Hi, David, Generally speaking, iterators will make a snapshot of key space of RocksDB. Hence, it associates with some memory overhead. More severe performance issue we saw before is that if you insert and delete tons of sessions in a short time period, the iterator seek function can be extremely slow, due to the need to traverse through tons of tombstone (i.e. deleted records) before hitting the next live record. The suggested "queue" mechanism helps to avoid the issue, since all deletions happen sequentially in a continuous block and all insertions also happen sequentially in a continuous block. This gives greater opportunity for the compaction thread to come in and cleanup all the tombstone records and make iterator faster, again.
In your use case, if you can make sure that the newly inserted session's sessionId is *alway* at the tail of your session table, and your session expiration order is the same as the order determined by the sessionId, that should work as well. -Yi On Mon, Jun 6, 2016 at 3:17 PM, David Yu <david...@optimizely.com> wrote: > Hi, Yi, > > Yes, the sessions are keyed by the sessionId. > > In our case, iterating through all OPEN sessions is inevitable, since that > is precisely where we evaluate (base on timestamp) and close sessions. In > other words, the closed session queue you suggested cannot be constructed > without going through all the sessions periodically. > > Can you explain (on a higher level) why iteration through the entries can > be a slow process? > > Thanks, > David > > On Mon, Jun 6, 2016 at 2:34 PM, Yi Pan <nickpa...@gmail.com> wrote: > > > Hi, David, > > > > I would recommend to keep a separate table of closed sessions as a > "queue", > > ordered by the time the session is closed. And in your window method, > just > > create an iterator in the "queue" and only make progress toward the end > of > > the "queue", and do a point deletion in the sessionStore, which I assume > > that would be keyed by the sessionId. > > > > The reason for that is: > > 1) RocksDB is a KV-store and it is super efficient in read/write by key, > > not by iterator > > 2) If you have to use iterator, making sure that the iterator only goes > > toward the "tail" where all meaningful work items will be is important to > > achieve fast and efficient operation. Please refer to this blog from > > RocksDB team: > > > > > https://github.com/facebook/rocksdb/wiki/Implement-Queue-Service-Using-RocksDB > > > > -Yi > > > > On Mon, Jun 6, 2016 at 2:25 PM, David Yu <david...@optimizely.com> > wrote: > > > > > We use Samza RocksDB to keep track of our user event sessions. The task > > > periodically calls window() to update all sessions in the store and > purge > > > all closed sessions. > > > > > > We do all of this in the same iterator loop. > > > > > > Here's how we are doing it: > > > > > > > > > public void window(MessageCollector collector, TaskCoordinator > > coordinator) > > > throws Exception { > > > > > > KeyValueIterator<String, Session> it = sessionStore.all(); > > > > > > while (it.hasNext()) { > > > > > > Entry<String, Session> entry = it.next(); > > > Session session = entry.getValue(); > > > > > > update(session); > > > > > > if (session.getStatus() == Status.CLOSED) { > > > sessionStore.delete(entry.getKey()); > > > } else { > > > sessionStore.put(entry.getKey(), session); > > > } > > > } > > > } > > > > > > > > > The question is: is this the correct/efficient way to do a read+update > > for > > > RocksDB? > > > > > > Thanks, > > > David > > > > > >
