Health checks

Health checks ensure that only healthy servers are kept in the load balancing rotation. They check the status of each server by using one of the health checking modes described in this section.

Active health checks Jump to heading #

An active health check attempts to connect to a server or send it an HTTP request at a regular interval. If the connection cannot be established or the HTTP request fails, the health checks fails.

If the number of consecutive failed checks meets the failure threshold, the server is taken out of rotation. Health checks continue while the server is down, however. If the server resumes service and responds successfully to the health checks, and if the number of consecutive successful responses meets the success threshold, the server is restored to rotation.

TCP health checks Jump to heading #

A basic TCP-layer health check tries to connect to the server’s TCP port. The check is valid when the server answers with a SYN/ACK packet. Enable it by adding a check argument to each server line that you would like to monitor.

In the following example, the load balancer tries to connect to port 80 on each server:

haproxy
backend be_myapp
  server srv1 10.0.0.1:80 check
  server srv2 10.0.0.2:80 check

haproxy
backend be_myapp
  server srv1 10.0.0.1:80 check
  server srv2 10.0.0.2:80 check

To send health check probes to a port other than the one to which normal traffic is sent, add the port argument. In the following example, the health check is sent to port 8080.

haproxy
backend be_myapp
  server srv1 10.0.0.1:80 check port 8080
  server srv2 10.0.0.2:80 check port 8080

haproxy
backend be_myapp
  server srv1 10.0.0.1:80 check port 8080
  server srv2 10.0.0.2:80 check port 8080

Define a send/expect sequence Jump to heading #

Use option tcp-check to define a sequence of messages to send and responses to expect back. Below, we send the string PING and expect to receive the string PONG:

haproxy
backend be_mayapp
  option tcp-check
  tcp-check send PING\r\n
  tcp-check expect string PONG
  server srv1 10.0.0.1:80 check

haproxy
backend be_mayapp
  option tcp-check
  tcp-check send PING\r\n
  tcp-check expect string PONG
  server srv1 10.0.0.1:80 check

HTTP health checks Jump to heading #

An HTTP-layer health check sends an HTTP OPTIONS request to the server and expects to get a successful response. To enable it, add option httpchk to the backend section:

haproxy
backend be_myapp
  option httpchk
  server srv1 192.168.1.5:80 check

haproxy
backend be_myapp
  option httpchk
  server srv1 192.168.1.5:80 check

Checks send an OPTIONS request to the URL / by default.

You can change the HTTP method and URL by specifying them on the option httpchk line. In the following example, we send checks using GET instead of OPTIONS to the URL /healthz:

haproxy
backend be_myapp
  option httpchk GET /healthz
  server srv1 10.0.0.1:80 check
  server srv2 10.0.0.2:80 check

haproxy
backend be_myapp
  option httpchk GET /healthz
  server srv1 10.0.0.1:80 check
  server srv2 10.0.0.2:80 check

If the response status code is in the 2xx or 3xx range, the server is healthy.

Expect a response status Jump to heading #

Use http-check expect to specify which HTTP status code indicates a healthy server. In the following example, the server must return a 200 OK response:

haproxy
backend bk_myapp
  option httpchk
  http-check expect status 200
  server srv1 10.0.0.1:80 check
  server srv2 10.0.0.2:80 check

haproxy
backend bk_myapp
  option httpchk
  http-check expect status 200
  server srv1 10.0.0.1:80 check
  server srv2 10.0.0.2:80 check

As an alternative to http-check expect status, where you specify one explicit status value, you can use rstatus to specify a regular expression to match multiple status codes. In the next example, the health check uses rstatus in conjunction with the negation operator (!) to consider all statuses as valid except for 5xx responses:

haproxy
backend bk_myapp
  option httpchk
  http-check expect ! rstatus ^5
  default-server inter 3s fall 3 rise 2
  server srv1 10.0.0.1:80 check
  server srv2 10.0.0.2:80 check

haproxy
backend bk_myapp
  option httpchk
  http-check expect ! rstatus ^5
  default-server inter 3s fall 3 rise 2
  server srv1 10.0.0.1:80 check
  server srv2 10.0.0.2:80 check

