Here we give some recipes how to submit HTTP and FTP requests to servers. There is a quite simple interface representing the hierarchy of remote files, which is recommended for all occasional uses. If performance is important, however, the protocol-specific interfaces will give you more options.
Let's start with an example: We want to get the public files
let fs1 = Http_fs.http_fs "http://foo.org"
The same for FTP looks like:
let fs2 = Ftp_fs.ftp_fs "ftp://foo.org"
fs2 provide now a number of methods for accessing
files. These do not only cover downloads, but also listing directories,
writing files, renaming files, and a number of further operations. The
commonly available methods are those of
Netfs.stream_fs. The incarnations
of this interface for concrete protocols usually define more methods. It
is guaranteed that you can coerce the types to
let my_filesystems = [ (fs1 :> Netfs.stream_fs); (fs2 :> Netfs.stream_fs) ]
The method we use here is
method read : read_flags -> string -> Netchannels.in_obj_channel
All access methods take a list of flags as first argument. For example,
a possible flag here is
`Binary switching to binary mode for the protcols
where it makes a difference (like FTP).
The second argument is the file path, using slashes as separators, and
always starting with a slash. The path is appended to the base URL
given when creating the
fs2 objects. Note that the path must
not contain any URL-specific encodings like "%xx".
We get a an
Netchannels.in_obj_channel back we can read the data from:
let c1 = fs1 # read  "/bar" let s1 = Netchannels.string_of_in_obj_channel c1 let () = c1 # close_in() let c2 = fs2 # read [`Binary] "/baz" let s2 = Netchannels.string_of_in_obj_channel c2 let () = c2 # close_in()
It depends very much on the implementation what actually happens:
By default, the downloaded data are cached in a temporary file. Some
implementations support the streaming mode to avoid that (like HTTP),
and you are directly connected with the reading socket when reading from
`Streaming as flag to
enable this. In streaming mode, however, neither retries nor redirects
writeworks very much like
read, only that you get a
Netchannels.out_obj_channelback. The network write operation normally starts first when this channel is closed, and the so-far cached data are uploaded to the server. For HTTP there is also a streaming mode. The
writeoperation takes also flags that look like normal open flags, i.e. whether you want to create a file, truncate a file, or ensure the unique creation. Not all protocols support every combination, though. For HTTP a
writeis translated to sending a
PUTmethod to the server.
readdirreads the names of a directory. For FTP this is clearly an NLST command. For HTTP the implementation just extracts the names from the hyperlinks contained in the page - this works well for applying
readdirto automatically generated file indexes.
removetranslates to the
DELETEmethod for HTTP. This method is defined in the HTTP standard, but usually not available on servers, though.
sizegets the size of a file. This may work for HTTP or may not - depending on whether the server knows the size (which is often not the case for dynamically generated content). For FTP there is the SIZE command. However, this is a later addition to the protocol, and may not be available on ancient servers.
test_listallow it to test properties of files (existence, type, non-empty, accessibility). This is only partially implemented for HTTP and FTP.
There is a full implementation of
Netfs.stream_fs for accessing
Netfs.local_fs. There are more definitions inside
and outside Ocamlnet, see Other impementations of stream_fs for a list. It also mentions
a WebDAV implementation extending the HTTP definition explained here,
and which covers a larger set of access operations.
When creating the access object, one can set a callback that allows almost arbitrary configurations:
let fs1 = Http_fs.http_fs ~config_pipeline:(fun p -> ...) "http://foo.org" let fs2 = Ftp_fs.ftp_fs ~config_client:(fun c -> ...) "ftp://foo.org"
c are the underlying protocol implementations.
Do it like this in the
let user = "user" in let password = "secret" in let realm = "the realm string" in let domain = [ "http://foo.org " ] in let keys = new Http_client.key_ring() in keys # add_key (Http_client.key ~user ~password ~realm ~domain); let ah = new Http_client.unified_auth_handler keys in p # add_auth_handler ah
This works for both "basic" and "digest" authentication.
This is not done in the
config_client callback, but directly when
creating the filesystem object. The user string is always taken from
the URL (as normally the accessed file space depends on the user).
Passwords and account names (if needed) are supplied by callbacks:
let fs2 = Ftp_fs.ftp_fs ~get_password:(fun () -> "secret") ~get_account:(fun () -> "account") "ftp://email@example.com"
Do it like this in the
p # set_proxy "proxy.company.net" 8080; p # set_proxy_auth "user" "secret"; p # avoid_proxy_for [ ".company.net"; "localhost" ]
Or you can just import this data from the environment variables "http_proxy" and "no_proxy":
p # set_proxy_from_environment()
Web proxies often also support FTP URLs, but only for a limited set
of operations (often only
let fs1 = Http_fs.http_fs ~config_pipeline:(fun p -> p # set_proxy "proxy.company.net" 8080; p # set_proxy_auth "user" "secret"; ) ~enable_ftp:true "ftp://foo.org"
In this configuration, the web proxy is contacted via HTTP, and the proxy talks FTP with the content server.
