Subgraph support for query batching (#4661)

This project is an extension of the existing work to support [client
side batching in the
router](#126).
The current implementation is experimental and is publicly
[documented](https://www.apollographql.com/docs/router/executing-operations/query-batching/).
The additional work to enable batching requests to subgraphs is captured
in [this issue](#2002).
Currently the concept of a batch is preserved until the end of the
RouterRequest processing. At this point we convert each batch request
item into a separate SupergraphRequest. These are then planned and
executed concurrently within the router and re-assembled into a batch
when they complete. It's important to note that, with this
implementation, the concept of a batch, from the perspective of an
executing router, now disappears and each request is planned and
executed separately.
This extension will modify the router so that the concept of a batch is
preserved, at least outwardly, so that multiple subgraph requests are
"batched" (in exactly the same format as a client batch request) for
onward transmission to subgraphs. The goal of this work is to provide an
optimisation by reducing the number of round-trips to a subgraph from
the router.
Additionally, the work will address an [unresolved
issue](#4019) from the
existing experimental implementation and promote the existing
implementation from experimental to fully supported.

Fixes #2002 

<!-- start metadata -->
---

**Review Guidance**

This is a fairly big PR, so I've written these notes to help make the
review more approachable.

1. The most important files to review are (in order):

-
[.changesets/feat_garypen_2002_subgraph_batching.md](https://github.com/apollographql/router/pull/4661/files#diff-6376c91cfdd47332a662c760ac849bb5449a1b6df6891b30b72b43f041bd836f)
-
[docs/source/executing-operations/query-batching.mdx](https://github.com/apollographql/router/pull/4661/files#diff-617468db3057857f71c387eaa0d1a6161e3c1b8bf9fcb2de6fc6eafedc147277)
-
[apollo-router/src/services/router/service.rs](https://github.com/apollographql/router/pull/4661/files#diff-544579a213fda1bff6313834d30fe1746a8a28ffd7c0d6dfa1081fa36a487355)
-
[apollo-router/src/services/supergraph/service.rs](https://github.com/apollographql/router/pull/4661/files#diff-5d72a88a68962a5926fb5bb115ea3efc186904612f74e697d72e3f009669c733)
-
[apollo-router/src/query_planner/plan.rs](https://github.com/apollographql/router/pull/4661/files#diff-21a82d277d12e8f21b6b71398d62e95303a117130cc4a27510b85ebfceeb8208)
-
[apollo-router/src/services/subgraph_service.rs](https://github.com/apollographql/router/pull/4661/files#diff-6ef5a208ca8622f30eef88f75c18566e0304d59856b66293dcd6811555e6382e)
-
[apollo-router/src/batching.rs](https://github.com/apollographql/router/pull/4661/files#diff-3e884074ecad8176341159a2382aa81c49d74b851894b8ade9fa4718c434dec6)

First read the documentation. Hopefully that will make clear how this
feature works. I've picked these files as being most important (and
ordered them for review) because:

router service => This is where we spot incoming batches and create
context `BatchQuery` items to manage them through the router. We also
re-assemble them on the way back and identify any batches which may need
to be cancelled.

supergraph service => Here we pick up the information about how many
fetches we believe each `BatchQuery` will need to make.

plan => The new `query_hashes()` does this fetch identification for us.
This is the most important function in this feature.

subgraph service => Here's is where we intercept the calls to subgraphs
and park threads to wait for batch execution to be performed. We do a
lot of work here, so this is where most of the intrusive changes are:
assembling and dis-assembling batches and managing the co-ordination
between a number of parked tasks.

batching => This is the implementation of batch co-ordination. Each
batch has a task which manages a variety of channels to facilitate
communication between the incoming batches, waiting tasks and outgoing
(to subgraph) batches. I'm suggesting reading this *after* reading
through the service changes because it should mainly just be
implementation details and you will be able to follow what is happening
without knowing all this detail initially. Once you understand the
changes to the services, you will need to read this code. Feel free to
peek ahead though if that's how you like to review stuff.

