SWI-Prolog HTTP support
Jan Wielemaker
HCS,
University of Amsterdam
The Netherlands
E-mail: wielemak@science.uva.nl
Abstract
This article documents the package HTTP, a series of libraries for accessing data on HTTP servers as well as providing HTTP server capabilities from SWI-Prolog. Both server and client are modular libraries. The server can be operated from the Unix inetd super-daemon as well as as a stand-alone server that runs on all platforms supported by SWI-Prolog.

# Table of Contents

## 1 Introduction

The HTTP (HyperText Transfer Protocol) is the W3C standard protocol for transferring information between a web-client (browser) and a web-server. The protocol is a simple envelope protocol where standard name/value pairs in the header are used to split the stream into messages and communicate about the connection-status. Many languages have client and or server libraries to deal with the HTTP protocol, making it a suitable candidate for general purpose client-server applications. It is the basis of popular agent protocols such as SOAP and FIPA.

In this document we describe a modular infra-structure to access web-servers from SWI-Prolog and turn Prolog into a web-server. The server code is designed to allow the same `body' to be used from an interactive server for debugging or providing services from otherwise interactive applications, run the body from an inetd super-server or as a CGI script behind a generic web-server.

The design of this module is different from the competing XPCE-based HTTP server located in `library(http/httpd.pl)`, which intensively uses XPCE functionality to reach its goals. Using XPCE is not very suitable for CGI or inetd-driven servers due to required X11 connection and much larger footprint.

### Acknowledgements

This work has been carried out under the following projects: GARP, MIA, IBROW and KITS. The following people have pioneered parts of this library and contributed with bug-report and suggestions for improvements: Anjo Anjewierden, Bert Bredeweg, Wouter Jansweijer and Bob Wielinga.

## 2 The HTTP client libraries

This package provides two packages for building HTTP clients. The first, `library(http/http_open)` is a very lightweight library for opening a HTTP URL address as a Prolog stream. It can only deal with the HTTP GET protocol. The second, `library(http/http_client)` is a more advanced library dealing with keep-alive, chunked transfer and a plug-in mechanism providing conversions based on the MIME content-type.

### 2.1 The `library(http/http_open)` library

The library `library(http/http_open)` provides a very simple mechanism to read data from an HTTP server using the HTTP 1.0 protocol and HTTP GET access method. It defines one predicate:

http_open(+URL, -Stream, +Options)
  Open the data at the HTTP server as a Prolog stream. After this predicate succeeds the data can be read from Stream. After completion this stream must be closed using the built-in Prolog predicate close/1. Options provides additional options:
  timeout(+Timeout)
    If provided, set a timeout on the stream using set_stream/2. With this option if no new data arrives within Timeout seconds the stream raises an exception. Default is to wait forever (`infinite`).
  header(+Name, -AtomValue)
    If provided, AtomValue is unified with the value of the indicated field in the reply header. Name is matched case-insensitive and the underscore (`_`) matches the hyphen (`-`). Multiple of these options may be provided to extract multiple header fields. If the header is not available AtomValue is unified to the empty atom ('').
  size(-Size)
    If provided Size is unified with the value of the `Content-Length` fields of the reply-header.
  proxy(+Host, +Port)
    Use an HTTP proxy to connect to the outside world.
  user_agent(+Agent)
    Defines the value of the `User-Agent` field of the HTTP header. Default is `SWI-Prolog (http://www.swi-prolog.org)`.
  request_header(+Name = +Value)
    Additional name-value parts are added in the order of appearance to the HTTP request header. No interpretation is done.
  method(Method)
    One of `get` (default) or `head`. The `head` message can be used in combination with the `header(Name, Value)` option to access information on the resource without actually fetching the resource itself. The returned stream must be closed immediately.

  Here is a simple example:

      ?- http_open('http://www.swi-prolog.org/news.html', In, []),
         copy_stream_data(In, user_output),
         close(In).
      <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">

      <HTML>
      <HEAD>
      <TITLE>News</TITLE>
      </HEAD>
      ...

  The example below fetches the modification time of a web-page. Note that Modified is `''` if the web-server does not provide a time-stamp for the resource. See also parse_time/2.

      modified(URL, Stamp) :-
              http_open(URL, In,
                        [ method(head),
                          header(last_modified, Modified)
                        ]),
              close(In),
              Modified \== '',
              parse_time(Modified, Stamp).
      close(In).

### 2.2 The `library(http/http_client)` library

The `library(http/http_client)` library provides more powerful access to reading HTTP resources, providing keep-alive connections, chunked transfer and conversion of the content, such as breaking down multipart data, parsing HTML, etc. The library announces itself as providing `HTTP/1.1`.

http_get(+URL, -Reply, +Options)
  Performs a HTTP GET request on the given URL and then reads the reply using http_read_data/3. Defined options are:
  connection(ConnectionType)
    If `close` (default) a new connection is created for this request and closed after the request has completed. If `'Keep-Alive'` the library checks for an open connection on the requested host and port and re-uses this connection. The connection is left open if the other party confirms the keep-alive and closed otherwise.
  http_version(Major-Minor)
    Indicate the HTTP protocol version used for the connection. Default is `1.1`.
  proxy(+Host, +Port)
    Use an HTTP proxy to connect to the outside world.
  user_agent(+Agent)
    Defines the value of the `User-Agent` field of the HTTP header. Default is `SWI-Prolog (http://www.swi-prolog.org)`.
  request_header(Name = Value)
    Add a line "Name: Value" to the HTTP request header. Both name and value are added uninspected and literally to the request header. This may be used to specify accept encodings, languages, etc. Please check the RFC2616 (HTTP) document for available fields and their meaning.
  reply_header(Header)
    Unify Header with a list of Name=Value pairs expressing all header fields of the reply. See http_read_request/2 for the result format.

