# `DdDataStreams.Extractor.GooglePubSub`
[🔗](https://github.com/rockneurotiko/dd_data_streams/blob/main/lib/dd_data_streams/extractor/google_pub_sub.ex#L1)

The extractor for `BroadwayCloudPubSub` v2.

A Pub/Sub message names its consume edge by its subscription, and v2 stamps it
as `metadata.subscription` together with `metadata.attributes`,
`metadata.orderingKey` and `metadata.receivedAt`, which is what a checkpoint
needs to name the edge. A message that carries no subscription - the shape
`BroadwayCloudPubSub` v1 produces - records no consume checkpoint, because
there is no name to report its edge under.

Consume checkpoints therefore require `broadway_cloud_pub_sub` 2.0.0-rc or
newer, tested with 2.0.0-rc.3, the first to stamp the receive time as
`metadata.receivedAt`, a `%DateTime{}`. On stable 1.0.0 the v1 metadata has no
`:subscription`, so `consume/1` returns `:error` for every message and a v1
pipeline records nothing.

Both edges are reported as `type:google-pubsub`, the technology Datadog files
these streams under. Pub/Sub names its subscriptions and topics as full
resource names (`projects/<project>/subscriptions/<id>`); this extractor
reduces each to its short id, which is what dd-trace-java reports for Pub/Sub
(both edges). dd-trace-py reports the full resource path of the subscription
under `topic:` instead, so a Py consumer appears as a different node; the
pathway still connects, because the link between two nodes is the hash
carried on the message. An already-short name is reported as it is.

A Pub/Sub message metadata carries only its subscription, so the topic is read
from the `dd-topic` attribute the library's own producer writes next to the
pathway header. When that attribute is present the consume edge reports both
`subscription:<subscription>` and `topic:<topic>`, because Datadog derives the
queues in the middle of a pathway - `data_streams.latency`'s `visited_queues`
and `detailed_pathway` - from the `topic` tag; with only a subscription the
queue between two services is missing. A message from a producer that did not
write the attribute, or from outside this library, keeps the subscription-only
tags: nothing is invented from the subscription's name, because naming is the
application's convention.

The publish time Pub/Sub stamps on a message - `metadata.publishTime`, a
`%DateTime{}` in v2 - starts the consume checkpoint of a message that starts a
pathway, so a pipeline that only consumes reports how long the message waited
in the bus instead of zero. A message that carries a pathway header keeps the
start its producer wrote, because a first-hop start is only offered to a
pathway that has not started. A publish time that is absent, is not a
`%DateTime{}`, or falls outside the window a clock could plausibly have
stamped is dropped rather than believed.

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