Expect a string in the response Jump to heading #

To specify a string to search for in the body of an HTTP or TCP response:

1. Set tune.chksize in the global section to set the number of bytes of the response to buffer. Below, we buffer 32 kilobytes (32,768 bytes) of the response:

   haproxy
   global
     tune.chksize 32768

   haproxy
   global
     tune.chksize 32768

2. Set the string that you expect to see in the body by adding the expect string directive to http-check or tcp-check. In the next example, the response must contain the string OK:

   haproxy
   backend be_myapp
     option httpchk
     http-check expect string OK
     server srv1 10.0.0.1:80   check
     server srv2 10.0.0.2:8080 check

   haproxy
   backend be_myapp
     option httpchk
     http-check expect string OK
     server srv1 10.0.0.1:80   check
     server srv2 10.0.0.2:8080 check

Use the expect rstring argument to specify a regular expression instead of an explicit string.

Customize with the send directive Jump to heading #

Another way to change the HTTP method and URL is by adding the http-check send line and specifying the new values there. In the following example, checks send GET requests to the URL /healthz:

haproxy
backend be_myapp
  option httpchk
  http-check send meth HEAD uri /healthz ver HTTP/1.1 hdr Host test.local
  server srv1 192.168.1.5:80 check

haproxy
backend be_myapp
  option httpchk
  http-check send meth HEAD uri /healthz ver HTTP/1.1 hdr Host test.local
  server srv1 192.168.1.5:80 check

You can send POST requests too:

haproxy
backend be_myapp
  option httpchk
  http-check send meth POST uri /health hdr Content-Type "application/json;charset=UTF-8" hdr Host www.mwebsite.com body "{\"id\": 1, \"field\": \"value\"}"
  server srv1 192.168.1.5:80 check

haproxy
backend be_myapp
  option httpchk
  http-check send meth POST uri /health hdr Content-Type "application/json;charset=UTF-8" hdr Host www.mwebsite.com body "{\"id\": 1, \"field\": \"value\"}"
  server srv1 192.168.1.5:80 check

Customize connect arguments Jump to heading #

Use the connect directive to enable SNI, connect over SSL/TLS, perform health checks over SOCKS4, and choose the protocol, such as HTTP/2 or FastCGI. Here’s an example where health checks are performed using HTTP/2 and SSL:

haproxy
backend be_myapp
  option httpchk
  http-check connect ssl alpn h2
  http-check send meth HEAD uri /health ver HTTP/2 hdr Host www.test.local
  server srv1 192.168.1.5:443 check

haproxy
backend be_myapp
  option httpchk
  http-check connect ssl alpn h2
  http-check send meth HEAD uri /health ver HTTP/2 hdr Host www.test.local
  server srv1 192.168.1.5:443 check

To close a connection cleanly instead of sending a RST, use the linger option.

Check multiple HTTP endpoints Jump to heading #

Additional power comes from the ability to query several endpoints during a single health check. In the following example, we make requests to two distinct services: one listening at port 8080 and the other at port 8081. We also use different URIs. If either endpoint fails to respond, the entire health check fails.

haproxy
backend servers
  option httpchk
  
  http-check connect port 8080
  http-check send meth HEAD uri /health
  
  http-check connect port 8081
  http-check send meth HEAD uri /up
  
  server server1 127.0.0.1:80 check

haproxy
backend servers
  option httpchk
  
  http-check connect port 8080
  http-check send meth HEAD uri /health
  
  http-check connect port 8081
  http-check send meth HEAD uri /up
  
  server server1 127.0.0.1:80 check

Change the interval Jump to heading #

By default, the load balancer sends a health check every two seconds. Change this by adding the inter argument to the server line. In the next example, we send a health check every four seconds:

haproxy
backend be_myapp
  server srv1 10.0.0.1:80 check inter 4s
  server srv2 10.0.0.2:80 check inter 4s

haproxy
backend be_myapp
  server srv1 10.0.0.1:80 check inter 4s
  server srv2 10.0.0.2:80 check inter 4s

