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<chapter id="statemgmt">
    <title>HTTP state management</title>
    <para>Originally HTTP was designed as a stateless, request / response oriented protocol that
        made no special provisions for stateful sessions spanning across several logically related
        request / response exchanges. As HTTP protocol grew in popularity and adoption more and more
        systems began to use it for applications it was never intended for, for instance as a
        transport for e-commerce applications. Thus, the support for state management became a
        necessity.</para>
    <para>Netscape Communications, at that time a leading developer of web client and server
        software, implemented support for HTTP state management in their products based on a
        proprietary specification. Later, Netscape tried to standardise the mechanism by publishing
        a specification draft. Those efforts contributed to the formal specification defined through
        the RFC standard track. However, state management in a significant number of applications is
        still largely based on the Netscape draft and is incompatible with the official
        specification. All major developers of web browsers felt compelled to retain compatibility
        with those applications greatly contributing to the fragmentation of standards
        compliance.</para>
    <section>
        <title>HTTP cookies</title>
        <para>An HTTP cookie is a token or short packet of state information that the HTTP agent and the
            target server can exchange to maintain a session. Netscape engineers used to refer to it
            as a "magic cookie" and the name stuck.</para>
        <para>HttpClient uses the <interfacename>Cookie</interfacename> interface to represent an
            abstract cookie token. In its simplest form an HTTP cookie is merely a name / value pair.
            Usually an HTTP cookie also contains a number of attributes such as version, a domain
            for which is valid, a path that specifies the subset of URLs on the origin server to
            which this cookie applies, and the maximum period of time for which the cookie is valid.</para>
        <para>The <interfacename>SetCookie</interfacename> interface represents a
                <literal>Set-Cookie</literal> response header sent by the origin server to the HTTP
            agent in order to maintain a conversational state.
            The <interfacename>SetCookie2</interfacename> interface extends SetCookie with
                <literal>Set-Cookie2</literal> specific methods.</para>
        <para>The <interfacename>ClientCookie</interfacename> interface extends
                <interfacename>Cookie</interfacename> interface with additional client specific
            functionality such as the ability to retrieve original cookie attributes exactly as they were
            specified by the origin server. This is important for generating the
                <literal>Cookie</literal> header because some cookie specifications require that the
                <literal>Cookie</literal> header should include certain attributes only if they were
            specified in the <literal>Set-Cookie</literal> or <literal>Set-Cookie2</literal>
            header.</para>
        <section>
            <title>Cookie versions</title>
            <para>Cookies compatible with Netscape draft specification but non-compliant with the
                official specification are considered to be of version 0. Standard compliant cookies
                are expected to have version 1. HttpClient may handle cookies differently depending
                on the version.</para>
            <para>Here is an example of re-creating a Netscape cookie:</para>
            <programlisting><![CDATA[
BasicClientCookie netscapeCookie = new BasicClientCookie("name", "value");
netscapeCookie.setVersion(0);
netscapeCookie.setDomain(".mycompany.com");
netscapeCookie.setPath("/");
]]></programlisting>
            <para>Here is an example of re-creating a standard cookie. Please note that standard
                compliant cookie must retain all attributes as sent by the origin server:</para>
            <programlisting><![CDATA[
BasicClientCookie stdCookie = new BasicClientCookie("name", "value");
stdCookie.setVersion(1);
stdCookie.setDomain(".mycompany.com");
stdCookie.setPath("/");
stdCookie.setSecure(true);
// Set attributes EXACTLY as sent by the server 
stdCookie.setAttribute(ClientCookie.VERSION_ATTR, "1");
stdCookie.setAttribute(ClientCookie.DOMAIN_ATTR, ".mycompany.com");
]]></programlisting>
            <para>Here is an example of re-creating a <literal>Set-Cookie2</literal> compliant
                cookie. Please note that standard compliant cookie must retain all attributes as
                sent by the origin server:</para>
            <programlisting><![CDATA[
BasicClientCookie2 stdCookie = new BasicClientCookie2("name", "value");
stdCookie.setVersion(1);
stdCookie.setDomain(".mycompany.com");
stdCookie.setPorts(new int[] {80,8080});
stdCookie.setPath("/");
stdCookie.setSecure(true);
// Set attributes EXACTLY as sent by the server 
stdCookie.setAttribute(ClientCookie.VERSION_ATTR, "1");
stdCookie.setAttribute(ClientCookie.DOMAIN_ATTR, ".mycompany.com");
stdCookie.setAttribute(ClientCookie.PORT_ATTR, "80,8080");
]]></programlisting>
        </section>
    </section>
    <section>
        <title>Cookie specifications</title>
        <para>The <interfacename>CookieSpec</interfacename> interface represents a cookie management
            specification. The cookie management specification is expected to enforce:</para>
        <itemizedlist>
            <listitem>
                <para>rules of parsing <literal>Set-Cookie</literal> and optionally
                        <literal>Set-Cookie2</literal> headers.</para>
            </listitem>
            <listitem>
                <para>rules of validation of parsed cookies.</para>
            </listitem>
            <listitem>
                <para>formatting of <literal>Cookie</literal> header for a given host, port and path
                    of origin.</para>
            </listitem>
        </itemizedlist>
        <para>HttpClient ships with several <interfacename>CookieSpec</interfacename>
            implementations:</para>
        <itemizedlist>
            <listitem>
                <formalpara>
                    <title>Netscape draft:</title>
                    <para>This specification conforms to the original draft specification published
                        by Netscape Communications. It should be avoided unless absolutely necessary
                        for compatibility with legacy code.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>Standard:</title>
                    <para>RFC 2965 HTTP state management specification.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>Browser compatibility:</title>
                    <para>This implementation strives to closely mimic the (mis)behavior of common web
                        browser applications such as Microsoft Internet Explorer and Mozilla
                        FireFox.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>Best match:</title>
                    <para>'Meta' cookie specification that picks up a cookie policy based on the
                        format of cookies sent with the HTTP response. It basically aggregates all
                        above implementations into one class.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <title>Ignore cookies:</title>
                    <para>All cookies are ignored.</para>
                </formalpara>
            </listitem>
        </itemizedlist>
        <para>It is strongly recommended to use the <literal>Best Match</literal> policy and let
            HttpClient pick up an appropriate compliance level at runtime based on the execution
            context.</para>
    </section>
    <section>
        <title>Choosing cookie policy</title>
        <para>Cookie policy can be set at the HTTP client and overridden on the HTTP request level
            if required.</para>
        <programlisting><![CDATA[
RequestConfig globalConfig = RequestConfig.custom()
        .setCookieSpec(CookieSpecs.BEST_MATCH)
        .build();
CloseableHttpClient httpclient = HttpClients.custom()
        .setDefaultRequestConfig(globalConfig)
        .build();
RequestConfig localConfig = RequestConfig.copy(globalConfig)
        .setCookieSpec(CookieSpecs.BROWSER_COMPATIBILITY)
        .build();
HttpGet httpGet = new HttpGet("/");
httpGet.setConfig(localConfig);
]]></programlisting>
    </section>
    <section>
        <title>Custom cookie policy</title>
        <para>In order to implement a custom cookie policy one should create a custom implementation
            of the <interfacename>CookieSpec</interfacename> interface, create a
                <interfacename>CookieSpecProvider</interfacename> implementation to create and
            initialize instances of the custom specification and register the factory with
            HttpClient. Once the custom specification has been registered, it can be activated the
            same way as a standard cookie specification.</para>
        <programlisting><![CDATA[
CookieSpecProvider easySpecProvider = new CookieSpecProvider() {

