WASM extensions

WASM extensions

The host runs WASM extensions of the extism-kernel-v1 profile: an Extism guest that speaks the Morphir Extension Protocol (MEP) 0.2.0-draft.1. Extension settings says how to name a module and how the host checks it before it runs.

Experimental morphir compile and morphir generate run a configured Python module through the same runtime as the internal morphir extension worker. The bounds below apply to each session.

One instance for each session

Each session gets its own instance of the guest and its own instance of the Extism kernel, in a store of its own. Two sessions of the same module share no memory and no state. A guest that fails ends its own session only.

Bounds of a session

The worker takes --memory-pages, --instructions and --call-ms to set other values for one run. A page is 64 KiB, so the default ceiling is 4096 pages.

Memory

An Extism session has two linear memories: the guest’s own memory, and the memory of the Extism kernel, which holds the guest’s input, its output and what it allocates through the kernel. The ceiling applies to each of them separately. A session’s linear memory can therefore reach twice the ceiling: 512 MiB with the default of 256 MiB. The code the engine compiles from the module, and the host’s own memory, come on top of that. Plan for twice the ceiling for each WASM session that can run at the same time.

A memory cannot grow past its ceiling. The engine refuses the growth, and the guest decides what happens next:

• The guest stops with a trap. If the guest’s memory or the kernel’s memory is at its ceiling when the call ends, the session fails with extension.memory_limit.
• The guest reports the failure itself, for example with an Extism error or a non-zero result. The session fails with that failure, extension.incomplete_session, even when a memory is at its ceiling.
• The guest handles the refusal and answers. The call succeeds.

Every failure diagnostic states both memory sizes and the ceiling, so you can see which memory was full:

extension.memory_limit: the guest failed at the memory ceiling (the guest trapped, with the guest's memory at its ceiling): <what the engine reported>; guest memory 256 MiB (268435456 bytes), kernel memory 1 MiB (1048576 bytes), ceiling 256 MiB (268435456 bytes)

A module whose memory starts above the ceiling does not start. It fails with extension.memory_limit before any guest code runs.

In worker mode the same failure is a fault event on stderr with kind memory-limit, and the worker exits with status 1.

If an extension needs more memory, raise the ceiling for it. Each memory can then grow to the new ceiling, so the session can reach twice the new value.