Use any of the following time suffixes:

• us : microseconds
• ms : milliseconds
• s : seconds
• m : minutes
• h : hours
• d : days

Other arguments that affect the check interval are defined below:

Change the failure threshold Jump to heading #

Use the fall argument to change the number of failed health checks that will trigger removing the server from the load balancing rotation. By default, this is set to 3. In the following example, 5 failed checks will put the server into the DOWN state:

haproxy
backend be_myapp
  server srv1 10.0.0.1:80 check fall 5
  server srv2 10.0.0.2:80 check fall 5

haproxy
backend be_myapp
  server srv1 10.0.0.1:80 check fall 5
  server srv2 10.0.0.2:80 check fall 5

Use the rise argument to set how many successful checks are needed to bring a down server back up. The default is 2. In the following example, 10 successful health checks are needed before the server will return to the load balancing rotation:

haproxy
backend be_myapp
  server srv1 10.0.0.1:80 check fall 5 rise 10
  server srv2 10.0.0.2:80 check fall 5 rise 10

haproxy
backend be_myapp
  server srv1 10.0.0.1:80 check fall 5 rise 10
  server srv2 10.0.0.2:80 check fall 5 rise 10

Set check-scoped variables Jump to heading #

Use either tcp-check set-var or http-check set-var to set a variable scoped to a health check session. For example:

haproxy
backend be_myapp
  tcp-check set-var(check.port) int(1234)
  tcp-check connect port var(check.port)
  server srv1 10.0.0.1:80 check

haproxy
backend be_myapp
  tcp-check set-var(check.port) int(1234)
  tcp-check connect port var(check.port)
  server srv1 10.0.0.1:80 check

Passive health checks Jump to heading #

A passive health check monitors live traffic for errors. You can watch for either failed TCP connections or bad HTTP responses. Passive checks will detect errors returning from any part of your proxied service, but they require active traffic to monitor.

Monitor for TCP connection errors Jump to heading #

To monitor live traffic for TCP connection errors, follow these steps:

1. Add the check argument to the server lines you want to monitor.
2. Add the observe layer4 argument to each server line to activate passive health checking.
3. Add the error-limit and on-error arguments to set the threshold for failed passive health checks and the action to take when errors exceed that threshold.

In the following example, we monitor for TCP connection errors. When there are at least 10 of these errors, we mark the server as down by using the mark-down value for the on-error argument:

haproxy
backend servers
  server server1 192.168.0.10:80 check  inter 2m  observe layer4  error-limit 10  on-error mark-down

haproxy
backend servers
  server server1 192.168.0.10:80 check  inter 2m  observe layer4  error-limit 10  on-error mark-down

The check argument enables an active health check probe that will ping the server’s TCP port at an interval. The interval is 2 seconds by default, which you can change using the inter keyword. After a set number of successful active health check probes, this will bring the server back online after it has been removed from the load-balancing rotation from failed passive health checks. In the example above, the interval is increased to 2 minutes to ensure that the server can remain healthy for a longer period of time before returning to service.

Monitor for HTTP response errors Jump to heading #

To monitor live traffic for HTTP response errors, follow these steps:

1. Add the check argument to the server lines you want to monitor.
2. Add the observe layer7 argument to each server line to activate passive health checking.
3. Add the error-limit and on-error arguments to set the threshold for failed passive health checks and the action to take when errors exceed that threshold.

In the following example, we monitor for HTTP response errors. When there are at least 10 of these errors, we mark the server as down by using the mark-down value for the on-error argument:

haproxy
backend servers
  server server1 192.168.0.10:80 check  observe layer7  error-limit 10  on-error mark-down

haproxy
backend servers
  server server1 192.168.0.10:80 check  observe layer7  error-limit 10  on-error mark-down

The check argument enables an active health check probe that will ping the server’s TCP port at an interval. After a set number of successful active health check probes, this will bring the server back online after it has been removed from the load balancing rotation from failed passive health checks.

