# Security - First Steps { #security-first-steps }

Let's imagine that you have your **backend** API in some domain.

And you have a **frontend** in another domain or in a different path of the same domain (or in a mobile application).

And you want to have a way for the frontend to authenticate with the backend, using a **username** and **password**.

We can use **OAuth2** to build that with **FastAPI**.

# First Steps { #first-steps }

The simplest FastAPI file could look like this:

{* ../../docs_src/first_steps/tutorial001.py *}

Copy that to a file `main.py`.

Run the live server:

<div class="termy">

```console
$ <font color="#4E9A06">fastapi</font> dev <u style="text-decoration-style:solid">main.py</u>

  <span style="background-color:#009485"><font color="#D3D7CF"> FastAPI </font></span>  Starting development server 🚀

                return mockResource1;
            }
        };

        // When/Then
        assertTrue(iterator.hasNext(), "Should have first element");
        SmbResource first = iterator.next();
        assertNotNull(first, "First element should not be null");
        assertEquals("file1.txt", first.getName());

        assertTrue(iterator.hasNext(), "Should have second element");
        SmbResource second = iterator.next();

   * the first are ignored.
   */
  public static <E> ImmutableSet<E> of(E e1, E e2, E e3, E e4, E e5) {
    return new RegularSetBuilderImpl<E>(5).add(e1).add(e2).add(e3).add(e4).add(e5).review().build();
  }

  /**
   * Returns an immutable set containing the given elements, minus duplicates, in the order each was

        NetServerFileEntryAdapterIterator itr = new NetServerFileEntryAdapterIterator(parent, delegate, null);

        // Act & Assert
        assertTrue(itr.hasNext(), "Should have first element ready");
        SmbResource r1 = itr.next();
        assertNotNull(r1, "First adapted resource must not be null");
        // For workgroup/server entries, names end with '/'
        assertTrue(r1.getName().endsWith("/"), "Name should end with '/'");

        } else if (first != null && second != null) {
            // Both non-null - should be same instance
            assertSame(first, second, "Second getSubject should return same cached instance as first");
        } else {
            fail("Inconsistent behavior: first=" + first + ", second=" + second);
        }

        // Third call should return the same cached Subject instance

    void testNextElementFirstCall() {
        Object element = transaction.nextElement();
        assertSame(transaction, element, "First nextElement should return the transaction itself");
    }

    @Test
    @DisplayName("Test nextElement() changes command on second call")
    void testNextElementSecondCall() {
        // First call - primary

  @Override
  public UnmodifiableIterator<C> descendingIterator() {
    return new AbstractSequentialIterator<C>(last()) {
      final C first = first();

      @Override
      protected @Nullable C computeNext(C previous) {
        return equalsOrThrow(previous, first) ? null : domain.previous(previous);
      }
    };
  }

    int len = sequence.length();
    int first;
    int last;

    for (first = 0; first < len; first++) {
      if (!matches(sequence.charAt(first))) {
        break;
      }
    }
    for (last = len - 1; last > first; last--) {
      if (!matches(sequence.charAt(last))) {
        break;
      }
    }

    return sequence.subSequence(first, last + 1).toString();
  }

  /**

        return array;
    }

    /**
     * Returns the index of the first occurrence of the specified object in the array.
     *
     * @param <T>
     *            the type of the array elements
     * @param array
     *            the array
     * @param obj
     *            the object to search for
     * @return the index of the first occurrence of the object in the array
     */