If you do not configure the proxy, any accesses will fail (no transport error).
This is an alternative to a web proxy.
Do it like this in the
p # set_socks5_proxy "proxy.company.net" 1080
Do it like this in the
c # set_socks5_proxy "proxy.company.net" 1080
The current implementation is limited to file transfers in passive mode, though. This is nowadays not a problem anymore, because almost all FTP servers support it.
Support for TLS (SSL) is not available by default. Ocamlnet must be compiled with support for TLS, and a certain configuration must be applied to the HTTP pipeline.
Https_client module for a recipe (this module is part of
Netglob module can be used to interpret wildcards in filenames.
let files = Netglob.glob ~fsys:(Netglob.of_stream_fs (fs2 :> Netfs.stream_fs)) (`String "/dir/*.gif")
This would return paths to all gif files in /dir on the FTP server
Caveat: Globbing works only well if the server provides the operations for recognizing directories. Most FTP servers don't - only the recently (1) added MLST command allows it to safely recognize directories.
(1) recently = many years ago, but existing FTP deployments seem only to be very slowly upgraded.
Test whether an FTP server supports MLST: There must be a line for MLST in the output for the FEAT command, like in
$ ftp localhost Connected to localhost. 220---------- Welcome to Pure-FTPd [privsep] [TLS] ---------- ... ftp> quote feat 211-Extensions supported: EPRT IDLE MDTM SIZE REST STREAM MLST type*;size*;sizd*;modify*;UNIX.mode*;UNIX.uid*;UNIX.gid*;unique*; MLSD AUTH TLS PBSZ PROT UTF8 TVFS ESTA PASV EPSV SPSV ESTP 211 End.
For HTTP servers, the recognition of directories is even worse. Don't rely on it.
For example, let's copy the file "/xyz" from
fs2, i.e. from
an HTTP server to an FTP server:
Netfs.copy (fs1 :> Netfs.stream_fs) "/xyz" (fs2 : Netfs.stream_fs) "/xyz"
There is the generic file iterator
Netfs.iter, which walks through
the directory hierarchy on the server:
Netfs.iter ~pre:(fun name kind symkind -> ...) (fs2 :> Netfs.stream_fs) "/"
Note that you may run into problems in conjunction with HTTP and FTP:
Http_client module is the real implementation of HTTP. It
is asynchronous, which means it can do many tasks in parallel, but
also needs special care when using it.
The tasks are organized as pipelines. This is actually an HTTP protocol feature - one can send the next request(s) to an HTTP server without having to wait for the response of the prior request. The pipeline is available as Ocaml class:
let p = new Http_client.pipeline
By adding requests, the pipeline is told to send them to the right
server. If the server allows pipelining on HTTP level, this feature
is exploited to speed up the accesses. Here, we submit two different
let x1 = new Http_client.get "http://foo.org/bar" let x2 = new Http_client.get "http://foo.org/baz" p # add x1; p # add x2
x2 are instances of
They have lots of access methods for changing the request type and
getting the returned response.
Now, after just adding the objects, nothing is done yet. You also have to start the pipeline:
p # run()
(Or, alternatively, do
Unixqueue.run p#event_system, which is just
Now, you can get the fetched data with:
let d1 = x1 # response_body # value
Before looking at the value, you would normally check the status code of the response. There are a few possibilities:
Http_client.http_call.statusonly indicates the class of the response (success, redirect, client error, server error), or whether there was a socket or protocol error.
Http_client.http_call.response_statusreturns the code as variant
Http_client.http_call.response_status_codereturns the code numerically
p is actually several pipelines in one object. For each
p keeps a small number of parallel connections
(normally 2). Each connection is then driven in a pipelined way, if
p, all the connections to the servers are created in
parallel, and the communication is done in parallel.
Generally, a client can only connect to a single server, not to several at once. Also, there is no queue of pending requests - all submitted requests are immediately executed, and the next request can first be started when the previous has finished.
In synchrounous code, a file download looks like:
let client = new ftp_client() let () = client # exec (connect_method ~host:"foo.bar" ()) let () = client # exec (login_method ~user:"anonymous" ~get_password:(fun () -> "") ~get_account:(fun () -> "") ()) let buffer = Buffer.create 1000 let ch = new Netchannels.output_buffer buffer let () = client # exec (get_method ~file:(`NVFS "dir/baz") ~representation:`Image ~store:(fun _ -> `File_structure ch) ()); let s = Buffer.contents buffer
As you see, this is just a sequence of
exec calls. There is also an
exec_e method allowing to start these operations as
Uq_engines.engine, allowing asynchronous execution.
FTP has a number of subtle protocol options - like file transfer in
several modes. Please refer to
Ftp_client where these details are
You close the connection using the
QUIT command with
let () = client # exec (quit_method())
or just run
client # reset() to just shut the TCP connection down.