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Hello, web services
Nowadays, numerous web applications offer their features as _web services_, APIs
that can be used by other web applications or native clients. This is how
Twitter, GitHub, Google Maps or countless others can be scripted by third-party
applications, using cleanly defined and easily accessible protocols.
With Opa, offering a web service is just as simple as creating any other form of
web application. In this chapter, instead of writing a new application, we will
extend our wiki to make it accessible through such a web API. This task will lead
us through REST web service design, command-line testing of Opa services, management
of URI queries and more.
Several protocols share the landscape of web services, in particular _REST_
(Representational State Transfer, a simple standard which does not specify how
messages should be formated, only how they should be exchanged), _SOAP_ (a more
complex standard imposing conventions on the formatting of messages) and _WSDL_
(a higher-level protocol). In this chapter, we'll only cover _REST_.
In this chapter, we will modify our wiki to make it accessible by a _REST_
API. This involves few changes from the original wiki, only the addition of a
few cases to differentiate between several kinds of requests that can be sent by
a client -- which does not need to be a browser anymore.
If you are curious, this is the full source code of the REST wiki server:
We will now walk through the concepts introduced in this listing.
Removing topics
In the rest of this chapter (pun intended), we will want to be able to delete a
topic previously added to the wiki. Adding this feature (without showing it in
the user interface) is just the matter of one line, as follows:
function remove_topic(topic) {
In this extract, we use function `Db.remove`, a function whose sole role is to
remove the contents of a database path. Notice the `@` before `/wiki[topic]`?
This symbol signifies that we are not working with the _value_ `/wiki[topic]` but
with the _path_ itself. If we had omitted this symbol, the Opa compiler would
have complained that `Db.remove` cannot work with a `string` -- which is
absolutely true.
Resting a little
A web service behaves much like a web application, without the client part. In other word, as any Opa
web application, it starts with a server declared with `Server.start`:
###### The server, with an entry point for rest
function topic_of_path(path) {
String.capitalize(String.to_lower(List.to_string_using("", "", "::", path)));
function start(url) {
match (url) {
case {path: [] ... }: display("Hello");
case {path: ["_rest_" | path] ...}: rest(topic_of_path(path));
case {~path ...}: display(topic_of_path(path));
[ {bundle: @static_include_directory("resources")}
, {dispatch: start}
In this version of `start`, we have slightly altered our pattern-matching to
handle the case of paths starting with `"_rest_"`. We decide that such paths
are actually entry points for REST-based requests and handle them as such. Here,
we delegate the management to function `rest`, which we write immediately:
As you may see, this function is also quite simple:
###### Handling rest requests
function rest(topic) {
match (HttpRequest.get_method()) {
case {some: method} :
match (method) {
case {post}:
_ = save_source(
match (HttpRequest.get_body()) {
case ~{some}: some;
case {none}: "";
case {delete}:
case {get}:
Resource.raw_response(load_source(topic), "text/plain", {success});
First, notice that `rest` is based on pattern-matching. Expect to meet pattern-matching
constantly in Opa. The first three patterns are built from some of to the distinct verbs
of the standard vocabulary of REST (these verbs are called _Http methods_):
* `{post}` is used to place information on a server, here to add some content to the wiki;
* `{delete}` is used to remove information from the server, here remove a topic from the wiki;
* `{get}` is used to get information from a server, here to download the source code of an entry.
From these verbs, we build the following patterns:
* `{some: {post}}`, i.e. the Http method is defined and is _post_;
* `{some: {delete}}`, i.e. the Http method is defined and is _delete_;
* `{some: {get}}`, i.e. the Http method is defined and is _get_;
* `_`, i.e. any other case, whether the Http method is not defined or whether it is a method that we do not
wish to handle.
Everything else in `rest` is simply function calls. You can find the definition of each function in the API
documentation, so we will just introduce quickly the functions you have not seen yet:
* Function `HttpRequest.get_method` has type `-> option(method)`. If the function is called from a request and this request has a method _m_, it produces `{some: m}`. Otherwise, it produces `{none}`.
* Similarly, `HttpRequest.get_body` has type `-> option(string)`. If the function is called from a request containing a body _b_, it produces `{some: b}`. Otherwise, it produces `{none}`.
* Function `Resource.raw_response` has type `string, string, status -> resource`. It produces a resource with a body from its body, its MIME type, and a status. This function is commonly used to reply to REST requests.
* Finally, function `Resource.raw_status` has type `status -> resource`. It produces an empty resource, and is generally used to return an error to a REST request.
As pattern-matching against an `option` is very common, Opa provides an operator `?` that can be used to make
the above extract shorter and more readable.
Expression `a?b` is equivalent to the following three lines:
match a with
| {none} -> b
| ~{some} -> some
With this expression, we may rewrite our extract as follows:
###### Handling rest requests (shorter variant)
function rest(topic) {
match (HttpServer.get_method()) {
case {some : {post}} :
_ = save_source(topic, HttpServer.get_body() ? "");
case {some : {delete}} :
case {some : {get}} :
Resource.raw_response(load_source(topic), "text/plain", {success});
default :
And with this, we are done! Our wiki can now be scripted by external web applications:
All in all, the changes required a dozen lines of code.
