The maximum number of outstanding messages per streaming pull.
The Cloud Pub/Sub C++ client library uses streaming pull requests to receive messages from the service. The service will stop delivering messages if this many messages or more have neither been acknowledged nor rejected.
If a negative or 0 value is supplied, the number of messages will be unlimited.
Example
namespace pubsub = ::google::cloud::pubsub;
using ::google::cloud::future;
using ::google::cloud::Options;
using ::google::cloud::StatusOr;
auto sample = [](std::string project_id, std::string subscription_id) {
// Change the flow control watermarks, by default the client library uses
// 0 and 1,000 for the message count watermarks, and 0 and 10MiB for the
// size watermarks. Recall that the library stops requesting messages if
// any of the high watermarks are reached, and the library resumes
// requesting messages when *both* low watermarks are reached.
auto constexpr kMiB = 1024 * 1024L;
auto subscriber = pubsub::Subscriber(pubsub::MakeSubscriberConnection(
pubsub::Subscription(std::move(project_id), std::move(subscription_id)),
Options{}
.set<pubsub::MaxOutstandingMessagesOption>(1000)
.set<pubsub::MaxOutstandingBytesOption>(8 * kMiB)));
auto session = subscriber.Subscribe(
[](pubsub::Message const& m, pubsub::AckHandler h) {
std::move(h).ack();
std::cout << "Received message " << m << "\n";
PleaseIgnoreThisSimplifiesTestingTheSamples();
});
return std::make_pair(subscriber, std::move(session));
};
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-03-21 UTC."],[[["This webpage details the `MaxOutstandingMessagesOption` within the Google Cloud Pub/Sub C++ client library, which controls the maximum number of unacknowledged messages during streaming pull operations."],["The latest version available is 2.37.0-rc, and the page provides documentation for versions ranging from 2.11.0 to 2.37.0-rc, showing historical updates and changes."],["Setting the `MaxOutstandingMessagesOption` to a negative or zero value results in an unlimited number of outstanding messages, effectively disabling the message limit."],["The `MaxOutstandingMessagesOption` works in conjunction with `MaxOutstandingBytesOption` to manage message flow control, where message requests pause if the high watermark is met and resume only when both low watermarks are reached."],["The `MaxOutstandingMessagesOption` parameter is represented by a type alias of `std::int64_t`."]]],[]]
