The Runtime API allows changing the settings in real-time mode. You will have no downtime and no need to restart/reload the HAProxy service while using the Runtime API.

You could face the following issue: if you disable a frontend or a backend server, the changes will not be saved; this server will be rolled back on the next restart of HAproxy. If you tick the Save changes checkbox, the changes will be saved but won't be displayed in the configuration files so you may have some difficulties with debugging (or be confused at least).

Save changes checkbox

Use the interface that works fine with the HAProxy Runtime API to avoid these problems. It allows editing the runtime configuration and save changes in the config file without downtime.

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Note that if you make changes on the master server, these changes will take effect on the slave server too.
Short instrauction

Actual commands for HAProxy v2

Action&Keyword: Description:
prompt toggle interactive mode with prompt on/off
quit disconnect
show tls-keys [id|*] show TLS keys references or remove TLS ticket keys when id is specified
set ssl tls-key [id|keyfile] tlskey set the next TLS key for the id or keyfile listener to tlskey
show sess [id] report the list of current sessions or dump this exact session
shutdown session kill a specific session
shutdown sessions server kill sessions on a server
clear counters clear max statistics counters (add 'all' for all counters)
show info report information about the running process
show stat report counters for each proxy and server
show schema json report the schema used for stats
disable agent disable agent checks (use 'set server' instead)
disable health disable health checks (use 'set server' instead)
disable server disable a server for maintenance (use 'set server' instead)
enable agent enable agent checks (use 'set server' instead)
enable health enable health checks (use 'set server' instead)
enable server enable a disabled server (use 'set server' instead)
set maxconn server change the frontend's maxconn setting
set server change a server's state, weight or address
get weight report a server's current weight
set weight change a server's weight (deprecated)
show resolvers [id] dump counters from all resolvers section
associated name servers clear table remove an entry from a table
set table [id] update or create a table entry
show table [id] report table usage stats or dump this table's contents
show peers [peers section] dump some information about all the peers or this peers section
disable frontend temporarily disable specific frontend
enable frontend re-enable specific frontend
set maxconn frontend change a frontend's maxconn setting
show servers state [id] dump volatile server information (for backend id)
show backend list backends in the current running config
shutdown frontend stop a specific frontend
set dynamic-cookie-key backend change a backend secret key for dynamic cookies
enable dynamic-cookie backend enable dynamic cookies on a specific backend
disable dynamic-cookie backend disable dynamic cookies on a specific backend
show errors report last request and response errors for each proxy
set maxconn global change the per-process maxconn setting
set rate-limit change a rate limiting value
set severity-output [none|number|string] set presence of severity level in feedback information
set timeout change a timeout setting
show env [var] dump environment variables known to the process
show cli sockets dump list of cli sockets
show cli level display the level of the current CLI session
show fd [num] dump list of file descriptors in use
show activity show per-thread activity stats (for support/developers)
operator lower the level of the current CLI session to operator
user lower the level of the current CLI session to user
show startup-logs report logs emitted during HAProxy startup
show cache show cache status
add acl add acl entry
clear acl clear the content of this acl
del acl delete acl entry
get acl report the patterns matching a sample for an ACL
show acl [id] report available acls or dump an acl's contents
add map add map entry
clear map id clear the content of this map
del map delete map entry
get map report the keys and values matching a sample for a map
set map modify map entry
show map [id] report available maps or dump a map's contents
show pools report information about the memory pools usage
show profiling show CPU profiling options
set profiling enable/disable CPU profiling
show threads show some threads debugging information
runtimeapi

Maxconn global

Dynamically change the global maxconn setting within the range defined by the initial global maxconn setting.

Increase the global maxconn setting value to accept pending connections immediately. Or set it below the current number of connections to delay this process until the threshold is reached. Set this value to zero to restore the initial setting.

Maxconn frontend

Dynamically change the specified frontend's maxconn setting. Any positive value including zero is allowed, but setting values larger than the global maxconn do not make any sense. Increase the setting value to accept pending connections immediately. Or set it below the current number of connections to delay this process until the threshold is reached.

Roxy-WI allows to change IP address and port number for backends without reloading or restarting the HAProxy service.

How you may do it:

Changes will take effect for the running configuration and will be saved in the configuration file as well. This means changes will not be lost if you restart the HAProxy service.

HTTP requests are stateless by design. However, this raises some questions about tracking users activities, including malicious ones, in order you can collect metrics, block users, and make other decisions based on state. The only way to track user activities between requests is to add a mechanism for storing events and categorizing them (by client's IP address or other key metrics).

HAProxy gives you a fast in-memory storage which is called stick tables out of the box. Roxy-WI allows managing it via its GUI.

Block or allow the specified IP addresses with the help of a simple Access Control List and HAProxy.