wondering what the best strategy to achieve atomic updates with LMDB. what i am trying to achieve is a read then update atomic action given a highly concurrent use case, for example, if a key/value pair represents a counter, how does one increment or decrement the counter atomically. i am pretty sure mdb_get -> mdv_set sequence is not atomic, wondering if mdb_cursor_get -> mdv_cursor_put sequence is? perhaps a certain flag is required on the get action to achieve a lock? in my bdb implementation i used lockers to achieve this.

howard - thank you for the prompt reply and for putting this wonderful library together. to clarify my understanding, can you elaborate a little on the locking lmdb tx provide cross threads and processes? in particular 1. do they provide a writer lock by default and one needs to flag MDB_TXN_RDONLY when a writer lock is not required?

2. given that mdb_txn_begin is a precursor to most other actions, are these locks "data store" wide? is there some other mechanism to lock a key/value pair or cursor so not to delay access to other keys? or is the basic assumption such is unnecessary?

thank again! tomer On Jul 24, 2013, at 1:52 PM, Howard Chu <hyc(a)symas.com <mailto:hyc@symas.com>> wrote: > Tomer Doron wrote: >> wondering what the best strategy to achieve atomic updates with LMDB. >> >> what i am trying to achieve is a read then update atomic action given a >> highly concurrent use case, for example, if a key/value pair represents a >> counter, how does one increment or decrement the counter atomically. >> >> i am pretty sure mdb_get -> mdv_set sequence is not atomic, wondering if >> mdb_cursor_get -> mdv_cursor_put sequence is? perhaps a certain flag is >> required on the get action to achieve a lock? in my bdb implementation i >> used lockers to achieve this. > > Transactions are atomic. Whatever operations you perform in a single > transaction will occur atomically. BDB-style locking is unnecessary. > > -- > -- Howard Chu > CTO, Symas Corp. http://www.symas.com > Director, Highland Sun http://highlandsun.com/hyc/ > Chief Architect, OpenLDAP http://www.openldap.org/project/