  Remaining options are passed to http_read_data/3.

http_post(+URL, +In, -Reply, +Options)
  Performs a HTTP POST request on the given URL. It is equivalent to http_get/3, except for providing an input document, which is posted using http_post_data/3.
http_read_data(+Header, -Data, +Options)
  Read data from an HTTP stream. Normally called from http_get/3 or http_post/4. When dealing with HTTP POST in a server this predicate can be used to retrieve the posted data. Header is the parsed header. Options is a list of `Name(Value)` pairs to guide the translation of the data. The following options are supported:
  to(Target)
    Do not try to interpret the data according to the MIME-type, but return it literally according to Target, which is one of:
    stream(Output)
      Append the data to the given stream, which must be a Prolog stream open for writing. This can be used to save the data in a (memory-)file, XPCE object, forward it to process using a pipe, etc.
    atom
      Return the result as an atom. Though SWI-Prolog has no limit on the size of atoms and provides atom-garbage collection, this options should be used with care.1Currently atom-garbage collection is activated after the creation of 10,000 atoms.
    codes
      Return the page as a list of character-codes. This is especially useful for parsing it using grammar rules.
  content_type(Type)
    Overrule the `Content-Type` as provided by the HTTP reply header. Intented as a work-around for badly configured servers.

  If no `to(Target)` option is provided the library tries the registered plug-in conversion filters. If none of these succeed it tries the built-in content-type handlers or returns the content as an atom. The builtin content filters are described below. The provided plug-ins are described in the following sections.

  application/x-www-form-urlencoded
    This is the default encoding mechanism for POST requests issued by a web-browser. It is broken down to a list of Name = Value terms.

  Finally, if all else fails the content is returned as an atom.

http_post_data(+Data, +Stream, +ExtraHeader)
  Write an HTTP POST request to Stream using data from Data and passing the additional extra headers from ExtraHeader. Data is one of:
  html(+HTMLTokens)
    Send an HTML token string as produced by the library `library(html_write)` described in section section 3.9.
  file(+File)
    Send the contents of File. The MIME type is derived from the filename extension using file_mime_type/2.
  file(+Type, +File)
    Send the contents of File using the provided MIME type, i.e. claiming the `Content-type` equals Type.
  cgi_stream(+Stream, +Len)
    Read the input from Stream which, like CGI data starts with a partial HTTP header. The fields of this header are merged with the provided ExtraHeader fields. The first Len characters of Stream are used.
  form(+ListOfParameter)
    Send data of the MIME type `application/x-www-form-urlencoded` as produced by browsers issuing a POST request from an HTML form. ListOfParameter is a list of Name=Value or Name(Value) .
  form_data(+ListOfData)
    Send data of the MIME type `multipart/form-data` as produced by browsers issuing a POST request from an HTML form using `enctype` `multipart/form-data`. This is a somewhat simplified MIME `multipart/mixed` encoding used by browser forms including file input fields. ListOfData is the same as for the List alternative described below. Below is an example from the SWI-Prolog Sesame interface. Repository, etc. are atoms providing the value, while the last argument provides a value from a file.

                ...,
                http_post([ protocol(http),
                            host(Host),
                            port(Port),
                            path(ActionPath)
                          ],
                          form_data([ repository = Repository,
                                      dataFormat = DataFormat,
                                      baseURI    = BaseURI,
                                      verifyData = Verify,
                                      data       = file(File)
                                    ]),
                          _Reply,
                          []),
                ...,

  List
    If the argument is a plain list, it is sent using the MIME type `multipart/mixed` and packed using mime_pack/3. See mime_pack/3 for details on the argument format.

#### 2.2.1 The MIME client plug-in

This plug-in library `library(http/http_mime_plugin)` breaks multipart documents that are recognised by the `Content-Type: multipart/form-data` or `Mime-Version: 1.0` in the header into a list of Name = Value pairs. This library deals with data from web-forms using the `multipart/form-data` encoding as well as the FIPA agent-protocol messages.

#### 2.2.2 The SGML client plug-in

This plug-in library `library(http/http_sgml_plugin)` provides a bridge between the SGML/XML/HTML parser provided by `library(sgml)` and the http client library. After loading this hook the following mime-types are automatically handled by the SGML parser.

text/html
  Handed to `library(sgml)` using W3C HTML 4.0 DTD, suppressing and ignoring all HTML syntax errors. Options is passed to load_structure/3.
text/xml
  Handed to `library(sgml)` using dialect `xmlns` (XML + namespaces). Options is passed to load_structure/3. In particular, `dialect(xml)` may be used to suppress namespace handling.
text/x-sgml
  Handled to `library(sgml)` using dialect `sgml`. Options is passed to load_structure/3.

## 3 The HTTP server libraries

The HTTP server library consists of two parts. The first deals with connection management and has three different implementation depending on the desired type of server. The second implements a generic wrapper for decoding the HTTP request, calling user code to handle the request and encode the answer. This design is summarised in figure 1.

Figure 1 : Design of the HTTP server

The functional body of the user's code is independent from the selected server-type, making it easy to switch between the supported server types. Especially the XPCE-based event-driven server is comfortable for debugging but less suitable for production servers. We start the description with how the user must formulate the functionality of the server.

### 3.1 The `Body'

The server-body is the code that handles the request and formulates a reply. To facilitate all mentioned setups, the body is driven by http_wrapper/5. The goal is called with the parsed request (see section 3.4) as argument and `current_output` set to a temporary buffer. Its task is closely related to the task of a CGI script; it must write a header declaring holding at least the `Content-type` field and a body. Here is a simple body writing the request as an HTML table.

    reply(Request) :-
            format('Content-type: text/html~n~n', []),
            format('<html>~n', []),
            format('<table border=1>~n'),
            print_request(Request),
            format('~n</table>~n'),
            format('</html>~n', []).

    print_request([]).
    print_request([H|T]) :-
            H =.. [Name, Value],
            format('<tr><td>~w<td>~w~n', [Name, Value]),
            print_request(T).

#### 3.1.1 Returning special status codes

Besides returning a page by writing it to the current output stream, the server goal can raise an exception using throw/1 to generate special pages such as `not_found`, `moved`, etc. The defined exceptions are:

http_reply(+Reply, +HdrExtra)
  Return a result page using http_reply/3. See http_reply/3 for details.
http_reply(+Reply)
  Equivalent to `http_reply(Reply,[])`.
http(not_modified)
  Equivalent to `http_reply(not_modified,[])`. This exception is for backward compatibility and can be used by the server to indicate the referenced resource has not been modified since it was requested last time.

### 3.2 Session management

The library `library(http/http_session.pl)` provides cookie-based session management. The library installes a session-id cookie using the hook http:request_expansion/2. It allows querying the session and provides a simple assert/retract based store to store information related to a session. Note that session management only works with the threaded and XPCE based server frameworks as the inetd based server starts a server for each request.

