I am running an Apache server that receives HTTP requests and connects to a daemon script over ZeroMQ. The script implements the Multithreaded Server pattern (http://zguide.zeromq.org/page:all#header-73), it successfully receives the request and dispatches it to one of its worker threads, performs the action, responds back to the server, and the server responds back to the client. Everything is done synchronously as the client needs to receive a success or failure response to its request.
As the number of users is growing into a few thousands, I am looking into potentially improving this. The first thing I looked at is the different patterns of ZeroMQ, and whether what I am using is optimal for my scenario. I've read the guide but I find it challenging understanding all the details and differences across patterns. I was looking for example at the Load Balancing Message Broker pattern (http://zguide.zeromq.org/page:all#header-73). It seems quite a bit more complicated to implement than what I am currently using, and if I understand things correctly, its advantages are:
Actual load balancing vs the round-robin task distribution that I currently have
Asynchronous requests/replies
Is that everything? Am I missing something? Given the description of my problem, and the synchronous requirement of it, what would you say is the best pattern to use? Lastly, how would the answer change, if I want to make my setup distributed (i.e. having the Apache server load balance the requests across different machines). I was thinking of doing that by simply creating yet another layer, based on the Multithreaded Server pattern, and have that layer bridge the communication between the web server and my workers.
Some thoughts about the subject...
Keep it simple
I would try to keep things simple and "plain" ZeroMQ as long as possible. To increase performance, I would simply to change your backend script to send request out from dealer socket and move the request handling code to own program. Then you could just run multiple worker servers in different machines to get more requests handled.
I assume this was the approach you took:
I was thinking of doing that by simply creating yet another layer, based on the Multithreaded Server pattern, and have that layer bridge the communication between the web server and my workers.
Only problem here is that there is no request retry in the backend. If worker fails to handle given task it is forever lost. However one could write worker servers so that they handle all the request they got before shutting down. With this kind of setup it is possible to update backend workers without clients to notice any shortages. This will not save requests that get lost if the server crashes.
I have the feeling that in common scenarios this kind of approach would be more than enough.
Mongrel2
Mongrel2 seems to handle quite many things you have already implemented. It might be worth while to check it out. It probably does not completely solve your problems, but it provides tested infrastructure to distribute the workload. This could be used to deliver the request to be handled to multithreaded servers running on different machines.
Broker
One solution to increase the robustness of the setup is a broker. In this scenario brokers main role would be to provide robustness by implementing queue for the requests. I understood that all the requests the worker handle are basically the same type. If requests would have different types then broker could also do lookups to find correct server for the requests.
Using the queue provides a way to ensure that every request is being handled by some broker even if worker servers crashed. This does not come without price. The broker is by itself a single point of failure. If it crashes or is restarted all messages could be lost.
These problems can be avoided, but it requires quite much work: the requests could be persisted to the disk, servers could be clustered. Need has to be weighted against the payoffs. Does one want to use time to write a message broker or the actual system?
If message broker seems a good idea the time which is required to implement one can be reduced by using already implemented product (like RabbitMQ). Negative side effect is that there could be a lot of unwanted features and adding new things is not so straight forward as to self made broker.
Writing own broker could covert toward inventing the wheel again. Many brokers provide similar things: security, logging, management interface and so on. It seems likely that these are eventually needed in home made solution also. But if not then single home made broker which does single thing and does it well can be good choice.
Even if broker product is chosen I think it is a good idea to hide the broker behind ZeroMQ proxy, a dedicated code that sends/receives messages from the broker. Then no other part of the system has to know anything about the broker and it can be easily replaced.
Using broker is somewhat developer time heavy. You either need time to implement the broker or time to get use to some product. I would avoid this route until it is clearly needed.
Some links
Comparison between broker and brokerless
RabbitMQ
Mongrel2
Related
I was writing a POC on long-polling using go.
I see the general package to be used is https://github.com/jcuga/golongpoll .
But assuming that I would want to publish an event to the golongpoll.SubscriptionManager from a general context, especially when there is a possibility that the long poll API request is being served by one machine, while the Kafka event for that particular consumer group is consumed by another instance in the cluster.
The examples given in the documentation did not talk of such a scenario at all, even though this seems like a common scenario. One way I can think of is have a distributed cache like Redis in between and have all the services poll this for a change? But that sounds a bit dumb to me.
Should a websocket connection be general or specific?
e.g. If I was building a stock trading system, I'd likely to have real time stock prices, real time trade information, real time updates to the order book, perhaps real time chat to enable traders to collude and manipulate the market. Should I have one websocket to handle all the above data flow or is it better to have several websocket to handle different topics?