Exercises will show you how to introduce more complex forms of scripting.
Testing it
The simplest way of testing a REST API is to use a command-line tool that lets
you place requests directly, for instance `curl` or `wget`. Assuming
that `curl` is installed on your system, the following command-line will test
the result of placing a `{get}` request at address `_rest_/hello`:
curl localhost:8080/_rest_/hello
Execute this command-line and `curl` will show you the result of the call.
Similarly, the following command-line will test the result of placing a `{post}`
request at the same address:
curl localhost:8080/_rest_/hello -d "I've just POSTed to change the contents of my wiki"
Now, we are not here to learn about `curl`, but to learn about Opa. And what best way to test
the REST API of a wiki than by writing a web front-end that does not rely on its own database
but on that of the wiki we have just defined?
We will do just this in the next chapter.
### When is a method or a body not defined?
As mentioned, functions `HttpServer.get_method` and `HttpServer.get_body` can
produce result `{none}` if the http method (respectively the body) does not
This may be surprising, as, by definition of the protocols, every request has a
method (not all have a body). Indeed, the only case in which `HttpServer.get_method`
returns `{none}` is when there is _no request_, i.e. when the function has been
called by the server for its own use and not during the execution of a request
on behalf of a web browser or a distant web server.
On the other hand, many requests do not have a body. Function `HttpServer.get_body`
returns `{none}` when there is no body, or when there is no request, as above.
### Only one server?
If you have started thinking about large applications, at this stage, you might
start worrying about having to centralize all your path management into only one
pattern-matching, which could hurt modularity and hamper your work.
No need to worry, though, as we've already seen with Opa, you may combine any
number of servers in an application. Take a look at all the variants in
`Server.handler`, which is the type of the second argument to `Server.start`,
to see other ways of constructing servers.
### Rest for chat
Add a REST API to your chat, with the following feature:
* use a `{post}` request send a message for immediate display into the chat (for the moment, we will assume that the message has been written by author "ghost").
To deal with several entry points, you will have to rewrite your `server` and replace `one_page_bundle` by a dispatcher. For these
exercises, we decide that any request placed on path `_rest_` is a REST request.
For testing, use the following command-line (assuming that `curl` is installed on your system):
curl localhost:8080/_rest_ -d "Whispers..."
### Rest for chat, with logs
If you have not done so yet, update your chat to maintain conversation logs in the database.
Now, add the following REST API:
* use a `{get}` request to get the log of messages as `string` containing one message per line.
Remember, use function `List.to_string_using` to convert a list to a `string`.
### Rest for chat, with queries
For this exercise, we wish to extend the REST API for the chat to be able to
send a message _and_ give a name to the author of the message. For this purpose,
we need to send more informations than simply `{post}`. In the REST world, there
are typically two ways of passing additional informations: either in the URI
itself or in the body of the request. For this exercise, we will see the first
* if a `{post}` request is received on `_rest_` and if the _query_ of the request contains a pair `("author", x)`, use the value of `x` as the author name;
* otherwise, use name "ghost", as above.
### About queries
A _query_ is an element of a URI. From the user's perspective, queries look like
`?author=name&arg2=val2&arg3=val3`. From the developer's perspective, the query
is contained in field `query` of the URI, just as `path`. This field contains
a list of pairs with the name of the argument and its value. So, for the
previous query, the list will look like:
[("author", "name"), ("arg2", "val2"), ("arg3", "val3")]
Note that the order of these arguments is meaningless.
### About association lists
Lists of pairs containing a name and a value (or, more generally, a key and a
value) are generally called "association lists".
In Opa, the most common function to extract a value from an association list is
`List.assoc`. This function takes two arguments: the key to search and the list in which
to search. Its result is an `option` which may contain either `{none}` (if the key
does not appear in the list) or `{some: v}` (if the key appears in the list, associated
to value `v`).
### Rest for chat, with JSON
Another common technique used among REST services is to pass additional
information as part of the body of the request, often formated using the
JavaScript Object Notation language (or _JSON_). The objective of this exercise
is to use JSON instead of the URI to send the author name to the server.
* if a `{post}` request is received on `_rest_` and if the body of the request is a valid JSON construction containing a field `"author"`, use the value associated to this field as the author name;
* otherwise, use name "ghost", as above.
### JSON requests
To obtain the JSON body of a request, use function
### About JSON
JSON is a format of strings which can be interpreted as simple data structures.
In Opa, a string in JSON format can be transformed into a value with type
`RPC.Json.json` by using function
Json.deserialize: string -> option(RPC.Json.json)
Note that this function can return `{none}` if the `string` was incorrectly formated.
The opposite operation is implemented by function
Json.serialize: RPC.Json.json -> string
Type `RPC.Json.json` is defined as follows:
type RPC.Json.json =
{ int Int }
or { float Float }
or { string String }
or { bool Bool }
or { list(RPC.Json.json) List }
or { list((string, RPC.Json.json)) Record }
As above, case `Record` corresponds to a list of associations.
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