### Crear un Engine

Un `engine` de SQLModel (en el fondo, realmente es un `engine` de SQLAlchemy) es lo que **mantiene las conexiones** a la base de datos.

Tendrías **un solo objeto `engine`** para todo tu código para conectar a la misma base de datos.

{* ../../docs_src/sql_databases/tutorial001_an_py310.py ln[14:18] hl[14:15,17:18] *}

  }

  /**
   * All of the infrastructure of split and splitToString is identical, so we do one test of
   * splitToString. All other cases should be covered by testing of split.
   *
   * <p>TODO(user): It would be good to make all the relevant tests run on both split and
   * splitToString automatically.
   */
  public void testCharacterSimpleSplitToList() {
    String simple = "a,b,c";

 *     the same lambda function as passed into {@link #using(Function)}.
 *     <p>This class is scheduled to be removed in October 2019.
 */
// TODO(b/68134636): Remove by 2019-10
@Deprecated
@Beta
@GwtCompatible
public abstract class TreeTraverser<T> {
  /** Constructor for use by subclasses. */
  public TreeTraverser() {}

  /**

   * bits, returns the low 32 bits of the result.
   *
   * @since 14.0
   */
  @J2ktIncompatible
  @GwtIncompatible // Does not truncate correctly
  public UnsignedInteger times(UnsignedInteger val) {
    // TODO(lowasser): make this GWT-compatible
    return fromIntBits(value * checkNotNull(val).value);
  }

  /**
   * Returns the result of dividing this by {@code val}.
   *

 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained">immutable collections</a>.
 *
 * @author Jared Levy
 * @author Louis Wasserman
 * @since 2.0 (implements {@code NavigableSet} since 12.0)
 */
// TODO(benyu): benchmark and optimize all creation paths, which are a mess now
@GwtCompatible
@SuppressWarnings("serial") // we're overriding default serialization
public abstract class ImmutableSortedSet<E> extends ImmutableSet<E>

     * @param thumbnailId The thumbnail ID.
     * @param value The thumbnail value to update.
     */
    protected void updateThumbnailField(final String thumbnailId, final String value) {
        // TODO bulk
        final FessConfig fessConfig = ComponentUtil.getFessConfig();
        try {
            ComponentUtil.getIndexingHelper()

 * support {@code remove()}.
 *
 * @author Jared Levy
 */
@GwtCompatible
@NullMarked
public class MapsTransformValuesUnmodifiableIteratorTest extends MapInterfaceTest<String, String> {
  // TODO(jlevy): Move shared code of this class and MapsTransformValuesTest
  // to a superclass.

  public MapsTransformValuesUnmodifiableIteratorTest() {
    super(true, true, false /*supportsPut*/, true, true, false);
  }

En este caso, podrías querer documentar cómo esa API externa *debería* verse. Qué *path operation* debería tener, qué cuerpo debería esperar, qué response debería devolver, etc.

## Una aplicación con callbacks

Veamos todo esto con un ejemplo.

Imagina que desarrollas una aplicación que permite crear facturas.

Estas facturas tendrán un `id`, `title` (opcional), `customer`, y `total`.

 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible
public abstract class ForwardingMap<K extends @Nullable Object, V extends @Nullable Object>
    extends ForwardingObject implements Map<K, V> {
  // TODO(lowasser): identify places where thread safety is actually lost

  /** Constructor for use by subclasses. */
  protected ForwardingMap() {}

  @Override
  protected abstract Map<K, V> delegate();

  @Override

   * @param reason Reason for shutting down, no longer than 123 bytes of UTF-8 encoded data (**not** characters) or null.
   * @throws IllegalArgumentException if [code] is invalid or [reason] is too long.
   */
  fun close(
    code: Int,
    reason: String?,
  ): Boolean

  /**
   * Immediately and violently release resources held by this web socket, discarding any enqueued