It all depends. Let's look at your options, assuming your stock trader, your chat, and your order book are built as separate servers/micro-services.
One WebSocket for each server
You can have each server running their own WebSocket server, streaming events relevant to that server.
Pros
It is a simple approach. Each server is independent.
Cons
Scales poorly. The number of open TCP connections will come at a price as the number of concurrent users increases. Increased complexity when you need to replicate the servers for redundancy, as all replicas needs to broadcast the same events. You also have to build your own fallback for recovering from client data going stale due to lost WebSocket connection. Need to create event handlers on the client for each type of event. Might have to add version handling to prevent data races if initial data is fetched over HTTP, while events are sent on the separate WebSocket connection.
Publish/Subscribe event streaming
There are many publish/subscribe solutions available, such as Pusher, PubNub or SocketCluster. The idea is often that your servers publish events on a topic/subject to a message queue, which is listened to by WebSocket servers that forwards the events to the connected clients.
Pros
Scales more easily. The server only needs to send one message, while you can add more WebSocket servers as the number of concurrent users increases.
Cons
You most likely still have to handle recovery from events lost during disconnect. Still might require versioning to handle data races. And still need to write handlers for each type of event.
Realtime API gateway
This part is more shameless, as it covers Resgate, an open source project I've been involved in myself. But it also applies to solutions such as Firebase. With the term "realtime API gateway", I mean an API gateway that not only handles HTTP requests, but operates bidirectionally over WebSocket as well.
With web clients, you are seldom interested in events - you are interested in change of state. Events are just means to either describe the changes. By fetching the data through a gateway, it can keep track on which resources the client is currently interested in. It will then keep the client up to date for as long as the data is being used.
Pros
Scales well. Client requires no custom code for event handling, as the system updates the client data for you. Handles recovery from lost connections. No data races. Simple to work with.
Cons
Primarily for client rendered web sites (using React, Vue, Angular, etc), as it works poorly with sites with server-rendered pages. Harder to apply to already existing HTTP API's.
We need to deliver real-time messages to our clients, but their servers are behind a proxy, and we cannot initialize a connection; webhook variant won't work.
What is the best way to deliver real-time messages considering that:
client that is behind a proxy
client can be off for a long period of time, and all messages must be delivered
the protocol/way must be common enough, so that even a PHP developer could easily use it
I have in mind three variants:
WebSocket - client opens a websocket connection, and we send messages that were stored in DB, and messages comming in real time at the same time.
RabbitMQ - all messages are stored in a durable, persistent queue. What if partner will not read from a queue for some time?
HTTP GET - partner will pull messages by blocks. In this approach it is hard to pick optimal pull interval.
Any suggestions would be appreciated. Thanks!
Since you seem to have to store messages when your peer is not connected, the question applies to any other solution equally: what if the peer is not connected and messages are queueing up?
RabbitMQ is great if you want loose coupling: separating the producer and the consumer sides. The broker will store messages for you if no consumer is connected. This can indeed fill up memory and/or disk space on the broker after some time - in this case RabbitMQ will shut down.
In general, RabbitMQ is a great tool for messaging-based architectures like the one you describe:
Load balancing: you can use multiple publishers and/or consumers, thus sharing load.
Flexibility: you can configure multiple exchanges/queues/bindings if your business logic needs it. You can easily change routing on the broker without reconfiguring multiple publisher/consumer applications.
Flow control: RabbitMQ also gives you some built-in methods for flow control - if a consumer is too slow to keep up with publishers, RabbitMQ will slow down publishers.
You can refactor the architecture later easily. You can set up multiple brokers and link them via shovel/federation. This is very useful if you need your app to work via multiple data centers.
You can easily spot if one side is slower than the other, since queues will start growing if your consumers can't read fast enough from a queue.
High availability and fault tolerance. RabbitMQ is very good at these (thanks to Erlang).
So I'd recommend it over the other two (which might be good for a small-scale app, but you might grow it out quickly is requirements change and you need to scale up things).
Edit: something I missed - if it's not vital to deliver all messages, you can configure queues with a TTL (message will be discarded after a timeout) or with a limit (this limits the number of messages in the queue, if reached new messages will be discarded).
I am planning to inegrate messaging middleware in my web application. Right now I am tesing different messaging middleware software like RabbitMQ,JMS, HornetQ, etc..
Examples provided with this softwares are working but its not giving as desired results.
So, I want to know that which are the factors which are responsible to improve peformance that one should keep in eyes?
