distributed_mutex.cxx

• example of distributed mutex object, that backed by a document inside collection.
  • example of distributed mutex object, that backed by a document inside collection.

/* -*- Mode: C++; tab-width: 4; c-basic-offset: 4; indent-tabs-mode: nil -*- */
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 *   Copyright 2021-Present Couchbase, Inc.
 *
 *   Licensed under the Apache License, Version 2.0 (the "License");
 *   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.
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#include <couchbase/best_effort_retry_strategy.hxx>
#include <couchbase/cluster.hxx>
#include <couchbase/codec/tao_json_serializer.hxx>
#include <couchbase/fmt/error_context.hxx>
 
#include <fmt/chrono.h>
#include <fmt/format.h>
 
#include <tao/json.hpp>
 
#include <system_error>
#include <tao/json/from_string.hpp>
#include <thread>
 
static constexpr auto connection_string{ "couchbase://127.0.0.1" };
static constexpr auto username{ "Administrator" };
static constexpr auto password{ "password" };
static constexpr auto bucket_name{ "default" };
static constexpr auto scope_name{ couchbase::scope::default_name };
static constexpr auto collection_name{ couchbase::collection::default_name };
 
class lock_aware_retry_strategy : public couchbase::best_effort_retry_strategy
{
public:
  lock_aware_retry_strategy(couchbase::backoff_calculator calculator)
    : couchbase::best_effort_retry_strategy(calculator)
    , calculator_{ calculator }
  {
  }
 
  auto retry_after(const couchbase::retry_request& request,
                   couchbase::retry_reason reason) -> couchbase::retry_action override
  {
    if (reason == couchbase::retry_reason::key_value_locked) {
      // here we use the same calculator as best_effort_retry_strategy, but it could be different
      // one
      auto backoff_duration = calculator_(request.retry_attempts());
      fmt::print("retrying in {} because of \"key_value_locked\", attempt {}\n",
                 backoff_duration,
                 request.retry_attempts());
      return couchbase::retry_action{ backoff_duration };
    }
    return couchbase::best_effort_retry_strategy::retry_after(request, reason);
  }
 
private:
  couchbase::backoff_calculator calculator_;
};
 
class couchbase_mutex
{
public:
  static constexpr std::chrono::seconds default_lock_duration{ 15 };
  static constexpr std::chrono::seconds default_timeout{ 10 };
 
  couchbase_mutex(couchbase::collection collection,
                  std::string document_id,
                  std::chrono::seconds lock_duration = default_lock_duration,
                  std::chrono::seconds timeout = default_timeout)
    : collection_{ std::move(collection) }
    , document_id_{ std::move(document_id) }
    , lock_duration_{ lock_duration }
    , timeout_{ timeout }
  {
    auto options = couchbase::upsert_options{}.retry_strategy(retry_strategy_).timeout(timeout_);
    auto [err, resp] = collection_.upsert(document_id_, content_, options).get();
    std::size_t retry_attempts = retry_attemps_from_context(err.ctx().to_json());
 
    if (err.ec()) {
      throw std::system_error(
        err.ec(),
        fmt::format(R"(unable to create mutex "{}" (retries: {}))", document_id_, retry_attempts));
    }
    cas_ = resp.cas();
    fmt::print("[created ] \"{}\", cas: {}, retries: {}, lock_duration: {}\n",
               document_id_,
               cas_.value(),
               retry_attempts,
               lock_duration);
  }
 
  void lock()
  {
    std::scoped_lock lock(mutex_);
    auto options =
      couchbase::get_and_lock_options{}.retry_strategy(retry_strategy_).timeout(timeout_);
    auto [err, resp] = collection_.get_and_lock(document_id_, lock_duration_, options).get();
    std::size_t retry_attempts = retry_attemps_from_context(err.ctx().to_json());
    if (err) {
      throw std::system_error(
        err.ec(),
        fmt::format(R"(unable to lock mutex "{}" (retries: {}))", document_id_, retry_attempts));
    }
    cas_ = resp.cas();
    fmt::print(
      "[locked  ] \"{}\", cas: {}, retries: {}\n", document_id_, cas_.value(), retry_attempts);
  }
 
  void unlock()
  {
    std::scoped_lock lock(mutex_);
    auto options = couchbase::unlock_options{}.timeout(timeout_);
    auto err = collection_.unlock(document_id_, cas_, options).get();
    std::size_t retry_attempts = retry_attemps_from_context(err.ctx().to_json());
    if (err) {
      throw std::system_error(
        err.ec(),
        fmt::format(R"(unable to unlock mutex "{}" (retries: {}))", document_id_, retry_attempts));
    }
    fmt::print(
      "[unlocked] \"{}\", cas: {}, retries: {}\n", document_id_, cas_.value(), retry_attempts);
  }
 
private:
  static auto retry_attemps_from_context(const std::string& context_json) -> std::size_t
  {
    auto ctx = tao::json::from_string(context_json);
    if (const auto* attempts = ctx.find("retry_attempts"); attempts != nullptr) {
      return attempts->get_unsigned();
    }
    return 0;
  }
 
  couchbase::collection collection_;
  std::string document_id_;
  std::chrono::seconds lock_duration_;
  std::chrono::seconds timeout_;
  couchbase::cas cas_;
  const std::string content_{ "__couchbase_mutex__" };
  mutable std::mutex mutex_{}; // regular mutex to protect internal state
  std::shared_ptr<lock_aware_retry_strategy> retry_strategy_{
    std::make_shared<lock_aware_retry_strategy>(couchbase::controlled_backoff)
  }; // see also couchbase::exponential_backoff calculator
};
 
int
main()
{
  auto options = couchbase::cluster_options(username, password);
  options.apply_profile("wan_development");
  auto [connect_err, cluster] = couchbase::cluster::connect(connection_string, options).get();
  auto collection = cluster.bucket(bucket_name).scope(scope_name).collection(collection_name);
 
  // Obtain thread_id for simplicity. Could be pid_id, if it was more portable.
  auto writer_id =
    fmt::format("thread:{}", std::hash<std::thread::id>()(std::this_thread::get_id()));
 
  // Create distributed mutex to protect modification of document "order:42".
  couchbase_mutex mutex(collection, "demo_mutex");
  {
    std::scoped_lock lock(mutex);
 
    // while lock is kept, other process cannot modify "order:42"
    const std::string document_id{ "order:42" };
    const tao::json::value basic_doc{ { "type", "book" },
                                      { "name", "Alice in Wonderland" },
                                      { "author", "Lewis Carroll" },
                                      { "price_usd", 4.0 },
                                      { "writer_id", writer_id } };
    auto [err, resp] = collection.upsert(document_id, basic_doc, {}).get();
    fmt::print("[stored  ] \"{}\", ec: {}, id: \"{}\", CAS: {}, writer_id: \"{}\"\n",
               document_id,
               err.ec() ? err.ec().message() : "success",
               document_id,
               resp.cas().value(),
               writer_id);
 
    fmt::print(
      stderr,
      "[wait    ] pretend to do some work for 7 seconds (distributed mutex still locked)\n");
    std::this_thread::sleep_for(std::chrono::seconds{ 7 });
  }
 
  cluster.close().get();
 
  return 0;
}