I'm working on a protocol handling data exchange that somehow a little complex, then I found TLV is the one I need. Is there a formal way to read and write TLV in erlang? or some lib / code example handling this? thanks.
The "default" in Erlang is LTV rather than TLV, but it is rather easy to handle:
case gen_tcp:recv(Socket, 8) of
<<Type:32/integer, Len:32/integer>> ->
Payload = gen_tcp:recv(Socket, Len),
{type_of(Type), Payload};
...
end,
You will need passive sockets to get this to work, but it is rather easy to do. If you have the freedom to pick your format, the LTV encoding is better because you can then put the socket in {active, once} mode which means the underlying layer decodes stuff for you.
I haven't actually used it, but how about this one: https://github.com/essiene/smpp34pdu/blob/master/src/tlv.erl
Related
Suppose I have a method
timer:apply_after(timeout, module, method, [Hosts])
Is there a tool that can help format this into something like:
timer:apply_after(
timeout,
module,
method,
[Hosts]
)
I don't think erl_tidy does this.
You could give prettypr a try. I've never used it myself, and might not give the exact output you specified, but it does format the source code according to the available horizontal space.
It should be possible to do this through erl_tidy, by passing an option {printer, fun (Tree, Opts) -> erl_prettypr:format(Tree, [{paper, P}, {ribbon, R} | Opts]) end}.
See http://erlang.org/doc/man/erl_prettypr.html#format-2 for details about the ribbon and paper options, and http://erlang.org/doc/man/erl_tidy.html#file-2 for info about the printer option.
Is there any way to read whole lines from a socket in Erlang, or do I need to implement line buffering manually on top of gen_tcp:recv?
Have you tried using
inet:setopts(Socket, [{packet, line}])
See: http://www.erlang.org/doc/man/inet.html#setopts-2
Cheers!
There is no need to implement line buffering yourself.
gen_tcp:listen/2 accepts {packet, line} for its Options argument, which will put the socket into line mode and so calls to gen_tcp:recv will block until a complete line has been read.
gen_tcp:listen(Port, [{packet, line}])
Make sure that your buffer size set via the {buffer, Size} option to the same call (or inet:setopts/2) is big enough that it will fit all of your lines, otherwise they will be truncated.
Or, if using Elixir, this should get you started:
:gen_tcp.listen(port, [packet: :line, buffer: 1024])
Mainly I want to know if I can send a function in a message in a distributed Erlang setup.
On Machine 1:
F1 = Fun()-> hey end,
gen_server:call(on_other_machine,F1)
On Machine 2:
handler_call(Function,From,State) ->
{reply,Function(),State)
Does it make sense?
Here's an interesting article about "passing fun's to other Erlang nodes". To resume it briefly:
[...] As you might know, Erlang distribution
works by sending the binary encoding
of terms; and so sending a fun is also
essentially done by encoding it using
erlang:term_to_binary/1; passing the
resulting binary to another node, and
then decoding it again using
erlang:binary_to_term/1.[...]
This is pretty obvious
for most data types; but how does it
work for function objects?
When you encode a fun, what is encoded
is just a reference to the function,
not the function implementation.
[...]
[...]the definition of the function is not passed along; just exactly enough information to recreate the fun at an other node if the module is there.
[...] If the module containing the fun has not yet been loaded, and the target node is running in interactive mode; then the module is attempted loaded using the regular module loading mechanism (contained in the module error_handler); and then it tries to see if a fun with the given id is available in said module. However, this only happens lazily when you try to apply the function.
[...] If you never attempt to apply the function, then nothing bad happens. The fun can be passed to another node (which has the module/fun in question) and then everybody is happy.
Maybe the target node has a module loaded of said name, but perhaps in a different version; which would then be very likely to have a different MD5 checksum, then you get the error badfun if you try to apply it.
I would suggest you to read the whole article, cause it's extremely interesting.
You can send any valid Erlang term. Although you have to be careful when sending funs. Any fun referencing a function inside a module needs that module to exist on the target node to work:
(first#host)9> rpc:call(second#host, erlang, apply,
[fun io:format/1, ["Hey!~n"]]).
Hey!
ok
(first#host)10> mymodule:func("Hey!~n").
5
(first#host)11> rpc:call(second#host, erlang, apply,
[fun mymodule:func/1, ["Hey!~n"]]).
{badrpc,{'EXIT',{undef,[{mymodule,func,["Hey!~n"]},
{rpc,'-handle_call_call/6-fun-0-',5}]}}}
In this example, io exists on both nodes and it works to send a function from io as a fun. However, mymodule exists only on the first node and the fun generates an undef exception when called on the other node.
