenario that are not supported by TiCDC
  + TiKV clusters that use RawkV alone are not supported
  + DDL operations and SEQUENCE function that create SEQUENCE in TiDB are not supported
  + Partial support for scenario with larger upstream transactions (greater than 5 GB in size)

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When synchronizing with TiCDC, please be aware of the following restrictions and unsupported scenario.

Requirements for a valid index
TiCDC can only synchronize tables that have at least one valid index. Valid indexes are defined as follows:
The primary key (PRIMARY KEY) is a valid index.
A unique index (UNIQUE INDEX) that also satisfies the following conditions is a valid index:
Each column in an index is explicitly defined as not null (NOT NULL) in the table schema.
There are no VIRTUAL GENERATED COLUMNS in the index.
Starting with version 4.0.8, TiCDC can synchronize tables that do not have a valid index by modify the task configuration, but the guarantee of data consistency is reduced. Refer to Synchronize tables without valid indexes for specific usage and precautions.

scenario that are not currently supported
scenario currently not supported by TiCDC are as follows:
TiKV clusters that use RawkV alone are not supported at this time.
DDL operations and SEQUENCE function that create SEQUENCE in TiDB are not supported at this time. When using SEQUENCE in upstream TiDB, TiCDC ignores the SEQUENCE DDL operations/functions executed upstream, but DML operations that use the SEQUENCE function can be synchronized correctly.
Partial support is provided for scenario with large upstream transaction. For details, see Does TiCDC support synchronous transactions? Are there any risks?
notice
Starting with v5.3.0, TiCDC no longer supports the ring sync feature.