The examples contain the file `calc.pl`, which realises a simple calculator with internal state.

http_set_session_options(+Options)
  Set options for the session manager. Defined options are:
  timeout(+Seconds)
    Max idle time of a session. Session cookies are deleted if no request is received within the specified time. The value `0' disables timeout handling.
  cookie(+Atom)
    Name of the cookie to use for session management. The default is `swipl_session`.
  path(+Atom)
    Path with which to associate the session management. Default is `/`, associating it with the entire server.
http_session_id(-Id)
  Returns an identifier for the current session. The identifier is an atom.
http_current_session(?Id, ?Data)
  Enumerate sessions and associated data. All sessions have the Data item `idle(Seconds)`, describing the current idle-time of the session. Other data elements are added by the user using http_session_assert/1 and friends.
http_session_asserta(+Term)
  Associate Term with the current session, before any other associated term.
http_session_assert(+Term)
  Associate Term with the current session, after any other associated term.
http_session_retract(?Term)
  Non-deterministically retract terms associated with the current session.
http_session_retractall(+Term)
  Retract all matching terms from associated with the current session.
http_session_data(?Term)
  Enumerate all associated terms that unify with Term.

### 3.3 Get parameters from HTML forms

The library `library(http/http_parameters)` provides two predicates to fetch HTTP request parameters as a type-checked list easily. The library transparently handles both GET and POST requests. It builds on top of the low-level request representation described in section 3.4.

http_parameters(+Request, ?Parameters)
  The predicate is passes the Request as provided to the handler goal by http_wrapper/5 as well as a partially instantiated lists describing the requested parameters and their types. Each parameter specification in Parameters is a term of the format Name(-Value, +Options) . Options is a list of option terms describing the type, default, etc. If no options are specified the parameter must be present and its value is returned in Value as an atom. If a parameter is missing the exception `error(`existence_error(form_data, Name)`, _)` is thrown. Defined options are:
  default(Default)
    If the named parameter is missing, Value is unified to Default.
  optional(true)
    If the named parameter is missing, Value is left unbound and no error is generated.
  zero_or_more
    The same parameter may not appear or appear multiple times. If this option is present, `default` and `optional` are ignored and the value is returned as a list. Type checking options are processed on each value.
  oneof(List)
    Succeeds if the value is member of the given list.
  length > N
    Succeeds if value is an atom of more than N characters.
  length >= N
    Succeeds if value is an atom of more or than equal to N characters.
  length < N
    Succeeds if value is an atom of less than N characters.
  length =< N
    Succeeds if value is an atom of length than or equal to N characters.
  number
    Convert value to a number. Throws a type-error otherwise.
  integer
    Convert value to an integer. Throws a type-error otherwise.
  float
    Convert value to a float. Integers are tranformed into fload. Throws a type-error otherwise.

  Below is an example

      reply(Request) :-
              http_parameters(Request,
                              [ title(Title, [optional(true)]),
                                name(Name, [ length >= 2 ]),
                                age(Age, [integer])
                              ]),
              ...

  Same as `http_parameters(Request, Parameters,[])`

http_parameters(+Request, ?Parameters, +Options)
  In addition to http_parameters/2, the following options are defined.
  form_data(-Data)
    Return the entire set of provided Name=Value pairs from the GET or POST request. All values are returned as atoms.
  attribute_declarations(:Goal)
    If a parameter specification lacks the parameter options, call `call(Goal, +ParamName, -Options)` to find the options. Intended to share declarations over many calls to http_parameters/3. Using this construct the above can be written as below.

        reply(Request) :-
                http_parameters(Request,
                                [ title(Title),
                                  name(Name),
                                  age(Age)
                                ],
                                [ attribute_declarations(param)
                                ]),
                ...

        param(title, [optional(true)]).
        param(name,  [length >= 2 ]).
        param(age,   [integer]).

### 3.4 Request format

The body-code (see section 3.1) is driven by a Request. This request is generated from http_read_request/2 defined in `library(http/http_header)`.

http_read_request(+Stream, -Request)
  Reads an HTTP request from Stream and unify Request with the parsed request. Request is a list of `Name(Value)` elements. It provides a number of predefined elements for the result of parsing the first line of the request, followed by the additional request parameters. The predefined fields are:
  host(Host)
    If the request contains `Host:` Host, Host is unified with the host-name. If Host is of the format <host>:<port> Host only describes <host> and a field `port(Port)` where Port is an integer is added.
  input(Stream)
    The Stream is passed along, allowing to read more data or requests from the same stream. This field is always present.
  method(Method)
    Method is one of `get`, `put` or `post`. This field is present if the header has been parsed successfully.
  path(Path)
    Path associated to the request. This field is always present.
  peer(Peer)
    Peer is a term `ip(A,B,C,D)` containing the IP address of the contacting host.
  port(Port)
    Port requested. See `host` for details.
  search(ListOfNameValue)
    Search-specification of URI. This is the part after the `?`, normally used to transfer data from HTML forms that use the ``GET`' protocol. In the URL it consists of a www-form-encoded list of Name=Value pairs. This is mapped to a list of Prolog Name=Value terms with decoded names and values. This field is only present if the location contains a search-specification.
  http_version(Major-Minor)
    If the first line contains the `HTTP/`Major.Minor version indicator this element indicate the HTTP version of the peer. Otherwise this field is not present.
  cookie(ListOfNameValue)
    If the header contains a `Cookie` line, the value of the cookie is broken down in Name=Value pairs, where the Name is the lowercase version of the cookie name as used for the HTTP fields.
  set_cookie(set_cookie(Name, Value, Options))
    If the header contains a `SetCookie` line, the cookie field is broken down into the Name of the cookie, the Value and a list of Name=Value pairs for additional options such as `expire`, `path`, `domain` or `secure`.

  If the first line of the request is tagged with `HTTP/`Major.Minor, http_read_request/2 reads all input upto the first blank line. This header consists of Name:Value fields. Each such field appears as a term `Name(Value)` in the Request, where Name is canonised for use with Prolog. Canonisation implies that the Name is converted to lower case and all occurrences of the `-` are replaced by `_`. The value for the `Content-length` fields is translated into an integer.

