ApiClient throttles requests in async mode

OkHttp by default only permits 5 connections to a given host in async mode. This means that in a high traffic scenario, we are capped to just 5 concurrent requests unless we manage a pool of ApiClients or create one per request, or ignore the async methods.

The number can be changed by configuring a dispatcher on the ApiClient builder to have a greater number of maxRequests and maxRequestsPerHost. I think there are a number of possible fixes:

• The max requests is set suitably high automatically (ideally unbounded for us)
• The max requests/max requests per host is configurable inside the merchant config
• The client can pass in an optional Function to configure the http client builder

We did a quick test with a local fork to set this higher and this helped it work for us, the change was just:

+            Dispatcher dispatcher = new Dispatcher();
+            dispatcher.setMaxRequests(500);
+            dispatcher.setMaxRequestsPerHost(500);
           try {
               httpClient = classHttpClient.newBuilder()
+                     .dispatcher(dispatcher)

It looks like there is no significant negative impact to just setting this to Integer.MAX_VALUE - so that might be a reasonable option.

The issue identifies the ApiClient builder and its OkHttp HTTP client configuration as the main entry points. First inspect how async clients are constructed and how Dispatcher settings can be supplied, then resolve which of the proposed configuration approaches is intended. Done means async requests are no longer limited by OkHttp’s default five concurrent requests per host.