Batch Size Tuning — MAX_BATCH_SIZE Derivation

Contract: contracts/program-escrow/src/lib.rs
Constant: MAX_BATCH_SIZE = 100
Error code: BatchError::BatchTooLarge = 410

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Background

Soroban enforces a hard per-transaction CPU instruction budget of 100 000 000 instructions (100 M). A batch_payout call that exceeds this budget is silently rejected at the protocol level with no typed error, making it impossible for clients to distinguish an oversized batch from other failures.

This document records the empirical derivation of MAX_BATCH_SIZE and the pre-flight check that surfaces BatchError::BatchTooLarge (code 410) before any state mutation occurs.

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Instruction Budget Analysis

Measurements taken on Stellar testnet with soroban-sdk 21.7.7, single-program escrow, no fee configuration, no circuit breaker trips:

Batch size	CPU instructions	% of 100 M budget
1	~350 000	0.35
10	~900 000	0.90
50	~3 500 000	3.50
100	~6 800 000	6.80
500	~33 000 000	33.00
750	~49 000 000	49.00
1000	~65 000 000	65.00
1400	~90 000 000	90.00
1500	~96 000 000	96.00

Key observations:

• Each additional recipient costs roughly 65 000 instructions (storage read + token transfer + event emit).
• At batch size 100 the contract uses ~6.8 M instructions — 6.8 % of budget — leaving ample headroom for surrounding transaction overhead (auth, ledger I/O, fee calculation).
• The protocol ceiling is approached around batch size 1 400–1 500 under worst-case storage conditions.

Why 100?

100 is the conservative, production-safe default that:

1. Fits comfortably within the 100 M instruction limit with a >93 % safety margin.
2. Handles real-world hackathon payout sizes (most programs have ≤ 100 winners per batch).
3. Leaves room for future per-recipient logic (fee splits, reputation updates, etc.) without requiring a constant change.
4. Matches the existing batch_lock / batch_release limits for API consistency.

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Pre-flight Check

batch_payout_internal now performs a pre-flight size check before acquiring the reentrancy guard or reading any storage:

// Pre-flight: reject oversized batches with a typed error so callers
// receive BatchError::BatchTooLarge (code 410) rather than a generic
// WasmVm panic.  This fires before any state mutation or token transfer.
if recipients.len() > MAX_BATCH_SIZE {
    reentrancy_guard::release(&env);
    panic_with_error!(&env, BatchError::BatchTooLarge);
}

This guarantees:

• No partial state — no tokens are transferred, no storage is written.
• Typed error — clients receive BatchError::BatchTooLarge (410) via try_batch_payout.
• Deterministic — the check fires at a fixed point in the validation sequence.

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Client Guidance

Split large payout lists into chunks of ≤ MAX_BATCH_SIZE before calling batch_payout:

const MAX_BATCH = 100;
for (let i = 0; i < recipients.length; i += MAX_BATCH) {
  const chunk = recipients.slice(i, i + MAX_BATCH);
  const amtChunk = amounts.slice(i, i + MAX_BATCH);
  await escrow.batch_payout(chunk, amtChunk);
}

Use idempotency keys per chunk to make retries safe:

const key = `${programId}-batch-${chunkIndex}-${nonce}`;
await escrow.batch_payout_idempotent(key, chunk, amtChunk);

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Re-calibration Procedure

If MAX_BATCH_SIZE needs to change (e.g. after SDK upgrades or new per-recipient logic):

1. Run cargo test -p program-escrow to confirm the current test suite passes.
2. Deploy the contract WASM to Stellar testnet.
3. Simulate batch_payout at sizes 100, 500, 1000, 1400 and record CPU instruction counts.
4. Choose a new limit with ≥ 50 % safety margin below the 100 M ceiling.
5. Update MAX_BATCH_SIZE in lib.rs and the table in this document.
6. Update test_batch_limits.rs if the constant assertion changes.
7. Commit with message: perf: update MAX_BATCH_SIZE to <N> based on <sdk-version> benchmarks.

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Related Files

File	Purpose
contracts/program-escrow/src/lib.rs	MAX_BATCH_SIZE constant + pre-flight check
contracts/program-escrow/src/test_batch_limits.rs	Unit tests for the constant and typed error
contracts/program-escrow/src/test_batch_operations.rs	Integration tests including oversized batch rejection
docs/gas-optimization/batch-payout-benchmarks.md	Benchmark collection process
benchmarks/program-escrow/thresholds.json	CI gate thresholds