1 files changed, 78 insertions, 0 deletions
diff --git a/src/seastar/demos/sharded_parameter_demo.cc b/src/seastar/demos/sharded_parameter_demo.cc
new file mode 100644
index 000000000..da6da789a
--- /dev/null
+++ b/src/seastar/demos/sharded_parameter_demo.cc
@@ -0,0 +1,78 @@
+/*
+ * This file is open source software, licensed to you under the terms
+ * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
+ * distributed with this work for additional information regarding copyright
+ * ownership.  You may not use this file except in compliance with the License.
+ *
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+/*
+ * Copyright 2020 ScyllaDB
+ */
+
+
+// Demonstration of seastar::sharded_parameter
+
+#include <seastar/core/sharded.hh>
+#include <seastar/core/app-template.hh>
+#include <seastar/core/thread.hh>
+#include <seastar/util/defer.hh>
+#include <cassert>
+
+// This is some service that we wish to run on all shards.
+class service_one {
+    int _capacity = 7;
+public:
+    // Pretend that this int is some important resource.
+    int get_capacity() const { return _capacity; }
+};
+
+// Another service that we run on all shards, that depends on service_one.
+class service_two {
+    int _resource_allocation;
+public:
+    service_two(service_one& s1, int resource_allocation) : _resource_allocation(resource_allocation) {}
+    int get_resource_allocation() const { return _resource_allocation; }
+};
+
+int main(int ac, char** av) {
+    seastar::app_template app;
+    return app.run(ac, av, [&] {
+        // sharded<> setup code is typically run in a seastar::thread
+        return seastar::async([&] {
+
+            // Launch service_one
+            seastar::sharded<service_one> s1;
+            s1.start().get();
+            auto stop_s1 = seastar::defer([&] { s1.stop().get(); });
+
+            auto calculate_half_capacity = [] (service_one& s1) {
+                return s1.get_capacity() / 2;
+            };
+
+            // Launch service_two, passing it per-shard dependencies from s1
+            seastar::sharded<service_two> s2;
+            // Start s2, passing two parameters to service_two's constructor
+            s2.start(
+                    // Each service_two instance will get a reference to a service_one instance on the same shard
+                    std::ref(s1),
+                    // This calculation will be performed on each shard
+                    seastar::sharded_parameter(calculate_half_capacity, std::ref(s1))
+            ).get();
+            seastar::defer([&] { s2.stop().get(); });
+
+            s2.invoke_on_all([] (service_two& s2) {
+                assert(s2.get_resource_allocation() == 3);
+            }).get();
+        });
+    });
+}