Frequently asked questions

For a general overview of the built-in proxy, see Built-in proxy.

Why Lua?

I thought ya’ll could use a break from YAML :) We use Lua and its extensibility like glue: nearly everyone has different methods of managing their configuration, and in most cases a simple lua script would be able to parse output from such services or talk to them directly. A handful of for loops and object reuses can remove tens of thousands of lines of generated configuration.

For route handling our usage of Lua is more bold, but again it’s simply glue. Configurations for routes will tend to be wide but shallow: just a couple small function calls based on the contents of the request, and then the request is handed back off to the C side. Object methods written in C further avoid copying strings/data to/from Lua for common operations.

Since the proxy integrates with memcached’s existing systems, we can (over time) ensure all large memory allocations are handled outside of Lua. The rest of the tiny allocations, if any, are tied to a coroutine and released immediately after the (typically under a millisecond) request/response.

Why not LuaJIT?

For half the answer to this, please see Why lua? - for loading configuration the performance difference isn’t going to matter much. For route handling very little work is done from Lua, which will be reduced or removed as development continues.

When development started LuaJIT had a much more rocky status. Also Lua 5.4 has a good number of language and performance improvements on its own.

Why not use a mesh router?

Memcached’s proxy is not intended to replace a mesh router; its scope is much smaller and more performance focused. A mesh router may be highly configurable, with broad support, but will be very slow. Caching services (and in this case a caching proxy) can be used to restore performance to a service migrated to a mesh router; for cost or practicality reasons.

Many data queries don’t or shouldn’t go through a mesh router anyway. If you want to speed up access to data storage from an application implemented as an endpoint on a mesh router, a caching service is what sits behind that endpoint but before its actual data storage.

What happens to in-flight requests on config reload?

When the configuration is reloaded, mcp_config_routes gets called again. During this process routes are assembled into functions and mcp.attach() is called with these top level hooks. Any time a request comes in these top level hooks are checked for their associated function.

This means once a request fires off its hook, it is immune from configuration changes until it completes. Any new request coming in will see what’s presently configured in a hook and get the new code even as older requests are being processed.

This does mean you have to be careful with using global variables in Lua: we suggest the use of function closures for routes to avoid configurations clashing.