Here is an example:

    ?- http_read_request(user, X).
    |: GET /mydb?class=person HTTP/1.0
    |: Host: gollem
    |:
    X = [ input(user),
          method(get),
          search([ class = person
                 ]),
          path('/mydb'),
          http_version(1-0),
          host(gollem)
        ].

#### 3.4.1 Handling POST requests

Where the HTTP `GET` operation is intended to get a document, using a path and possibly some additional search information, the `POST` operation is intended to hand potentially large amounts of data to the server for processing.

The Request parameter above contains the term `method(post)`. The data posted is left on the input stream that is available through the term `input(Stream)` from the Request header. This data can be read using http_read_data/3 from the HTTP client library. Here is a demo implementation simply returning the parsed posted data as plain text (assuming pp/1 pretty-prints the data).

    reply(Request) :-
            member(method(post), Request), !,
            http_read_data(Request, Data, []),
            format('Content-type: text/plain~n~n', []),
            pp(Data).

If the POST is initiated from a browser, content-type is generally either `application/x-www-form-urlencoded` or `multipart/form-data`. The latter is broken down automatically if the plug-in `library(http/http_mime_plugin)` is loaded.

### 3.5 Running the server

The functionality of the server should be defined in one Prolog file (of course this file is allowed to load other files). Depending on the wanted server setup this `body' is wrapped into a small Prolog file combining the body with the appropriate server interface. There are three supported server-setups. For most applications we advice the multi-threaded server. Examples of this server archirecture are the PlDoc documentation system and the SeRQL Semantic Web server infrastructure.

All the server setups may be wrapped in a reverse proxy to make them available from the public web-server as described in section 3.5.7.

- Using `library(xpce_httpd)` for an event-driven server
  This approach provides a single-threaded event-driven application. The clients talk to XPCE sockets that collect an HTTP request. The server infra-structure can talk to multiple clients simultaneously, but once a request is complete the wrappers call the user's goal and blocks all further activity until the request is handled. Requests from multiple clients are thus fully serialised in one Prolog process.

  This server setup is very suitable for debugging as well as embedded server in simple applications in a fairly controlled environment.

- Using `library(thread_httpd)` for a multi-threaded server
  This server exploits the multi-threaded version of SWI-Prolog, running the users body code parallel from a pool of worker threads. As it avoids the state engine and copying required in the event-driven server it is generally faster and capable to handle multiple requests concurrently.

  This server is harder to debug due to the involved threading. It can provide fast communication to multiple clients and can be used for more demanding servers.

- Using `library(inetd_httpd)` for server-per-client
  In this setup the Unix inetd user-daemon is used to initialise a server for each connection. This approach is especially suitable for servers that have a limited startup-time. In this setup a crashing client does not influence other requests.

  This server is very hard to debug as the server is not connected to the user environment. It provides a robust implementation for servers that can be started quickly.

#### 3.5.1 Common server interface options

All the server interfaces provide `http_server(:Goal, +Options)` to create the server. The list of options differ, but the servers share common options:

port(?Port)
  Specify the port to listen to for stand-alone servers. Port is either an integer or unbound. If unbound, it is unified to the selected free port.
after(:Goal)
  Specify a goal to be run on the query just like the first argument of http_server/2. This goal however is started after the request has been answered. It is called using `call(Goal, Request)`. This extension was added to support the FIPA-HTTP protocol, which issues HTTP POST requests on the server. The server answers these requests with an empty document before starting processing. The `after`-option is used for the processing:

      :- http_server(reply, [after(action), ...]).

      reply(Request) :-
              format('Content-type: text/plain\r\n\r\n').

      action(Request) :-
              <start agent work on request>

#### 3.5.2 From an interactive Prolog session using XPCE

The `library(http/xpce_httpd.pl)` provides the infrastructure to manage multiple clients with an event-driven control-structure. This version can be started from an interactive Prolog session, providing a comfortable infra-structure to debug the body of your server. It also allows the combination of an (XPCE-based) GUI with web-technology in one application.

http_server(:Goal, +Options)
  Create an instance of interactive_httpd. Options must provide the `port(?Port)` option to specify the port the server should listen to. If Port is unbound an arbitrary free port is selected and Port is unified to this port-number. The only other option provided is the `after(:Goal)` option.

The file `demo_xpce` gives a typical example of this wrapper, assuming `demo_body` defines the predicate reply/1.

    :- use_module(xpce_httpd).
    :- use_module(demo_body).

    server(Port) :-
            http_server(reply, Port, []).

The created server opens a server socket at the selected address and waits for incoming connections. On each accepted connection it collects input until an HTTP request is complete. Then it opens an input stream on the collected data and using the output stream directed to the XPCE socket it calls http_wrapper/5. This approach is fundamentally different compared to the other approaches:

- Server can handle multiple connections
  When inetd will start a server for each client, and CGI starts a server for each request, this approach starts a single server handling multiple clients.

- Requests are serialised
  All calls to Goal are fully serialised, processing on behalf of a new client can only start after all previous requests are answered. This easier and quite acceptable if the server is mostly inactive and requests take not very long to process.

- Lifetime of the server
  The server lives as long as Prolog runs.

#### 3.5.3 Multi-threaded Prolog

The `library(http/thread_httpd.pl)` provides the infrastructure to manage multiple clients using a pool of worker-threads. This realises a popular server design, also seen in SUN JavaBeans and Microsoft .NET. As a single persistent server process maintains communication to all clients startup time is not an important issue and the server can easily maintain state-information for all clients.

