Hi all,
First, I'm having trouble finding resources to answer a question like this myself, so please forgive me if I've missed something.
I'm considering using LMDB (versus LevelDB) for a project I'm working on where I'll be receiving a high volume (hundreds per second) of high priority requests (over HTTP) and issuing multiple (<10) database queries per request.
I'll also have a separate process receiving updates for the data and writing to the database. ÂThis will happen often (several times a minute, perhaps), but the priority is much lower than the read requests.
LMDB appealed to me because of the read performance and that I could have one processing reading data from LMDB and another process writing data updates to LMDB.
For proof of concept, I hacked up the following (I'll use pseudocode since I used the Go bindings for my actual programs, and hopefully my question is sufficiently abstract not to matter):
Process 1, the writer, simply writes a random integer (from 0 to 1000) to a defined set of keys:
env = NewEnv()
env.Open("/tmp/foo", 0, 0664)
txn = env.BeginTxn(nil, 0)
dbi = txn.DBIOpen(nil, 0)
txn.Commit()
txn = env.BeginTxn(nil, 0)
n_entries = 5
for i = 0; i < n_entries; i++ {
  key = sprintf("Key-%d", i)
  val = sprintf("Val-%d", rand.Int(1000))
  txn.Put(dbi, key, val, 0)
}
txn.Commit()
env.DBIClose(dbi)
env.Close()
Process 2, the reader, simply loops forever and does random access reads on the data from process 1 (I won't benefit from a cursor for my actual problem), and prints out that data occasionally:
env = NewEnv()
env.Open("/tmp/foo", 0, 0664)
while {
  txn = BeginTxn(nil, 0)
  dbi = txn.DBIOpen(nil, 0)
  txn.Commit()
  for i = 0; i < n_entries; i++ {
    key = sprintf("Key-%d", i)
    val = txn.Get(dbi, key)
    print("%s: %s", key, value)
  }
  env.DBIClose(dbi)
  sleep(5)
}
So my high level question is: What am I doing wrong? ÂThis seems to work OK, but a lot of it was guesswork, so I'm sure I'm doing some silly things.
For example, first I put the BeginTxn() and DBIOpen() calls in process 2 outside of the while loop, but when I did that, I never saw the updates values upon running process 1 simultaneously. ÂIn my real-world application, it seems like adding these calls to every request (to be sure the data being read is up-to-date) could be an unnecessary performance penalty.
I was suspect there are flags that I can/should be using, but I'm not sure.
Thanks for any input.
    Brian