Assessment Shape Sizing Only

When to dispatch here

Use this recipe when the user asks one of:

• “What instance class should I use for X GB of data / Y QPS?”
• “We have N nodes of r5.4xlarge on self-managed — what’s the AOS equivalent?”
• “Size this cluster for 200 GB of logs / 50M vectors at dim 768.”
• “How many r7g.large.search do I need for 80 GB indexed?”
• “What’s the right node count for <workload>?”

The hallmark: there is a workload to size, but no migration question, no schema paste, no ‘should I use OpenSearch’ framing. The user already chose AOS — they want a baseline today.

Detection signals

If the user pastes an _cat/indices, traffic numbers, AND asks for a migration plan → that is FULL_ASSESSMENT, not SIZING_ONLY. If they ask “Managed vs Serverless” → COMPARATIVE_DECISION. If they ask “should I even use OpenSearch for 200 MB of Postgres rows” → ANTI_PATTERN_PUSHBACK.

Required output template

Produce exactly these four blocks. No headings beyond what is shown — keep the response tight.

1. Detected shape line (one sentence)

Detected shape: SIZING_ONLY — baseline for <source_size> <workload_type> on Amazon OpenSearch Service.

2. Baseline (one sentence + bullets)

Lead with a single concrete recommendation:

Run 3 × r7g.large.search data nodes + 3 × m7g.large.search dedicated cluster managers across 3 AZs, 1 replica, EBS gp3 sized to <storage_number> GiB per data node.

Then 3-5 bullets with numeric justification — instance choice rationale, replica setting, cluster-manager sizing rationale, storage rounding, AZ count.

3. Storage math (inline derivation)

ALWAYS show the formula and substitute numbers, even when inputs are estimated:

If source data is unknown, present the tiered band instead (see below) — never invent a single number.

4. References (one line, max ~3 URLs)

References: bp-instances (opens in a new tab) · bp-sharding (opens in a new tab) · https://calculator.aws (opens in a new tab).

That’s it. No other sections.

Match-source rule (CRITICAL)

When the user names their existing self-managed/EC2 cluster, match the source profile instead of falling back to a greenfield baseline:

• 6 × r5.2xlarge self-managed → recommend 6 × r7g.2xlarge.search (Graviton equivalent), not “3 × r7g.large is our default.”
• 4 × m5.xlarge self-managed → recommend 4 × m7g.xlarge.search.
• The customer has already proven their working set fits that RAM-to-data ratio. Downsize only if you can show the source was over-provisioned (e.g., JVMMemoryPressure consistently <40%).

Only fall back to “3 × r7g.large.search“ greenfield baseline when the source size is <100 GB AND the user provided no source cluster.

Tiered band sizing (UNKNOWN inputs)

When source size is not specified, do NOT guess. Present three bands and ask the user to confirm:

Always close the tiered version with: “Confirm source size and I will collapse to one recommendation.”

NOT REQUIRED — explicitly OMIT

Do NOT produce any of the following in a SIZING_ONLY answer:

• Timeline & Resourcing — REMOVED FROM SUITE. Never write “engineer-weeks”, “Phase 1 / Phase 2”, “T+0 / T+30 days”, or any sequencing language.
• Executive Summary, Source Inventory, Target Topology, Migration Path, Readiness Checklist, Risk Register, Citations section, Open Questions.
• Migration verbs: “cutover”, “Historical Data Migration”, “snapshot”, “_reindex from remote”, “Migration Assistant for Amazon OpenSearch Service”.
• Customer-specific trade-off narrative (“if your team prefers…”, “if you have RIs in…”).
• Cost discussion beyond the calculator.aws link. Never write “$X/month”.
• JVM heap tuning advice (heap is service-managed; mention only in passing if the user asks).
• Schema conversion, query DSL examples, FGAC setup.
• Long instance-family compendium — pick one family and move on.

If the user asks for any of the above, route them: “That’s outside SIZING_ONLY — for migration path see FULL_ASSESSMENT, for cost open https://calculator.aws (opens in a new tab).”

Worked exemplar (~250 words)

User asks: “We have 80 GB of product-search data on a self-managed Elasticsearch 7.10 cluster running 4 × r5.xlarge. What should we run on AOS?”

Response:

Detected shape: SIZING_ONLY — baseline for 80 GiB product-search workload on Amazon OpenSearch Service, sized to match an existing 4 × r5.xlarge source.

Run 4 × r7g.xlarge.search data nodes + 3 × m7g.large.search dedicated cluster managers across 3 AZs, 1 replica, EBS gp3 at 80 GiB per data node.

• Match-source: the existing 4 × r5.xlarge already proves the working set fits that RAM-to-data ratio; mapping to Graviton (r7g.xlarge.search) preserves the profile and gains ~20% price/perf.
• Master nodes: required because data-node count ≥ 3; m7g.large.search × 3 covers up to 30 nodes / 15K shards on OS 2.17+ — well above this footprint.
• Replicas = 1 (one primary copy + one replica); 3 AZs gives plain Multi-AZ (99.9% SLA). Multi-AZ-with-Standby (99.99% SLA) is NOT enabled here — it requires a multiple-of-3 data-node count and ≥ 2 replicas (see gotcha #19); this 4-node / 1-replica layout does not qualify. Re-shape to 3 (or 6) data nodes with 2 replicas if Standby is required.
• Shards: for a search workload, target 10–30 GiB per shard. 80 GiB / 20 GiB → 4 primary shards, aligned to data-node count. Final layout: 4 primary × 2 (1 replica) = 8 shards across 4 nodes.
• Heap is service-managed — sized automatically at 50% of RAM, capped at 32 GiB. Not a customer knob.

References: bp-instances (opens in a new tab) · bp-sharding (opens in a new tab) · https://calculator.aws (opens in a new tab).

Pre-emit checklist (specific to SIZING_ONLY)

Tick every box before sending. If any box is unticked, fix the response — don’t ship it.

• Not done: First sentence is the shape declaration, not a greeting, not a restatement of the question.
• Not done: Baseline is one sentence with instance class + count + AZ count + replica count + storage number.
• Not done: Storage formula is shown with numbers substituted, not just stated abstractly. Even if source size is a band, at least one band has the math worked.
• Not done: Match-source rule applied when user named their current cluster — Graviton equivalent of their current family at the same size, not a greenfield default.
• Not done: Tiered bands used (and only used) when source size is genuinely unknown.
• Not done: Cluster managers explicitly addressed — present when ≥3 data nodes or ≥10 indexes; called out as m7g.large.search × 3 (or larger per the cluster-manager-sizing table in sizing.md).
• Not done: Current-generation Graviton by default (r7g/r8g family). r6g only with explicit user justification.
• Not done: No dollar figures. Single calculator.aws link is the only cost reference.
• Not done: No Timeline & Resourcing. No “engineer-weeks”, no phased rollout, no “T+N days”.
• Not done: No migration content. No Historical Data Migration, snapshot, _reindex.remote, Migration Assistant for Amazon OpenSearch Service.
• Not done: No 9-section scaffold. No Executive Summary, no Risk Register, no Readiness Checklist.
• Not done: References footer is one line with at most three URLs (bp-instances, bp-sharding, calculator.aws).
• Not done: Heap mentioned (if at all) as service-managed, never as a customer-tunable knob.
• Not done: Total response ≤ ~300 words unless tiered bands forced expansion. If you wrote more, you drifted into FULL_ASSESSMENT — trim back.