In addition to the functionality provided by the other (XPCE and inetd) servers, the threaded server can also be used to realise an HTTPS server exploiting the `library(ssl)` library. See option `ssl(+SSLOptions)` below.

http_server(:Goal, +Options)
  Create the server. Options must provide the `port(?Port)` option to specify the port the server should listen to. If Port is unbound an arbitrary free port is selected and Port is unified to this port-number. The server consists of a small Prolog thread accepting new connection on Port and dispatching these to a pool of workers. Defined Options are:
  port(?Port)
    Port the server should listen to. If unbound Port is unified with the selected free port.
  workers(+N)
    Defines the number of worker threads in the pool. Default is to use two workers. Choosing the optimal value for best performance is a difficult task depending on the number of CPUs in your system and how much resources are required for processing a request. Too high numbers makes your system switch too often between threads or even swap if there is not enough memory to keep all threads in memory, while a too low number causes clients to wait unnecessary for other clients to complete. See also http_workers/2.
  timeout(+SecondsOrInfinite)
    Determines the maximum period of inactivity handling a request. If no data arrives within the specified time since the last data arrived the connection raises an exception, the worker discards the client and returns to the pool-queue for a new client. Default is `infinite`, making each worker wait forever for a request to complete. Without a timeout, a worker may wait forever on an a client that doesn't complete its request.
  keep_alive_timeout(+SecondsOrInfinite)
    Maximum time to wait for new actity on Keep-Alive connections. Choosing the correct value for this parameter is hard. Disabling Keep-Alive is bad for performance if the clients request multiple documents for a single page. This may ---for example-- be caused by HTML frames, HTML pages with images, associated CSS files, etc. Keeping a connection open in the threaded model however prevents the thread servicing the client servicing other clients. The default is 5 seconds.
  local(+KBytes)
    Size of the local-stack for the workers. Default is taken from the commandline option.
  global(+KBytes)
    Size of the global-stack for the workers. Default is taken from the commandline option.
  trail(+KBytes)
    Size of the trail-stack for the workers. Default is taken from the commandline option.
  after(:Goal)
    After replying a request, execute Goal providing the request as argument.
  ssl(+SSLOptions)
    Use SSL (Secure Socket Layer) rather than plan TCP/IP. A server created this way is accessed using the `https://` protocol. SSL allows for encrypted communication to avoid others from tapping the wire as well as improved authentication of client and server. The SSLOptions option list is passed to ssl_init/3. The port option of the main option list is forwarded to the SSL layer. See the `library(ssl)` library for details.
http_current_server(?:Goal, ?Port)
  Query the running servers. Note that http_server/3 can be called multiple times to create multiple servers on different ports.
http_workers(:Port, ?Workers)
  Query or manipulate the number of workers of the server identified by Port. If Workers is unbound it is unified with the number of running servers. If it is an integer greater than the current size of the worker pool new workers are created with the same specification as the running workers. If the number is less than the current size of the worker pool, this predicate inserts a number of `quit' requests in the queue, discarding the excess workers as they finish their jobs (i.e. no worker is abandoned while serving a client).

  This can be used to tune the number of workers for performance. Another possible application is to reduce the pool to one worker to facilitate easier debugging.

http_current_worker(?Port, ?ThreadID)
  True if ThreadID is the identifier of a Prolog thread serving Port. This predicate is motivated to allow for the use of arbitrary interaction with the worker thread for development and statistics.

#### 3.5.4 From (Unix) inetd

All modern Unix systems handle a large number of the services they run through the super-server inetd. This program reads `/etc/inetd.conf` and opens server-sockets on all ports defined in this file. As a request comes in it accepts it and starts the associated server such that standard I/O refers to the socket. This approach has several advantages:

- Simplification of servers
  Servers don't have to know about sockets and -operations.

- Centralised authorisation
  Using tcpwrappers simple and effective firewalling of all services is realised.

- Automatic start and monitor
  The inetd automatically starts the server `just-in-time' and starts additional servers or restarts a crashed server according to the specifications.

The very small generic script for handling inetd based connections is in `inetd_httpd`, defining http_server/1:

http_server(:Goal, +Options)
  Initialises and runs http_wrapper/5 in a loop until failure or end-of-file. This server does not support the Port option as the port is specified with the inetd configuration. The only supported option is After.

Here is the example from `demo_inetd`

    #!/usr/bin/pl -t main -q -f
    :- use_module(demo_body).
    :- use_module(inetd_httpd).

    main :-
            http_server(reply).

With the above file installed in `/home/jan/plhttp/demo_inetd`, the following line in `/etc/inetd` enables the server at port 4001 guarded by tcpwrappers. After modifying inetd, send the daemon the `HUP` signal to make it reload its configuration. For more information, please check inetd.conf(5).

    4001 stream tcp nowait nobody /usr/sbin/tcpd /home/jan/plhttp/demo_inetd

#### 3.5.5 MS-Windows

There are rumours that inetd has been ported to Windows.

#### 3.5.6 As CGI script

To be done.

#### 3.5.7 Using a reverse proxy

There are three options for public deployment of a service. One is to run it on a dedicated machine on port 80, the standard HTTP port. The machine may be a virtual machine running ---for example--- under VMWARE or XEN. The (virtual) machine approach isolates security threads and allows for using a standard port. The server can also be hosted on a non-standard port such as 8000, or 8080. Using non-standard ports however may cause problems with intermediate proxy- and/or firewall policies. Isolation can be achieved using a Unix chroot environment. Another option, also recommented for Tomcat servers, is the use of Apache reverse proxies. This causes the main web-server to relay requests below a given URL location to our Prolog based server. This approach has several advantages:

- We can access the server on port 80, just as for a dedicated machine. We do not need a machine though and we only need access to the apache configuration.
- As Apache is doing the front-line service, the Prolog server is normally protected from malformed HTTP requests that could result in denial of service or otherwise comprimise the server. In addition, Apache can provide encodings such as compression to the outside world.

Note that the proxy technology can be combined with isolation methods such as dedicated machines, virtual machines and chroot jails. The proxy can also provide load balancing.

Setting up a reverse proxy

The Apache reverse proxy setup is really simple. Ensure the modules `proxy` and `proxy_http` are loaded. The add two simple rules to the server configuration. Below is an example that makes a PlDoc server on port 4000 available from the main Apache server at port 80.

    ProxyPass        /pldoc/ http://localhost:4000/pldoc/
    ProxyPassReverse /pldoc/ http://localhost:4000/pldoc/

Apache rewrites the HTTP headers passing by, but using the above rules it does not examine the content. This implies that URLs embedded in the (HTML) content must use relative addressing. If the locations on the public and Prolog server are the same (as in the example above) it is allowed to use absolute locations. I.e. `/pldoc/search` is ok, but `http://myhost.com:4000/pldoc/search` is not. If the locations on the server differ, locations must be relative (i.e. not start with `/`.

This problem can also be solved using the contributed Apache module `proxy_html` that can be instructed to rewrite URLs embedded in HTML documents. In our experience, this is not troublefree as URLs can appear in many places in generated documents. JavaScript can create URLs on the fly, which makes rewriting virtually impossible.

