## Composition by architecture modules

Each [architecture module and platform](platforms.md) can contribute code to any of the runtimes.
Not every module contributes to every runtime.

The core-runtime module defines each runtime:

- The target JVM for the runtime. Each runtime has its own JVM compatibility constraints.
- Some base services that are available to code hosted by the runtime. This varies by runtime.

  newline-terminated lines in the byte slice s.
- [SplitSeq] returns an iterator over
  all substrings of s separated by sep.
- [SplitAfterSeq] returns an iterator
  over substrings of s split after each instance of sep.
- [FieldsSeq] returns an iterator over
  substrings of s split around runs of whitespace characters,
  as defined by unicode.IsSpace.
- [FieldsFuncSeq] returns an iterator

    /**
     * Returns model source identified by a path relative to this model source POM. Implementation <strong>MUST</strong>
     * be able to accept <code>relPath</code> parameter values that
     * <ul>
     * <li>use either / or \ file path separator</li>
     * <li>have .. parent directory references</li>
     * <li>point either at file or directory, in the latter case POM file name 'pom.xml' needs to be used by the
     * requested model source.</li>

# Using the Request Directly

Up to now, you have been declaring the parts of the request that you need with their types.

Taking data from:

* The path as parameters.
* Headers.
* Cookies.
* etc.

And by doing so, **FastAPI** is validating that data, converting it and generating documentation for your API automatically.

But there are situations where you might need to access the `Request` object directly.

   * Reads a count corresponding to a serialized map, multiset, or multimap. It returns the size of
   * a map serialized by {@link #writeMap(Map, ObjectOutputStream)}, the number of distinct elements
   * in a multiset serialized by {@link #writeMultiset(Multiset, ObjectOutputStream)}, or the number
   * of distinct keys in a multimap serialized by {@link #writeMultimap(Multimap,
   * ObjectOutputStream)}.
   */

# Custom Request and APIRoute class

In some cases, you may want to override the logic used by the `Request` and `APIRoute` classes.

In particular, this may be a good alternative to logic in a middleware.

For example, if you want to read or manipulate the request body before it is processed by your application.

/// danger

This is an "advanced" feature.

If you are just starting with **FastAPI** you might want to skip this section.

///

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