faq/README.md

## Frequently asked questions

### I'm modifying a statement and the changes get ignored, why?

In driver 4, statement classes are **immutable**. All mutating methods return a new instance, so
make sure you don't accidentally ignore their result:

```java
BoundStatement boundSelect = preparedSelect.bind();

// This doesn't work: setInt and setPageSize don't modify boundSelect in place:
boundSelect.setInt("k", key);
boundSelect.setPageSize(1000);
session.execute(boundSelect);

// Instead, reassign the statement every time:
boundSelect = boundSelect.setInt("k", key).setPageSize(1000);
```

All of these mutating methods are annotated with `@CheckReturnValue`. Some code analysis tools --
such as [ErrorProne](https://errorprone.info/) -- can check correct usage at build time, and report
mistakes as compiler errors.

The driver also provides builders:

```java
BoundStatement boundSelect =
    preparedSelect.boundStatementBuilder()
        .setInt("k", key)
        .setPageSize(1000)
        .build();
```

### Why do asynchronous methods return `CompletionStage<T>` instead of `CompletableFuture<T>`?

Because it's the right abstraction to use. A completable future, as its name indicates, is a future
that can be completed manually; that is not what we want to return from our API: the driver
completes the futures, not the user.

Also, `CompletionStage` does not expose a `get()` method; one can view that as an encouragement to
use a fully asynchronous programming model (chaining callbacks instead of blocking for a result).

At any rate, `CompletionStage` has a `toCompletableFuture()` method. In current JDK versions, every
`CompletionStage` is a `CompletableFuture`, so the conversion has no performance overhead.

### Where is `DowngradingConsistencyRetryPolicy`?

That retry policy was deprecated in driver 3.5.0, and does not exist anymore in 4.0.0. The main
motivation is that this behavior should be the application's concern, not the driver's.

We recognize that there are use cases where downgrading is good -- for instance, a dashboard
application would present the latest information by reading at QUORUM, but it's acceptable for it to
display stale information by reading at ONE sometimes. 

But APIs provided by the driver should instead encourage idiomatic use of a distributed system like
Apache Cassandra, and a downgrading policy works against this. It suggests that an anti-pattern such
as "try to read at QUORUM, but fall back to ONE if that fails" is a good idea in general use cases, 
when in reality it provides no better consistency guarantees than working directly at ONE, but with
higher latencies. 

We therefore urge users to carefully choose upfront the consistency level that works best for their
use cases. If there is a legitimate reason to downgrade and retry, that should be handled by the
application code.

### I want to set a date on a bound statement, where did `setTimestamp()` go?

The driver now uses Java 8's improved date and time API. CQL type `timestamp` is mapped to
`java.time.Instant`, and the corresponding getter and setter are `getInstant` and `setInstant`.

See [Temporal types](../manual/core/temporal_types/) for more details.

### Why do DDL queries have a higher latency than driver 3?

If you benchmark DDL queries such as `session.execute("CREATE TABLE ...")`, you will observe a
noticeably higher latency than driver 3 (about 1 second).

This is because those queries are now *debounced*: the driver adds a short wait in an attempt to
group multiple schema changes into a single metadata refresh. If you want to mitigate this, you can
either adjust the debouncing settings, or group your schema updates while temporarily disabling the
metadata; see the [performance](../manual/core/performance/#debouncing) page.

This only applies to DDL queries; DML statements (`SELECT`, `INSERT`...) are not debounced.