parallel-reconcile-merge

Parallel Reconcile-Merge

Overview

Parallelism itself is not the enemy — uncoordinated parallelism is. The chapter's safe pattern embeds a parallel window inside a sequential control flow: planning verifies the branches are truly independent, the branches run simultaneously reading different data sources and writing to separate channels, and only the merge node combines them.

This skill implements the execution + reconciliation half:

• Per-branch error isolation (tree-pipeline error aggregation): each branch runs in isolation; an exception is caught and recorded in that branch's own result. "One branch's failure shouldn't cascade to others or corrupt shared state." Branches receive a copy of context and never write shared state — only the merge node combines them.
• Reducer-style deterministic merge (Example 5-8): merge successful branch values with a reducer (add, extend, union, max, min) so the result is "deterministic merging regardless of execution order."
• Completion policy: all_or_nothing (every branch must succeed) vs partial_coverage (a quorum suffices — "sometimes three successful branches out of four provide sufficient coverage").
• Flag union: any red flag from any branch is surfaced; a failed sibling never silently drops another branch's flag.

In the DevOps latency investigation (account 123456789012), the planner spawns parallel hypothesis tests — database connection-pool metrics and payment-service memory profiles — reading different data sources. The pool branch raises a pool_exhausted flag; the memory branch comes back normal. Reconcile surfaces the flag, merges both findings, and (under all-or-nothing for a confirmation phase) reports completion only if both branches succeeded. The analogous claims case runs fraud / provider-credentials / medical-necessity branches the same way: any flag from any branch affects the final decision.

When to Use

• A planning node has verified branch independence (different data sources, no shared decision, no shared mutable state)
• Parallel hypothesis tests, multi-aspect verification (fraud / pricing / credentials), parallel research tracks
• You need deterministic merge + a quorum/all-or-nothing completion policy + flag aggregation