I have one app. that consists of "Manager" and "Worker". Currently, the worker always initiates the connection, says something to the manager, and the manager will send the response.
Since there is a LOT of communication between the Manager and the Worker, I'm considering to have a socket open between the two and do the communication. I'm also hoping to initiate the interaction from both sides - enabling the manager to say something to the worker whenever it wants.
However, I'm a little confused as to how to deal with "collisions". Say, the manager decides to say something to the worker, and at the same time the worker decides to say something to the manager. What will happen? How should such situation be handled?
P.S. I plan to use Netty for the actual implementation.
"I'm also hoping to initiate the interaction from both sides - enabling the manager to say something to the worker whenever it wants."
Simple answer. Don't.
Learn from existing protocols: Have a client and a server. Things will work out nicely. Worker can be the server and the Manager can be a client. Manager can make numerous requests. Worker responds to the requests as they arrive.
Peer-to-peer can be complex with no real value for complexity.
I'd go for a persistent bi-directional channel between server and client.
If all you'll have is one server and one client, then there's no collision issue... If the server accepts a connection, it knows it's the client and vice versa. Both can read and write on the same socket.
Now, if you have multiple clients and your server needs to send a request specifically to client X, then you need handshaking!
When a client boots, it connects to the server. Once this connection is established, the client identifies itself as being client X (the handshake message). The server now knows it has a socket open to client X and every time it needs to send a message to client X, it reuses that socket.
Lucky you, I've just written a tutorial (sample project included) on this precise problem. Using Netty! :)
Here's the link: http://bruno.linker45.eu/2010/07/15/handshaking-tutorial-with-netty/
Notice that in this solution, the server does not attempt to connect to the client. It's always the client who connects to the server.
If you were thinking about opening a socket every time you wanted to send a message, you should reconsider persistent connections as they avoid the overhead of connection establishment, consequently speeding up the data transfer rate N-fold.
I think you need to read up on sockets....
You don't really get these kinds of problems....Other than how to responsively handle both receiving and sending, generally this is done through threading your communications... depending on the app you can take a number of approaches to this.
The correct link to the Handshake/Netty tutorial mentioned in brunodecarvalho's response is http://bruno.factor45.org/blag/2010/07/15/handshaking-tutorial-with-netty/
I would add this as a comment to his question but I don't have the minimum required reputation to do so.
If you feel like reinventing the wheel and don't want to use middleware...
Design your protocol so that the other peer's answers to your requests are always easily distinguishable from requests from the other peer. Then, choose your network I/O strategy carefully. Whatever code is responsible for reading from the socket must first determine if the incoming data is a response to data that was sent out, or if it's a new request from the peer (looking at the data's header, and whether you've issued a request recently). Also, you need to maintain proper queueing so that when you send responses to the peer's requests it is properly separated from new requests you issue.
Related
I know this question has been asked partially before (How to Scale Node.js WebSocket Redis Server?) but I am wondering if there is any alternatives to redis for rapidly sharing websocket objects between node instances, specifically ws type sockets (https://github.com/einaros/ws). I've tried redis and ran into issues with the fact that the web socket objects are cyclic and difficult to serialise. I then used Crockford's cycle.js (https://github.com/douglascrockford/JSON-js/blob/master/cycle.js), however it seems to strip out the websocket objects methods, as I get an error from node saying "Object object has no method send" after I have read the socket back from redis and retrocycled it. Any help would be much appreciated.
Thanks in advance, James.
IMO you should use messaging queue for that.. e.g (RabbitMQ)
Application starts on Node A and Node B and connects to RabbitMQ
Client A connects to Node A and subscribe to Queue named XXX Client
Client B connects to Node B and subscribe to Queue named XXX
Client A sendsmessage to websocket server Websocket Server sends message to Node A
Node A publishes messages to RabbitMQ queue XXX
Node B receives the message from RabbitMQ as it is subscribed to queue XXX
Node B sends message to Client B or publishes the messages to all connected clients on node B
So, all you need is to put Messaging queue in your architecture (RabbitMQ, ZeroMQ) etc
There is a library which allows easily scale WebSocket across node.js processes and machines, you can check out it:
https://github.com/ClusterWS/ClusterWS
When we speak of scalability we expect or want to hear the words linear performance gains. To be honest though this is not the case most setups as their reliance on another server/service is too great and thus bottle-necks form up within the network you're trying to host for users.