Which are the areas, a developer should take care of to improve the performance of middleware messaging software?
I'm the project lead for HornetQ but I will try to give you a generic answer that could be applied to any message system you choose.
A common question that I see is people asking why a single producer / single consumer won't give you the expected performance.
When you send a message, and are asking confirmation right away, you need to wait:
The message transfer from client to server
The message being persisted on the disk
The server acknowledging receipt of the message by sending a callback to the client
Similarly when you are receiving a message, you ACK to the server:
The ACK is sent from client to server
The ACK is persisted
The server sends back a callback saying that the callback was achieved
And if you need confirmation for all your message-sends and mesage-acks you need to wait these steps as you have a hardware involved on persisting the disk and sending bits on the network.
Message Systems will try to scale up with many producers and many consumers. That is if many are producing they should all use the resources available at the server shared for all the consumers.
There are ways to speed up a single producer or single consumer:
One is by using transactions. So, you minimize the blocks and syncs you perform on disk while persisting at the server and roundtrips on the network. (This is actually the same on any database)
Another one, is by using Callbacks instead of blocking at the consumer. (JMS 2 is proposing a Callback similar to the ConfirmationHandler on HornetQ).
Also: most providers I know will have a performance section on their docs with requirements and suggestions for that specific product. You should look individually at each product
Will web sockets when used in all web browsers make ajax obsolete?
Cause if I could use web sockets to fetch data and update data in realtime, why would I need ajax? Even if I use ajax to just fetch data once when the application started I still might want to see if this data has changed after a while.
And will web sockets be possible in cross-domains or only to the same origin?
WebSockets will not make AJAX entirely obsolete and WebSockets can do cross-domain.
AJAX
AJAX mechanisms can be used with plain web servers. At its most basic level, AJAX is just a way for a web page to make an HTTP request. WebSockets is a much lower level protocol and requires a WebSockets server (either built into the webserver, standalone, or proxied from the webserver to a standalone server).
With WebSockets, the framing and payload is determined by the application. You could send HTML/XML/JSON back and forth between client and server, but you aren't forced to. AJAX is HTTP. WebSockets has a HTTP friendly handshake, but WebSockets is not HTTP. WebSockets is a bi-directional protocol that is closer to raw sockets (intentionally so) than it is to HTTP. The WebSockets payload data is UTF-8 encoded in the current version of the standard but this is likely to be changed/extended in future versions.
So there will probably always be a place for AJAX type requests even in a world where all clients support WebSockets natively. WebSockets is trying to solve situations where AJAX is not capable or marginally capable (because WebSockets its bi-directional and much lower overhead). But WebSockets does not replace everything AJAX is used for.
Cross-Domain
Yes, WebSockets supports cross-domain. The initial handshake to setup the connection communicates origin policy information. The wikipedia page shows an example of a typical handshake: http://en.wikipedia.org/wiki/WebSockets
I'll try to break this down into questions:
Will web sockets when used in all web browsers make ajax obsolete?
Absolutely not. WebSockets are raw socket connections to the server. This comes with it's own security concerns. AJAX calls are simply async. HTTP requests that can follow the same validation procedures as the rest of the pages.
Cause if I could use web sockets to fetch data and update data in realtime, why would I need ajax?
You would use AJAX for simpler more manageable tasks. Not everyone wants to have the overhead of securing a socket connection to simply allow async requests. That can be handled simply enough.
Even if I use ajax to just fetch data once when the application started I still might want to see if this data has changed after a while.
Sure, if that data is changing. You may not have the data changing or constantly refreshing. Again, this is code overhead that you have to account for.
And will web sockets be possible in cross-domains or only to the same origin?
You can have cross domain WebSockets but you have to code your WS server to accept them. You have access to the domain (host) header which you can then use to accept / deny requests. This can, however, be spoofed by something as simple as nc. In order to truly secure the connection you will need to authenticate the connection by other means.
Websockets have a couple of big downsides in terms of scalability that ajax avoids. Since ajax sends a request/response and closes the connection (..or shortly after) if someone stays on the web page it doesn't use server resources when idling. Websockets are meant to stream data back to the browser, and they tie up server resources to do so. Servers have a limit in how many simultaneous connections they can keep open at one time. Not to mention depending on your server side technology, they may tie up a thread to handle the socket. So websockets have more resource intensive requirements for both sides per connection. You could easily exhaust all of your threads servicing clients and then no new clients could come in if lots of users are just sitting on the page. This is where nodejs, vertx, netty can really help out, but even those have upper limits as well.
