HAProxy

HAProxy (High Availability Proxy) is a powerful, open-source load balancer and proxy server used to improve the performance and reliability of web services by distributing network traffic across multiple servers. It is widely used to enhance the scalability, availability, and reliability of web applications by balancing client requests among backend servers.

HAProxy architecture is optimized to move data as fast as possible with the least possible operations. It focuses on optimizing the CPU cache’s efficiency by sticking connections to the same CPU as long as possible.

How HAProxy works

HAProxy operates as a reverse proxy, which means it accepts client requests and distributes them to one or more backend servers using the configured load-balancing algorithm. This ensures efficient use of server resources and prevents any single server from becoming overloaded.

• Client request processing:

  1. A client application connects to HAProxy instead of directly to the server.
  2. HAProxy analyzes the requests and determines what server to route it to for further processing.
  3. HAProxy forwards the request to the selected server using the routing algorithm defined in its configuration. It can be round robin, least connections, and others.
  4. HAProxy receives the response from the server and forwards it back to the client.
  5. After sending the response, HAProxy either closes the connection or keeps it open, depending on the configuration.

• Load balancing: HAProxy distributes incoming traffic using various algorithms such as round-robin, least connections, and IP hash.

• Health checks: HAProxy continuously monitors the health of backend servers to ensure requests are only routed to healthy servers.
• SSL termination: HAProxy offloads SSL/TLS encryption and decryption, reducing the workload on backend servers.
• Session persistence: HAProxy ensures that requests from the same client are routed to the same server for session consistency.
• Traffic management: HAProxy supports rate limiting, request queuing, and connection pooling for optimal resource utilization.
• Security: HAProxy supports SSL/TLS, IP filtering, and integration with Web Application Firewalls (WAF).

Role in a HA Patroni cluster

HAProxy plays a crucial role in managing PostgreSQL high availability in a Patroni cluster. Patroni is an open-source tool that automates PostgreSQL cluster management, including failover and replication. HAProxy acts as a load balancer and proxy, distributing client connections across the cluster nodes.

Client applications connect to HAProxy, which transparently forwards their requests to the appropriate PostgreSQL node. This ensures that clients always connect to the active primary node without needing to know the cluster’s internal state and topology.

HAProxy monitors the health of PostgreSQL nodes using Patroni’s API and routes traffic to the primary node. If the primary node fails, Patroni promotes a secondary node to a new primary, and HAProxy updates its routing to reflect the change. You can configure HAProxy to route write requests to the primary node and read requests - to the secondary nodes.

Redundancy for HAProxy

A single HAProxy node creates a single point of failure. If HAProxy goes down, clients lose connection to the cluster. To prevent this, set up multiple HAProxy instances with a failover mechanism. This way, if one instance fails, another takes over automatically.

To implement HAProxy redundancy:

1. Set up a virtual IP address that can move between HAProxy instances.

2. Install and configure a failover mechanism to monitor HAProxy instances and move the virtual IP to a backup if the primary fails.

3. Keep HAProxy configurations synchronized across all instances.

Note

In this reference architecture we focus on the on-premises deployment and use Keepalived as the failover mechanism.

If you use a cloud infrastructure, it may be easier to use the load balancer provided by the cloud provider to achieve high-availability for HAProxy.

How Keepalived works

Keepalived manages failover by moving the virtual IP to a backup HAProxy node when the primary fails.

No matter how many HAProxy nodes you have, only one of them can be a primary and have the MASTER state. All other nodes are BACKUP nodes. They monitor the MASTER state and take over when it is down.

To determine the MASTER, Keepalived uses the priority setting. Every node must have a different priority.

The node with the highest priority becomes the MASTER. Keepalived periodically checks every node’s health.

When the MASTER node is down or unavailable, it’s priority is lowered so that the next highest priority node becomes the new MASTER and takes over. The priority is adjusted by the value you define in the weight setting.

You must carefully define the priority and weight values in the configuration. When a primary node is down, its priority must be adjusted to be lower than the active node with the lowest priority by at least 1.

For example, your nodes have priority 110 and 100. The node with priority 110 is MASTER. When it is down, its priority must be lower than the priority of the remaining node (100).

When a failed node restores, its priority adjusts again. If it is the highest one among the nodes, this node restores its MASTER state, holds the virtual IP address and handles the client connections.