As we explore options we hear things like Databases, Message Queues, and Brokers; These are fine to use but as mentioned above if reliance on any of them is far too great you will destroy your setup in sure time.
Design the WSS Server to act solo (unless requirements are exceeded). You determine and set limits and let API server know this. So if I have 10 chat-rooms and they hold maximum 100 users and benching my WSS server proved I could hold 400-500 of them. With that information I'd set 4-5 rooms per server. So if two people enter room#1 they are on WSS server#1; If all 10 chat-rooms are full then WSS server #2 is now full and 11th room will need a WSS Server#3 up to 15th room.
The slowest part of the network would now just be your API server handling requests but this may include database as well.
If your requirements are for more users than the example, you can increase core power first or add a second server with help of an MQ or Redis Pub/Sub type setup.
Unfortunately there's no way to properly sort users, so if 3 rooms had 20 users and all were sitting on WSS server#1 that'd still leave a room left with hundreds of user slots available but is this really a problem?
It's possible this room could fill right up so leave them the spot, but still could be days till they max so programming something spicy for your needs will improve how cost effective you make it.
I'm new websocket streaming application.
I'm trying to evaluate kaazing and solace streaming vendor products.
I'm trying to put a layer or interface application infornt of kaazing publisher before it creates the socket connection.
The client would make request to the interface for the socket connection , the interface plays the role to authenticate and authorize and do some business changes before the creation of socket.
The interface establish the secured socket connection with kazzing or streaming application and transfer the connection object to the client in response.
The client use the established connection from the reponse and retains the connection for streaming the data to client from the streaming server.
The objective is to hide the topic info and connection established from the client side for secured process.
So the infterace creates the secured connection and transfers the connection to authorized client which continue the streaming from the streaming server until the session expires.
Please let me designing such an application is possible.
The client receives data but it doesn't establish connection of its own, its created from server side and transfered to client side after all necessary validation.
Guide me to proceed further with the design and I'm inneed of expert's valuable suggestion.
Thanks in advance and appriciated for directing me to the right path in designing the streaming appliation.
My aim is not to introduce a new layer between the gateway and client. Altimate aim is to customize the gateway for my product, For example the user(client) connects to gateway and tries to access the topic say stock(eligible to subscribe) then he will be able to get the data streaming. If he needs share he would register is some place and after approval only he will be eligible to view, So authorization would play a role and maintaince a session and loads the customized data and allows to stream.
Typically am trying to have a dashboard of data streaming. So only authorized streaming is allowed for user. Also he will be able to see all the topic name. whether its possible to use proxy name , example 1, or 2 might be the value he would use from client side, when it reaches the gateway it verify the authorization and replace the value with real topic name and establish the data.
Please let me know whether I've my question clear. Your valuable suggestions and guidance will be more helpful to continue with my research.
Thanks
Krish
in case you wonder how the world has changed over the last five year to address this kind of need, now you could give a try to streamdata.io, a proxy which uses SSE protocol (unidirectional) and is available in minutes. It would perfectly fit what's needed in the example given, because Im not sure WebSocket is the ideal answer here. Tho it should work, it's basically like buying a harvester to go get your kids at school.
As a disclaimer, I need to say that I work for Streamdata, and we see this kind of thing every day.
WebSocket is great if you need bidirectional streams of data, like in chats or games. Most of the time, when the clients mainly expect updated data from the server, instead of polling AF or deploying WebSocket, SSE is the best alternative as it's easier to deploy (http, etc.). The rest of the settings (for security) can be done through the proxy interface.
Boom: it's available before you even realize it, it's secure, easy to maintain and more efficient than before. You have more time available to answer questions from the community over here; your team is happy; your boss loves you more than before and might even give you a raise. Life can be so simple sometimes!
I am basically writing an almost purely clientside application (there is a webserver which can be used to store some persistent data, but its easier to forget about it), but as part of this I was looking to add some functionality akin to hosting a game.
The scenario would be 1 person would host the game via their browser (open a TCP socket awaiting connections), then X other people would connect to that server and join. The server would be in charge of receiving and sending data between clients.
