Scale-out generally involves increasing the number of application instances and placing a load balancer in front of them. This approach presents challenges when using SignalR, as each instance will only maintain records of its connected clients.
In the image above, clients #1 and #2 are connected to server #1, while client #3 is connected to server #2. Therefore, when calling Clients.All.SendAsync(), messages are only sent to clients connected to the respective instance. Fortunately, the Redis backplane provides a solution to this problem.
The Redis backplane employs the publish/subscribe pattern to synchronize the servers, working as follows:
All servers subscribe to the Redis backplane.
Whenever a new message arrives, the server holding the connection sends the message to the Redis backplane.
The Redis backplane publishes the message.
The servers subscribed to the Redis backplane receive the message and forward it to their connected clients.
In our previous article, Getting Started with SignalR in .NET 6, we developed a basic SignalR application. And now we intend to scale it out. To accomplish this, we will use Terraform, AWS Elastic Beanstalk, and Amazon ElastiCache for Redis.
Pre-requisites
Have an IAM User with programmatic access.
Set up your AWS CLI locally.
Install Terraform CLI.
Have an AWS EC2 Key Pair.
Have an Amazon VPC and Subnets.
The Application
Download the code from here. Open the solution and within the WebAPI project, add a Procfile file with the following content:
web: dotnet exec ./WebAPI.dll --urls http://0.0.0.0:5000/
Run the following command to create the artifact to deploy:
mkdir terraform/publish
dotnet publish ./WebAPI/WebAPI.csproj --output "terraform/publish" --configuration "Release" --framework "net6.0" /p:GenerateRuntimeConfigurationFiles=true --runtime linux-x64 --no-self-contained
Compress-Archive -Path terraform/publish/* -DestinationPath terraform/app.zip
The Terraform script
Under the terraform folder, create a variables.tf file with the following content:
variable "aws_region" {
type = string
default = "us-east-2"
}
variable "aws_profile" {
type = string
default = "default"
}
variable "application" {
type = string
default = "<MY_APP_NAME>"
}
variable "vpc" {
type = string
default = "<MY_VPC>"
}
variable "subnets" {
type = list
default = ["<MY_SUBNET>"]
}
variable "keypair" {
type = string
default = "<MY_KEY_PAIR>"
}
Create a main.tf file as follow:
terraform {
required_providers {
aws = {
source = "hashicorp/aws"
version = "4.22.0"
}
}
}
provider "aws"{
region = var.aws_region
profile = var.aws_profile
}
resource "aws_s3_bucket" "bucket" {
bucket = "${var.application}-bucket"
}
resource "aws_s3_object" "bucket_object" {
bucket = aws_s3_bucket.bucket.id
key = "app-${uuid()}.zip"
source = "app.zip"
}
resource "aws_security_group" "security_group" {
vpc_id = var.vpc
name = var.application
description = "Security group for elastic beanstalk app ${var.application}"
ingress {
from_port = 3389
to_port = 3389
protocol = "tcp"
cidr_blocks = ["0.0.0.0/0"]
}
egress {
from_port = 0
to_port = 0
protocol = "-1"
cidr_blocks = ["0.0.0.0/0"]
}
}
data "aws_iam_policy_document" "assume_service_role" {
statement {
effect = "Allow"
principals {
type = "Service"
identifiers = ["elasticbeanstalk.amazonaws.com"]
}
actions = ["sts:AssumeRole"]
condition {
test = "StringEquals"
variable = "sts:ExternalId"
values = [
"elasticbeanstalk"
]
}
}
}
resource "aws_iam_role" "service_role" {
name = "aws-elasticbeanstalk-service-role-${var.application}"
assume_role_policy = data.aws_iam_policy_document.assume_service_role.json
}
resource "aws_iam_role_policy_attachment" "AWSElasticBeanstalkEnhancedHealth-attach" {
