HTTP Client API — cpp-netlib v0.11.0

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# HTTP Client API

## General

`cpp-netlib` includes and implements a number of HTTP clients that you can use and embed in your own applications. All of the HTTP client implementations:

> - Cannot be copied. This means you may have to store instances of the clients in dynamic memory if you intend to use them as function parameters or pass them around in smart pointers or by reference.
> - Assume that requests made are independent of each other. There currently is no cookie or session management system built-in to cpp-netlib’s HTTP client implementations.

The HTTP clients all share the same API, but the internals are documented in terms of what is different and what to expect with the different implementations.

As of 0.9.1 the default implementation for the `cpp-netlib` HTTP client is asynchronous.

As of 0.11 the Synchronous Clients are now DEPRECATED and will be removed in subsequent releases.

## Implementations

There is a single user-facing template class named `basic_client` which takes three template parameters:

> - Tag - which static tag you choose that defines the behavior of the client.
> - http_version_major - an unsigned int that defines the HTTP major version number, this directly affects the HTTP messages sent by the client.
> - http_version_minor - an unsigned int that defines the HTTP minor version number.

There are two major different class of implementations of the `basic_client` template that depend on which tag you choose: Synchronous Clients and Asynchronous Clients. These two different classes are described in their own sections following this one. What follows is a table of all tags supported by the HTTP client implementation provided by `cpp-netlib`.

Tag | Description
--- | ---
http_default_8bit_tcp_resolve | This is the default HTTP implementation tag that resolves addresses with a TCP resolver and provides a synchronous/blocking HTTP client interface.
http_default_8bit_udp_resolve | This is similar to the above tag except that it specifies the HTTP client to use a UDP resolver. It also provides a synchronous/ blocking HTTP client interface.
http_keepalive_8bit_tcp_resolve | This tag specifies that the HTTP client by default will keep connections to the server alive. It only makes sense if the `version_major` and `version_minor` are both `1`, to indicate HTTP 1.1. This tag causes the HTTP client to resolve using a TCP resolver and provides a synchronous/ blocking HTTP client interface.
http_keepalive_8bit_udp_resolve | This is similar to the above tag except that it specifies the HTTP client to use a UDP resolver. It also provides a synchronous/ blocking HTTP client interface.
http_async_8bit_tcp_resolve | This tag provides an active HTTP client object implementation that uses a TCP resolver. Response objects returned will encapsulate a number of Boost.Thread shared futures to hold values. Users don’t have to see this as they are implementation details.
http_async_8bit_udp_resolve | This is similar to the above tag except that specifies the HTTP client to use a UDP resolver.

In the above table the tags follow a pattern for describing the behavior introduced by the tags. This pattern is shown below:

> <protocol>_<modifier>_<character-width>_<resolve-strategy>

For example, the tag `http_default_8bit_tcp_resolve` indicates the protocol `http`, a modifier `default`, a character width of `8bit`, and a resolve strategy of `tcp_resolve`.

### Synchronous Clients

Of the client tags shown in the table, the following makes the `basic_client` behave as a fully synchronous client.

> - http_default_8bit_tcp_resolve
> - http_default_8bit_udp_resolve
> - http_keepalive_8bit_tcp_resolve
> - http_keepalive_8bit_udp_resolve

The synchronous client implements all the operations of the client underneath the interface all block to wait for I/O to finish. All the member methods are synchronous and will block until the response object is ready or throws if erros are encountered in the performance of the HTTP requests.

Warning

The synchronous clients are NOT thread safe. You will need to do external synchronization to use synchronous client implementations.

Note

As of version 0.11, all the synchronous client implementations are deprecated. They will be removed in the next version of the library.

### Asynchronous Clients

The following tags specify the `basic_client` to behave in an asynchronous manner:

> - http_async_8bit_tcp_resolve
> - http_async_8bit_udp_resolve

An asynchronous client implementation means that``basic_client<...>`` is an Active Object. This means that the client has and manages its own lifetime thread, and returns values that are asynchronously filled in. The response object encapsulates Boost.Thread futures which get filled in once the values are available.

The asynchronous clients implement all operations asynchronously which are hidden from the user. The interface is still synchronous but the fetching of data happens on a different thread.

Note

The asynchronous clients are thread safe, and can be shared across many threads. Each request starts a sequence of asynchronous operations dedicated to that request. The client does not re-cycle connections and uses a one-request-one-connection model.

When an asynchronous client object is destroyed, it waits for all pending asynchronous operations to finish. Errors encountered during operations on retrieving data from the response objects cause exceptions to be thrown – therefore it is best that if a client object is constructed, it should outlive the response object or be outside the try-catch block handling the errors from operations on responses. In code, usage should look like the following:

    http::client client;
    try {
      http::client::response response = client.get("http://www.example.com/");
      std::cout << body(response);
    } catch (std::exception& e) {
      // deal with exceptions here
    }

A common mistake is to declare the client inside the try block which invokes undefined behavior when errors arise from the handling of response objects.

