cdk/crates/cdk-wasm/wasm_usage.md

WASM Binary Size Analysis

Analysis of cdk-wasm release build (cargo build --target wasm32-unknown-unknown --release).

Binary size: ~6.6 MB raw (before gzip/brotli compression).

Tooling used: twiggy for symbol-level profiling.

Per-component breakdown

Component	Size (KB)	%	Notes
[data/rodata]	1554.8	23.1%	String literals, lookup tables, constants
cdk (wallet core)	944.1	14.0%	Wallet sagas, swap, receive, restore, OIDC
core/alloc/std	841.9	12.5%	Rust stdlib (formatting, collections, etc.)
[function names]	793.3	11.8%	Debug symbol names — stripped by wasm-pack
[other]	597.9	8.9%	Mostly secp256k1 C symbols + serde monomorphs
serde (all)	425.9	6.3%	JSON/CBOR deserialization monomorphization
tracing	251.7	3.7%	Logging framework
cashu	223.9	3.3%	Protocol types + their serde impls
secp256k1	221.8	3.3%	Elliptic curve crypto (C code compiled to WASM)
wasm-bindgen	189.6	2.8%	JS glue layer
cdk_wasm + cdk_common	191.7	2.9%	FFI wrappers + shared types
url/idna	66.3	1.0%	URL parsing + international domain names
lightning	50.5	0.8%	Invoice parsing
bitcoin_hashes	31.2	0.5%	SHA256, SHA512, RIPEMD160
Everything else	~110	1.6%	bip39, cbor, encoding, tokio, etc.

Key observations

1. Biggest wins would come from: reducing cdk code size (14%), trimming serde monomorphization (6.3%), and evaluating if tracing (3.7%) can be compiled out for WASM.

2. .rodata at 23% is large — this is string literals (error messages, format strings, tracing span names) and lookup tables (unicode normalization, secp256k1 precomputed tables). Hard to reduce without removing features.

3. [function names] (11.8%) disappears when wasm-pack uses --release with name stripping, so the real production binary is closer to ~5.9 MB raw (before gzip).

4. secp256k1 + bitcoin_hashes (3.8% combined) is the cost of doing cryptography in WASM — this is the C secp256k1 library compiled to WASM. Can't avoid it.

5. serde monomorphization (6.3%) is the classic Rust WASM bloat problem — every Deserialize impl for each type generates a separate copy of the JSON parser. miniserde or hand-written parsing would shrink this but at massive effort.

6. jsonwebtoken/ring is completely gone from the WASM build — zero cost, replaced by the browser's built-in Web Crypto API via SubtleCrypto.

Potential size reduction strategies

• Strip tracing on WASM: feature-gate tracing instrumentation behind cfg(not(target_arch = "wasm32")) or use console.log directly. Saves ~250 KB.
• wasm-opt: run wasm-opt -Oz on the output. Typically 10-20% reduction.
• gzip/brotli: the final served .wasm file compresses well (~60-70% reduction).
• Reduce serde monomorphization: use #[serde(deserialize_with)] to share deserializer instances across similar types, or use serde_json::Value as an intermediate.

Reproducing this analysis

# Install twiggy
cargo install twiggy

# Build release WASM
cargo build --target wasm32-unknown-unknown -p cdk-wasm --release

# Analyze
twiggy top target/wasm32-unknown-unknown/release/cdk_wasm.wasm -n 50

# Dominator tree (what retains what)
twiggy dominators target/wasm32-unknown-unknown/release/cdk_wasm.wasm -d 2 | head -80

# CSV export for scripting
twiggy top target/wasm32-unknown-unknown/release/cdk_wasm.wasm -n 30000 --format csv > twiggy.csv