MixT Prototype: Code

DSL for mixed-consistency transactions: website, paper

Hello! This is the prototype for MixT. This is not intended for distribution or serious end-user experience; there are some very platform-specific assumptions in this code. If you are brave or curious, welcome!

Building MixT

• The transactions directory is the code's proper home. Build from there.
• the pg_env.sh script will configure your environment for MixT. source pg_env.sh before attempting to build!
• clang++-5.0 is the Officially Supported Compiler™; ≥g++-7.2 should also work.
• 64-bit Gentoo Linux is the only tested build and runtime environment for MixT.
• dev-libs/libpqxx is a required dependency for building MixT with postgres (as is default)
• the build does not produce a library; link the object files explicitly please.
• there is no configure; just source pg_env.sh; make <target>

Using MixT

MixT's transaction compiler is implemented entirely in compile-time C++ through the use of constexpr and, yes, some templates. To write a MixT transaction, just #include mtl/mtl.hpp. There are some example transactions in the root directory; look for the TRANSACTION(...) invocations. MixT includes bindings for a postgres backend and a simple in-memory backend; use the in-memory backend if you're just trying out some transaction code. The in-memory backend is also a good thing to copy when writing your own bindings.

Interpreting errors

C++ is famously bad at giving reasonable error messages, especially when those errors involve templates. First, I must strongly recommend clang here; g++ is not quite there yet with error messages. If you are building under clang and MixT gives you an error when compiling your transaction, there are a few common cases to look for:

• Always build with -ferror-limit=1 set. clang and gcc both tend to treat type errors as "pretend it's an integer and move on", which makes errors after the first one likely to be spurious or misleading.
• constexpr variable 'prev' must be initialized by a constant expression indicates an exception has been thrown in constexpr code. Scroll down until you see subexpression not valid, which will tell you the exact exception the code tried to throw. This is usually enough to understand the error.
• if you're getting an error in constexpr code, you can explicitly call the constexpr function that's throwing the error. This will now execute at runtime, and give you normal exception behavior.
• static_assert failed: errors usually pertain to typechecking failures or flow violations. In either case, you'll get an error message directly.

Notes on compilers with MixT

This project is actively pushing the boundaries of compiler support for modern C++. Some things to watch out for:

Things clang really doesn't like:

• concept-style auto, which means the mixt_method syntax only builds on g++ until clang adds support.

Things that g++ really doesn't like:

• exceptions thrown from constexpr contexts unless guarded by explicit conditionals
• anonymous structs used as template parameters
• deeply-nested constexpr; you need a lot of RAM to build large examples under g++. My largest example took 45GB on g++-8.1