I have a server which supports web sockets. Browsers connect to my site and each one opens a web socket to www.mydomain.example. That way, my social network app can push messages to the clients.
Traditionally, using just HTTP requests, I would scale up by adding a second server and a load balancer in front of the two web servers.
With web sockets, the connection has to be directly with the web server, not the load balancers, because if a machine has a physical limit of say 64k open ports, and the clients were connecting to the load balancer, then I couldn't support more than 64k concurrent users.
So how do I:
get the client to connect directly to the web server (rather than the load balancer) when the page loads? Do I simply load the JavaScript from a node, and the load balancers (or whatever) randomly modifies the URL for the script, every time the page is initially requested?
handle a ripple start? The browser will notice that the connection is closed as the web server shuts down. I can write JavaScript code to attempt to reopen the connection, but the node will be gone for a while. So I guess I would have to go back to the load balancer to query the address of the next node to use?
I did wonder about the load balancers sending a redirect on the initial request, so that the browser initially requests www.mydomain.example and gets redirected to www34.mydomain.example. That works quite well, until the node goes down - and sites like Facebook don't do that. How do they do it?
Put a L3 load-balancer that distributes IP packets based on source-IP-port hash to your WebSocket server farm. Since the L3 balancer maintains no state (using hashed source-IP-port) it will scale to wire speed on low-end hardware (say 10GbE). Since the distribution is deterministic (using hashed source-IP-port), it will work with TCP (and hence WebSocket).
Also note that a 64k hard limit only applies to outgoing TCP/IP for a given (source) IP address. It does not apply to incoming TCP/IP. We have tested Autobahn (a high-performance WebSocket server) with 200k active connections on a 2 core, 4GB RAM VM.
Also note that you can do L7 load-balancing on the HTTP path announced during the initial WebSocket handshake. In that case the load balancer has to maintain state (which source IP-port pair is going to which backend node). It will probably scale to millions of connections nevertheless on decent setup.
Disclaimer: I am original author of Autobahn and work for Tavendo.
Note that if your websocket server logic runs on nodejs with socket.io, you can tell socket.io to use a shared redis key/value store for synchronization.
This way you don't even have to care about the load balancer, events will propagate among the server instances.
var io = require('socket.io')(3000);
var redis = require('socket.io-redis');
io.adapter(redis({ host: 'localhost', port: 6379 }));
See: Socket IO - Using multiple nodes
But at some point I guess redis can become the bottleneck...
You can also achieve layer 7 load balancing with inspection and "routing functionality"
See "How to inspect and load-balance WebSockets traffic using Stingray Traffic Manager, and when necessary, how to manage WebSockets and HTTP traffic that is received on the same IP address and port." https://splash.riverbed.com/docs/DOC-1451
Related
I have an AWS Application load balancer to distribute the http(s) traffic.
Problem 1:
Suppose I have a target group with 2 EC2 instances: micro and xlarge. Obviously they can handle different traffic levels. Does the load balancer manage traffic proportionally to instance sizes or just round robin? If only round robin is used and no other factors taken into account, then it's not really balancing load, because at some point the micro instance will be suffering from the traffic, while xlarge will starve.
Problem 2:
Suppose I have target group with 2 EC2 instances, both are same size. But my service is not using a classic http request/response flow. It is using HTTP websockets, i.e. a client makes HTTP request just once, to establish a socket, and then keeps the socket open for longer time, sending and receiving messages (e.g. a chat service). Let's suppose my load balancer is using round robin and both EC2 instances have 1000 clients connected each. Now suppose one of the EC2 instances goes down and 1000 connected clients drop their socket connections. The instance gets back up quickly and is ready to accept websocket calls again. The 1000 clients who dropped are trying to reconnect. Now, if the load balancer would use pure round robin, I'll end up with 1500 clients connected to instance #1 and 500 clients connected to instance #2, thus not really balancing the load correctly.
Basically, I'm trying to find out if some more advanced logic is being used to select a target in a group, or is it just a naive round robin selection. If it's round robin only, then how can I really balance the websocket connections load?
Websockets start out as http or https connections, so a load balancer can dispatch them to a server. Once the server accepts the http connection, both the server and the client "upgrade" the connection to use the websocket protocol. They then leave the connection open to use for websocket traffic. As far as the load balancer can tell, the connection is simply a long-lasting http connection.
