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eBay Compatible Application Check process requires that any server-side web application using the eBay API should not make more than 18 simultaneous API calls. Meaning, at any given point of time, the number of active requests on eBay, from a particular AppID should not be more than 18. This limit is not just about the number of threads you create within your application. For example, if an application makes API calls through a Java Servlet, the webserver will create a separate thread for each client request, even though the servlet is not multi-threaded.
How to enforce a limit on maximum number of requests depends on the Application Server/ Webserver that you are using. Usually, there should be a server configuration where you can set this limit.
Some examples are given below. Please go through your web server documentation for more details and the implications of putting this limit. For example in some cases, changing the number of threads will affect other applications running on the web server.
In Apache Tomcat, the number of simultaneous request are defined by setting maxThreads in
Apache HTTP Server
In Apache HTTP Server, MPM (Multi-Processing Module) handles the number of threads. The parameters to be set and the configuration file names are different for different operating systems.
On Unix based systems, the parameters are defined in a configuration file called httpd.conf and the parameters to be defined are ThreadsPerChild, MaxRequestsPerChild and MaxClients.
In IBM WebSphere the WAS administrative console is used to tune thread pools.
In IIS, you can limit the number of requests by setting Threads Per Processor Limit through Internet Information Services (IIS) Manager.