# `Nous.HTTP.StreamBackend.Req`
[🔗](https://github.com/nyo16/nous/blob/v0.17.1/lib/nous/http/stream_backend/req.ex#L1)

`Nous.HTTP.StreamBackend` implementation backed by `Req` (Finch
underneath).

Default streaming backend. Drives `Req.post/1` with the `:into`
callback so chunks are pushed into a `Task`, which forwards them to
the consuming `Stream.resource` via `send/2`.

## Backpressure

Req's `:into` callback runs in the spawned `Task`, which forwards each
chunk to the consumer with `send/2`. BEAM mailboxes are unbounded, so
the pair share an `:atomics` counter of **in-flight bytes**: the
producer adds `byte_size(chunk)` before sending, the consumer subtracts
it on receipt.

Above the 8 MB high-water mark the producer stops calling `send/2` and
parks in a `receive`; the consumer signals `{ref, :resume}` once the
counter falls below the 1 MB low-water mark. Because the producer is
Req's `:into` callback, parking it stops draining the socket, so
backpressure propagates all the way to the wire. Resident memory per
stream is bounded by the byte watermark rather than by chunk *count*,
and the steady-state cost is one local `:atomics` read per chunk — no
polling and no cross-process `Process.info/2`.

If the consumer is truly unresponsive (the counter stays above the
high-water mark for longer than `:backpressure_max_wait_ms`, default
30s), the producer aborts the request and the stream yields
`{:stream_error, %{reason: :backpressure_overflow, inflight_bytes: n}}`
rather than wedging forever.

The consumer process is resolved when enumeration starts, not when the
stream is built, so a stream may be constructed in one process and
enumerated in another (task, GenServer, LiveView).

Callers whose downstream consumers reliably block per chunk (LiveView
fan-out under load, persistence-on-every-chunk, slow IO) can still
prefer `Nous.HTTP.StreamBackend.Hackney`, which provides strict
pull-based backpressure via `:hackney`'s `{:async, :once}` mode: one
chunk is read from the socket per consumer request, with no in-flight
window at all.

## TLS verification

Req's defaults handle TLS verification via Mint/Finch (system CAs
with peer verification). No additional configuration needed.

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*Consult [api-reference.md](api-reference.md) for complete listing*