As for anonymous functions, it seems they can be sent and work as expected.
t1#localhost:
(t1#localhost)7> register(shell, self()).
true
(t1#localhost)10> A = me, receive Fun when is_function(Fun) -> Fun(A) end.
hello me you
ok
t2#localhost:
(t2#localhost)11> B = you.
you
(t2#localhost)12> Fn2 = fun (A) -> io:format("hello ~p ~p~n", [A, B]) end.
#Fun<erl_eval.6.54118792>
(t2#localhost)13> {shell, 't1#localhost'} ! Fn2.
I am adding coverage logic to an app built on riak-core, and the merge of results gathered can be tricky if anonymous functions cannot be used in messages.
Also check out riak_kv/src/riak_kv_coverage_filter.erl
riak_kv might be using it to filter result, I guess.
I'm making a application in Erlang, with a GUI in Java.
I've managed to establish a connection between the to languages, but now i need to (i guess) send a message from Java to Erlang, every time I e.g press a button.
Is that the right way to go?
How would such a message look?
I've found a few good sites about this form of integration, but I feel like im not getting everything.
http://www.trapexit.org/How_to_communicate_java_and_erlang
Besides classic Java-Erlang communication via OTP jinterface you can research such methods like:
- thrift
- ice from zeroC (no official erlang binding)
- maybe two http servers on both sides (I like this approach)
- protocol buffers (rather not, it is better for larger data transfers)
You need to learn the shape of your traffic and choose the best solution.
Jinterface is not so bad, tho.. (here is official doc: http://www.erlang.org/doc/apps/jinterface/jinterface_users_guide.html)
If jinterface is too complicated you might just use the packet option on open_port and use
byte[] in_buf = new byte[256];
byte[] out_buf = new byte[256];
int in_count = System.in.read ();
int offset = 0;
do
{
int c = System.in.read (in_buf, offset, in_count-offset);
offset += c;
}
while (offset < in_count);
To read packets from erlang and to write use:
System.out.write(out_count);
System.out.write(out_buf, 0, out_count);
On the erlang side this would match with
open_port({spawn, "<path-to-java> -cp <classpath> your-java-prog",
[{packet, 1}]).
If you need larger packets use {packet, 2} or {packet, 4} and adapt the java.
Inside the packets you can run whatever protocol you like on both sides.
I am working on an application similar to yours: C++ GUI and Erlang server. I use TCP sockets to exchange messages between the GUI and server, and Erlang server patterns for handling requests (I may have more than one GUI hooked up to the server at the same time).
I have Ejabberd up and running with test users, and its working fine. I want to write a module that can intercept messages and modify them, as follows :
intercept "messages"
send them to a php file
get the result from the same php file (immediate)
Modify the message stanza and send it down the wire to the recipient
The ejabberd documentation is weak and tutorials are non-existent. Can you give me some sample code that does this. I can then figure how to configure it for my needs.
Thanks a bundle!
Adil
Here's the basic example of such module:
-module(packet_interceptor).
-behaviour(gen_mod).
-export([start/2,
stop/1]).
-export([on_filter_packet/1]).
start(Host, _Opts) ->
ejabberd_hooks:add(filter_packet, global, ?MODULE, on_filter_packet, 0).
on_filter_packet({From, To, XML} = Packet) ->
%% does something with a packet
%% should return modified Packet or atom `drop` to drop the packet
Packet.
And make sure to add this module into ejabberd's configuration into module section:
{modules,
[...
...
...
{packet_interceptor, []}
]}.
Just extend on_filter_packet/1 the way you want and return appropriately modified packet.
gleber's example is excellent. I also wrote a more complex example of packet manipulation that I built for Chesspark called
mod_sunshine.
How can send this XML data to php (via ?xml=) and retrieve the resulting XML and then pass it to the recipient?
If you have a recent version of ejabberd that uses exmpp, you can use exmpp_xml:node_to_binary/1 or exmpp_xml:node_to_list/1 to turn the internal representation of the XML to a binary or a string respectively.
There were similarly named functions in the older ejabberd, but within the xml module.
It seems that what you want to do there is to create an XMPP server component. Lots of things has already been said on that post Which XMPP server to experiment developing a server component.
I can link you some useful links:
Jabber Component Protocol
An Echo-Bot in Python from metajack.im, a very nice blog from an XMPP guru. This bot listen for any message stanzas and reply to some of them.
Gleber's solution is really "ejabberd" oriented (certainly the easiest one in your case), whereas this one can scale with other XMPP servers.
There is the interface:
ejabberd_router:register_route(MyHost)
which I have used in the past and works well. Once the MyHost is registered with ejabberd, the module will receive the communications intended to MyHost through info messages (assuming gen_server).
As noted by #gleber, don't forget to add your module to the configuration file.