# 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 🚀

# 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**.

        } 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

        // Then: Will create new connection since real transport has no socket
        assertNotSame(first, second, "Should create new connection when first is disconnected");
    }

    @Test
    @DisplayName("Should create new connection when force signing differs")
    void testNoReuseWithDifferentSigning() throws Exception {
        // Given: An existing connection without signing enforced

	if err := DB.Where("roles2 IS NULL AND roles3 = ?", "").First(&result, data.ID).Error; err != nil {
		t.Fatalf("failed to query data, got error %v", err)
	}

	AssertEqual(t, result, data)

	if err := DB.Model(&result).Update("roles", "").Error; err != nil {
		t.Fatalf("failed to update data's roles, got error %v", err)
	}

	if err := DB.First(&result, data.ID).Error; err != nil {

	}

	var result2 User
	if err := DB.Where("name = ?", "create_from_map_2").First(&result2).Error; err != nil || result2.Age != 19 {
		t.Fatalf("failed to query data after create from slice of map, got error %v", err)
	}

	var result3 User
	if err := DB.Where("name = ?", "create_from_map_3").First(&result3).Error; err != nil || result3.Age != 20 {

        // Should alternate between channels
        ChannelInfo first = loadBalancer.selectChannel(mockMessage);
        ChannelInfo second = loadBalancer.selectChannel(mockMessage);

        assertNotNull(first);
        assertNotNull(second);
        // Due to round-robin, should get different channels or same order
        assertTrue(Arrays.asList(channel1, channel2).contains(first));

		}
		lastUpdatedAt = n
	}

	checkOtherData := func(name string) {
		var first, last User
		if err := DB.Where("id = ?", users[0].ID).First(&first).Error; err != nil {
			t.Errorf("errors happened when query before user: %v", err)
		}
		CheckUser(t, first, *users[0])

		if err := DB.Where("id = ?", users[2].ID).First(&last).Error; err != nil {
			t.Errorf("errors happened when query after user: %v", err)
		}

                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();

    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