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Hello, wiki

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Hello, wiki

Wikis, and other form of user-editable pages, are components commonly encountered on the web. In itself, developing a simple wiki is error-prone but otherwise not very difficult. However, developing a rich wiki that both scales up and does not suffer from vulnerabilities caused by the content provided by users is quite harder. Once again, Opa makes it simple.

In this chapter, we will see how to program a complete, albeit simple, wiki application in Opa. Along the way, we will introduce the Opa database, the client-server security policy, the mechanism used to incorporate user-defined pages without breaking security, as well as more user interface manipulation.


Let us start with a picture of the wiki application we will develop in this chapter:

Final version of the Hello wiki application

This web application stores pages and lets users edit them, using Markdown syntax, which is a popular markup language that supports headings, links, lists, images etc.

If you are curious, here is the full source code of the application. You can view the running application at or fork it.

In this listing, we define a database for storing the content of the pages in the Markdown syntax format, we define the user interface and finally, the main application. In the rest of the chapter, we will walk you through all the concepts and constructions introduced.

As for the chat, we use Bootstrap CSS from Twitter with a single import line.

Setting up storage

A wiki is all about modifying, storing and displaying pages. This means that we need to set up some storage to receive these pages, as follows:

database stringmap(string) /wiki

This defines a database path, i.e. a place where we can store information, and specifies the type of information that will be stored there. Here, we use type stringmap(string). This is a stringmap (i.e. an association from string keys to values) containing values of type string (used to denote Markdown code).

TIP: About database paths

In Opa, the database is composed of paths. A path is a named slot where you can store exactly one value at a time. Of course, this value can be a list, or a map, for instance, both of which act as containers for several values.

As everything else in Opa, paths are typed, with the type of the value that can be stored. Paths can be defined for any Opa type, except types containing functions or real-time web constructions such as networks. To guard against subtle incompatibilities between successive versions of an application, the type must be given during the definition of the path.

TIP: HTML VS markup

On the web, letting users directly write HTML code that can be seen at a later stage by other users is not a good idea, as it opens the way for numerous forms of attacks, sometimes quite subtle and hard to detect.

Opa offers at least two ways to circumvent this problem. Firstly, there is a templating mechanism (see package stdlib.web.template), which is an XML-based markup that covers a safe subset of HTML. Moreover this templating mechanism is designed for full extensibility -- indeed, our website, including our own on-line editor, is developed using templating.

Another, more lightweight approach, and one that we are using here, is to use Markdown (package, which is a very popular markup language that converts easy to read and write plain text format into structurally valid and completely safe XHTML (and supports headings, links, images, code snippets etc.).

It is generally a good idea to associate a default value to each path, as this makes manipulation of data easier:

database /wiki[_] = "This page is empty. Double-click to edit."

The square brackets [_] are a convention to specify that we are talking about the contents of a map and, more precisely, providing a default value. Here, the default value is "This page is empty. Double-click to edit.", i.e. a simple text in Markdown syntax (here it's just plain text).

With these two lines, the database is set. Any data written to the database will be kept persistent. Should you stop and restart your application, the data will be checked and made accessible to your application as it was at the point you stopped it.

Loading, parsing and writing

Reading data from or writing data to the database is essentially transparent. For clarity and performance, we may start by defining two loading functions. One, load_source, will load some content from the database and present it as source code that the user can edit, while a second one, load_rendered will load the same content and present it as xhtml that can be displayed immediately:

function load_source(topic) {

function load_rendered(topic) {
  source = load_source(topic)
  Markdown.xhtml_of_string(Markdown.default_options, source)

In this extract, topic is the topic we wish to display or edit -- i.e. the name of the page. The content associated with topic can be found in the database at path /wiki[topic]. Once we have this content, depending on our needs, we just return it as a string, or convert to xhtml data structure for display, using the Markdown.xhtml_of_string function, which parses the string and builds its corresponding xhtml representation, with respect to the Markdown syntax.

Saving data is equally simple:

function save_source(topic, source) {
    /wiki[topic] <- source;

This function takes two arguments: a topic, with the same meaning as above, and a source, i.e. a string, which is a representation of the page with Markdown syntax. The instruction after do writes source to the database at path /wiki[topic].

User interface

As previously, we define a function to produce the user interface:

function display(topic) {
   xhtml =
     <div class="navbar navbar-fixed-top"><div class="navbar-inner"><div class="container"><div id=#logo></div></div></div></div>
     <div class="content container">
       <div class="page-header"><h1>About {topic}</h1></div>
       <div class="well" id=#show_content ondblclick={function(_) { edit(topic) }}>{load_rendered(topic)}</div>
       <textarea rows="30" id=#edit_content onblur={function(_) { save(topic) }}></textarea>
    Resource.styled_page("About {topic}", ["/resources/css.css"], xhtml);

This time, instead of producing a xhtml result, we have embedded this result in a resource, i.e. a representation for anything that the server can send to the client, whether it is a page, an image, or anything else. In practice, most applications produce a set of resources, as this is more powerful and more flexible than plain xhtml. Of course that will require different variant of Server.start, which we will see later.

