STMX is an actively maintained, high-performance concurrency library providing Transactional Memory for Common Lisp.

Home page and downloads:

Main features

  • Extremely intuitive to use and to write correct, thread-safe concurrent code
  • Brings database-style transactions to Common Lisp by introducing transactional memory
  • High performance implementation, benchmarked to exceed 7 millions transactions per CPU core per second on commodity PC hardware
  • Support for hardware memory transactions (requires 64-bit SBCL and CPUs with Intel TSX instructions - currently Core i5 4570, Core i7 4670, Core i7 4770 and some others). They increase STMX performance up to almost 40 millions transactions per CPU core per second
  • Removes the need for traditional locks, mutexes and conditions - writing correct concurrent code with them is well known to be hard
  • Transactional code is intrinsically deadlock-free: if two transactions conflict one of them will be re-executed
  • Automatic commit and rollback: if a transaction completes normally it will be committed, if it signals an error it will be rolled back
  • Transactions are composable: they can be executed in a larger transaction, either in sequence (all-or-nothing) or as alternatives (try them until one succeeds)
  • Offers freedom of choice between blocking and non-blocking transactional functions: given either behaviour, it is trivial to transform it into the other.
  • Features transactional versions of popular data structures: hash tables, red-black trees, stack, fifo
  • Includes transactional data structure for multicast publish/subscribe
  • Creating new transactional data structures is easy
  • Extensive test suite
  • Tested on SBCL, ABCL, CCL, CMUCL and ECL.
  • Very simple to install with Quicklisp
A quick-start guide and installation instructions are provided in file

License: LLGPL

What STMX is NOT

In order not to confuse programmers - less experienced ones in particular - and to avoid rising unrealistic hopes, the author himself stated the following about STMX:
  • it is NOT a quick hack to automagically transform existing, slow, single-threaded programs into fast, concurrent ones. No matter how much transactions can help, writing concurrent code still requires careful design and implementation - and testing. And refactoring takes time too.
  • it is NOT for optimization-focused programmers trying to squeeze the last cycle from their Common Lisp programs. STMX records an in-memory transaction log containing all reads and writes from/to transactional memory, then later (during commit) validates the transaction log against the latest data present in transactional memory and finally copies the transaction log onto the transactional memory while holding locks. STMX is quite optimized, but this machinery comes at an obvious performance cost with respect to hand-made, highly optimized locking code (but a good reality check is to ask yourself how many people have the skill and patience to write such code without bugs).
  • it is NOT supposed to be used for all data structures in a Common Lisp program. STMX is intended only for the data accessed concurrently by multiple threads while being modified by at least one thread. And even in that case, transactional memory is not always needed: it depends on the kinds of modifications.
  • it is NOT a serialization or persistence framework. Rather, messing with metaclasses and playing (allowed) tricks with slots contents as STMX does, quite likely does not mix well with serialization or persistence libraries such as CL-STORE, because they typically need full control on the slots of the objects to be serialized and de-serialized.
  • it is NOT a million dollar library from some deep-pocket company. At the moment, it is the work of a single person.