docs: Propose build batching for Bazel.

docs-v2/design_proposals/concurrent-bazel-builds.md

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+# Concurrent Bazel Builds
+
+* Author(s): Seth Nelson
+* Design Shepherd: 
+* Date: 2024/05/21
+* Status: [Reviewed/Cancelled/Under implementation/Complete]
+
+## Background
+
+Bazel supports high levels of concurrency in its action graph model; however, Skaffold is not currently able to 
+take advantage of that concurrency to build multiple targets because it is modeled around a separate builder
+invocation per artifact. This does not play nicely with Bazel's workspace lock model; invoking multiple builds
+in parallel as separate processes forces them to serialize. This effectively renders Skaffold's `--build-concurrency`
+null and void (see https://github.com/GoogleContainerTools/skaffold/issues/6047), and results in longer overall
+build times than if multiple target's action execution could interleave.
+
+This document proposes changes to Skaffold which will enable multiple targets to be build in a single Bazel invocation.
+
+
+## Design
+
+### Where to break the "one artifact per build" abstraction
+
+As part of this design, I considered three places to introduce the concept of multi-artifact builds:
+
+1. For all builds - in the `builder_mux`, group builds before scheduling.
+2. For the local builder pipeline - allow any underlying artifactBuilder to implement a `BuildMultiple` method,
+   and wrap that in a `BatchingBuilder` which can queue up multiple incoming builds before sending groups into `BuildMultiple`.
+3. For Bazel only, where batching happens as an implementation detail with `bazel/build.go`.
+
+Ultimately, this design proposes #3. 
+
+* While #1 results (in my opinion) in the cleanest abstraction model (after all, builder_mux is the demux point for 
+  turning a list of artifacts into a set of builds to be scheduled), it was rejected due to its invasiveness and 
+  complexity. Implementing this at the `builder_mux` level results in a
+  ton of method forking (into multi-artifact variants); it also introduces significant scheduling complexity (to avoid 
+  cycles introduced by batching build targets). This complexity did not feel warranted for a feature that is likely
+  to only be used by Bazel.
+* #2 was rejected because it would incorrectly (continue to) couple the compilation vs load/push phases of Bazel's `artifactBuilder` implementation.
+  We only want to group up the actual `bazel build` invocation, not the subsequent `docker.Push`/`b.loadImage` which
+  is also part of the `Build` implementation. Batching at the `Build` implementation level would (a) reduces the speed gains, because we would serialize the loads/pushes,
+  and (b) adds implementation complexity, because we need to handle BatchingBuilders with different `artifactBuilder`
+  constructor parameters (like loadImages).
+
+
+### Design Overview
+
+This design proposes implementing build batching for Bazel builds by altering the Bazel Builder to build multiple
+JARs, and then multiplexing contemporaneous builds (e.g. waiting for a period of time for builds targets to 
+"collect" and then building them all together). 
+
+* This changes no existing externally-visible methods/interfaces.
+* This does not _require_ config changes, though we could consider (a) initially implementing this functionality hidden behind a configuration flag, for rollout safety, and (b) configuring the batch window.
+* This design naturally respects the `--build_concurrency` flag, which limits how many builds will be scheduled contemporaneously.
+
+### Design Elements
+
+#### Bazel Builder can build multiple TARs
+
+This is an in-place change; we replace `buildTar` with `buildTars`, which builds up a list of `buildTargets`.
+
+#### computeBatchKey function
+
+A `computeBatchKey` function which returns the same value if two different builds can be grouped together. Elements of
+the Bazel batch key include:
+* The sorted list of build args
+* The (currently, single) bazel platform flag
+* The artifact's workspace
+
+#### batchingBuilder
+
+A BatchingBuilder wraps an underlying multi-artifact build function (the `buildTars` function), and maintains a map of
+non-started `MultiArtifactBuild` objects (per batch key).
+
+```go
+type batchingBuilder struct {
+	builder   multiJarBuilder
+	batchSize time.Duration
+	builds    map[string]*MultiArtifactBuild
+	mu        sync.Mutex
+}
+```
+
+```go
+type MultiArtifactBuild struct {
+	ctx       context.Context
+	artifacts []*latest.Artifact
+	platforms platform.Matcher
+	workspace string
+	mu        sync.Mutex
+	doneCond  chan bool
+	running   bool
+	out       io.Writer
+	digest    map[*latest.BazelArtifact]string
+	err       error
+}
+```
+
+When it is asked to build an artifact, it first checks to see if there is an existing build for the batch key.
+* If there is, it simply adds the artifact to the build's `artifacts` list. 
+* If there is not, it creates a new MultiArtifact build, and kicks off a goroutine that will wait `batchSize` before
+  invoking its `multiJarBuilder`.
+
+#### How this impacts logging and metrics:
+
+Given our choice in where to break the "one artifact per build" abstraction, this design presents some rough edges
+around logging and metrics that are worth acknowledging:
+* If any build in a batch fails, they are all reflected as failed to the Events API / console output.
+  * This is fundamentally unavoidable in any implementation that involves a single bazel invocation building multiple 
+  targets.
+* Only one build's `OutputWriter` is actually used (the first one to create the `MultiArtifactBuild` object). 
+  * This
+  presents some rough edges in my POC that I could use some help ironing out - depending on scheduling order, sometimes
+  it fails to stream output to the console until the build is complete.
+
+### Open Questions
+
+**\<Should we include either of the configuration knobs listed in the Design Overview>**
+
+Resolution: __Not Yet Resolved__
+
+
+## Implementation plan
+
+I believe this change is well-contained enough to be implemented in a single PR, please let me know what you think. 
+You can find a functional proof-of-concept here: https://github.com/GoogleContainerTools/skaffold/compare/main...sethnelson-tecton:skaffold:sethn/parallel-builds
+
+___
+
+
+## Integration test plan
+
+New test cases (in addition to status quo test cases of unbatched builds):
+
+1. Batched builds that can all batch together (2 artifacts, no dependencies, build-concurrency>=2)
+1. Batched builds that should not batch together due to concurrency limits (4 artifacts, build-concurrency>=2)
+1. Batched builds that should not batch together due to scheduling (2 artifacts, one depends on the other, build-concurrency>=2)
+1. Batched builds with different build args