## Member Functions

In this section we assume that the following typedef is in effect:

    typedef boost::network::http::basic_client<
        boost::network::http::tags::http_default_8bit_udp_resolve
        , 1
        ,1
        >
        client;

Also, that code using the HTTP client will have use the following header:

    #include <boost/network/include/http/client.hpp>

Note

Starting version 0.9, cpp-netlib clients and server implementations by default now have an externally-linked component. This is a breaking change for code that used to rely on cpp-netlib being a header-only library, but can inhibited by defining the `BOOST_NETWORK_NO_LIB` preprocessor macro before including any cpp-netlib header.

Note

Starting version 0.11, cpp-netlib clients and server implementations no longer support the `BOOST_NETWORK_NO_LIB` option.

### Constructors

The client implementation can be default constructed, or customized at initialization.

`client()`
  Default constructor.
`explicit client(client::options const &)`
  Constructor taking a `client_options<Tag>` object. The following table shows the options you can set on a `client_options<Tag>` instance.

Parameter Name | Type | Description
--- | --- | ---
follow_redirects | `bool` | Boolean to specify whether the client should follow HTTP redirects. Default is `false`.
cache_resolved | `bool` | Boolean to specify whether the client should cache resolved endpoints. The default is `false`.
io_service | `shared_ptr<io_service>` | Shared pointer to a Boost.Asio `io_service`.
openssl_certificate | `string` | The filename of the certificate to load for the SSL connection for verification.
openssl_verify_path | `string` | The directory from which the certificate authority files are located.
always_verify_peer | `bool` | Boolean to specify whether the client should always verify peers in SSL connections

To use the above supported named parameters, you’ll have code that looks like the following:

    using namespace boost::network::http; // parameters are in this namespace
    client::options options;
    options.follow_redirects(true)
           .cache_resolved(true)
           .io_service(boost::make_shared<boost::asio::io_service>())
           .openssl_certificate("/tmp/my-cert")
           .openssl_verify_path("/tmp/ca-certs");
    client client_(options);
    // use client_ as normal from here on out.

### HTTP Methods

The client implementation supports various HTTP methods. The following constructs assume that a client has been properly constructed named `client_` and that there is an appropriately constructed request object named `request_` and that there is an appropriately constructed response object named `response_` like the following:

    using namespace boost::network::http;  // parameters are here
    client client_();
    client::request request_("http://cpp-netib.github.com/");
    client::response response_;

