Add Axon.deferred/2 - #651
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Adds `Axon.deferred/2`, which defers construction of a subgraph until
the parent template shapes are known at initialization time. This
covers cases where the structure of one part of a graph depends on the
shape of another part that isn't statically derivable at the call site
— decoders mirroring an encoder's input dim, projection heads matching
a backbone's hidden size, adapters wrapping an opaque inner model.
The factory receives a `{parent, template}` tuple per parent and must
return an `%Axon{}` rooted at those parents. It runs once, the first
time `init_fn` is called, and the resulting subgraph is compiled and
reused for the lifetime of the `build/2` result. Like blocks, deferred
subgraphs namespace their parameters under the layer name.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
The deferred node memoized its materialized subgraph in an ETS table created per node per Axon.build/2 call. Nothing ever deleted it, so every build leaked a table: 200 builds left 200 live :axon_deferred tables, which survive garbage collection. A process that rebuilds a deferred model in a loop would eventually exhaust ERL_MAX_ETS_TABLES and take down the VM. Drop the cache and rebuild the subgraph from the parent templates in both init and predict. The factory is a pure function of its templates, so both paths agree on the subgraph and the parameters line up. This trades the cache for extra tracing work. Under a caching Nx.Defn compiler, or Axon.compile/4, the subgraph is built once per input signature and not at all on subsequent calls. Under the default evaluator, which re-traces on every call, it is rebuilt per call - the same order of work the evaluator already does for the parent graph. Since the invocation count is now compiler-dependent, the factory is documented as needing to be pure, and the test asserts that repeated traces agree rather than pinning an exact count. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
polvalente
approved these changes
Aug 16, 2026
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