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High Availability and Scalability
Scalability & High Availability
- Scalability means that an application / system can handle greater loads by adapting
- There are two kinds of scalability:Vertical ScalabilityHorizontal Scalability (= elasticity)
- Scalability is linked but different to High Availability
- Let's deep dive into the distinction, using a call center as an example
Vertical Scalability
- Vertically scalability means increasing the size of the instance
- For example, your application runs on a t2.micro
- Scaling that application vertically means running it on a t2.large
- Vertical scalability is very common for non distributed systems, such as a database
- RDS, ElastiCache are services that can scale vertically
- There's usually a limit to how much you can vertically scale (hardware limit)
Horizontal Scalability
- Horizontal Scalability means increasing the number of instances / systems for your application
- Horizontal scaling implies distributed systems
- This is very common for web applications / modern applications
- It's easy to horizontally scale thanks the cloud offerings such as Amazon EC2
High Availability
- High Availability usually goes hand in hand with horizontal scaling
- High availability means running your application / system in at least 2 data centers (==Availability Zones)
- The goal of high availability is to survive a data center loss
- The high availability can be passive (for RDS Multi AZ for example)
- The high availability can be active (for horizontal scaling)
High Availability & Scalability For EC2
- Vertical Scaling: Increase instance size (= scale up / down)From: t2.nano - 0.5G of RAM, 1 vCPUTo: u-12tb1.metal - 12.3TB of RAM, 448vCPUs
- Horizontal Scaling: Increase number of instances (= scale out / in)Auto Scaling GroupLoad Balancer
- High Availability: Run instances for the same application across multi AZ Auto Scaling Group multi AZLoad Balancer multi AZ
Elastic Load Balancing (ELB) Overview
What is load balancing?
- Load Balances are servers that forward traffic to multiple servers (e.g., EC2 instances) downstream
Why use a load balancer?
- Spread load across multiple downstream instances
- Expose a single point of access (DNS) to your application
- Seamlessly handle failures of downstream instances
- Do regular health checks to your instances
- Provide SSL termination (HTTPS) for your websites
- Enforce stickiness with cookies
- High availability across zones
- Separate public traffic from private traffic
Why use an Elastic Load Balance?
- An Elastic Load Balancer is a managed load balancerAWS guarantees that it will be workingAWS takes care of upgrades, maintenance, high availabilityAWS provides only a few configuration knobs
- It costs less to setup your own load balancer but it will be a lot more effort on your end
- It is integrated with many AWS offerings / services EC2, EC2 Auto Scaling Groups, Amazon ECS AWS Certificate Manager (ACM), Cloud Watch Route 53, AWS WAF, AWS Global Accelerator
Health Checks
- Health Checks are crucial for Load Balancers
- They enable the load balancer to know if instances it forwards traffic to are available to reply to requests
- The health check is done on a port and a route (/health is common)
- lf the response is not 200 (OK), then the instance is unhealthy
Types of load balancer on AWS
- AWS has 4 kinds of managed Load Balancers
- Classic Load Balancer (v1 - old generation) - 2009 - CLB HTTP, HTTPS, TCP, SSL (secure TCP)
- Application Load Balancer (v2 - new generation) - 2016 - ALBHTTP, HTTPS, Web Socket
- Network Load Balancer (v2 - new generation) - 2017 - NLB TCP, TLS (secure TCP), UDP
- Gateway LoadBalancer - 2020 - GWLBOperates at layer 3 (Network layer) - IP Protocol
- Overal, it is recommended to use the newer generation load balancers as they provide more features
- Some load balancers can be setup as internal (private) or external (public) ELBs
Load Balancer Security Groups
Classic Load Balancer (CLB)
Classic Load Balancers (v1)
- Supports TCP (Layer 4), HTTP & HTTPS (Layer 7)
- Health checks are TCP or HTTP based
- Fixed hostname http://XXX.region.elb.amazonaws.com
Application Load Balancer (ALB)
Application Load Balancer (v2)
- Application load balancers is Layer 7 (HTTP)
- Load balancing to multiple HTTP applications across machines (target groups)
- Load balancing to multiple applications on the same machine (ex: containers)
- Support for HTTP/2 and WebSocket
