docs: add "How to deal with cookies" in FAQ

Author: cedev935

src/site/markdown/faq.md

@@ -0,0 +1,123 @@
+# Frequently asked questions
+
+<!-- MACRO{toc} -->
+
+## How to deal with cookies
+
+In order to store the cookies sent by the server and include them in all subsequent requests, you need to create an OkHttpClient with a custom [cookie jar](https://square.github.io/okhttp/4.x/okhttp/okhttp3/-cookie-jar/).
+
+You can either implement your own cookie jar:
+
+```java
+public class MyApp {
+
+    public static void main(String[] argz) throws Exception {
+        IO.Options options = new IO.Options();
+
+        OkHttpClient okHttpClient = new OkHttpClient.Builder()
+                .cookieJar(new MyCookieJar())
+                .build();
+
+        options.callFactory = okHttpClient;
+        options.webSocketFactory = okHttpClient;
+
+        Socket socket = IO.socket(URI.create("https://example.com"), options);
+
+        socket.connect();
+    }
+
+    private static class MyCookieJar implements CookieJar {
+        private Set<WrappedCookie> cache = new HashSet<>();
+
+        @Override
+        public void saveFromResponse(HttpUrl url, List<Cookie> cookies) {
+            for (Cookie cookie : cookies) {
+                this.cache.add(new WrappedCookie(cookie));
+            }
+        }
+
+        @Override
+        public List<Cookie> loadForRequest(HttpUrl url) {
+            List<Cookie> cookies = new ArrayList<>();
+            Iterator<WrappedCookie> iterator = this.cache.iterator();
+            while (iterator.hasNext()) {
+                Cookie cookie = iterator.next().cookie;
+                if (isCookieExpired(cookie)) {
+                    iterator.remove();
+                } else if (cookie.matches(url)) {
+                    cookies.add(cookie);
+                }
+            }
+            return cookies;
+        }
+
+        private static boolean isCookieExpired(Cookie cookie) {
+            return cookie.expiresAt() < System.currentTimeMillis();
+        }
+    }
+
+    private static class WrappedCookie {
+        private final Cookie cookie;
+
+        public WrappedCookie(Cookie cookie) {
+            this.cookie = cookie;
+        }
+
+        @Override
+        public boolean equals(Object o) {
+            if (!(o instanceof WrappedCookie)) return false;
+            WrappedCookie that = (WrappedCookie) o;
+            return that.cookie.name().equals(this.cookie.name())
+                    && that.cookie.domain().equals(this.cookie.domain())
+                    && that.cookie.path().equals(this.cookie.path())
+                    && that.cookie.secure() == this.cookie.secure()
+                    && that.cookie.hostOnly() == this.cookie.hostOnly();
+        }
+
+        @Override
+        public int hashCode() {
+            int hash = 17;
+            hash = 31 * hash + cookie.name().hashCode();
+            hash = 31 * hash + cookie.domain().hashCode();
+            hash = 31 * hash + cookie.path().hashCode();
+            hash = 31 * hash + (cookie.secure() ? 0 : 1);
+            hash = 31 * hash + (cookie.hostOnly() ? 0 : 1);
+            return hash;
+        }
+    }
+}
+```
+
+Or use a package like [PersistentCookieJar](https://github.com/franmontiel/PersistentCookieJar):
+
+```java
+public class MyApp {
+
+    public static void main(String[] argz) throws Exception {
+        IO.Options options = new IO.Options();
+
+        ClearableCookieJar cookieJar = new PersistentCookieJar(new SetCookieCache(), new SharedPrefsCookiePersistor(context));
+
+        OkHttpClient okHttpClient = new OkHttpClient.Builder()
+                .cookieJar(cookieJar)
+                .build();
+
+        options.callFactory = okHttpClient;
+        options.webSocketFactory = okHttpClient;
+
+        Socket socket = IO.socket(URI.create("https://example.com"), options);
+
+        socket.connect();
+    }
+}
+```
+
+## How to use with AWS Load Balancing
+
+When scaling to multiple Socket.IO servers, you must ensure that all the HTTP requests of a given session reach the same server (explanation [here](https://socket.io/docs/v4/using-multiple-nodes/#why-is-sticky-session-required)).
+
+Sticky sessions can be enabled on AWS Application Load Balancers, which works by sending a cookie (`AWSALB`) to the client.
+
+Please see [above](#how-to-deal-with-cookies) for how to deal with cookies.
+
+Reference: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/sticky-sessions.html

src/site/site.xml

@@ -30,6 +30,7 @@
             <item name="Listening to events" href="./listening_to_events.html"/>
             <item name="The Socket instance" href="./socket_instance.html"/>
             <item name="Migrating from 1.x" href="./migrating_from_1_x.html"/>
+            <item name="FAQ" href="./faq.html"/>
         </menu>
 
         <menu name="Miscellaneous">