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Re: LMDB and multiple processes

Brian G. Merrell wrote:

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.


http://symas.com/mdb/doc/


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.  This 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?  This seems to work OK,
but a lot of it was guesswork, so I'm sure I'm doing some silly things.


Your reader process should be using read transactions.


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.  In 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.
In the actual LMDB API read transactions can be reused by their creating thread, so they are zero-cost after the first time. I don't know if any of the other language wrappers leverage this fact.
Opening a DBI only needs to be done once per process. Opening per transaction would be stupid, like reopening a file handle on every request. 


I was suspect there are flags that I can/should be using, but I'm not sure.

Thanks for any input.

         Brian



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