- Support redirects (from HTTP to HTTPS for example)
- Routing tables to different target groups:Routing based on path in URL (http://example.com/users & http://example.com/posts)Routing based on hostname in URL (http://one.example.com & http://other.example.com)Routing based on Query String, Headers (http://example.com/users?id=l23&order=false)
- ALB are a great fit for micro services & container-based application (example: Docker & Amazon ECS)
- Has a port mapping feature to redirect to a dynamic port in ECS
- In comparison, we'd need multiple Classic Load Balancer per application
Application Load Balancer (v2) HTTP Based Traffic
Application Load Balancer (v2) Target Groups
- EC2 instances (can be managed by an Auto Scaling G Group) - HTTP
- ECS tasks (managed by ECS itself) - HTTP
- Lambda functions - HTTP request is translated into a JSON event
- IP Addresses - must be private IPs
- ALB can route to multiple target groups
- Health checks are at the target group level
Application Load Balancer (v2) Query Strings/Parameters Routing
Application Load Balancer (v2) Good to Know
- Fixed hostname (http://XXX.region.elb.amazonaws.com)
- The application servers don't see the IP of the client directly The true IP of the client is inserted in the header X-Forwarded-For We can also get Port (X-Forwarded-Port) and proto (X-Forwarded-Proto)
Network Load Balancer (NLB)
Network Load Balancer (v2)
- Network load balancers (Layer4) allow to:Forward TCP & UDP traffic to your instances Handle millions of request per secondsLess latency ~ 100ms (vs 400 ms for ALB)
- NLB has one static IP per AZ, and supports assigning Elastic IP (helpful for whitelisting specific IP)
- NLB are used for extreme performance, TCP or UDP traffic
- Not included in the AWS free tier
Network Load Balancer (v2) TCP (Layer 4) Based Traffic
Network Load Balancer - Target Groups
- EC2 instances
- IP Addresses - must be private IPs
- Application Load Balancer
- Health Checks support the TCP, HTTP and HTTPS Protocols
Gateway Load Balancer (GWLB)
Gateway Load Balancer
- Deploy, scale, and manage a fleet of 3rd party network virtual appliances in AWS
- Example: Firewalls, Intrusion Detection and Prevention Systems, Deep Packet Inspection Systems, payload manipulation, ...
- Operates at Layer 3 (Network Layer) - IP Packets
- Combines the following functions:Transparent Network Gateway - single entry/exit for all trafficLoad Balancer - distributes traffic to your virtual appliances
- Uses the GENEVE protocol on port 6081
Gateway Load Balancer - Target Groups
- EC2 instances
- IP Addresses - must be private IPs
Elastic Load Balancer - Sticky Sessions
Sticky Sessions (Session Affinity)
- It is possible to implement stickiness so that the same client is always redirected to the same instance behind a load balancer
- This works for Classic Load Balancers & Application Load Balancers
- The "cookie" used for stickiness has an expiration date you control
- Usecase: make sure the user doesn't Iose his session data
- Enabling stickiness may bring imbalance to the load over the backend EC 2 instances
Sticky Sessions - Cookie Names
- Application-based Cookies Custom cookieGenerated by the targetCan include any custom attributes required by the application Cookie name must be specified individually for each target groupDon't use AWSALB, AWSALBAPP, or AWSALBTG (reserved for use by the ELB)Application cookieGenerated by the load balancer 、Cookie name is AWSALBAPP
- Duration-based CookiesCookie generated by the load balancer Cookie name is AWSALB for ALB, AWSELB for CLB
Elastic Load Balancer - Cross Zone Load Balancing
Cross-Zone Load Balancing
Cross-Zone Load Balancing
- Application Load Balancer Always on (can't be disabled)No charges for inter AZ data
- Network Load BalancerDisabled by defaultYou pay charges ($) for inter AZ data if enabled
- Classic Load BalancerDisabled by default No charges for inter AZ data if enabled
Elastic Load Balancer - SSL Certificates
SSL/TLS - Basics
- An SSL Certificate allows traffic between your clients and your load balancer to be encrypted in transit (in-flight encryption)
- SSL refers to Secure Sockets Layer, used to encrypt connections
- TLS refers to Transport Layer Security, which is a newer version
- Nowadays, TLS certificates are mainly used, but people still refer as SSL
- Public SSL certificates are issued by Certificate Authorities (CA)
- Comodo, Symantec, GoDaddy, GlobalSign, Digicert, Letsencrypt, etc...
