ASP.NET and
SignalR have some improvements. For example SignalR is now being used in the
Visual Studio development experience making it possible for edits to be propagated
across browsers without referh. Here are some notes from Damian Edwards presentation
:
3-502
Scaling the real time web with ASP.NET SignalR
See www.asp.net/signalr
for getting started
· Scaling real-time traffic shares
many considerations with traditional web traffic eg CPU, bandwidth, memory
· Application scenarios have huge
impact on scaling patterns.
· Big difference is in concurrency,
supporting many long running idle and active connections vs short requests
· Different SignalR transports have
different overheads
General
things to watch out for:
Blocking
calls eg block I/O
· Never ever block a Hub method, it
jams up pipes
· Use 4.5 async where possible
Sending
large messages
· Memory leaks caused by
misunderstanding SignalR object lifetime eg Hub instances
· Session - don't use it from SignalR.
Instead use Hub state, cookies, browser storage, database etc. instead
Remember the
secret of scale "Have your app do as little as possible. If you do
nothing, you can scale infinitely" - Scott Hanselman
SignalR core architecture: Pub/Sub
1. Publisher
Message
serialized and saved to cache associated with Signal, topic is marked for delivery
2. Message Cache
3. Worker
Worker is
scheduled for signal, selects a waiting subscriber, retrieves message from
cache
Worker sends
message to client as bytes over transport
4. Client
Pattern 1 Sever broadcast
Low rate,
message to all clients
Low rate
broadcast of the same payload to clients
One message
buss sends maps to many users (fan out)
More clients
don't increase bus traffic
eg
application wide alerts
Pattern 2 Server Push
Low rate,
message to unique clients)
Low rate
broadcast of the unique payload to each client
One message
bus sends maps to one user (no fan out)
More clients
means more message bus traffic
eg Job
monitor
Pattern 3 User event driven
Broadcast on
client action
Broadcast on
client actions
One message
bus send maps too many users (fan out)
More clients
means more message traffic
eg Chat
Pattern 4 High frequency real-time
Fixed high
rate, unique message
Fixed high
rate broadcast from servers and clients (don't go above 25Hz
One message
bus sends maps to one user (no fan out)
More clients
means more message traffic
eg Gaming
Demo
There are
command line utilities for Signal R in Microsoft.AspNt.SignalR.Utils. These
include things like
signalR
[args]
· ipc Installs SignalR performance
counters
· upc Uninstalls SignalR performance
counters
· ghp Generates HubProxy JavaScript
files for server Hub classes
To generate
load use crank.exe /url:http://localhost:29573/TestConnection
/Clients:100 /BatchSize:200
For Isolated
scenarios in-proc use Stress (Microsoft.AspNet.SignalR.Stress\bin\Debug)
Adjust IIS
Settings for concurrency limits
There was
something said about Max concurrent requests per CPU
appcmd.exe
set config /section:system.webserver/serverRuntime
/appConcurrentRequestLimit:100000
Scale-out Issues
· Client transience: How many messages
one server gets to the other servers in my web farm
· Client transience: When does a
client disconnect from my App
· Client transience: How do I avoid
duplicate and missed messages as I move from one server to another
· Client distribution: What happens if
one function on one server is called many more times than others
To solve
these issues use SQL Server, Redis & Windows Azure Service Bus (from
NuGet). This works well for Server Broadcast pattern but limited for others because
every message goes to every server, therefor as traffic increases you are
limited by how fast any web server can pull messages off the backplane.
Back planes
are much slower than single server
microsoft.aspnet.signalr.redis
public
static void Start()
{
GlobalHost.DependencyResolver.UseRedis("localhost",
6379, "", "build");
RouteTable.Routes.MapHubs();
}
Custom scale-out
· Common Server
· Specific Server
· Filtered message bus
· Server transition
· Hybrid
2.0 Scale out improvements
· Support for pre serialized messages
· Support for single serialization
when sending multiple messages
Resources
· github.com/signalr/signalr
· twitter.com/damianedwards
