I am having the data in Modbus TCP/IP. I have to read the available data with Apache NiFi. I don't know, which processor have to use exactly (Ex. GetTCP, ListenTCP, Plc4xSourceProcessor). Can you help me on this? Is there any feasibility with Apache NiFi?
the Plc4xSourceProcessor is what you are looking for. The Apache PLC4X project provides drivers for accessling PLCs using various protocols. One of the is the Modbus protocol. So if you use the Plc4xSourceProcessor and configure a modbus connection string and list the addresses you want to collect, then you will be able to do so.
I happen to have written the PLC4X-NiFi Integration documentation on our website just a couple of days ago: https://plc4x.apache.org/users/integrations/apache-nifi.html
I think this will be helpful.
Chris
I don't really know what Modbus TCP/IP is, but it basically comes down to whether you want NiFi to be a client or a server.
ListenTCP creates a TCP server that is waiting for some client to make a connection and start sending data. The most common case would be a log forwarding system like syslog which can be configured to forward logs to a host/port over TCP.
GetTCP is a client that connects to some host/port which is the server, and starts reading data.
Plc4xSourceProcessor is not part of the official Apache NiFi code, but from quickly looking at it, it seems like more of a client processor similar to GetTCP since you give it a connection string telling it where to connect to.
Related
Currently we receive on port 9020 from a client. That client uses port switching on their end and sometimes we wind up with multiple "Established" connections on the one port - all with different remote ports. We can manually end each established connection and our job will connect again after a few seconds. We can also run ENDTCPCNN for each remote port listed. We are looking for a way to programmatically see if there are multiple remote ports connected to the local port and if so end the established connections (while leaving the Listener running). Does anyone know of a way to get the information?
To answer your question, assuming you are on a supported version of the OS, take a look at the QSYS2.NETSTAT_INFO SQL view.
select local_port, remote_port, protocol, tcp_state, idle_time, network_connection_type
from qsys2.netstat_info
where local_port = 9020;
Otherwise, you'd need to use the List Network Connections and Retrieve Network Connection Data APIs yourself.
But are you sure you need to do this? Sounds like the client is leaving the connection open for re-use. That's a good thing for performance. You server code should be automatically timing out and closing idle connections.
I have been writing my own version of the 802.11 protocol with network stack. This is mostly a learning experience to see more in depth on how networks work.
My question is, is there a standard for replying to client devices that a certain protocol is unsupported?
I have an android device connecting to my custom wifi device and immediately sending a TON of requests at the DNS port of my UDP protocol. Since I would like to test out other protocols I would very much like a way for my wifi device to tell the android device that DNS is not available and get it to quite down a little.
Thanks in advance!
I don't see a possibility to send a reply that a service is not available.
I can't find anything about this case in the UDP specification.
One part of the DNS specification assumes that there are multiple DNS servers and defines how to handle communication with them. This explains part of the behavior in your network, but does not provide much information how to handle it.
4.2.1 Messages - format - UDP usage
The optimal UDP retransmission policy will vary with performance of the
Internet and the needs of the client, but the following are recommended:
The client should try other servers and server addresses
before repeating a query to a specific address of a server.
The retransmission interval should be based on prior
statistics if possible. Too aggressive retransmission can
easily slow responses for the community at large. Depending
on how well connected the client is to its expected servers,
the minimum retransmission interval should be 2-5 seconds.
7.2 Resolver Implementation - sending the queries
If a resolver gets a server error or other bizarre response
from a name server, it should remove it from SLIST, and may
wish to schedule an immediate transmission to the next
candidate server address.
According to this you could try to send garbage back to the client, but this is rather a hack, or an error, but how does an error look like? Such a solution assumes that you have knowledge about the service that you don't support.
I believe that the DNS - requests can be avoided by using DHCP. DHCP allows to specify DNS-servers as listed in the linked page. This is the usual way that I know for a DNS-resolver in a LAN to get initial DNS servers although I don't find anything about this in the DNS specification. You can give the Android - device a DNS-server with DHCP so that it does to need to try to query your device. Querying your device could be a fallback.
Additionally to DNS there is mDNS which uses multicasts in the network to send queries. This seems not to be the protocol you have to do with because it uses the special port 5353.
Not possible to stop DNS in the way you intend. However, only for your tests you can check the UDP messages and find out the names the device is looking for. Then you update the hosts file (google how to do it: http://www.howtogeek.com/140576/how-to-edit-the-hosts-file-on-android-and-block-web-sites/) and add those names with some localoop IP address. That might work for your test.
Other possibility is to change DNS server to some localloop IP address: http://xslab.com/2013/08/how-to-change-dns-settings-on-android/
Again, this is only to avoid having all the DNS messages through the wifi connection.