`response_ = client_.get(request_)`
  Perform an HTTP GET request.
`response_ = client_.get(request_, callback)`
  Perform an HTTP GET request, and have the body chunks be handled by the `callback` parameter. The signature of `callback` should be the following: `void(iterator_range<char const *> const &, boost::system::error_code const &)`.
`response_ = client_.head(request_)`
  Perform an HTTP HEAD request.
`response_ = client_.post(request_)`
  Perform an HTTP POST, use the data already set in the request object which includes the headers, and the body.
`response_ = client_.post(request_, callback)`
  Perform an HTTP POST request, and have the body chunks be handled by the `callback` parameter. The signature of `callback` should be the following: `void(iterator_range<char const *> const &, boost::system::error_code const &)`.
`response_ = client_.post(request_, body)`
  Body is a string of type `boost::network::string<Tag>::type` where `Tag` is the HTTP Client’s `Tag`. The default content-type used is `x-application/octet-stream`.
`response_ = client_.post(request_, body, callback)`
  Body is a string of type `boost::network::string<Tag>::type` where `Tag` is the HTTP Client’s `Tag`. The default content-type used is `x-application/octet-stream`. Have the response body chunks be handled by the `callback` parameter. The signature of `callback` should be the following: `void(iterator_range<char const *> const &, boost::system::error_code const &)`.
`response_ = client_.post(request_, body, content_type)`
  The body and content_type parameters are of type `boost::network::string<Tag>::type` where `Tag` is the HTTP Client’s `Tag`. This uses the request object’s other headers.
`response_ = client_.post(request_, body, content_type, callback)`
  The body and content_type parameters are of type `boost::network::string<Tag>::type` where `Tag` is the HTTP Client’s `Tag`. This uses the request object’s other headers. Have the response body chunks be handled by the `callback` parameter. The signature of `callback` should be the following: `void(iterator_range<char const *> const &, boost::system::error_code const &)`.
`response_ = client_.post(request_, streaming_callback)`
  Perform and HTTP POST request, and have the request’s body chunks be generated by the `streaming_callback` which has a signature of the form: `bool(string_type&)`. The provided `string_type&` will be streamed as soon as the function returns. A return value of `false` signals the client that the most recent invocation is the last chunk to be sent.
`response_ = client_.post(request_, callback, streaming_callback)`
  Perform an HTTP POST request, and have the body chunks be handled by the `callback` parameter. The signature of `callback` should be the following: `void(iterator_range<char const *> const &, boost::system::error_code const &)`. This form also has the request’s body chunks be generated by the `streaming_callback` which has a signature of the form: `bool(string_type&)`. The provided `string_type&` will be streamed as soon as the function returns. A return value of `false` signals the client that the most recent invocation is the last chunk to be sent.
`response_ = client_.put(request_)`
  Perform an HTTP PUT, use the data already set in the request object which includes the headers, and the body.
`response_ = client_.put(request_, callback)`
  Perform an HTTP PUT request, and have the body chunks be handled by the `callback` parameter. The signature of `callback` should be the following: `void(iterator_range<char const *> const &, boost::system::error_code const &)`.
`response_ = client_.put(request_, body)`
  Body is a string of type `boost::network::string<Tag>::type` where `Tag` is the HTTP Client’s `Tag`. The default content-type used is `x-application/octet-stream`.
`response_ = client_.put(request_, body, callback)`
  Body is a string of type `boost::network::string<Tag>::type` where `Tag` is the HTTP Client’s `Tag`. The default content-type used is `x-application/octet-stream`. Have the response body chunks be handled by the `callback` parameter. The signature of `callback` should be the following: `void(iterator_range<char const *> const &, boost::system::error_code const &)`.
`response_ = client_.put(request_, body, content_type)`
  The body and content_type parameters are of type `boost::network::string<Tag>::type` where `Tag` is the HTTP Client’s `Tag`. This uses the request object’s other headers.
`response_ = client_.put(request_, body, content_type, body_handler=callback)`
  The body and content_type parameters are of type `boost::network::string<Tag>::type` where `Tag` is the HTTP Client’s `Tag`. This uses the request object’s other headers. Have the response body chunks be handled by the `callback` parameter. The signature of `callback` should be the following: `void(iterator_range<char const *> const &, boost::system::error_code const &)`.
`response_ = client_.put(request_, streaming_callback)`
  Perform and HTTP PUT request, and have the request’s body chunks be generated by the `streaming_callback` which has a signature of the form: `bool(string_type&)`. The provided `string_type&` will be streamed as soon as the function returns. A return value of `false` signals the client that the most recent invocation is the last chunk to be sent.
`response_ = client_.put(request_, callback, streaming_callback)`
  Perform an HTTP PUT request, and have the body chunks be handled by the `callback` parameter. The signature of `callback` should be the following: `void(iterator_range<char const *> const &, boost::system::error_code const &)`. This form also has the request’s body chunks be generated by the `streaming_callback` which has a signature of the form: `bool(string_type&)`. The provided `string_type&` will be streamed as soon as the function returns. A return value of `false` signals the client that the most recent invocation is the last chunk to be sent.
`response_ = client_.delete_(request_)`
  Perform an HTTP DELETE request.
`response_ = client_.delete_(request_, body_handler=callback)`
  Perform an HTTP DELETE request, and have the response body chunks be handled by the `callback` parameter. The signature of `callback` should be the following: `void(iterator_range<char const *> const &, boost::system::error_code const &)`.

### Client-Specific

`client_.clear_resolved_cache()`
  Clear the cache of resolved endpoints.

### Streaming Body Handler

As of v0.9.1 the library now offers a way to support a streaming body callback function in all HTTP requests that expect a body part (GET, PUT, POST, DELETE). A convenience macro is also provided to make callback handlers easier to write. This macro is called `BOOST_NETWORK_HTTP_BODY_CALLBACK` which allows users to write the following code to easily create functions or function objects that are compatible with the callback function requirements.

An example of how to use the macro is shown below:

    struct body_handler {
        explicit body_handler(std::string & body)
        : body(body) {}

        BOOST_NETWORK_HTTP_BODY_CALLBACK(operator(), range, error) {
            // in here, range is the Boost.Range iterator_range, and error is
            // the Boost.System error code.
            if (!error)
                body.append(boost::begin(range), boost::end(range));
        }

        std::string & body;
    };

    // somewhere else
    std::string some_string;
    response_ = client_.get(request("http://cpp-netlib.github.com/"),
                            body_handler(some_string));

You can also use if for standalone functions instead if you don’t want or need to create a function object.

    BOOST_NETWORK_HTTP_BODY_CALLBACK(print_body, range, error) {
        if (!error)
            std::cout << "Received " << boost::distance(range) << "bytes."
                      << std::endl;
        else
            std::cout << "Error: " << error << std::endl;
    }

    // somewhere else
    response_ = client_.get(request("http://cpp-netlib.github.com/"),
                            print_body);

The `BOOST_NETWORK_HTTP_BODY_CALLBACK` macro is defined in `boost/network/protocol/http/client/macros.hpp`.

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