# Consistency

A setting that defines a successful write or read by the number of cluster
replicas that acknowledge the write or respond to the read request,
respectively.

## Default consistency

The default consistency is `CASS_CONSISTENCY_LOCAL_ONE`.

The default serial consistency is `CASS_CONSISTENCY_ANY`, which means no serial consistency specified.
This may result in an error upon execution of a statement that requires serial consistency (Lightweight Transaction, LWT).
In order to avoid this, you should set the serial consistency level to `CASS_CONSISTENCY_SERIAL` or `CASS_CONSISTENCY_LOCAL_SERIAL`
before executing such statement.

## Consistency Levels

### Read and Write Consistency Levels

The consistency level determines the number of replicas on which the read/write
must respond/succeed before returning an acknowledgment to the client
application. Descriptions for each read/write consistency level can be found
[here](https://docs.scylladb.com/manual/stable/cql/consistency.html).

<table class="table table-striped table-hover table-condensed">
  <thead>
  <tr>
   <th>Level</th>
   <th>Driver</th>
  </tr>
  </thead>
  <tbody>
  <tr>
   <td>All</td>
   <td><code>CASS_CONSISTENCY_ALL</code></td>
  </tr>
  <tr>
   <td>Each Quorum</td>
   <td><code>CASS_CONSISTENCY_EACH_QUORUM</code></td>
  </tr>
  <tr>
   <td>Quorum</td>
   <td><code>CASS_CONSISTENCY_QUORUM</code></td>
  </tr>
  <tr>
   <td>Local Quorum</td>
   <td><code>CASS_CONSISTENCY_LOCAL_QUORUM</code></td>
  </tr>
  <tr>
   <td><b>One</b></td>
   <td><code><b>CASS_CONSISTENCY_ONE</b></code></td>
  </tr>
  <tr>
   <td>Two</td>
   <td><code>CASS_CONSISTENCY_TWO</code></td>
  </tr>
  <tr>
   <td>Three</td>
   <td><code>CASS_CONSISTENCY_THREE</code></td>
  </tr>
  <tr>
   <td>Local One</td>
   <td><code>CASS_CONSISTENCY_LOCAL_ONE</code></td>
  </tr>
  <tr>
   <td>Any</td>
   <td><code>CASS_CONSISTENCY_ANY</code></td>
  </tr>
  <tr>
   <td>Serial</td>
   <td><code>CASS_CONSISTENCY_SERIAL</code></td>
  </tr>
  <tr>
   <td>Local Serial</td>
   <td><code>CASS_CONSISTENCY_LOCAL_SERIAL</code></td>
  </tr>
  </tbody>
</table>

**NOTE:** Consistency level `CASS_CONSISTENCY_ANY` is only valid for write operation statements.

## Setting Consistency Level

A [‘CassStatement’](https://cpp-rs-driver.docs.scylladb.com/stable/api/struct.CassFuture/) object
can have its consistency level altered at anytime before the statement is
executed by the session.

```c
CassStatement* statement = NULL;

/* Create a simple or prepared statment */

/* Ensure the session executed statement has strong consistency */
cass_statement_set_consistency(statement, CASS_CONSISTENCY_QUORUM);

cass_statement_free(statement);
```

**NOTE:** Consistency level is ignored for `USE`, `TRUNCATE`, `CREATE` and `ALTER`
statements, and some, like `CASS_CONSISTENCY_ANY`, aren’t allowed in all situations.