Set the on-error action Jump to heading #

The on-error argument on the server line determines what action to take when errors exceed the threshold you set with the error-limit. It accepts any of the following values:

Agent checks Jump to heading #

An agent check is one where the load balancer connects to an agent program running on a backend server. In response to the agent check probe, the agent program sends back a string of ASCII text that triggers a change in the load balancer. The change could be:

• put the server into the down state
• put the server into the up state
• put the server into maintenance mode
• take the server out of maintenance mode
• change the server’s weight
• change the server’s maxconn setting

Create an agent program Jump to heading #

An agent program can be written in any programming language, as long as it allows you to listen on a TCP port. When the program detects that the load balancer has connected, it should return a string of ASCII text that makes a change to the load balancer or keeps it at its current state.

The program running on the server can send back to the load balancer any of the following commands (note the trailing newline character \n):

Below is a program written in the Go programming language. It returns the string 50%\n if the server’s CPU idle time is less than 10, which would indicate it is near to maxing out its CPU. Otherwise, it returns the string 100%\n. Notice that the string should end with a line feed character (\n):

agent-program.go
go
package main
import (
    "fmt"
    "time"
    "github.com/firstrow/tcp_server"
    "github.com/mackerelio/go-osstat/cpu"
)
func main() {
    server := tcp_server.New(":9999")
    
    server.OnNewClient(func(c *tcp_server.Client) {
        fmt.Println("Client connected")
        cpuIdle, err := getIdleTime()
        
        if err != nil {
            fmt.Println(err)
            c.Close()
            return
        }
        
        if cpuIdle < 10 {
            // Set server weight to half
            c.Send("50%\n")
        } else {
            c.Send("100%\n")
        }
        
        c.Close()
    })
    
    server.Listen()
}
func getIdleTime() (float64, error) {
    before, err := cpu.Get()
    if err != nil {
        return 0, err
    }
    time.Sleep(time.Duration(1) * time.Second)
    after, err := cpu.Get()
    if err != nil {
        return 0, err
    }
    total := float64(after.Total - before.Total)
    cpuIdle := float64(after.Idle-before.Idle) / total * 100
    return cpuIdle, nil
}

agent-program.go
go
package main
import (
    "fmt"
    "time"
    "github.com/firstrow/tcp_server"
    "github.com/mackerelio/go-osstat/cpu"
)
func main() {
    server := tcp_server.New(":9999")
    
    server.OnNewClient(func(c *tcp_server.Client) {
        fmt.Println("Client connected")
        cpuIdle, err := getIdleTime()
        
        if err != nil {
            fmt.Println(err)
            c.Close()
            return
        }
        
        if cpuIdle < 10 {
            // Set server weight to half
            c.Send("50%\n")
        } else {
            c.Send("100%\n")
        }
        
        c.Close()
    })
    
    server.Listen()
}
func getIdleTime() (float64, error) {
    before, err := cpu.Get()
    if err != nil {
        return 0, err
    }
    time.Sleep(time.Duration(1) * time.Second)
    after, err := cpu.Get()
    if err != nil {
        return 0, err
    }
    total := float64(after.Total - before.Total)
    cpuIdle := float64(after.Idle-before.Idle) / total * 100
    return cpuIdle, nil
}

Configure the backend Jump to heading #

Configure the servers in the backend to send agent checks.

Here, the load balancer sends an agent-based check probe every five seconds to a program listening at 192.168.0.10 at port 8080:

haproxy
backend servers
  server server1 192.168.0.10:80 check  weight 100  agent-check  agent-addr 192.168.0.10  agent-port 8080  agent-inter 5s  agent-send ping\n

haproxy
backend servers
  server server1 192.168.0.10:80 check  weight 100  agent-check  agent-addr 192.168.0.10  agent-port 8080  agent-inter 5s  agent-send ping\n

Use the following arguments on each server line to enable agent-based checks:

LDAP health checks Jump to heading #

You can health check LDAPv3 servers. The load balancer uses the Anonymous Authentication Mechanism of Simple Bind to connect. The check is valid if the server responds with a successful result message.

1. Configure the LDAP servers accordingly to allow anonymous binding. You can do this with an IP alias on the server side that allows only the load balancer’s IP addresses to bind to it.

