ClickHouse/libs/libzkutil/src/ZooKeeperNodeCache.cpp

#include <zkutil/ZooKeeperNodeCache.h>

namespace zkutil
{

ZooKeeperNodeCache::ZooKeeperNodeCache(GetZooKeeper get_zookeeper_)
	: get_zookeeper(std::move(get_zookeeper_))
	, context(std::make_shared<Context>())
{
}

std::experimental::optional<std::string> ZooKeeperNodeCache::get(const std::string & path)
{
	zkutil::ZooKeeperPtr zookeeper;
	std::unordered_set<std::string> invalidated_paths;
	{
		std::lock_guard<std::mutex> lock(context->mutex);

		if (!context->zookeeper)
		{
			/// Possibly, there was a previous session and it has expired. Clear the cache.
			nonexistent_nodes.clear();
			node_cache.clear();

			context->zookeeper = get_zookeeper();
		}
		zookeeper = context->zookeeper;

		invalidated_paths.swap(context->invalidated_paths);
	}

	if (!zookeeper)
		throw DB::Exception("Could not get znode: `" + path + "'. ZooKeeper not configured.", DB::ErrorCodes::NO_ZOOKEEPER);

	for (const auto & path : invalidated_paths)
	{
		nonexistent_nodes.erase(path);
		node_cache.erase(path);
	}

	if (nonexistent_nodes.count(path))
		return std::experimental::nullopt;

	auto watch_callback = [context=context](zkutil::ZooKeeper & zookeeper, int type, int state, const char * path)
	{
		if (!(type != ZOO_SESSION_EVENT || state == ZOO_EXPIRED_SESSION_STATE))
			return;

		bool changed = false;
		{
			std::lock_guard<std::mutex> lock(context->mutex);
			if (&zookeeper != context->zookeeper.get())
				return;

			if (type != ZOO_SESSION_EVENT)
				changed = context->invalidated_paths.emplace(path).second;
			else if (state == ZOO_EXPIRED_SESSION_STATE)
			{
				context->zookeeper = nullptr;
				context->invalidated_paths.clear();
				changed = true;
			}
		}
		if (changed)
			context->changed_event.set();
	};

	std::string contents;

	auto cache_it = node_cache.find(path);
	if (cache_it != node_cache.end())
	{
		return cache_it->second;
	}

	if (zookeeper->tryGetWatch(path, contents, /* stat = */nullptr, watch_callback))
	{
		node_cache.emplace(path, contents);
		return contents;
	}

	/// Node doesn't exist. Create a watch on node creation.
	nonexistent_nodes.insert(path);

	if (!zookeeper->existsWatch(path, /* stat = */nullptr, watch_callback))
		return std::experimental::nullopt;

	/// Node was created between the two previous calls, try again. Watch is already set.
	if (zookeeper->tryGet(path, contents))
	{
		nonexistent_nodes.erase(path);
		node_cache.emplace(path, contents);
		return contents;
	}

	return std::experimental::nullopt;
}

}