If an email sent on XXXX#YYYY.ZZZ, I want to put the contents of the email in a database, but how may realize it?
I use postfix to MTA.
If you definitely must, write a program to process the input (say /usr/bin/your_program) and place this in /etc/aliases:
intended-local-address: "|/usr/bin/your_program"
I have used fetchmail. If You configure it right, it can make a blocking call to a remote IMAP server (it will hang until a mail is received, how cool is that, no active polling!). It handles many kinds of mail protocols. It puts the email with it's headers into a program of Your choice (I used python and it's builtin mail parsing libary).
I must say I am proud of this solution, as it was relatively easy to setup and very effective on the end.
Few more things worth noting:
The connection times out or resets eventually (after several hours, sometimes several weeks). I suspect that the IMAP server was being restarted.
Don't try to parse the email by Yourself. I gave up after debugging the 5th way of sending email body and then used the existing lib.
After the poll finishes, do a sleep 5 or something before You poll again. My mail program once felt into an infinite loop with another one and the sleep 5 saved me.
Related
I would want to send a message from the server actively, such as using UDP/TCPIP to a client using an arduino. It is known that this is possible if the user has port forward the specific port to the device on local network. However I wouldn't want to have the user to port forward manually, perhaps using another protocol, will this be possible?
1 Arduino Side
I think the closest you can get to this is opening a connection to the server from the arduino, then use available to wait for the server to stream some data to the arduino. Your code will be polling the open connection, but you are avoiding all the back and forth communications to open and close the connection, passing headers back and forth etc.
2 Server Side
This means the bulk of the work will be on the server side, where you will need to manage open connections so you can instantly write to them when a user triggers some event which requires a message to be pushed to the arduino. How to do this varies a bit depending on what type of server application you are running.
2.1 Node.js "walk-through" of main issues
In Node.js for example, you can res.write() on a connection, without closing it - this should give a similar effect as having an open serial connection to the arduino. That leaves you with the issue of managing the connection - should the server periodically check a database for messages for the arduino? That simply removes one link from the arduino -> server -> database polling link, so we should be able to do better.
We can attach a function triggered by the event of a message being added to the database. Node-orm2 is a database Object Relational Model driver for node.js, and it offers hooks such as afterSave and afterCreate which you can utilize for this type of thing. Depending on your application, you may be better off not using a database at all and simply using javascript objects.
The only remaining issue then, is: once the hook is activated, how do we get the correct connection into scope so we can write to it? Well you can save all the relevant data you have on the request to some global data structure, maybe a dictionary with an arduino ID as index, and in the triggered function you fetch all the data, i.e. the request context and you write to it!
See this blog post for a great example, including node.js code which manages open connections, closing them properly and clearing from memory on timeout etc.
3 Conclusion
I haven't tested this myself - but I plan to since I already have an existing application using arduino and node.js which is currently implemented using normal polling. Hopefully I will get around to it soon and return here with results.
Typically in long-polling (from what I've read) the connection is closed once data is sent back to the client (arduino), although I don't see why this would be necessary. I plan to try keeping the same connection open for multiple messages, only closing after a fixed time interval to re-establish the connection - and I hope to set this interval fairly high, 5-15 minutes maybe.
We use Pubnub to send notifications to a client web browser so a user can know immediately when they have received a "message" and stuff like that. It works great.
This seems to have the same constraints that you are looking at: No static IP, no port forwarding. User can theoretically just plug the thing in...
It looks like Pubnub has an Arduino library:
https://github.com/pubnub/arduino
I am writing a Client/Server application in C++ with the help of Boost Asio. I have a working server, and the server workflow is something I understand well.
My client application handles the connect gracefully as shown in Asio examples, after which, it exchanges a handshake with the server. After that however, the users should be able to send requests to the server when and how they want, which is where I have a problem understanding the paradigm.