### 3.6 The wrapper library

The body is called by the module `library(http/http_wrapper.pl)`. This module realises the communication between the I/O streams and the body described in section 3.1. The interface is realised by http_wrapper/5:

http_wrapper(:Goal, +In, +Out, -Connection, +Options)
  Handle an HTTP request where In is an input stream from the client, Out is an output stream to the client and Goal defines the goal realising the body. Connection is unified to `'Keep-alive'` if both ends of the connection want to continue the connection or `close` if either side wishes to close the connection.

  This predicate reads an HTTP request-header from In, redirects current output to a memory file and then runs `call(Goal, Request)`, watching for exceptions and failure. If Goal executes successfully it generates a complete reply from the created output. Otherwise it generates an HTTP server error with additional context information derived from the exception.

  http_wrapper/5 supports the following options:

  request(-Request)
    Return the executed request to the caller.
  peer(+Peer)
    Add peer(Peer) to the request header handed to Goal. The format of Peer is defined by tcp_accept/3 from the clib package.
http:request_expansion(+RequestIn, -RequestOut)
  This multifile hook predicate is called just before the goal that produces the body, while the output is already redirected to collect the reply. If it succeeds it must return a valid modified request. It is allowed to throw exceptions as defined in section 3.1.1. It is intended for operations such as mapping paths, deny access for certain requests or manage cookies. If it writes output, these must be HTTP header fields that are added before header fields written by the body. The example below is from the session management library (see section 3.2) sets a cookie.

              ...,
              format('Set-Cookie: ~w=~w; path=~w~n', [Cookie, SessionID, Path]),
              ...,

http_current_request(-Request)
  Get access to the currently executing request. Request is the same as handed to Goal of http_wrapper/5 after applying rewrite rules as defined by http:request_expansion/2. Raises an existence error if there is no request in progress.
http_relative_path(+AbsPath, -RelPath)
  Convert an absolute path (without host, fragment or search) into a path relative to the current page, defined as the path component from the current request (see http_current_request/1). This call is intended to create reusable components returning relative paths for easier support of reverse proxies.

  If ---for whatever reason--- the conversion is not possible it simply unifies RelPath to AbsPath.

### 3.7 Debugging Servers

The library `library(http/http_error.pl)` defines a hook that decorates uncaught exceptions with a stack-trace. This will generate a 500 internal server error document with a stack-trace. To enable this feature, simply load this library. Please do note that providing error information to the user simplifies the job of a hacker trying to compromise your server. It is therefore not recommended to load this file by default.

The example program `calc.pl` has the error handler loaded which can be triggered by forcing a divide-by-zero in the calculator.

### 3.8 Handling HTTP headers

The library `library(http/http_header)` provides primitives for parsing and composing HTTP headers. Its functionality is normally hidden by the other parts of the HTTP server and client libraries. We provide a brief overview of http_reply/3 which can be accessed from the reply body using an exception as explain in section 3.1.1.

http_reply(+Type, +Stream, +HdrExtra)
  Compose a complete HTTP reply from the term Type using additional headers from HdrExtra to the output stream Stream. ExtraHeader is a list of `Field(Value)`. Type is one of:
  html(+HTML)
    Produce a HTML page using print_html/1, normally generated using the `library(http/html_write)` described in section 3.9.
  file(+MimeType, +Path)
    Reply the content of the given file, indicating the given MIME type.
  tmp_file(+MimeType, +Path)
    Similar to `File(+MimeType, +Path)`, but do not include a modification time header.
  stream(+Stream, +Len)
    Reply using the next Len characters from Stream. The user must provides the MIME type and other attributes through the ExtraHeader argument.
  cgi_stream(+Stream, +Len)
    Similar to `stream(+Stream, +Len)`, but the data on Stream must contain an HTTP header.
  moved(+URL)
    Generate a ``301 Moved Permanently'' page with the given target URL.
  see_other(+URL)
    Generate a ``303 See Other'' page with the given target URL.
  not_found(+URL)
    Generate a ``404 Not Found'' page.
  forbidden(+URL)
    Generate a ``403 Forbidden'' page, denying access without challenging the client.
  authorise(+Method, +Realm)
    Generate a ``401 Authorization Required'', requesting the client to retry using proper credentials (i.e. user and password).
  not_modified
    Generate a ``304 Not Modified'' page, indicating the requested resource has not changed since the indicated time.
  server_error(+Error)
    Generate a ``500 Internal server error'' page with a message generated from a Prolog exception term (see print_message/2).

### 3.9 The `library(http/html_write)` library

Producing output for the web in the form of an HTML document is a requirement for many Prolog programs. Just using format/2 is satisfactory as it leads to poorly readable programs generating poor HTML. This library is based on using DCG rules.

The `library(http/html_write)` structures the generation of HTML from a program. It is an extensible library, providing a DCG framework for generating legal HTML under (Prolog) program control. It is especially useful for the generation of structured pages (e.g. tables) from Prolog data structures.

The normal way to use this library is through the DCG html/1. This grammar-rule provides the central translation from a structured term with embedded calls to additional translation rules to a list of atoms that can then be printed using print_html/[1,2].

html(:Spec)`-->`
  The DCG rule html/1 is the main predicate of this library. It translates the specification for an HTML page into a list of atoms that can be written to a stream using print_html/[1,2]. The expansion rules of this predicate may be extended by defining the multifile DCG html_write:expand/1. Spec is either a single specification or a list of single specifications. Using nested lists is not allowed to avoid ambiguity caused by the atom `[]`

  - Atomic data
    Atomic data is quoted using the html_quoted/1 DCG.

  - Fmt - Args
    Fmt and Args are used as format-specification and argument list to sformat/3. The result is quoted and added to the output list.

  - `\`List
    Escape sequence to add atoms directly to the output list. This can be used to embed external HTML code.

  - `\`Term
    Invoke the grammar rule Term in the calling module. This is the common mechanism to realise abstraction and modularisation in generating HTML.

  - Module:Term
    Invoke the grammar rule <Module>:<Term>. This is similar to `\`Term but allows for invoking grammar rules in external packages.

  - &(Entity)
    Emit `&<Entity>;`. As Prolog understands Unicode and automatically inserts appropriate entity declarations, this is normally not needed.