Also there is the issue of state of the underlying socket, and writing the code on both sides that carry on the stateful conversation which isn't something you have to do with ajax style because it's stateless. Websockets require you create a low level protocol which is solved for you with ajax. Things like heart beating, closing idle connections, reconnection on errors, etc are vitally important now. These are things you didn't have to solve when using AJAX because it was stateless. State is very important to the stability of your app and more importantly the health of your server. It's not trivial. Pre-HTTP we built a lot of stateful TCP protocols (FTP, telnet, SSH), and then HTTP happened. And no one did that stuff much anymore because even with its limitations HTTP was surprisingly easier and more robust. Websockets bring back the good and the bad of stateful protocols. You'll learn soon enough if you didn't get a dose of that last go around.
If you need streaming of realtime data this extra overhead is warranted because polling the server to get streamed data is worse, but if all you are doing is user interaction->request->response->update UI, then ajax is easier and will use less resources because once the response is sent the conversation is over and no additional server resources are used. So I think it's a tradeoff and the architect has to decide which tool fits their problem. AJAX has its place, and websockets have their place.
Update
So the architecture of your server is what matters when we are talking about threads. If you are using a traditionally multi-threaded server (or processes) where a each socket connection gets its own thread to respond to requests then websockets matter a lot to you. So for each connection we have a socket, and eventually the OS will fall over if you have too many of these, and the same goes for threads (more so for processes). Threads are heavier than sockets (in terms of resources) so we try and conserve how many threads we have running simultaneously. That means creating a thread pool which is just a fixed number of threads that is shared among all sockets. But once a socket is opened the thread is used for the entire conversation. The length of those conversations govern how quickly you can repurpose those threads for new sockets coming in. The length of your conversation governs how much you can scale. However if you are streaming this model doesn't work well for scaling. You have to break the thread/socket design.
HTTP's request/response model makes it very efficient in turning over threads for new sockets. If you are just going to use request/response use HTTP its already built and much easier than reimplementing something like that in websockets.
Since websockets don't have to be request/response as HTTP and can stream data if your server has a fixed number of threads in its thread pool and you have the same number of websockets tying up all of your threads with active conversations, you can't service new clients coming in! You've reached your maximum capacity. That's where protocol design is important too with websockets and threads. Your protocol might allow you to loosen the thread per socket per conversation model that way people just sitting there don't use a thread on your server.
That's where asynchronous single thread servers come in. In Java we often call this NIO for non-blocking IO. That means it's a different API for sockets where sending and receiving data doesn't block the thread performing the call.
So traditional in blocking sockets when you call socket.read() or socket.write() they wait until the data is received or sent before returning control to your program. That means your program is stuck waiting for the socket data to come in or go out until you can do anything else. That's why we have threads so we can do work concurrently (at the same time). Send this data to client X while I wait on data from client Y. Concurrencies is the name of the game when we talk about servers.
In a NIO server we use a single thread to handle all clients and register callbacks to be notified when data arrives. For example
socket.read( function( data ) {
// data is here! Now you can process it very quickly without waiting!
});
The socket.read() call will return immediately without reading any data, but our function we provided will be called when it comes in. This design radically changes how you build and architect your code because if you get hung up waiting on something you can't receive any new clients. You have a single thread you can't really do two things at once! You have to keep that one thread moving.
NIO, Asynchronous IO, Event based program as this is all known as, is a much more complicated system design, and I wouldn't suggest you try and write this if you are starting out. Even very Senior programmers find it very hard to build a robust systems. Since you are asynchronous you can't call APIs that block. Like reading data from the DB or sending messages to other servers have to be performed asynchronously. Even reading/writing from the file system can slow your single thread down lowering your scalability. Once you go asynchronous it's all asynchronous all the time if you want to keep the single thread moving. That's where it gets challenging because eventually you'll run into an API, like DBs, that is not asynchronous and you have to adopt more threads at some level. So a hybrid approaches are common even in the asynchronous world.
The good news is there are other solutions that use this lower level API already built that you can use. NodeJS, Vertx, Netty, Apache Mina, Play Framework, Twisted Python, Stackless Python, etc. There might be some obscure library for C++, but honestly I wouldn't bother. Server technology doesn't require the very fastest languages because it's IO bound more than CPU bound. If you are a die hard performance nut use Java. It has a huge community of code to pull from and it's speed is very close (and sometimes better) than C++. If you just hate it go with Node or Python.
Yes, yes it does. :D
The earlier answers lack imagination. I see no more reason to use AJAX if websockets are available to you.