axelar-gmp

Axelar GMP (Hedera)

GMP pattern: encode a payload on the source chain, pay Axelar gas in native token, call the Gateway, then decode and act on the destination behind an allowlisted source.

Source app → BridgeSender (pay gas + gateway.callContract)
                 → Axelar relayers
                      → AxelarExecutable._execute → Handler

On Hedera, orchestration often pairs GMP with HSS (0x16b) so a contract can self-reschedule and dispatch each cycle. Keep bridge transport behind a thin sender interface so the orchestrator stays bridge-agnostic.

Quick Reference

See references/hedera-axelar.md for chain-name conventions and SDK imports. See references/examples.md for sender / receiver sketches.

Critical: Gas Then Gateway

Always pay gas before callContract. Use msg.value > 0 for relay fees; refund address should be the funded orchestrator (or plan owner), not a random EOA.

gasService.payNativeGasForContractCall{ value: msg.value }(
    address(this),       // sender contract
    destinationChain,    // Axelar name string
    destinationAddress,  // receiver 0x… as string
    payload,
    refundAddress        // usually authorizedCaller / orchestrator
);
gateway.callContract(destinationChain, destinationAddress, payload);

Critical: Authorize And Allowlist

Compare Axelar chain/address strings with a shared lowercase hash — Axelar may vary casing.

require(
    keccak256(bytes(_toLower(srcChain))) == keccak256(bytes(_toLower(expectedSourceChain))),
    "invalid source chain"
);
require(
    keccak256(bytes(_toLower(srcAddress))) == keccak256(bytes(_toLower(expectedSourceAddress))),
    "invalid source address"
);

Architecture: Split Transport From Business Logic

Source chain                    Destination chain
─────────────────────────────   ─────────────────────────────
Orchestrator ──send──►          MessageReceiver (_execute)
     │ BridgeSender iface              │ Handler iface
BridgeSender                    Handler (swap / settle / …)

1. Deploy handler first (no bridge knowledge).
2. Deploy receiver with gateway + expected source + handler.
3. Deploy sender with gateway + gas service + destination chain/address.
4. Deploy orchestrator with sender address; authorize orchestrator on sender.
5. Wire after both deploys: set destination on sender; set expected source on receiver.
6. Fund orchestrator (native for Axelar gas) and handler (tokens/capital for destination work).

Payload Contract

Agree a single ABI encode/decode on both sides. Keep payloads versionable if you evolve fields — mismatched decode bricks the route.

bytes memory payload = abi.encode(/* agreed fields… */);
// receiver:
(/* fields… */) = abi.decode(payload, (/* matching types… */));

Hedera Orchestration Hook (HSS)

When the source app schedules itself via HSS (0x16b), each cycle should:

1. Enforce interval / plan active
2. Call the bridge sender with native value for Axelar gas
3. Reschedule next expiry with hasScheduleCapacity + scheduleCall (SUCCESS = 22)

If scheduling fails, mark a recovery flag and expose a manual reschedule for the plan owner.

Checklist

• [ ] Gas paid with native value before callContract
• [ ] Destination chain name matches Axelar’s ID (string, not numeric chain ID)
• [ ] Peer addresses set after both contracts deploy (wire step)
• [ ] Source allowlist + authorized callers on both ends
• [ ] Payload encode/decode identical
• [ ] Orchestrator funded for relay gas; destination handler funded for work
• [ ] Demo vs production: clarify who owns destination proceeds

References

• references/examples.md — sender / receiver / allowlist sketches
• references/hedera-axelar.md — chain names vs IDs, SDK imports
• Axelar GMP overview
• axelar-gmp-sdk-solidity