2. There are still a couple of TODOs which will be resolved early next
week. They are both related to how we handle context cloning, so a
decision is still pending there.

Obviously all the files need to be reviewed, but the remaining files
should be fairly mechanical/straight-forward.

---

**Checklist**

Complete the checklist (and note appropriate exceptions) before the PR
is marked ready-for-review.

- [x] Changes are compatible[^1]
- [x] Documentation[^2] completed
- [x] Performance impact assessed and acceptable
- Tests added and passing[^3]
    - [x] Unit Tests
    - [x] Integration Tests
    - ~[ ] Manual Tests~

**Exceptions**

*Note any exceptions here*

**Notes**

[^1]: It may be appropriate to bring upcoming changes to the attention
of other (impacted) groups. Please endeavour to do this before seeking
PR approval. The mechanism for doing this will vary considerably, so use
your judgement as to how and when to do this.
[^2]: Configuration is an important part of many changes. Where
applicable please try to document configuration examples.
[^3]: Tick whichever testing boxes are applicable. If you are adding
Manual Tests, please document the manual testing (extensively) in the
Exceptions.

---------

Co-authored-by: Nicholas Cioli <nicholas.cioli@apollographql.com>
Co-authored-by: Edward Huang <edward.huang@apollographql.com>

.changesets/feat_garypen_2002_subgraph_batching.md

@@ -0,0 +1,41 @@
+### Subgraph support for query batching ([Issue #2002](https://github.com/apollographql/router/issues/2002))
+
+As an extension to the ongoing work to support [client-side query batching in the router](https://github.com/apollographql/router/issues/126), the router now supports batching of subgraph requests. Each subgraph batch request retains the same external format as a client batch request. This optimization reduces the number of round-trip requests from the router to subgraphs.
+
+Also, batching in the router is now a generally available feature: the `experimental_batching` router configuration option has been deprecated and is replaced by the `batching` option.
+
+Previously, the router preserved the concept of a batch until a `RouterRequest` finished processing. From that point, the router converted each batch request item into a separate `SupergraphRequest`, and the router planned and executed those requests concurrently within the router, then reassembled them into a batch of `RouterResponse` to return to the client. Now with the implementation in this release, the concept of a batch is extended so that batches are issued to configured subgraphs (all or named). Each batch request item is planned and executed separately, but the queries issued to subgraphs are optimally assembled into batches which observe the query constraints of the various batch items.
+
+To configure subgraph batching, you can enable `batching.subgraph.all` for all subgraphs. You can also enable batching per subgraph with `batching.subgraph.subgraphs.*`. For example:
+
+```yaml
+batching:
+  enabled: true
+  mode: batch_http_link
+  subgraph:
+    # Enable batching on all subgraphs
+    all:
+      enabled: true
+```
+
+```yaml
+batching:
+  enabled: true
+  mode: batch_http_link
+  subgraph:
+    # Disable batching on all subgraphs
+    all:
+      enabled: false
+    # Configure(over-ride) batching support per subgraph
+    subgraphs:
+      subgraph_1:
+        enabled: true
+      subgraph_2:
+        enabled: true
+```
+
+Note: `all` may be over-ridden by `subgraphs`. This applies in general for all router subgraph configuration options.
+
+To learn more, see [query batching in Apollo docs](https://www.apollographql.com/docs/router/executing-operations/query-batching/).
+
+By [@garypen](https://github.com/garypen) in https://github.com/apollographql/router/pull/4661

apollo-router/feature_discussions.json

@@ -2,10 +2,9 @@
   "experimental": {
     "experimental_retry": "https://github.com/apollographql/router/discussions/2241",
     "experimental_response_trace_id": "https://github.com/apollographql/router/discussions/2147",
-    "experimental_when_header": "https://github.com/apollographql/router/discussions/1961",
-    "experimental_batching": "https://github.com/apollographql/router/discussions/3840"
+    "experimental_when_header": "https://github.com/apollographql/router/discussions/1961"
   },
   "preview": {
     "preview_entity_cache": "https://github.com/apollographql/router/discussions/4592"
   }
-}
+}