The initial workflow goes like a little like this:
OnConnected() { SendHandshake() }
SendHandshake() { async.write_some(handshake...), async_read_some(&OnRead) }
OnRead() { ReadServerHandshake() *** }
And users would send messages by using Write(msg):
Write (msg) { async_write_some(msg,&OnWrite), async_Read_some(&OnRead) }
OnWrite() {}
EDIT: Rephrasing the question to be clearer, here is the scenario:
After the initial handshaking is complete, the Client is only used to send requests to the server, on which it will get a reply. So, for instance, a user sends a write. Client waits for the read operation to complete, reads the reply and does something with it. The next user write will only come after, say, 5 minutes. Will the io_service stop working in the meanwhile because there are no outstanding asynchronous operations in between the last reply read and the next write?
On an informative note, you can provide it with io_service::work to stop an io_service from running out of work. This will ensure that the io_service::run never returns until the work object is destroyed.
To control the lifetime of the work object, you can use a shared_ptr pointer and reset it once the work is done, or you can use boost::optional as outlined here.
Of course you still need to handle the case where either the server closes the TCP connection, or the connection dies for whatever reason. To handle this case, one solution would be to have an outstanding async_read on the socket to the server. The read handler should be called with an error_code when/if something goes wrong with the connection. If you have the outstanding read on the connection, you do not need to use the work object.
If you want the IO service to complete a read, you must start a read. If you want to read data any time the client sends it, you must have an asynchronous read operation pending at all times. Otherwise, how would the library know what to do with the data?
Short summary: How to accept content that may be endless and not uploaded at once (the connection needs to be kept alive), in a scenario where I'm the server and I'd like the clients to make those uploads in a RESTful (or something close) way ?
In the same way that I can make an http server that keeps the connection alive with a client and may continue sending content that the client reads and parses intantly (probably using a browser), I need to keep a connection opened with a client that will send me data that may not end or be continuously uploaded.
One (simple) way to do this would be simply to have a TCP server and then clients would write data to a socket.
But how do I do this with an HTTP PUT request ? This answers half of the question: "How will I parse a file upload continuously, without the upload finishing ?" But how will clients proceed to upload something that is not even a file and are separate blocks of data, like if they would be writing those blocks of data to a socket ? Is it even possible ?
If your data isn't going to have a discrete end, then you're not really performing an upload; you're doing a streaming scenario. For a streaming scenario, socket handling is much more appropriate.
First I think Sonier is right. But I found this solution by Felix called "Streaming file uploads with node.js" which might be useful.
Furthermore I think node.js might not be best fit for this, because everything has to be kept in memory and with big file-size you can hit a very hard wall. Some other popular node.js file upload solutions are:
https://github.com/felixge/node-formidable
https://github.com/rootslab/formaline
https://github.com/FooBarWidget/multipart-parser
I have a tcpip socket interface to a third party software app. I've implemented this interface for several customer sites with no problem. The latest customer, though... problems. We've turned on logging in the apps on either end, and also installed Wireshark on the PC to log raw tcpip traffic. With that, we've proved that my server app successfully sends the message out, the pc receives the message, but the client app doesn't see it. (This is a totally intermittent problem, which is why it's such a pain to troubleshoot.)
The socket details are as simple as they come: one socket handling two way communications between the server and the pc. The messages are plain ascii text and fairly short (not XML). The server initiates communications by sending the first message, and then the client responds with several messages. The socket is kept open at all times while the apps are running. The client app is designed so that the end user can only process one case at a time, which prevents message collisions from happening. They have some sort of polling set up, their app "hibernates" until it sees the initiating message from the server.
The third party vendor has advised me to add a few second delay before I send them the initiating message. I can't see how that helps. If the client is "sleeping", just polling the socket waiting for a message, how does adding a delay before the first message help? It's not like we send two messages and the second one gets lost. It's losing the first message. So I don't see how it matters if we send that message now or two seconds from now.
I've asked them and they haven't given me details. It could be some proprietary details in their coding that they don't want to disclose to me, and that's fair. So I'm asking here because I'm always learning new things about socket programming. Maybe you guys can shed some light on how polling a tcpip socket can be affected by message timing?
Since its someone else's client and they won't tell you what its doing (other than saying 'insert a delay'), the answer is probably that their client is reading and discarding the message because its not yet in a state to deal with it. The delay will allow the client time to get into a state where it can respond to the message properly.