My job is to write a distributed client/server application with some concurrent tasks. So i decided to use akka.net for the concurrency issues. To implement the ipc between server and client akka remote is used. For some reasons there may run more than one client of the same type on a workstation. So i configured these clients for dynamic assignment of a tcp port. This worked fine for sending messages to the server.
My problem is to push some information to the clients. To accomplish this task an actor on the client exist. Now the server creates a reference for this actor. Therefor it needs the port the client is listening on . My idea is to send the tcp port the client uses to the server in some sort of connection procedure using a actor on the server.
After searching for some hours I didn't find any hint where to find the dynamically assigned tcp port. So how would the client get the assigned tcp port?
Ok, I could use akka.cluster. But using akka.cluster is breaking a fly on the wheel, I think. And if it solves my issue reamins to be seen.
Two suggestions, assuming that it is your client that makes the first contact with the server.
I'd have the server keep track of which clients are connected. I'd probably have a heartbeat message that gets sent once every few seconds from each client system. This way you can store an IActorRef for each alive client and send messages back without the need for finding the port. IActorRefs are preferable wherever possible for location transparency.
If you actually need to explicitly find the port, you may be able to extract it from the Path property of the IActorRef of one of the actors on the client system.
Thanks to patricks suggestions my issue is solved.
The solution is to extract the needed information from the senders path available while executing the hello message. With this information the server is able to maintain a list of all connected clients and theire network address.
Thanks a lot # patrick.
Regards Gregor
I am trying to create a Web Server of my own and there are several questions about working of Web servers we are using today. Questions are:
After receiving a HTTP request from a client through port 80, does server respond using same port 80?
If yes then while sending a large file say a pic in MB's, webserver will be unable to receive requests from other clients?
Is a computer port duplex or simplex? (Can it send and receive at the same time)?
If another port on server side is used to send response to client, then (if TCP is used, which is generally used), again 3-way handshaking will be done which will be overhead...
http://beej.us/guide/bgnet/output/html/singlepage/bgnet.html here is a good guide on what's going on with webservers, although it's in c but the concepts are all there. This will explain the whole client server relationship as well as some implementation details.
I'll just give a high level on what's going on:
Usually what happens is when your server gets a new request that comes in it creates a fork that will process it, that way you are not bogged down by each request, when the request comes in the child process is handed a new file to write to(again this is all implementation details).
So really you have one server waiting for requests and for each request it received it spawns a child to process to deal with this request. I'm sure there are much easier languages to implement this stuff than c(I had to do both a c and java server serving to either one in my past) but c really gets you to understand the things that are going on and I'm betting that is what you are looking for here
Now there are a couple of things to think about:
how you want the webserver to work. The example explains the parent child process.
Do you want to use tcp/UDP there are differences in the way to payload gets delivered.
You don't have to connect on port 80. that's just the default for web.
Hopefully the guide will help you.
Yes. The server sends the response using the TCP connection established by the client, so it also responds using the same port. The server can handle connections from multiple clients using the same port because TCP connections are identified by (local-ip, local-port, remote-ip, remote-port), so the server can even handle multiple connections from same client provided that the source ports are different.
There are different techniques you can use to be able to serve multiple clients at the same time. These include
using multiple processes or threads: when one is busy serving a client the others can serve other clients.
using events: the server listens for events from the OS: when it can write a block of data to a connection it writes it, when a new client connects it accepts the connection, ...
Frequently both approaches are be combined.
A TCP connection is duplex: you can send and receive at the same time. The HTTP protocol is based on a simple request-response model though: at any given time only one party is "talking."
when I type wget http://yahoo.com:80 on unix shell. Can some one explain me what exactly happens from entering the command to reaching the yahoo server. Thank you very much in advance.
RFC provide you with all the details you need and are not tied to a tool or OS.
Wget uses in your case HTTP, which bases on TCP, which in turn uses IP, then it depends on what you use, most of the time you will encounter Ethernet frames.
In order to understand what happens, I urge you to install Wireshark and have a look at the dissected frames, you will get an overview of what data belongs to which network layer. That is the most easy way to visualize and learn what happens. Beside this if you really like (irony) funny documents (/irony) have a look at the corresponding RFCs HTTP: 2616 for example, for the others have a look at the external links at the bottom of the wikipedia articles.
The program uses DNS to resolve the host name to an IP. The classic API call is gethostbyname although newer programs should use getaddrinfo to be IPv6 compatible.
Since you specify the port, the program can skip looking up the default port for http. But if you hadn't, it would try a getservbyname to look up the default port (then again, wget may just embed port 80).
The program uses the network API to connect to the remote host. This is done with socket and connect
The program writes an http request to the connection with a call to write
The program reads the http response with one or more calls to read.