# 3.9 The Metric Channel

_Peios / Advanced Peios / PSPU / Observability Interfaces_

> The metric ingestion socket, separate from the log socket, and why the collector is a sink rather than a scraper.

A collector MUST expose an `AF_UNIX` `SOCK_DGRAM` socket for metric
ingestion, protected by a Security Descriptor as §3.3 requires,
separate from the log socket.

The channel works exactly as the log channel does, for the reasons given
there: a declared datagram ceiling, truncated datagrams discarded whole,
a receive queue of at most four times the ceiling, no backpressure, no
notification, and no way for a producer to discover the ceiling (§3.6).

> [!NOTE]
> §3.A gives the mainline value and adjustable range
> for this bound and every other in this chapter.

## 3.9.1 A sink, not a collector of its own

The collector is **pushed** to. It MUST NOT scrape an endpoint, read a
kernel interface, or poll anything to obtain metrics; every sample it
holds arrived on this socket because a producer sent it.

What gathers the measurements is a separate concern and a separate
program. A collection agent that reads system counters and submits them
is an ordinary producer here, with no privileged position and no
interface of its own.

> [!NOTE]
> The push direction follows from the loss model rather than from taste.
> A pulling collector must reach every producer on a schedule, which
> makes it responsible for their availability and makes a slow producer
> the collector's problem. Pushing keeps each producer responsible for
> its own submissions and keeps a slow or dead producer invisible to
> everything except the gap it leaves.

## 3.9.2 Batching

Batching matters more here than it does for logs. A collection sweep
produces many samples at once — every CPU core, every disk, every
interface — and they share a moment, so a producer SHOULD submit a sweep
as one batched datagram rather than as one datagram per sample.

The rules are the log rules: one map or an array of maps, per-record
validation, and the encoded datagram bounded by the ceiling (§3.8,
§3.12).