role = aws_iam_role.service_role.name
policy_arn = "arn:aws:iam::aws:policy/service-role/AWSElasticBeanstalkEnhancedHealth"
}
resource "aws_iam_role_policy_attachment" "AWSElasticBeanstalkManagedUpdatesCustomerRolePolicy-attach" {
role = aws_iam_role.service_role.name
policy_arn = "arn:aws:iam::aws:policy/AWSElasticBeanstalkManagedUpdatesCustomerRolePolicy"
}
data "aws_iam_policy_document" "assume_role" {
statement {
effect = "Allow"
principals {
type = "Service"
identifiers = ["ec2.amazonaws.com"]
}
actions = ["sts:AssumeRole"]
}
}
resource "aws_iam_role" "role" {
name = "aws-elasticbeanstalk-ec2-role-${var.application}"
assume_role_policy = data.aws_iam_policy_document.assume_role.json
}
resource "aws_iam_role_policy_attachment" "CloudWatchFullAccess-attach" {
role = aws_iam_role.role.name
policy_arn = "arn:aws:iam::aws:policy/CloudWatchFullAccess"
}
resource "aws_iam_role_policy_attachment" "AWSElasticBeanstalkWebTier-attach" {
role = aws_iam_role.role.name
policy_arn = "arn:aws:iam::aws:policy/AWSElasticBeanstalkWebTier"
}
resource "aws_iam_role_policy_attachment" "AWSElasticBeanstalkWorkerTier-attach" {
role = aws_iam_role.role.name
policy_arn = "arn:aws:iam::aws:policy/AWSElasticBeanstalkWorkerTier"
}
resource "aws_iam_role_policy_attachment" "AWSElasticBeanstalkMulticontainerDocker-attach" {
role = aws_iam_role.role.name
policy_arn = "arn:aws:iam::aws:policy/AWSElasticBeanstalkMulticontainerDocker"
}
resource "aws_iam_instance_profile" "instance_profile" {
name = aws_iam_role.role.name
role = aws_iam_role.role.name
}
resource "aws_elastic_beanstalk_application" "application" {
name = var.application
}
resource "aws_elastic_beanstalk_environment" "environment" {
name = "${var.application}-env"
application = aws_elastic_beanstalk_application.application.name
solution_stack_name = "64bit Amazon Linux 2 v2.5.2 running .NET Core"
tier = "WebServer"
setting {
namespace = "aws:ec2:vpc"
name = "VPCId"
value = var.vpc
}
setting {
namespace = "aws:ec2:vpc"
name = "Subnets"
value = join(",",var.subnets)
}
setting {
namespace = "aws:ec2:vpc"
name = "ELBScheme"
value = "internal"
}
setting {
namespace = "aws:ec2:vpc"
name = "ELBSubnets"
value = join(",",var.subnets)
}
setting {
namespace = "aws:autoscaling:launchconfiguration"
name = "IamInstanceProfile"
value = aws_iam_role.role.name
}
setting {
namespace = "aws:autoscaling:launchconfiguration"
name = "SecurityGroups"
value = aws_security_group.security_group.id
}
setting {
namespace = "aws:autoscaling:launchconfiguration"
name = "InstanceType"
value = "t2.small"
}
setting {
namespace = "aws:autoscaling:asg"
name = "MinSize"
value = 2
}
setting {
namespace = "aws:autoscaling:asg"
name = "MaxSize"
value = 2
}
setting {
namespace = "aws:autoscaling:launchconfiguration"
name = "EC2KeyName"
value = var.keypair
}
setting {
namespace = "aws:elasticbeanstalk:environment"
name = "LoadBalancerType"
value = "application"
}
setting {
namespace = "aws:elasticbeanstalk:environment"
name = "ServiceRole"
value = aws_iam_role.service_role.name
}
setting {
namespace = "aws:elasticbeanstalk:environment:process:default"
name = "HealthCheckPath"
value = "/swagger/index.html"
}
setting {
namespace = "aws:elasticbeanstalk:environment:process:default"
name = "StickinessEnabled"
value = "true"
}
setting {
namespace = "aws:elasticbeanstalk:environment:proxy"
name = "ProxyServer"
value = "none"
}
}
resource "aws_elastic_beanstalk_application_version" "version" {
bucket = aws_s3_bucket.bucket.id
key = aws_s3_object.bucket_object.id
application = aws_elastic_beanstalk_application.application.name
name = "${var.application}-app-${uuid()}"
}
resource "aws_security_group" "security_group_redis" {
