HAProxy

1. Overview

HAProxy is a reliable and high-performance load balancer that can operate on the TCP and HTTP networking layers it's a very popular free and open-source software load balancer that powers many distributed systems web applications and websites and is considered to be almost a standard for load balancers in the industry. It is actually very easy to set up but despite its simplicity and has a lot of very advanced features and capabilities which are so important for production systems. Naturally, it officially supports only Linux.

2. Configuration

HAProxy's logic comes from a config file (haproxy.cfg)

• Predefined location: /usr/local/etc/haproxy/haproxy.cfg
• Command line: haproxy -f haproxy.cfg

2.1 Sections

2.1.1 global section

Parameters for the entire load balancing process (OS-specific)

2.1.2 Proxies section

Parameters for proxying incoming traffic to our backend cluster

• defaults: optional default subsection where we can set default parameters for all of our networking proxies
• frontend: Describes the listening sockets for all the incoming requests from the client and the logic on how to handle
• backend: describes a set of servers that participate in our back-end cluster
• listen: optional frontend + backend

3. Load Balancing

global
	maxconn	512

defaults
	mode http
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms

frontend http-in
	bind *:80
    default_backend servers

backend servers
	balance roundrobin
	server server1 127.0.0.1:8000 weight 2
    server server2 127.0.0.1:8001 weight 2
    server server3 127.0.0.1:8002 weight 1

3. High Availability

global
	maxconn	512

defaults
	mode http
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms

frontend http-in
	bind *:80
    default_backend servers

backend servers
	balance roundrobin
    option httpchk GET /status
    http-check expect string "Server is alive"
	server server1 127.0.0.1:8000 check inter 1s
    server server2 127.0.0.1:8001 check	inter 1s
    server server3 127.0.0.1:8002 check	inter 1s

4. Monitoring

global
	maxconn	512

defaults
	mode http
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms

frontend http-in
	bind *:80
    default_backend servers

backend servers
	balance roundrobin
    option httpchk GET /status
    http-check expect string "Server is alive"
	server server1 127.0.0.1:8000 check inter 1s
    server server2 127.0.0.1:8001 check	inter 1s
    server server3 127.0.0.1:8002 check	inter 1s
    
listen stats_page
	bind *:83
    stats enable
    stats uri /

5. Advanced Routing (ACLs)

global
	maxconn	512

defaults
	mode http
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms

frontend http-in
	bind *:80
    acl even path_end -i /even
    acl odd path_end -i /odd
    
    use_backend even_application_nodes if even
    use_backend odd_application_nodes if odd

backend odd_application_nodes
	balance roundrobin
    option httpchk GET /status
    http-check expect string "Server is alive"
	server server1 127.0.0.1:8000 check
    server server3 127.0.0.1:8002 check
    
backend even_application_ndoes
	balance roundrobin
    option httpchk GET /status
    http-check expect string "Server is alive"
    server server2 127.0.0.1:8001 check
    
listen stats_page
	bind *:83
    stats enable
    stats uri /

6. Layer 4 and 7 Load Balancing

global
	maxconn	512

defaults
	mode tcp
    timeout connect 10s
    timeout client 50s
    timeout server 50s

frontend http-in
	bind *:80
    default_backend application_nodes

backend application_nodes
	balance roundrobin
    option httpchk GET /status
    http-check expect string "Server is alive"
	server server1 127.0.0.1:8000 check
    server server1 127.0.0.1:8001 check
    server server3 127.0.0.1:8002 check
    
listen stats_page
	bind *:83
    stats enable
    stats uri /

After switching to TCP, We open the browser and keep sending your requests to localhost suddenly all our requests are going to the same server instead of each request being sent to a different one. The reason for this behavior is any time we refresh the browser we actually send a new HTTP GET request to an HAProxy. However, all those requests are still sent on the same TCP connection. Since our load balancer now does not understand HTTP as far as its concerned all the TCP packets belong to the same stream and hence will be sent to the same server. To break the TCP connection and open a new one for each request we need to close the web browser entirely and once we open a new browser instance and send a new request to localhost and new TCP connection will be established. Now, this new connection will be routed to a new back-end server based on the round-robin policy.

7. Running HAProxy with Docker

The great part about docker for us is for platforms like Windows that do not have the same resource isolation features like Linux docker will run a single Linux virtual machine behind the scenes for us so we can run the same commands in the same containers on any platform.

7.1 Example

7.1.1 Docker Image for Sample Project

FROM maven:3.6.1-jdk-11 AS MAVEN_TOOL_CHAIN_CONTAINER
RUN mkdir src
COPY src /tmp/src
COPY ./pom.xml /tmp/
WORKDIR /tmp/
RUN mvn package
RUN ls -la /tmp

FROM openjdk:11
COPY --from=MAVEN_TOOL_CHAIN_CONTAINER /tmp/target/webapp-1.0-SNAPSHOT-jar-with-dependencies.jar /tmp/
WORKDIR /tmp/
ENTRYPOINT ["java","-jar", "webapp-1.0-SNAPSHOT-jar-with-dependencies.jar"]
CMD ["80", "Server Name"]

7.1.2 HAProxy Image

FROM haproxy:1.7
COPY haproxy.cfg /usr/local/etc/haproxy/haproxy.cfg
ENTRYPOINT ["haproxy", "-f", "/usr/local/etc/haproxy/haproxy.cfg"]

7.1.3 HAProxy Configuration

global
    maxconn 500

defaults
    mode http
    timeout connect 10s
    timeout client  50s
    timeout server  50s

frontend http-in
    bind *:80
    default_backend application_nodes

backend application_nodes
    balance roundrobin
    option httpchk GET /status
    http-check expect string "Server is alive"
    server server01 app1:9001 check inter 1s
    server server02 app2:9002 check inter 2s
    server server03 app3:9003 check inter 2s

listen stats 
    bind *:83
    stats enable
    stats uri /

8. Reference

http://www.haproxy.org/

https://en.wikipedia.org/wiki/HAProxy

https://cbonte.github.io/haproxy-dconv/1.7/configuration.html#7.1

https://hub.docker.com/