In other words, the client has a race condition. One easy way this can happen is if they have one thread for reading messages and another for dealing with them.
Short of running strace(1) on the client to see what system calls it is making, its tough to tell what the client is actually doing.
I'm writing a Comet-like app using Flex on the client and my own hand-written server.
I need to be able to send short bursts of data from the client at quite a high frequency (e.g. of the order of 10ms between sends).
I also need the server to push short bursts of data at a similarly high frequency.
I'm using NetConnection.call() to send the data to the server, and URLStream (with chunked encoding) to push the data from the server to the client.
What I've found is that the data isn't being sent/received as soon as it's available. For example, in IE, it seems the data is sent every 200ms rather than as soon as NetConnection.call() is called. Similarly, URLStream isn't making the data available as soon as the server is sending it.
Judging by the difference in behaviour between the browsers, it seems as though the Flash Player (version 10) is relying on the host browser to do all the comms. Can anyone confirm this? Update: This is very likely as only the host browser would know about the proxy settings that might be set.
I've tried using the Socket class and there's no problem with speed there: it works perfectly. However, I'd like to be able to use HTTP-based (port 80) connections so that my app can run in heavily fire-walled environments (I tried using a Socket over port 80, but that has its problems).
Incidentally, all development/testing has been done on an internal LAN, so bandwidth/latency is not an issue.
Update: The data being sent/received is in small packets and doesn't need to be in any particular format. For example, I might need to send a short array of Numbers, and this could either be encoded in AMF (e.g. via NetConnection.call()) or could be put into GET parameters (e.g. using sendToURL()). The main point of my question is really to see whether anyone else has experienced the same problem in calling NetConnection/URLStream frequently, and whether there is a workaround (it's also possible that the fault lies with my server code of course, rather than Flash).
Thanks.
Turns out the problem had nothing to do with Flash/Flex or any of the host browsers. The problem was in my server code (written in C++ on Linux), and without access to my source code the cause is hard to find (so I couldn't have hoped for an answer from this forum).
Still - thank you everyone who chipped in.
It was only after looking carefully at the output shown in Wireshark that I noticed the problem, which was twofold:
Nagle's algorithm
I was sending replies in multiple packets by calling write() multiple times (e.g. once for the HTTP response header, and again for the HTTP response body). The server's TCP/IP stack was waiting for an ACK for the first packet before sending the second, but because of Nagle's algorithm the client was waiting 200ms before sending back the ACK to the first packet, so the server took at least 200ms to send the full HTTP response.
The solution is to use send() with the flag MSG_MORE until all the logically connected blocks are written. I could also have used writev() or setsockopt() with TCP_CORK, but it suited my existing code better to use send().
Chunk-encoded streams
I'm using a never-ending HTTP response with chunk encoding to push data back to the client. Naggle's algorithm needs to be turned off here because even if each chunk is written as one packet (using MSG_MORE), the client OS TCP/IP stack will still wait up to 200ms before sending back an ACK, and the server can't push a subsequent chunk until it gets that ACK.
The solution here is to ask the server not to wait for an ACK for each sent packet before sending the next packet, and this is done by calling setsockopt() with the TCP_NODELAY flag.
The above solutions only work on Linux and aren't POSIX-compliant (I think), but that isn't a problem for me.
I'm almost 100% sure the player relies on the browser for such communications. Can't find an official page stating so atm, but check this out for example:
Applications hosting the Flash Player
ActiveX control or Flash Player
plug-in can use the
EnforceLocalSecurity and
DisableLocalSecurity API calls to
control security settings.
Which I think somehow implies the idea. Also, I've suffered some network related bugs on FF/IE only which again points out to the player using each browser for networking (otherwise there wouldn't be such differences).
And regarding your latency problem, I think that if speed is critical, your best bet is sockets. You have some work to do, but seems possible, check out the docs again:
This error occurs in SWF content.
Dispatched if a call to
Socket.connect() attempts to connect
either to a server outside the
caller's security sandbox or to a port
lower than 1024. You can work around
either problem by using a cross-domain
policy file on the server.
HTH,
Juan