name = "${var.application}-redis"
description = "Security group for ${var.application} redis"
vpc_id = var.vpc
ingress {
from_port = 0
to_port = 0
protocol = "-1"
cidr_blocks = ["0.0.0.0/0"]
}
egress {
from_port = 0
to_port = 0
protocol = "-1"
cidr_blocks = ["0.0.0.0/0"]
}
}
resource "aws_elasticache_subnet_group" "subnet_group" {
name = "${var.application}-redis-subnet"
subnet_ids = var.subnets
}
resource "aws_elasticache_replication_group" "cluster" {
replication_group_id = "${var.application}-redis"
description = "Redis for ${var.application}"
node_type = "cache.t2.small"
engine_version = "7.0"
engine = "redis"
port = 6379
parameter_group_name = "default.redis7.cluster.on"
automatic_failover_enabled = true
subnet_group_name = aws_elasticache_subnet_group.subnet_group.name
num_node_groups = 1
replicas_per_node_group = 1
security_group_ids = [aws_security_group.security_group_redis.id]
transit_encryption_enabled = true
}
The majority of the script's explanation can be found here. If we use Server-Sent Events or Long Polling, the option StickinessEnabled must be configured in the load balancer. About Redis, there are three configurations:
Redis Cluster: Single node, no replication.
Redis Replication Group Cluster mode disabled: One write node and multiple replica nodes.
Redis Replication Group Cluster mode enabled: Multiple write nodes (data partitioning) and multiple replica nodes.
For our use case, we are choosing the second configuration, which consists of one write node and one replica node.
The Deployment
Run the following commands:
cd terraform
terraform init
terraform plan -out app.tfplan
terraform apply 'app.tfplan'
Wait until the apply command ends, copy the outputs into the following script, and run it:
aws --region us-east-2 elasticbeanstalk update-environment --environment-name <OUTPUT_ENVIRONMENT_NAME> --version-label <OUTPUT_APPLICATION_VERSION_NAME>
Open a browser and navigate to http://<OUTPUT_ENVIRONMENT_CNAME>/swagger to see the application.
Adding the Redis backplane
At the solution level, run dotnet add WebAPI package Microsoft.AspNetCore.SignalR.StackExchangeRedis --version 6.0.16, then modify the Program.cs file as follows:
using WebAPI;
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddControllers();
builder.Services.AddCors();
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
builder.Services.AddSignalR().AddStackExchangeRedis("<OUTPUT_REDIS_CONFIGURATION_ENDPOINT>:6379,ssl=True,abortConnect=False"); ;
var app = builder.Build();
app.UseSwagger();
app.UseSwaggerUI();
app.UseCors(cp => cp
.AllowAnyHeader()
.SetIsOriginAllowed(origin => true)
.AllowCredentials()
);
app.MapHub<ChatHub>("/chathub");
app.MapControllers();
app.Run();
Remove the existing artifact and create a new one by executing:
dotnet publish ./WebAPI/WebAPI.csproj --output "terraform/publish" --configuration "Release" --framework "net6.0" /p:GenerateRuntimeConfigurationFiles=true --runtime linux-x64 --no-self-contained
Compress-Archive -Path terraform/publish/* -DestinationPath terraform/app.zip
Finally, redeploy the application with the new artifact:
cd terraform
terraform plan -out app.tfplan
terraform apply 'app.tfplan'
aws --region us-east-2 elasticbeanstalk update-environment --environment-name <OUTPUT_ENVIRONMENT_NAME> --version-label <OUTPUT_APPLICATION_VERSION_NAME>
Open the client.html file and change the connection URL with http://<OUTPUT_ENVIRONMENT_CNAME>/chathub, then open the file in a browser. In conclusion, scaling out SignalR applications can be achieved by using a Redis backplane to synchronize messages across multiple instances. All the code is available here. Thanks, and happy coding.