A number of functions can be used to construct a resource. Here, we use Resource.styled_page, a function which constructs a web page, from its title (first argument), a list of stylesheets (second argument) and xhtml content. At this stage, the xhtml content should not surprise you. We use <div> to display the contents of the page, or <textarea> (initially hidden) to modify them. When a user double-clicks on the content of the page (event dblclick), it triggers function edit, and when the user stops editing the source (event blur), it triggers function save.

Function edit is defined as follows:

function edit(topic) {
    Dom.set_value(#edit_content, load_source(topic));

This function loads the source code associated with topic, sets the content of #edit_content, replaces the <div> with the <textarea>, gives focus to the <textarea> (purely for comfort) and returns void.

Similarly, function save is defined as follows, and shouldn't surprise you:

function save(topic) {
    content = save_source(topic, Dom.get_value(#edit_content));
    #show_content = content;

With these three functions, the user interface is ready. It is now time to work on the server.

Serving the pages

For this application we will use a new variant of Server.start. Before we were using a construction for a single-page server; in practice most web appliactions have multiple pages. For that we can use Server.start(Server.http, {dispatch: dispatch_fun}), where dispatch_fun is a function that takes an Uri.relative and produces a resource.

Let's first construct such a function:

function start(url) {
  match (url) {
    case {path: {nil} ... } :
      { display("Hello") };
    case {path: path ...} :
      { display(String.concat("::", path)) };

This is another pattern-matching, built from some constructions that you have not seen yet. Pattern {path: [] ...} accepts requests for uris with empty path, such as "http://localhost:8080/". This is because ... accepts any number of fields in a record. In other words, the first case of our pattern-matching accepts any record containing at least one field named path, provided that this field contains exactly the empty list.

The second pattern accepts any record containing at least one field named path. From the definition of pattern-matching, it is executed only if the first pattern did not match, for instance on a request to "http://localhost:8080/hello".

In both cases, we execute function display. The first case is trivial, while in the second case, we first convert our list to a string, with separator "::".

Actually, we will make it a tad nicer by also ensuring that the first letter is uppercase, while the other letters are lowercase.

function start(url) {
  match (url) {
    case {path:[] ... } :
      { display("Hello") };
    case {~path ...} :
      { display(String.capitalize(String.to_lower(String.concat("::", path)))) };

In this new version, we use a shorter syntax for pattern-matching. We use [] for the empty list -- this is equivalent to {nil} -- and we write ~path -- which is equivalent to path = path.

Adding some style

As in the previous chapter, without style, this example looks somewhat bland.

As previously, we will fix this with an external stylesheet resources/css.css, with its content viewable here.

One last step: including the stylesheet. For this purpose, we need to extend our server to also include resources. We do it as follows:

   /** Statically embed a bundle of resources */
  [ {resources: @static_include_directory("resources")}
   /** Launch the [start] dispatcher */
  , {dispatch: start}

We provide Server.start here with a list of servers. Note that their order is important, as to serve a request they will be tried in that order. So we first put a resource bundle, which only handles requests to the defined resources and then we pass dispatching to our start function.

With this final line, we have a complete, working wiki. With a few additional images, we obtain:

Final version of the Hello wiki application

The complete application | Fork

This is a total of 30 effective lines of code + CSS.


What about user security?

As mentioned, one of the difficulties when developing a rich wiki is ensuring that it has no security vulnerabilities. Indeed, as soon as a user may edit content that will be displayed on another user's browser, the risk exists of letting one user hide some JavaScript code (or possibly Flash or Java code) that will be executed by another user. This is a well-known technique for stealing identities.

You may attempt to reproduce this with the wiki, the chat, or any other Opa application. This will fail. Indeed, while lower-level web technologies make no difference between JavaScript code, text, or structured data, Opa does, and ensures that data that has been provided as one can never be interpreted as the other one.

CAUTION: Careful with the <script>

There is, actually, one exception to this otherwise bullet-proof guarantee: if a developer manually introduces <script> tag containing a insert, as follows, the possibility exists that a malicious user could take advantage of this to inject arbitrary code.

The bottom line is therefore: do not introduce <script> tag containing an insert. This is the only case that, at the time of this writing, Opa cannot check.

What about database security?

Now that we have a database, it is high time to think about what can be modified and under what circumstances. By default, Opa takes a conservative approach and ensures that malicious clients have access to as few entry points as possible -- we call this publishing a function. By default, the only published functions are functions that users could trigger themselves by manipulation of the user interface, i.e. event handlers.