So in this scenario is it possible to host a websocket server within a webpage?
I was looking at trying to do something peer to peer style, but I don't think it is currently supported, but its not a major problem as its only going to be for sending small amounts of text and some update messages between clients.
The WebSocket browser API is client only (for the foreseeable future).
In some sense, WebRTC us peer-to-peer, but even if the WebRTC API adds the ability to send arbitrary data, you still need a STUN/TURN server to establish the initial connection.
First let me explain the data flow I need
Client connects and registers with server
Server sends initialization JSON to client
Client listens for JSON messages sent from the server
Now all of this is easy and straightforward to do manually, but I would like to leverage a server of some sort to handle all of the connection stuff, keep-alive, dead clients, etc. etc.
Is there some precedent set on doing this kind of thing? Where a client connects and receives JSON messages asynchronously from a server? Without using doing manual socket programming?
A possible solution is known as Comet, which involves the client opening a connection to the server that stays open for a long time. Then the server can push data to the client as soon as it's available, and the client gets it almost instantly. Eventually the Comet connection times out, and another is created.
Not sure what language you're using but I've seen several of these for Java and Scala. Search for comet framework and your language name in Google, and you should find something.
In 'good old times' that would be easy, since at the first connection the server gets the IP number of the client, so it could call back. So easy, in fact, that it was how FTP does it for no good reason.... But now we can be almost certain that the client is behind some NAT, so you can't 'call back'.
Then you can just keep the TCP connection open, since it's bidirectional, just make the client wait for data to appear. The server would send whatever it wants whenever it can.... But now everybody wants every application to run on top of a web browser, and that means HTTP, which is a strictly 'request/response' initiated by the client.
So, the current answer is Comet. Simply put, a JavaScript client sends a request, but the server doesn't answer for a looooong time. if the connection times out, the client immediately reopens it, so there's always one open pipe waiting for the server's response. That response will contain whatever message the server want's to send to the client, and only when it's pertinent. The client receives it, and immediately sends a new query to keep the channel open.
The problem is that HTTP is a request response protocol. The server cannot send any data unless a requests is submitted by the client.
Trying to circumvent this by macking a request and then continously send back responses on the same, original, requests is flawed as the behavior does not conform with HTTP and it does not play well with all sort of intermediaries (proxies, routers etc) and with the browser behavior (Ajax completion). It also doesn't scale well, keeping a socket open on the server is very resource intensive and the sockets are very precious resources (ordinarly only few thousand available).
Trying to circumvent this by reversing the flow (ie. server connects to the client when it has somehting to push) is even more flawed because of the security/authentication problems that come with this (the response can easily be hijacked, repudiated or spoofed) and also because often times the client is unreachable (lies behind proxies or NAT devices).
AFAIK most RIA clients just poll on timer. Not ideal, but this how HTTP works.
GWT provides a framework for this kind of stuff & has integration with Comet (at least for Jetty). If you don't mind writing at least part of your JavaScript in Java, it might be the easier approach.
I have a service that accepts callbacks from a provider.
Motivation: I do not want to EVER lose any callbacks (unless of course my network becomes unreachable).
Let's suppose the impossible happens and my mysql server becomes unreachable for some time,
I want to fallback to a secondary persistence store once I've retried several times and fail.
What are my options? Queues, in-memory cache ?
You say you're receiving "Callbacks" - you've not made clear what they are. What is the protocol? Is it over a network.
If it were HTTP, then I would say the best way is that if your application is unable to write the data into permanent storage, it should return an error ("Try again later" if that exists in the protocol) to the caller, who should try again later.
An asynchronous process like a callback should always be able to cope with failures downstream and queue its requests.
I've worked with a payment provider where this has been the case (Paypal). If you're unable to completely process the request, just send an error back to the caller.
I recommend some sort of job queue server. I personally use Starling and have had great results with it. It speaks the memcache protocol so it is easy to use as a persistent queue.
Starling on Github
I've put a queue in SQLite for this before. Though, in my case, it was to protect against loss of the network link to the MySQL server — the data was locally-generated.
You can have a backup MySQL server, and switch your connection to that one in case primary one breaks down. If it's going to be only fail-over store you could probably run it locally on the application server.