The maximum callback concurrency.
The Cloud Pub/Sub C++ client library will schedule parallel callbacks as long as the number of outstanding callbacks is less than this maximum.
Note that this controls the number of callbacks scheduled, not the number of callbacks actually executing at a time. The application needs to create (or configure) the background threads pool with enough parallelism to execute more than one callback at a time.
Some applications may want to share a thread pool across many subscriptions. The additional level of control (scheduled vs. running callbacks) allows applications, for example, to ensure that at most K threads in the pool are used by any given subscription.
Example
namespace pubsub = ::google::cloud::pubsub;
using ::google::cloud::future;
using ::google::cloud::GrpcBackgroundThreadPoolSizeOption;
using ::google::cloud::Options;
using ::google::cloud::StatusOr;
auto sample = [](std::string project_id, std::string subscription_id) {
// Create a subscriber with 16 threads handling I/O work, by default the
// library creates `std::thread::hardware_concurrency()` threads.
auto subscriber = pubsub::Subscriber(pubsub::MakeSubscriberConnection(
pubsub::Subscription(std::move(project_id), std::move(subscription_id)),
Options{}
.set<pubsub::MaxConcurrencyOption>(8)
.set<GrpcBackgroundThreadPoolSizeOption>(16)));
// Create a subscription where up to 8 messages are handled concurrently. By
// default the library uses `std::thread::hardware_concurrency()` as the
// maximum number of concurrent callbacks.
auto session = subscriber.Subscribe(
[](pubsub::Message const& m, pubsub::AckHandler h) {
// This handler executes in the I/O threads, applications could use,
// std::async(), a thread-pool, or any other mechanism to transfer the
// execution to other threads.
std::cout << "Received message " << m << "\n";
std::move(h).ack();
PleaseIgnoreThisSimplifiesTestingTheSamples();
});
return std::make_pair(subscriber, std::move(session));
};
Type Aliases
Type
Alias Of:
std::size_t