- SSL certificates have an expiration date (you set) and must be renewed
Load Balancer - SSL Certificates
- The load balancer uses an X.509 certificate (SSL/TLS server certifcate)
- You can manage certificates using ACM (AWS Certificate Manager)
- You can create upload your own certificates alternatively
- HTTPS listener:You must specify a default certificateYou can add an optional list of certs to support multiple domainsClients can use SNI (Server Name Indication) to specify the hostname they reach Ability to specify a security policy to support older versions of SSL/TLS (legacy clients)
SSL - Server Name Indication
- SNI solves the problem of loading multiple SSL certificates onto one web server (to serve multiple websites)
- It's a "newer" protocol, and requires the client to indicate the hostname of the target server in the initial SSL handshake
- The server will then find the correct certificate, or return the default one
Note:
- Only works for ALB & NLB (newer generation), Cloud Front
- Does not work for CLB(older gen)
Elastic Load Balancers - SSL Certificates
- Classic Load Balancer (v1)Support only one SSL certificate Must use multiple CLB for multiple hostname with multiple SSL certificates
- Application Load Balancer (v2)Supports multiple listeners with multiple SSL certificates Uses Server Name Indication (SNI) to make it work
- Network Load Balancer (v2)Supports multiple listeners with multiple SSL certificatesUses Server Name Indication (SNI) to make it work
Elastic Load Balancer - Connection Draining
Connection Draining
- Feature namingConnection Draining - for CLBDeregistration Delay - for ALB & NLB
- Time to complete "in-light requests" while the instance is de-registering or unhealthy
- Stops sending new requests to the EC2 instance which is de-registering
- Between 1 to 3600 seconds (default: 300 seconds)
- Can be disabled (set value to 0)
- Set to a low value if your requests are short
Auto Scaling Groups (ASG) Overview
What's an Auto Scaling Group?
- In real-life, the load on your websites and application can change
- In the cloud, you can create and get rid of servers very quickly
- The goal of an Auto Scaling Group (ASG) is to:Scale out (add EC2 instances) to match an increased loadScale in (remove EC2 instances) to match a decreased loadEnsure we have a minimum and a maximum number of EC2 instances running Automatically register new instances to a load balancerRe-create an EC2 instance in case a previous one is terminated (ex: if unhealthy)
- ASG are free (you only pay for the underlying EC2 instances)
Auto Scaling Group in AWS
Auto Scaling Group in AWS With Load Balancer
Auto Scaling Scaling Group Attributes
- A Launch Template (older "Launch Configurations" are deprecated)AMI + Instance TypeEC2 User Data EBS VolumesSecurity GroupsSSH Key PairIAM Roles for your EC2 InstancesNetwork + Subnets InformationLoad Balancer Information
- Min Size / Max Size / Initial Capacity
- Scaling Policies
Auto Scaling - CloudWatch Alarms & Scaling
- lt is possible to scale an ASG based on CloudWatch alarms
- An alarm monitors a metric (such as Average CPU, or a custom metric)
- Metrics such as Average CPU are computed for the overall ASG instances
- Based on the alarm:We can create scale-out policies (increase the number of instances)We can create scale-in policies (decrease the number of instances)
Auto Scaling Groups - Scaling Policies
Auto Scaling Groups - Dynamic Scaling Policies
- Target Tracking ScalingMost simple and easy to set-up Example: I want the average ASG CPU to stay at around 40%
- Simple / Step ScalingWhen a CloudWatch alarm is triggered (example CPU > 70%), then add 2 unitsWhen a CloudWatch alarm is triggered (example CPU < 30%), then remove 1
- Scheduled Actions Anticipate a scaling based on known usage patterns Example: increase the min capacity to 10 at 5 pm on Fridays
Auto Scaling Groups - Predictive Scaling
- Predictive scaling: continuously forecast load and schedule scaling ahead
Good metrics to scale on
- CPUUtilization: Average CPU utilization across your instances
- RequestCountPerTarget: to make sure the number of requests per EC2 instances is stable
- Average Network In / Out (if you're application is network bound)
- Any custom metric (that you push using CloudWatch)
Auto Scaling Groups - Scaling Cooldowns
- After a scaling activity happens, you are in the cooldown period (default 300 seconds)
- During the cooldown period, the ASG will not launch or terminate additional instances (to allow for metrics to stabilize)
- Advice: Use a ready-to-use AMI to reduce configuration time in order to be serving request faster and reduce the cooldown period