Taking a server down when it has websocket connections to clients requires your application to retry lost connections. Reconnecting on connection failure is one of the trickiest parts of websocket client programming. Your application cannot be robust without reconnect logic.
AWS's load balancer has no built-in knowledge of the capabilities of the servers behind it. You have observed that it sends requests equally to big and small servers. That can overwhelm the small ones.
I have managed this by building a /healthcheck endpoint in my servers. It's a straightforward https://example.com/heathcheck web page. You can put a little bit of content on the page announcing how many websocket connections are currently open, or anything else. Don't password protect it or require a session to hit it.
My /healthcheck endpoints, whenever hit, measure the server load. I simply use the number of current websocket connections, but you can use any metric you want. I compare the current load to a load threshold configured for each server. For example, on a micro instance I can handle 20 open websockets, and on a production instance I can handle 400.
If the server load is too high, my endpoint gives back a 503 http error status along with its content. 503 typically means "I am overloaded, please try again later." It can also mean "I will shut down when all my connections are closed. Please don't use me for any more connections."
Then I configure the load balancer to perform those health checks every couple of minutes on all the servers in the server pool (AWS calls the pool a "target group"). The health check operation detects "unhealthy" servers and temporarily takes them out of its rotation. (The health check also detects crashed servers, which is good.)
You need this loadbalancer health check for a large-scale production setup.
All that being said, you will get best results if all your server instances in your pool have roughly the same capacity as each other.
Our infrastructure is composed by
1 F5 load balancer
3 nodes
We have an application which uses websockets, so when a user goes to our site, it opens a websocket to the balancer which it connects to the first available node, and it works as expected.
Our truobles arrives with maintenance tasks, when we have to update our software, we need to turn offline 1 node at a time, deploy the new release and then turn it on again. Doing this task, the balancer drops the open websocket connections to the node and the clients retries to connect after few seconds to the first available nodes, creating an inconvenience for the client because he could miss a signal (or more).
How we can keep the connection between the client and the balancer, changing the backend websocket server? Is the load balancer enough to achieve our goal or we need to change our infrastructure?
To avoid this kind of problems I recommend to read about the Azure SignalR. With this you don't need to thing about stuff like load balancer, redis backplane and other infrastructures that you possibly need to a WebSockets connection.
Basically the clients will not connected to your node directly but redirected to Azure SignalR. You can read more about it here: https://learn.microsoft.com/en-us/azure/azure-signalr/signalr-overview
Since it is important to your application to maintain the connection, I don't see how any other way to archive no connection drop to your nodes, since you need to shut them down.
It's important to understand that the F5 is a full TCP proxy. This means that the F5 is the server to the client and the client to the server. If you are using the websockets protocol then you must apply a websockets profile to the F5 Virtual Server in order for the websockets application to be handled properly by the Load Balancer.
Details of the websockets profile can be found here: https://support.f5.com/csp/article/K14754
If a websockets and an HTTP profile are applied to the Virtual Server - meaning that you have websockets and web traffic using the same port and LB nodes - then the F5 will allow the websockets traffic as passthrough. Also keep in mind that if this is an HTTPS virtual sever that you will need to ensure a client and server side HTTPS profile (SSL offload) are applied to the Virtual Server.
While there are a variety of ways that you can fiddle with load balancers to minimize the downtime caused by a software upgrade, none of them solve the problem, which is that your application-layer protocol seems to not tolerate some small network outages.
Even if you have a perfect load balancer and your software deploys cause zero downtime, the customer's computer may be on flaky wifi which causes a network dropout for half a second - or going over ethernet and someone reconfigures some routing on their LAN, etc.
I'd suggest having your server maintain a queue of messages for clients (up to some size/time limit) so that when a client drops a connection - whether it be due to load balancers/upgrades - or any other reason, it can continue without disruption.
We are using 2 servers and have setup load balancer to redirect the trafic. Both servers are Compute engines.
We are also using websocket (socket.io) to keep the connection between users (online and offline status). When connection is established between users, it gets disconnected after few seconds. We concluded that it is load balancer configuration issue as if we use single server (without load balancer), connection remains alive until user goes offline.
