EventListener

EventListener

EventListener class contains a set of call-back functions that will be called when specific RocksDB event happens such as flush. It can be used as a building block for developing custom features such as stats-collector or external compaction algorithm.

How to use it?

In ColumnFamilyOptions, there's a variable called listeners, which allows developers to add custom EventListener to listen to the events of a specific rocksdb instance or a column family of a rocksdb instance.

// A vector of EventListeners which call-back functions will be called
// when specific RocksDB event happens.
std::vector<std::shared_ptr<EventListener>> listeners;

To listening to a rocksdb instance or a column family of a rocksdb instance, it can be done by simply adding a custom EventListener to ColumnFamilyOptions::listeners and use that options to open a DB:

// listen to a rocksdb instance
Options options;
...
options.listeners.emplace_back(new MyListener());

// listen to a column family of a rocksdb instance
ColumnFamilyOptions cf_options;
...
cf_options.listeners.emplace_back(new MyListener());

To listen to multiple column families of a rocksdb instance, one can either use a separate Listener instance for each column family, or use same Listener instance for all column families:

// one listener for each column family
for (size_t i = 0; i < cf_options.size(); ++i) {
  cf_options[i].listeners.emplace_back(new MyListener());
}

// one same listener for all column families.
EventListener my_listener = new MyListener();
for (size_t i = 0; i < cf_options.size(); ++i) {
  cf_options[i].listeners.emplace_back(my_listener);
}

Note that call-back functions should not run for an extended period of time before the function returns, otherwise RocksDB may be blocked. For example, it is not suggested to do DB::CompactFiles() (as it may run for a long while) or issue many of DB::Put() (as Put may be blocked in certain cases) in the same thread in the EventListener callback. However, doing DB::CompactFiles() and DB::Put() in another thread is considered safe.

[Threading] All EventListener callback will be called using the actual thread that involves in that specific event. For example, it is the RocksDB background flush thread that does the actual flush to call EventListener::OnFlushCompleted().