Why use thread pool isolation as the default isolation method?

[suanyi001]

Problem describe. I uesed spring-cloud-starter-circuitbreaker-resilience4j:2020.0.3 and resilience4j-bulkhead in my project dependencies, When I read the source code, I found the following code in org.springframework.cloud.circuitbreaker.resilience4j.Resilience4jBulkheadProvider：

   private <T> Supplier<CompletionStage<T>> decorateBulkhead(final String id, final Map<String, String> tags, final Supplier<T> supplier) {
        BulkheadConfiguration configuration = (BulkheadConfiguration)this.configurations.computeIfAbsent(id, this.defaultConfiguration);
        if (this.bulkheadRegistry.find(id).isPresent() && !this.threadPoolBulkheadRegistry.find(id).isPresent()) {
            Bulkhead bulkhead = this.bulkheadRegistry.bulkhead(id, configuration.getBulkheadConfig(), tags);
            CompletableFuture<T> asyncCall = CompletableFuture.supplyAsync(supplier);
            return Bulkhead.decorateCompletionStage(bulkhead, () -> {
                return asyncCall;
            });
        } else {
            ThreadPoolBulkhead threadPoolBulkhead = this.threadPoolBulkheadRegistry.bulkhead(id, configuration.getThreadPoolBulkheadConfig(), tags);
            return threadPoolBulkhead.decorateSupplier(supplier);
        }
    }

it makes Thread-pool isolation as the default isolation.And I did not find that semaphore isolation can be configured globally as the default isolation method. Can you provide a way like this.

[ryanjbaxter]

Originally we did not have support for bulkheads and this was added in this release. In order to not break people we kept the ThreadPool the default.

We can switch the default functionality in the main branch at this point.

I am also OK if you want to provide a configuration option to set a global bulkhead configuration.

Would you be interested in contributing that?