    public CookieSpec create(HttpContext context) {

        return new BrowserCompatSpec() {
            @Override
            public void validate(Cookie cookie, CookieOrigin origin)
                    throws MalformedCookieException {
                // Oh, I am easy
            }
        };
    }

};
Registry<CookieSpecProvider> r = RegistryBuilder.<CookieSpecProvider>create()
        .register(CookieSpecs.BEST_MATCH,
            new BestMatchSpecFactory())
        .register(CookieSpecs.BROWSER_COMPATIBILITY,
            new BrowserCompatSpecFactory())
        .register("easy", easySpecProvider)
        .build();

RequestConfig requestConfig = RequestConfig.custom()
        .setCookieSpec("easy")
        .build();

CloseableHttpClient httpclient = HttpClients.custom()
        .setDefaultCookieSpecRegistry(r)
        .setDefaultRequestConfig(requestConfig)
        .build();
]]></programlisting>
    </section>
    <section>
        <title>Cookie persistence</title>
        <para>HttpClient can work with any physical representation of a persistent cookie store that
            implements the <interfacename>CookieStore</interfacename> interface. The default
                <interfacename>CookieStore</interfacename> implementation called
                <classname>BasicCookieStore</classname> is a simple implementation backed by a
                <classname>java.util.ArrayList</classname>. Cookies stored in an
                <classname>BasicClientCookie</classname> object are lost when the container object
            get garbage collected. Users can provide more complex implementations if
            necessary.</para>
        <programlisting><![CDATA[
// Create a local instance of cookie store
CookieStore cookieStore = new BasicCookieStore();
// Populate cookies if needed
BasicClientCookie cookie = new BasicClientCookie("name", "value");
cookie.setVersion(0);
cookie.setDomain(".mycompany.com");
cookie.setPath("/");
cookieStore.addCookie(cookie);
// Set the store
CloseableHttpClient httpclient = HttpClients.custom()
        .setDefaultCookieStore(cookieStore)
        .build();
]]></programlisting>
    </section>
    <section>
        <title>HTTP state management and execution context</title>
        <para>In the course of HTTP request execution HttpClient adds the following state management
            related objects to the execution context:</para>
        <itemizedlist>
            <listitem>
                <formalpara>
                    <para><interfacename>Lookup</interfacename> instance representing the actual
                        cookie specification registry. The value of this attribute set in the local
                        context takes precedence over the default one.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <para><interfacename>CookieSpec</interfacename> instance representing the actual
                        cookie specification.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <para><classname>CookieOrigin</classname> instance representing the actual
                        details of the origin server.</para>
                </formalpara>
            </listitem>
            <listitem>
                <formalpara>
                    <para><interfacename>CookieStore</interfacename> instance representing the actual
                        cookie store. The value of this attribute set in the local context takes
                        precedence over the default one.</para>
                </formalpara>
            </listitem>
        </itemizedlist>
        <para>The local <interfacename>HttpContext</interfacename> object can be used to customize
            the HTTP state management context prior to request execution, or to examine its state after
            the request has been executed. One can also use separate execution contexts in order
            to implement per user (or per thread) state management. A cookie specification registry
            and cookie store defined in the local context will take precedence over the default
            ones set at the HTTP client level</para>
        <programlisting><![CDATA[
CloseableHttpClient httpclient = <...>

Lookup<CookieSpecProvider> cookieSpecReg = <...>
CookieStore cookieStore = <...>

HttpClientContext context = HttpClientContext.create();
context.setCookieSpecRegistry(cookieSpecReg);
context.setCookieStore(cookieStore);
HttpGet httpget = new HttpGet("http://somehost/");
CloseableHttpResponse response1 = httpclient.execute(httpget, context);
<...>
// Cookie origin details
CookieOrigin cookieOrigin = context.getCookieOrigin();
// Cookie spec used
CookieSpec cookieSpec = context.getCookieSpec();
]]></programlisting>
    </section>
</chapter>