# Usage Restrictions

## Functionality Limitations

* A single instance only provides a single node.
* Custom `remote_servers` configuration by users is not supported. There is only one `remote_servers` in a cluster named: ck_cluster.
* For dual replicas, DDL statements must be executed using the `ON CLUSTER ck_cluster` syntax.
* High availability must use the Replicated* table engine.
* To ensure cluster stability and data safety, horizontal scaling is not supported.

## Naming Restrictions

| Object    | Restrictions                                                   |
| --------- | -------------------------------------------------------------- |
| Database Name | Database names cannot be the same as built-in databases like "system." |
| Table Name    | Table names cannot contain quotes, exclamation marks (!), or spaces. Table names cannot be SQL reserved keywords. |
| Column Name   | Column names cannot contain quotes, exclamation marks (!), or spaces. Column names cannot be SQL reserved keywords. |
| Password      | Passwords must be 8-30 characters long. <br /> Cannot contain illegal characters outside [A-Z], [a-z], [0-9]. <br /> Must include both uppercase and lowercase letters and numbers. |