Writing an Erlang OTP File Poller


I have been playing with the idea to write a content management software for some time now. Reading files and pre-generating pages, but still be able to serve dynamic content. While having no Idea how the big players (joomla, wordpress, …) do it I thought it would be a good lesson on the problems such systems face.

As a first step I wanted to simply serve some Markdown files dynamically (i.e. changes and additions should be reloaded instantly), providing features similar to what pelican offers (which powers this blog).

Since Erlang is my current poison it only made sense to use it and Chicago Boss to build it but first, I needed to build a library to poll for files.

Erlang OTP - gen_server

Erlang OTP is a collection of design principles on how to implement common parts of processes. Erlang goes ahead and calls these abstractions Behaviours, in our case that would be a Server-Client behaviour, called gen_server. The idea is to divide the program into a generic part and a specific part, which gets executed via callbacks from the generic one.

Our File Polling Server, after being started, should go ahead and periodically (via a asynchronous call to itself) check if files have been modified lately. A client can then pull a list of files which have been found. Let’s see how it does that!

Code and Explanation

-include_lib("kernel/include/file.hrl"). % For FileInfo record used later
-record(state,{directory, regex, timeout=1000, files=[], poll=0}).


The header of the file is fairly standard for an Erlang program. First we define the name of the -module, include a -behaviour specification (which is somewhat familiar to interfaces in other languages) and follow it up with an -include_lib to use the FileInfo record as well as a definition of our state -record, which will keep track of our variables.

Then we export the public accessible functions for our module, splitting it up allows for easier reading. The first export handles all functions which should be used by clients, while the second one is used for gen_server callbacks.

%%%Client API
%%Starts Server
start_link(Directory, Regex, Timeout) ->
    gen_server:start_link(?MODULE, [Directory, Regex, Timeout], []).

%%Pull List of Files that changed
pull(Pid) ->
    gen_server:call(Pid, pull).
%%Stop Server
stop(Pid) ->
    gen_server:call(Pid, terminate).

This is the part which a client is supposed to interact with. scanner:start_link(Directory, Regex, Timeout) returns {ok,Pid} if successfully started, the Pid will be used by a Client to interact to the Server. For example scanner:pull(Pid) would pull a List of changed Files to the Client. Stop does exactly what is expected and terminates the server.

init([Directory, Regex, Timeout]) ->
    {ok, #state{directory=Directory, regex=Regex, timeout=Timeout, files=[], poll=0}}.

Now for the definition of functions which will be called by gen_server. Init will be called by start_link, sending the token scan towards itself (i.e.: self() ! scan) after 0 seconds, afterwards we build up our state record with the starting parameters and default values for the rest of them.

handle_call(pull,_From, State) ->
handle_call(terminate, _From, State) ->
    {stop, normal, ok, State}.

handle_call is responsible for synchronous messages, we use two of them. The first one sends the current contents of our state.files to the requesting client and sets an empty list into the record. The second one terminates the Server.

handle_cast(_, State) ->
    {noreply, State}.

handle_cast is for asynchronous messaging, which we don’t use. In future iterations we could implement changing of the watched directory or regular expression here.

handle_info(scan, State) ->
    {New_files,New_Poll} = get_new_files(
    MergedFiles = lists:usort(New_files ++ State#state.files),
handle_info(Msg,State) ->
    error_logger:error_msg("Unknown Message: ~p~n",[Msg]),

Now it gets really interesting! As mentioned above we use a timer to send a scan message to our-selfs after every Timeout. This triggers handle_info which is called by gen_server for every message that is not a synchronous or asynchronous request. This is where we check for new files via get_new_files (which we will talk about later). The results are merged with the existing file list, usorted (unique sort) and then a timer is setup, which will repeat the whole process in state.timeout milliseconds.

The other guard is just for debugging and catches other messages (and prints them via error_logger).

code_change(_OldVsn, Clients, _Extra) ->
    {ok, Clients}.

terminate(normal, State) ->
    error_logger:info_msg("Last state: ~w~n",[State]),

code_change is a feature for hot code reloading, which I did not touch yet, while as terminate is a function called gen_server wants to shut down. In out case it prints the current state record and quits.

get_new_files(Directory, Regex, LastPollTime) ->
    NewPollTime = {date(), time()},
    NewFiles = filelib:fold_files(
        fun(File, Acc) -> 
            {ok, FileInfo} = file:read_file_info(File),
            FileMtime = FileInfo#file_info.mtime,
                FileMtime>LastPollTime ->
                true ->
    { NewFiles, NewPollTime }.

Here is the private part of the Poller, which actually handles the logic of getting new files. I borrowed the idea from this blog post, though I have rewritten it somewhat. First we check the files mtime versus the current poll time. All this can be put into a single fold_files call, which we use to recursively (thats the true part in the call) search for all files matching Regex in Directory and add it to a list via an anonymous function. There we also test the files mtime and add the File to our NewFiles List if it is indeed newer.

The full source can be found here.


This part is fairly simple, since we abstracted that nicely!

[winlu@micronuke lib]$ erl
Erlang/OTP 17 [erts-6.1] [source] [async-threads:10] [hipe] [kernel-poll:false]

Eshell V6.1  (abort with ^G)
1> c(scanner).
2> {ok, Pid} = scanner:start_link(".",".*",5000).
3> scanner:pull(Pid).
4> scanner:pull(Pid).
5> scanner:pull(Pid).
["./New Text Document.txt"]
6> scanner:stop(Pid).

=INFO REPORT==== 27-Aug-2014::22:20:19 ===
Last state: {state,[46],[46,42],5000,[],63576397219}

Whats next

This implementation does not keep track of file deletion, so it would be a nice idea to actually have two lists of files in the state, as well as add, modify, delete responses. To stay inline with OTP we could spawn processes for every file we find and use those to send us a message if a statechange occurs.


I had a lot of fun playing around with Erlang today and hope this fact carried over in this post as well, also shoutout to the fine people in #erlang on freenode, who looked over my code.