  - `Tag(Content)`
    Emit HTML element Tag using Content and no attributes. Content is handed to html/1. See section 3.9.3 for details on the automatically generated layout.

  - `Tag(Attributes, Content)`
    Emit HTML element Tag using Attributes and Content. Attributes is either a single attribute of a list of attributes. Each attributes is of the format `Name(Value)` or Name=Value. Value is either atomic or a term Left+Right. The `+` operator implements concatenation.
page(:HeadContent, :BodyContent)`-->`
  The DCG rule page/2 generated a complete page, including the SGML `DOCTYPE` declaration. HeadContent are elements to be placed in the `head` element and BodyContent are elements to be placed in the `body` element.

  To achieve common style (background, page header and footer), it is possible to define DCG rules head/1 and/or body/1. The page/1 rule checks for the definition of these DCG rules in the module it is called from as well as in the `user` module. If no definition is found, it creates a head with only the HeadContent (note that the `title` is obligatory) and a `body` with `bgcolor` set to `white` and the provided BodyContent.

  Note that further customisation is easily achieved using html/1 directly as page/2 is (besides handling the hooks) defined as:

      page(Head, Body) -->
              html([ \['<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 4.0//EN">\n'],
                     html([ head(Head),
                            body(bgcolor(white), Body)
                          ])
                   ]).

page(:Contents)`-->`
  This version of the page/[1,2] only gives you the SGML `DOCTYPE` and the `HTML` element. Contents is used to generate both the head and body of the page.
html_begin(+Begin)`-->`
  Just open the given element. Begin is either an atom or a compound term, In the latter case the arguments are used as arguments to the begin-tag. Some examples:

              html_begin(table)
              html_begin(table(border(2), align(center)))

  This predicate provides an alternative to using the `\`Command syntax in the html/1 specification. The following two fragments are the same. The preferred solution depends on your preferences as well as whether the specification is generated or entered by the programmer.

      table(Rows) -->
              html(table([border(1), align(center), width('80%')],
                         [ \table_header,
                           \table_rows(Rows)
                         ])).

      % or

      table(Rows) -->
              html_begin(table(border(1), align(center), width('80%'))),
              table_header,
              table_rows,
              html_end(table).

html_end(+End)`-->`
  End an element. See html_begin/1 for details.

#### 3.9.1 Emitting HTML documents

The html/1 grammar rules translates a specification into a list of atoms and layout instructions. Currently the layout instructions are terms of the format `nl(N)`, requesting at least N newlines. Multiple consecutive `nl(1)` terms are combined to an atom containing the maximum of the requested number of newline characters.

To simplify handing the data to a client or storing it into a file, the following predicates are available from this library:

print_html(+List)
  Print the token list to the Prolog current output stream.
print_html(+Stream, +List)
  Print the token list to the specified output stream
html_print_length(+List, -Length)
  When calling html_print/[1,2] on List, Length characters will be produced. Knowing the length is needed to provide the `Content-length` field of an HTTP reply-header.

#### 3.9.2 Adding rules for html/1

In some cases it is practical to extend the translations imposed by html/1. When using XPCE for example, it is comfortable to be able defining default translation to HTML for objects. We also used this technique to define translation rules for the output of the SWI-Prolog `library(sgml)` package.

The html/1 rule first calls the multifile ruleset html_write:expand/1.

html_write:expand(+Spec)`-->`
  Hook to add additional translationrules for html/1.
html_quoted(+Atom)`-->`
  Emit the text in Atom, inserting entity-references for the SGML special characters `<&>`.
html_quoted_attribute(+Atom)`-->`
  Emit the text in Atom suitable for use as an SGML attribute, inserting entity-references for the SGML special characters `<&>'"`.

#### 3.9.3 Generating layout

Though not strictly necessary, the library attempts to generate reasonable layout in SGML output. It does this only by inserting newlines before and after tags. It does this on the basis of the multifile predicate html_write:layout/3

html_write:layout(+Tag, -Open, -Close)
  Specify the layout conventions for the element Tag, which is a lowercase atom. Open is a term Pre-Post. It defines that the element should have at least Pre newline characters before and Post after the tag. The Close specification is similar, but in addition allows for the atom `-`, requesting the output generator to omit the close-tag altogether or `empty`, telling the library that the element has declared empty content. In this case the close-tag is not emitted either, but in addition html/1 interprets Arg in `Tag(Arg)` as a list of attributes rather than the content.

  A tag that does not appear in this table is emitted without additional layout. See also print_html/[1,2]. Please consult the library source for examples.

#### 3.9.4 Examples

In the following example we will generate a table of Prolog predicates we find from the SWI-Prolog help system based on a keyword. The primary database is defined by the predicate predicate/5 We will make hyperlinks for the predicates pointing to their documentation.

    html_apropos(Kwd) :-
            findall(Pred, apropos_predicate(Kwd, Pred), Matches),
            phrase(apropos_page(Kwd, Matches), Tokens),
            print_html(Tokens).

    %       emit page with title, header and table of matches

    apropos_page(Kwd, Matches) -->
            page([ title(['Predicates for ', Kwd])
                 ],
                 [ h2(align(center),
                      ['Predicates for ', Kwd]),
                   table([ align(center),
                           border(1),
                           width('80%')
                         ],
                         [ tr([ th('Predicate'),
                                th('Summary')
                              ])
                         | \apropos_rows(Matches)
                         ])
                 ]).

    %       emit the rows for the body of the table.

    apropos_rows([]) -->
            [].
    apropos_rows([pred(Name, Arity, Summary)|T]) -->
            html([ tr([ td(\predref(Name/Arity)),
                        td(em(Summary))
                      ])
                 ]),
            apropos_rows(T).

    %       predref(Name/Arity)
    %
    %       Emit Name/Arity as a hyperlink to
    %
    %               /cgi-bin/plman?name=Name&arity=Arity
    %
    %       we must do form-encoding for the name as it may contain illegal
    %       characters.  www_form_encode/2 is defined in library(url).

    predref(Name/Arity) -->
            { www_form_encode(Name, Encoded),
              sformat(Href, '/cgi-bin/plman?name=~w&arity=~w',
                      [Encoded, Arity])
            },
            html(a(href(Href), [Name, /, Arity])).

