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#### http::async_write_some

Write part of a message to a stream asynchronously using a serializer.

##### Synopsis

Defined in header `<boost/beast/http/write.hpp>`

    template<
        class AsyncWriteStream,
        bool isRequest,
        class Body,
        class Fields,
        class WriteHandler = net::default_completion_token_t<            executor_type<AsyncWriteStream>>>
    DEDUCED
    async_write_some(
        AsyncWriteStream& stream,
        serializer< isRequest, Body, Fields >& sr,
        WriteHandler&& handler = net::default_completion_token_t< executor_type< AsyncWriteStream > >{});

##### Description

This function is used to write part of a message to a stream asynchronously using a caller-provided HTTP/1 serializer. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- One or more bytes have been transferred.
- The function `serializer::is_done` returns `true`
- An error occurs on the stream.

This operation is implemented in terms of zero or more calls to the stream's `async_write_some` function, and is known as a composed operation. The program must ensure that the stream performs no other writes until this operation completes.

The amount of data actually transferred is controlled by the behavior of the underlying stream, subject to the buffer size limit of the serializer obtained or set through a call to `serializer::limit`. Setting a limit and performing bounded work helps applications set reasonable timeouts. It also allows application-level flow control to function correctly. For example when using a TCP/IP based stream.

##### Parameters

Name | Description
--- | ---
`stream` | The stream to which the data is to be written. The type must support the AsyncWriteStream concept.
`sr` | The serializer to use. The object must remain valid at least until the handler is called; ownership is not transferred.
`handler` | The completion handler to invoke when the operation completes. The implementation takes ownership of the handler by performing a decay-copy. The equivalent function signature of the handler must be:
    void handler(
        error_code const & error,        // result of operation
        std::size_t bytes_transferred   // the number of bytes written to the stream
    );
If the handler has an associated immediate executor, an immediate completion will be dispatched to it. Otherwise, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using `net::post`.

##### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following net::cancellation_type values:

- `net::cancellation_type::terminal`

if the `stream` also supports terminal cancellation, `terminal` cancellation leaves the stream in an undefined state, so that only closing it is guaranteed to succeed.

##### See Also

`serializer`