2. Add option ldap-check to your backend section. In this example, we send the health check probes to alternative IP addresses specified with the addr argument on the server lines:

   haproxy
   backend be_myapp
     option ldap-check
     server srv1 10.0.0.1:389 check addr 10.0.0.11
     server srv2 10.0.0.2:389 check addr 10.0.0.12

   haproxy
   backend be_myapp
     option ldap-check
     server srv1 10.0.0.1:389 check addr 10.0.0.11
     server srv2 10.0.0.2:389 check addr 10.0.0.12

MySQL health checks Jump to heading #

You can health check MySQL database servers. The check is valid if the server responds with a successful result message. Two modes exist:

• check the MySQL handshake packet
• test Client Authentication

In the following example, we check a MySQL handshake by adding the option mysql-check directive:

haproxy
backend be_myapp
  option mysql-check
  server srv1 10.0.0.1:3306 check
  server srv2 10.0.0.2:3306 check

haproxy
backend be_myapp
  option mysql-check
  server srv1 10.0.0.1:3306 check
  server srv2 10.0.0.2:3306 check

Add a user argument to option mysql-check for the health check probe to send a Client Authentication packet:

haproxy
backend be_myapp
  option mysql-check user hapee-lb
  server srv1 10.0.0.1:3306 check
  server srv2 10.0.0.2:3306 check

haproxy
backend be_myapp
  option mysql-check user hapee-lb
  server srv1 10.0.0.1:3306 check
  server srv2 10.0.0.2:3306 check

PostgreSQL health checks Jump to heading #

You can perform a simple PostgreSQL check by sending a StartupMessage. The check is valid if the server responds with a successful Authentication request message rather than an error response. Add the option pgsql-check directive to your backend section and include a check argument on each server line.

haproxy
backend be_pgsql
  option pgsql-check
  server srv1 10.0.0.1:5432 check
  server srv2 10.0.0.2:5432 check

haproxy
backend be_pgsql
  option pgsql-check
  server srv1 10.0.0.1:5432 check
  server srv2 10.0.0.2:5432 check

Optionally, include the username that will be used to connect to the PostgreSQL server (here, hapee-lb):

haproxy
backend be_pgsql
  option pgsql-check hapee-lb
  server srv1 10.0.0.1:5432 check
  server srv2 10.0.0.2:5432 check

haproxy
backend be_pgsql
  option pgsql-check hapee-lb
  server srv1 10.0.0.1:5432 check
  server srv2 10.0.0.2:5432 check

Redis health checks Jump to heading #

You can monitor a Redis service by sending the PING command. The check is valid if the server responds with the string +PONG. Add the option redis-check directive to your backend section and include a check argument on each server line.

haproxy
backend be_redis
  option redis-check
  server srv1 10.0.0.1:6379 check
  server srv2 10.0.0.2:6379 check

haproxy
backend be_redis
  option redis-check
  server srv1 10.0.0.1:6379 check
  server srv2 10.0.0.2:6379 check

SMTP health checks Jump to heading #

You can monitor a Simple Mail Transfer Protocol (SMTP) service. Add the option smtpchk directive to your backend section and include a check argument on each server line. The check is valid if the server response code starts with the number 2.

haproxy
backend be_smtp
  option smtpchk
  server srv1 10.0.0.1:25 check
  server srv2 10.0.0.2:25 check

haproxy
backend be_smtp
  option smtpchk
  server srv1 10.0.0.1:25 check
  server srv2 10.0.0.2:25 check

You can also monitor an Extended Simple Mail Transfer Protocol (ESMTP) service. Add the hello command to use, which is HELO for SMTP and EHLO for ESMTP. Follow this with the domain name to present to the server:

haproxy
backend be_smtp
  option smtpchk EHLO mydomain.com
  server srv1 10.0.0.1:25 check
  server srv2 10.0.0.2:25 check

haproxy
backend be_smtp
  option smtpchk EHLO mydomain.com
  server srv1 10.0.0.1:25 check
  server srv2 10.0.0.2:25 check

See also Jump to heading #

• http-check connect
• http-check expect
• http-check send