    %       Find predicates from a keyword. '$apropos_match' is an internal
    %       undocumented predicate.

    apropos_predicate(Pattern, pred(Name, Arity, Summary)) :-
            predicate(Name, Arity, Summary, _, _),
            (   '$apropos_match'(Pattern, Name)
            ->  true
            ;   '$apropos_match'(Pattern, Summary)
            ).

#### 3.9.5 Remarks on the `library(http/html_write)` library

This library is the result of various attempts to reach at a more satisfactory and Prolog-minded way to produce HTML text from a program. We have been using Prolog for the generation of web pages in a number of projects. Just using format/2 never was a real option, generating error-prone HTML from clumsy syntax. We started with a layour on top of format/2, keeping track of the current nesting and thus always capable of properly closing the environment.

DCG based translation however naturally exploits Prolog's term-rewriting primitives. If generation fails for whatever reason it is easy to produce an alternative document (for example holding an error message).

The approach presented in this library has been used in combination with `library(http/httpd)` in three projects: viewing RDF in a browser, selecting fragments from an analysed document and presenting parts of the XPCE documentation using a browser. It has proven to be able to deal with generating pages quickly and comfortably.

In a future version we will probably define a goal_expansion/2 to do compile-time optimisation of the library. Quotation of known text and invocation of sub-rules using the `\`RuleSet and <Module>:<RuleSet> operators are costly operations in the analysis that can be done at compile-time.

## 4 Security

Writing servers is an inherently dangerous job that should be carried out with some considerations. You have basically started a program on a public terminal and invited strangers to use it. When using the interactive server or inetd based server the server runs under your privileges. Using CGI scripted it runs with the privileges of your web-server. Though it should not be possible to fatally compromise a Unix machine using user privileges, getting unconstrained access to the system is highly undesirable.

Symbolic languages have an additional handicap in their inherent possibilities to modify the running program and dynamically create goals (this also applies to the popular perl and java scripting languages). Here are some guidelines.

- Check your input
  Hardly anything can go wrong if you check the validity of query-arguments before formulating an answer.

- Check filenames
  If part of the query consists of filenames or directories, check them. This also applies to files you only read. Passing names as `/etc/passwd`, but also `../../../../../etc/passwd` are tried by experienced hackers to learn about the system they want to attack. So, expand provided names using absolute_file_name/[2,3] and verify they are inside a folder reserved for the server. Avoid symbolic links from this subtree to the outside world. The example below checks validity of filenames. The first call ensures proper canonisation of the paths to avoid an mismatch due to symbolic links or other filesystem ambiguities.

      check_file(File) :-
              absolute_file_name('/path/to/reserved/area', Reserved),
              absolute_file_name(File, Tried),
              atom_concat(Reserved, _, Tried).

- Check scripts
  Should input in any way activate external scripts using shell/1 or `open(pipe(Command), ...)`, verify the argument once more.

- Check meta-calling
  The attractive situation for you and your attacker is below:

      reply(Query) :-
              member(search(Args), Query),
              member(action=Action, Query),
              member(arg=Arg, Query),
              call(Action, Arg).              % NEVER EVER DO THIS!

  All your attacker has to do is specify Action as `shell` and Arg as `/bin/sh` and he has an uncontrolled shell!

## 5 Tips and tricks

- URL Locations
  With an application in mind, it is tempting to make all URL locations short and directly connected to the root (`/`). This is not a good idea. It is adviced to have all locations in a server below a directory with an informative name. Consider to make the root location something that can be changed using a global setting.

  - Page generating code can easily be reused. Using locations directly below the root however increases the likelyhood of conflicts.
  - Multiple servers can be placed behind the same public server as explained in section 3.5.7. Using a common and fairly unique root, redirection is much easier and less likely to lead to conflicts.

- Debugging
  Please check the section ``Thread-support library(threadutil)'' of the SWI-Prolog reference manual.

## 6 Status

The SWI-Prolog HTTP library is in active use in a large number of projects. It is considered one of the SWI-Prolog core libraries that is actively maintained and regulary extended with new features. This is particulary true for the multi-threaded server. The XPCE and inetd based servers are not widely used.

This library is by no means complete and you are free to extend it.

# Index

A
absolute_file_name/[2,3]
  4
B
body/1
  3.9
C
close/1
  2.1
F
file_mime_type/2
  2.2
format/2
  3.9 3.9.5 3.9.5
G
goal_expansion/2
  3.9.5
H
head/1
  3.9
html/1
  3.9 3.9 3.9 3.9 3.9 3.9.1 3.9.1 3.9.2 3.9.2 3.9.2 3.9.3
html_begin/1
  3.9
html_end/1
html_print/[1,2]
  3.9.1
html_print_length/2
html_quoted/1
  3.9
html_quoted_attribute/1
html_write:expand/1
html_write:layout/3
http_current_request/1
  3.6
http_current_server/2
http_current_session/2
http_current_worker/2
http_get/3
  2.2 2.2
http_open/3
http_parameters/2
  3.3
http_parameters/3
  3.3
http_post/4
  2.2
http_post_data/3
  2.2
http_read_data/3
  2.2 2.2 3.4.1
http_read_request/2
  2.2 3.4 3.4
http_relative_path/2
http_reply/3
  3.1.1 3.1.1 3.8
http:request_expansion/2
http_server/1
  3.5.4
http_server/2
  3.5.1
http_server/3
  3.5.3
http_session_assert/1
  3.2
http_session_asserta/1
http_session_data/1
http_session_id/1
http_session_retract/1
http_session_retractall/1
http_set_session_options/1
http_workers/2
  3.5.3
http_wrapper/5
  3.1 3.3 3.5.2 3.5.4 3.6 3.6 3.6
I
interactive_httpd
  3.5.2
L
load_structure/3
  2.2.2 2.2.2 2.2.2
M
mime_pack/3
  2.2 2.2
P
page/1
  3.9
page/2
  3.9 3.9
page/[1,2]
  3.9
parse_time/2
  2.1
pp/1
  3.4.1
predicate/5
  3.9.4
print_html/1
  3.8
print_html/2
print_html/[1,2]
  3.9 3.9 3.9.3
print_message/2
  3.8
R
reply/1
  3.5.2
S
set_stream/2
  2.1
sformat/3
  3.9
shell/1
  4
socket
  3.5.2
ssl_init/3
  3.5.3
T
tcp_accept/3
  3.6
throw/1
  3.1.1