We need help here if we need to do anything extra in load balancer configurations to work it smoothly with websocket.
Using ip addresses, not domain name (if that makes any difference)
I use a load balancer with my web site. The browser initiates a websocket connection to my app server. Does the open connection consume any resources on the LB or is it direct between the browser and the app server? If there is something open on the LB isn't it a bottleneck? I mean if my LB can handle X open connections then the X+1 user could not even open a connection.
It depends!
The most efficient load balancers listen for requests, do some analysis, then forward the requests; all the bits do not travel through the load balancer. The network forwarding happens at a lower network layer than http (e.g., it is not an http 302 redirect - the client never knows it happened, maintaining privacy around internal network configuration - this happens at OSI Level 4 I think).
However, some load balancers add more features, like acting as SSL endpoints or applying gzip compression. In these cases, they are processing bits as they pass through (encrypt/decrypt or compress in this case).
A picture may help. Compare the first diagram with the second & third here, noting redirection in the first that is absent in the others.
Is that called "clustering" of servers? When a web request is sent, does it go through the main server, and if the main server can't handle the extra load, then it forwards it to the secondary servers that can handle the load? Also, is one "server" that's up and running the application called an "instance"?
[...] Is that called "clustering" of servers?
Clustering is indeed using transparently multiple nodes that are seen as a unique entity: the cluster. Clustering allows you to scale: you can spread your load on all the nodes and, if you need more power, you can add more nodes (short version). Clustering allows you to be fault tolerant: if one node (physical or logical) goes down, other nodes can still process requests and your service remains available (short version).
When a web request is sent, does it go through the main server, and if the main server can't handle the extra load, then it forwards it to the secondary servers that can handle the load?
In general, this is the job of a dedicated component called a "load balancer" (hardware, software) that can use many algorithms to balance the request: round-robin, FIFO, LIFO, load based...
In the case of EC2, you previously had to load balance with round-robin DNS and/or HA Proxy. See Introduction to Software Load Balancing with Amazon EC2. But for some time now, Amazon has launched load balancing and auto-scaling (beta) as part of their EC2 offerings. See Elastic Load Balancing.
Also, is one "server" that's up and running the application called an "instance"?
Actually, an instance can be many things (depending of who's speaking): a machine, a virtual machine, a server (software) up and running, etc.
In the case of EC2, you might want to read Amazon EC2 Instance Types.
Here is a real example:
This specific configuration is hosted at RackSpace in their Managed Colo group.
Requests pass through a Cisco Firewall. They are then routed across a Gigabit LAN to a Cisco CSS 11501 Content Services Switch (eg Load Balancer). The Load Balancer matches the incoming content to a content rule, handles the SSL decryption if necessary, and then forwards the traffic to one of several back-end web servers.
Each 5 seconds, the load balancer requests a URL on each webserver. If the webserver fails (two times in a row, IIRC) to respond with the correct value, that server is not sent any traffic until the URL starts responding correctly.
Further behind the webservers is a MySQL master / slave configuration. Connections may be mad to the master (for transactions) or to the slaves for read only requests.
Memcached is installed on each of the webservers, with 1 GB of ram dedicated to caching. Each web application may utilize the cluster of memcache servers to cache all kinds of content.
Deployment is handled using rsync to sync specific directories on a management server out to each webserver. Apache restarts, etc.. are handled through similar scripting over ssh from the management server.
The amount of traffic that can be handled through this configuration is significant. The advantages of easy scaling and easy maintenance are great as well.
For clustering, any web request would be handled by a load balancer, which being updated as to the current loads of the server forming the cluster, sends the request to the least burdened server. As for if it's an instance.....I believe so but I'd wait for confirmation first on that.
You'd' need a very large application to be bothered with thinking about clustering and the "fun" that comes with it software and hardware wise, though. Unless you're looking to start or are already running something big, it wouldn't' be anything to worry about.
Yes, it can be required for clustering. Typically as the load goes up you might find yourself with a frontend server that does url rewriting, https if required and caching with squid say. The requests get passed on to multiple backend servers - probably using cookies to associate a session with a particular backend if necessary. You might have the database on a separate server also.
I should add that there are other reasons why you might need multiple servers, for instance there may be a requirement that the database is not on the frontend server for security reasons