💥 Standalone Activities for Java

[GregoryTravis]

Add standalone activity API (ActivityClient)

Introduces support for standalone activities — activities that execute independently of any workflow.

💥 Breaking Changes:

• ActivityInfo.getWorkflowId, .getRunId, .getWorkflowNamespace can now return null, in the case of standalone activities
• .getRunId now deprecated in favor of .getWorkflowRunId and .getActivityRunId.

New public API surface:

• ActivityClient — top-level client for starting, describing,
  listing, counting, cancelling, and terminating standalone activities
• ActivityHandle<R> / UntypedActivityHandle — typed and untyped
  handles returned by ActivityClient.start(); provide getResult(),
  getResult(long, TimeUnit), getResultAsync(), describe(),
  cancel(), and terminate()
• StartActivityOptions — builder-based options for starting an
  activity (id, task queue, timeouts, retry, priority, etc.)
• ActivityClientOptions — namespace / data converter / interceptor
  configuration for ActivityClient
• ActivityExecutionDescription — rich descriptor returned by
  describe() and list*(); extends ActivityExecutionMetadata
• ActivityExecutionMetadata — lightweight metadata used in list
  results
• ActivityExecutionCount — result of countActivities()
• ActivityAlreadyStartedException — thrown when a duplicate activity
  id is rejected by the server
• ActivityFailedException — thrown from getResult() when the
  activity fails

New interceptor API:

• ActivityClientCallsInterceptor — per-call interceptor for all
  ActivityClient operations
• ActivityClientCallsInterceptorBase — pass-through base
  implementation (delegates every method to the next interceptor)
• ActivityClientInterceptor — factory interceptor that wraps the
  client-level invoker
• ActivityClientInterceptorBase — no-op base implementation

ActivityInfo additions:

• getActivityRunId() — run-scoped id assigned by the server to each
  activity execution
• isInWorkflow() — distinguishes workflow-dispatched activities
  from standalone ones

ActivityCompletionClient additions:

• New overloads taking (String activityId, Optional<String> runId, …)
  for completing, failing, sending heartbeats, and cancelling standalone
  activities without a workflow id

ActivitySerializationContext fix:

• workflowId and workflowType are now @Nullable; removed
  requireNonNull guards that caused NPEs for standalone activities

Functions.java:

• Added Func and VFunc (zero-arg typed/void functional interfaces)
  needed by ActivityClient.start() method-reference overloads

CI:

• Enable standalone-activity server feature flags in the Temporal CLI
  dev server used by unit tests:
  frontend.activityAPIsEnabled, activity.enableStandalone,
  history.enableChasm, history.enableTransitionHistory

Tests:

• StandaloneActivityTest — integration tests against a real server
  covering the full activity lifecycle (start, poll, complete, cancel,
  terminate, describe, list, heartbeat, async completion)
• ActivityClientCallsInterceptorBaseTest — delegation tests for the
  base interceptor
• ActivityClientCallsInterceptorChainTest — chain ordering tests
  using a real server with two interceptors
• ActivityHandleImplTest — unit tests for handle dispatch
• ActivityCompletionClientImplTest — unit tests for completion client
• ActivityInfoImplTest — unit tests for ActivityInfo in the
  standalone context
• ActivitySerializationContextTest — confirms nullable workflow fields
• StartActivityOptionsTest, ActivityClientOptionsTest,
  ActivityExecutionDescriptionTest, ActivityExecutionMetadataTest,
  ActivityAlreadyStartedExceptionTest — options and value-type tests

[maciejdudko]

Looks good, but there are a couple things that need to be fixed before merging, most notably caching inside ActivityHandleImpl and how AsyncCompletionClient creates ManualActivityCompletionClient for standalone activities. See comments in code.

[maciejdudko]

LGTM!

Finished with other reviewers.