Qt License Server configuration

Define the Qt License Server configuration values in the configuration file (Linux: qt-license-server.service, Windows: qt-service.xml).

The table below summarizes the available configuration options.

Waiting for a free license seat

When a client asks for a floating license seat and every seat is taken, Qt License Server does not refuse the request. It keeps the client waiting, and hands it the next seat that another client releases. Clients are served oldest first.

By default Qt License Server holds the client's connection open for as long as this takes. The client waits quietly and never has to reconnect, which keeps the load on the server low even when thousands of clients are waiting at once.

Servers behind a reverse proxy

A reverse proxy, load balancer, or NAT device between the clients and Qt License Server closes connections that have been idle for too long. A connection held open while waiting for a seat looks idle to such a device, so the device closes it and the client reports a network error instead of waiting.

Set QLS_PROXY_IDLE_TIMEOUT to the idle timeout of that device:

Environment="QLS_PROXY_IDLE_TIMEOUT=75"

Use 75 for a default nginx configuration and 60 for a default AWS Application Load Balancer. Qt License Server then lets go of a waiting connection before the device does, and tells the client to reconnect and keep waiting. The client keeps its place in the queue, so waiting longer than one such window costs it nothing.

The shortest window Qt License Server uses is 30 seconds. If the device closes idle connections faster than that, Qt License Server logs a warning at start-up: raise the device's idle timeout.

To set the holding time yourself instead, use QLS_SEAT_WAIT_WINDOW. It overrides the derived value, and Qt License Server warns at start-up if it reaches the declared idle timeout.

If Qt License Server is holding connections indefinitely and sees the headers a proxy usually adds, it writes a warning to the log.

A client that can use seats from several full pools waits on one pool at a time, so it always gets a window of up to 60 seconds, even when the window is switched off. Only a reconnect lets it see a seat free on one of the other pools.

Host settings for many waiting clients

Every client waiting for a seat holds one connection, and therefore one file descriptor, for as long as it waits. Before raising QLS_MAX_QUEUE_SIZE, raise the host limits to match:

• The open file limit (ulimit -n) must be above QLS_MAX_QUEUE_SIZE. Qt License Server cannot accept a connection once the limit is reached, and the queue size does not protect against that.
• net.core.somaxconn bounds how many connections the operating system accepts at once. A large burst of clients starting together overruns a low value, and those clients see connection failures.
• net.ipv4.tcp_keepalive_intvl and net.ipv4.tcp_keepalive_probes decide how quickly Qt License Server reclaims the connection of a client whose host disappeared without closing it, for example after a power loss. With the Linux defaults this takes about 12 minutes, and until then that client still counts against QLS_MAX_QUEUE_SIZE. Lower them if clients are lost this way often.