"Mounting" means adding a completely "independent" application in a specific path, that then takes care of handling everything under that path, with the _path operations_ declared in that sub-application.

### Top-level application { #top-level-application }

First, create the main, top-level, **FastAPI** application, and its *path operations*:

{* ../../docs_src/sub_applications/tutorial001.py hl[3, 6:8] *}

### Sub-application { #sub-application }

import java.util.Iterator;
import java.util.List;
import java.util.stream.Stream;
import org.jspecify.annotations.Nullable;

/**
 * An accumulator that selects the "top" {@code k} elements added to it, relative to a provided
 * comparator. "Top" can mean the greatest or the lowest elements, specified in the factory used to
 * create the {@code TopKSelector} instance.
 *

# buildifier: disable=load-on-top

workspace(name = "org_tensorflow")

# buildifier: disable=load-on-top

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "rules_shell",
    sha256 = "bc61ef94facc78e20a645726f64756e5e285a045037c7a61f65af2941f4c25e1",
    strip_prefix = "rules_shell-0.4.1",
    url = "https://github.com/bazelbuild/rules_shell/releases/download/v0.4.1/rules_shell-v0.4.1.tar.gz",
)

        Exception level1 = new IOException("Level 1 error", level2);
        SsoLoginException exception = new SsoLoginException("Top level SSO error", level1);

        assertEquals("Top level SSO error", exception.getMessage());

        // Verify the exception chain
        Throwable cause1 = exception.getCause();
        assertNotNull(cause1);

        Throwable middleCause = new IllegalStateException("Middle cause", rootCause);
        ScheduledJobException exception = new ScheduledJobException("Top level error", middleCause);

        assertEquals("Top level error", exception.getMessage());
        assertEquals(middleCause, exception.getCause());
        assertEquals("Middle cause", exception.getCause().getMessage());

        RuntimeException middleCause = new RuntimeException("Middle cause", rootCause);
        FessSystemException topException = new FessSystemException("Top level", middleCause);

        assertEquals("Top level", topException.getMessage());
        assertEquals(middleCause, topException.getCause());
        assertEquals(rootCause, topException.getCause().getCause());
    }

        Exception intermediateCause = new IllegalStateException("Intermediate cause", rootCause);
        ScriptEngineException exception = new ScriptEngineException("Top level error", intermediateCause);

        assertEquals("Top level error", exception.getMessage());
        assertEquals(intermediateCause, exception.getCause());
        assertEquals("Intermediate cause", exception.getCause().getMessage());

Another big feature required by APIs is <abbr title="reading and converting to Python data">parsing</abbr> data from incoming requests.

Webargs is a tool that was made to provide that on top of several frameworks, including Flask.

It uses Marshmallow underneath to do the data validation. And it was created by the same developers.

It's a great tool and I have used it a lot too, before having **FastAPI**.

        Throwable intermediateCause = new RuntimeException("Intermediate problem", rootCause);
        GsaConfigException exception = new GsaConfigException("Top level GSA error", intermediateCause);

        assertNotNull(exception);
        assertEquals("Top level GSA error", exception.getMessage());

        // Check first level cause
        assertNotNull(exception.getCause());

## Star **FastAPI** in GitHub { #star-fastapi-in-github }

You can "star" FastAPI in GitHub (clicking the star button at the top right): <a href="https://github.com/fastapi/fastapi" class="external-link" target="_blank">https://github.com/fastapi/fastapi</a>. ⭐️