Constant memory adr (#1799)

fix #1717

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* [x] CHANGELOG updated or not needed
* [x] Documentation updated or not needed
* [x] Haddocks updated or not needed
* [x] No new TODOs introduced or explained herafter

docs/adr/2024-01-27_031-constant-memory.md

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+---
+slug: 31
+title: |
+  31. Achieve constant memory in hydra-node
+authors: [v0d1ch]
+tags: [Accepted]
+---
+
+## Status
+
+Accepted
+
+## Context
+
+When testing out hydra-node operation under heavy or increased load we are
+noticing that memory consumption is far from ideal. So far we didn't bother
+thinking about the performance so much but time has come to try and reduce
+memory footprint of a running hydra-node.
+
+There are some quick points to be scored here since our projections that are
+used to serve in-memory data are using a common haskell list as a data
+structure. We should stream the data keeping the memory bounded as the first
+optimisation.
+
+It is also not necesary to output the whole history of messages by default and
+only do that if clients request to see the whole history. Internally our
+`ServerOutput` type could be remapped to `StateChanged` since the two are
+almost identical. Any new information must be streamed to the clients
+automatically.
+
+## Decision
+
+- Re-map `ServerOutput` to `StateChanged` by adding any missing constructor to `StateChanged` (eg. `PeerConnected`).
+- Output new client messages on `newState` changes instead of using `ClientEffect`.
+- Use `StateChanged` in all projections we server from the API (re-use `eventId` as sequence number).
+- Make hydra-node output history of messages only on demand (breaking change is to be communicated in the changelog).
+- Use `conduit` library to achieve constant memory by streaming the data in our projections.
+
+## Consequences
+
+This should lead to much better performance of hydra-node in terms of used
+memory for the running process. This should be also confirmed by running the
+relevant [benchmarks](https://github.com/cardano-scaling/hydra/issues/1724) and
+do a test (even manual or a script) to assert that the memory consumption is
+actually reduced.