lmdb-safe/README.md

lmdb-safe

A safe modern & performant C++ wrapper of LMDB. For now briefly only available for C++17, will support C++ 11 again soon. MIT licensed.

LMDB is an outrageously fast key/value store with semantics that make it highly interesting for many applications. Of specific note, besides speed, is the full support for transactions and read/write concurrency. LMDB is also famed for its robustness.. when used correctly.

The design of LMDB is elegant and simple, which aids both the performance and stability. The downside of this elegant design is a plethora of rules that need to be followed to not break things. In other words, LMDB delivers great things but only if you use it exactly right.

Among the things to keep in mind when using LMDB natively:

• Never open a database file more than once anywhere in your process
• Never open more than one transaction within a thread
  • .. unless they are all Read Only and have MDB_NOTLS set
• When opening a named database, no other threads may do that at the same time
• Cursors within RO transactions need freeing, but cursors within RW transactions must not be freed.

Breaking these rules causes no errors, but does lead to silent data corruption, missing updates, or random crashes.

This LMDB library aims to deliver the full LMDB performance while programmatically making sure the LMDB semantics are adhered to, with very limited overhead.

Most common LMDB functionality is wrapped within this library but the native MDB handles are all available should you want to use functionality we did not (yet) cater for.

Example

The following example has no overhead compared to native LMDB, but already exhibits several ways in which lmdb-safe is easier and safer to use:

  auto env = getMDBEnv("./database", 0, 0600);
  auto dbi = env->openDB("example", MDB_CREATE);
  auto txn = env->getRWTransaction();

The first line requests an LMDB environment for a database hosted in ./database. Within LMDB, it is not allowed to open a database file more than once, not even from other threads, not even when using a different LMDB handle. getMDBEnv keeps a registry of LMDB environments, keyed to the exact inode. If another part of your process requests access to the same inode, it will get the same environment.

On the second line, a database is opened within our environment. The semantics of opening or creating a database within LMDB are tricky. With some loss of generality, MDBEnv::openDB will create a transaction for you to open the database, and close it too. Most of the time this is what you want. It is also possible to open a database within a transaction manually.

The third line opens a read/write transaction using the Resource Acquisition Is Initialization (RAII) technique. If txn goes out of scope, the transaction is aborted automatically. To commit or abort, use commit() or abort(), after which going out of scope has no further effect.

  txn.put(dbi, "lmdb", "great");

  string_view data;
  if(!txn.get(dbi, "lmdb", data)) {
    cout<< "Within RW transaction, found that lmdb = " << data <<endl;
  }
  else
    cout<<"Found nothing" << endl;

  txn.commit();

LMDB is so fast because it does not copy data unless it really needs to. Memory bandwidth is a huge determinant of performance on modern CPUs. This wrapper agrees and using modern C++, it is possible to seemlessly use 'views' on data without copying them. Using these techniques, the call to txn.put() sets the "lmdb" string to "great", without making additional copies.

We employ the same technique to request the value of "lmdb", which is made available to us as a read-only view, straight onto the memory mapped data on disk.

In the final line, we commit the transaction, after which it also becomes available for other threads and processes.