TIP: Published entry points

An entry point is a function that exists on the server but that could possibly be triggered by a user, possibly malicious.

Typically, every application contains at least one entry point, introduced by server -- in the wiki, this is function start. Most applications also feature a manner for the client to send information to the server and trigger some treatment upon this information.

More generally, any function can be used as an entry point if it is published. By default, Opa will automatically publish event handlers.

Here, this means functions edit and save. In our listing, no other functions are published. Consequently, the Markdown syntax analysis is invoked only on the server.

What about client-server performance?

At this stage, keeping the code in mind, you may start to wonder about performance and in particular about the number of requests involved in editing or saving a page. This is a very good point. Indeed, if your browser offers performance/request tracing tools, you will realize that calls to edit or save are quite costly.

This issue can be solved quite easily, but let us first detail the cost of saving:

  1. The client sends a save request to the server (1 request).
  2. The server prompts the client for the value of edit_content (2 requests).
  3. The server instructs the client to hide show_content (2 requests).
  4. The server instructs the client to show edit_content (2 requests).

Surely, this is not the best that Opa can do? Indeed, Opa can do better. To solve this, we only need to provide a little additional information to the compiler, namely to let it know that load_source, load_rendered and save_source have been designed to handle anything that can be thrown at them, and thus do not need to be kept hidden.

For this purpose, Opa offers a special directive: exposed, which indicates that a given function shuld be exposed to the client.


Plase note that from a security standpoint marking a function as exposed means that a compromised client can call such functions with arbitrary forged arugments; and not only those that would follow from the semantics of the application.

To apply it to our three functions, we simply add it where needed:

exposed load_source(topic) { ... }
exposed load_rendered(topic) { ... }
exposed save_source(topic, source) { ... }

And we are done!

With this simple modification, saving now requires only one request. We will discuss exposed and the details of client-server slicing in a later chapter.

Once again, we may take a look at the complete application:

The complete application, made faster

Run | Fork


Time to put your new knowledge to the test.

Changing the default content

Customize the wiki so that the database contains not a string but a option(string), i.e. a value that can be either {none} or {some: x}, where x is a string.

Use this change to ensure that the default content of a page with topic topic is "We have no idea about topic. Could you please enter some information?".

Inform users of changes

Drawing inspiration from the chat, add an on-screen zone to inform users of changes that take place while they are connected.

Template chat

Modify your "Hello, Chat" application so that users can enter rich text, not just raw text.

Chat log

Modify your "Hello, Chat" application to add the following features:

  • store the conversation as it takes place;
  • when a new user connects, display the log of the conversation.

For this purpose, you will need to maintain a list of messages in the database.

TIP: About lists

In Opa, lists are one of the most common data structures. They are immutable linked lists.

Lists have type list. More precisely, a list of elements of type t has type list(t), pronounced "list of t". The empty list is written []

or, equivalently, {nil}. It has type list('a), which means that it could be a list of anything. A list containing elements x, y, z is written [x,y,z]

or, equivalently,

  {hd: x, tl:
    {hd: y, tl:
      {hd: z; tl:

More generally, the definition of list in Opa is:

type list('a) = {nil} or {'a hd, list('a) tl}

If you have a list l and you wish to construct a list starting with element x and continuing with l, you can write either


or, equivalently,

{hd: x, tl: l}

or, equivalently,

List.cons(x, l)

TIP: About loops

If you have a list l and wish to apply a function f to all elements if l, use function List.iter. This is one of the many loop functions of Opa.

Yes, in Opa, loops are just regular functions.

For bonus points, make sure that the log is displayed on a slightly different background color.

Multi-room chat

Now that you know how to create multi-page servers, you can implement a multi-room chat, with the following definition:

  • visiting a page with path p connects you to a chatroom p;
  • each message also contains the name of the chatroom;
  • for a client visiting path p, only display messages for chatroom p.

TIP: Minimizing communications

The best place for deciding whether to display a message is inside the callback added with Network.add_callback. Fine-tuning this callback can help you minimize the amount of server-to-client communications. For this purpose, you can use directive server, which lets you force a function to be executed only on the server.

TIP: Scaling up

For optimal scalability, a better design is required.

You will need to maintain a family of networks, one for each room, typically as a stringmap of networks. Networks are real-time data structures and can therefore not be stored in the database, as it is meaningless to store information that becomes inconsistent and unsafe whenever a client disconnects. Rather, the stringmap should be maintained as part of the state of a distributed session. We will introduce this mechanism of distributed sessions, which is the powerful primitive used to implement networks, in a few chapters.

And more

Improve the wiki and the chat. Add features, make them nicer, make them better! And, once again, do not forget to show your changes to the community!

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