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docs/adr/0022-sharded-per-node-generation-cache.md

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+# ADR-0022: Sharded per-node generation cache for distributed PyTorch routes
+
+## Status: Accepted
+
+## Context
+
+The distributed PyTorch chat path previously recomputed the full prompt-so-far for
+every generated token. The head shard embedded the entire sequence each step, forwarded
+full-sequence activations through every downstream shard, and every shard called its
+decoder layers with `use_cache=False`. On a two-node Qwen2.5-0.5B route this produced
+the expected quadratic slowdown as output length grew.
+
+ADR-0020 and ADR-0021 fixed route construction and `start_layer` semantics. They did not
+define the per-request cache lifecycle needed for efficient decode.
+
+## Decision
+
+Distributed PyTorch generation now uses one stable route session id for an entire chat
+request. The wire protocol marks each activation hop with:
+
+- `X-Meshnet-Session`: stable per generation.
+- `X-Meshnet-Cache-Mode`: `prefill`, `decode`, or `stateless`.
+- `X-Meshnet-Seq-Len`: the total sequence length represented by the step.
+
+Step 0 is prefill: the head sends the full prompt activation through the planned route.
+Each shard stores only the opaque cache state returned by its own executed layer range.
+No shard receives or stores another shard's cache.
+
+Later cached decode steps send only the newest token activation (`[1, 1, hidden]`) with
+the full sequence length and newest position id. The backend deliberately treats layer
+cache state as opaque. Standard K/V tuples, HuggingFace cache objects, and hybrid
+linear-attention recurrent state are stored without shape assumptions.
+
+## Cache lifecycle
+
+Each `TorchModelShard` owns an in-memory LRU map keyed by
+`(session_id, effective_start_layer, shard_end)`. Entries expire by TTL and by a maximum
+session count (`MESHNET_SHARD_CACHE_TTL_SECONDS`, default 600;
+`MESHNET_SHARD_CACHE_MAX_SESSIONS`, default 16).
+
+If a decode step reaches a node after restart, eviction, TTL expiry, or route mismatch,
+the node returns an explicit `cache_miss` response. The head falls back to full prefill
+for the current prompt-so-far using the same session id, rebuilding the shard-local
+caches before continuing. Alpha route repair still does not migrate cache state across
+nodes; a true route change is treated as cache loss and recovered by re-prefill.
+
+## Consequences
+
+- Healthy decode sends O(1) activation payloads per token between nodes instead of
+  O(sequence length).
+- Cache internals stay behind the model backend boundary, which keeps Qwen3.6-style
+  hybrid recurrent cache state compatible with the same route protocol.
+- Restart and eviction degrade to slower stateless/full-prefill work rather than silent
+  output corruption.
+- Cross-node cache migration, batching cache state across sessions, and speculative
+  decoding remain future work.
+
+## Verification
+
+Unit coverage in `tests/test_real_model_backend.py` verifies opaque per-layer cache
+storage, cached one-token decode, explicit cache-miss errors, and LRU eviction. Live
+two-node Qwen2.5-0.5B TPS measurement still requires the physical two-machine topology
+used to observe the regression.