Error Handling

How to handle errors and recover from failed transactions.

State Machine States

Transactions can reach three terminal states:

State	Meaning	Recovery
`updated`	Success — DB synced	None needed
`failed`	DB sync failed after retries	Usually auto-heals; see below
`expired`	Not confirmed on-chain in time	Rebuild and resubmit

Common API Errors

Code	Meaning	Resolution
`400`	Bad request / validation error	Check request body against API Reference
`401`	Invalid or expired JWT	Re-authenticate user via wallet signature
`403`	Insufficient permissions	User lacks required role (e.g., not a teacher)
`409`	Duplicate registration	Transaction already registered — safe to ignore
`422`	Unprocessable entity	Semantic error (e.g., module doesn't exist)
`500`	Internal server error	Retry with exponential backoff
`502`	Upstream service error	Cardano node issue — retry after delay

Recovery Patterns

Expired Transaction

A transaction expires when it's not confirmed on-chain within the timeout window (typically 2 hours).

Before rebuilding, check if it actually confirmed:

async function handleExpired(txHash: string) {
  // Check Andamioscan to see if TX actually confirmed
  const scanRes = await fetch(
    `https://preprod.andamioscan.io/api/tx/${txHash}`
  );

  if (scanRes.ok) {
    // TX is on-chain! State machine missed it.
    // The self-healing reconciler will fix this on next read.
    console.log("TX confirmed but not detected — will auto-heal");
    return { action: "wait", reason: "auto-heal" };
  }

  // TX truly didn't confirm — safe to rebuild
  return { action: "rebuild", reason: "not-on-chain" };
}

Why this matters: If you rebuild without checking, you might submit a duplicate action (e.g., double-minting a credential).

Failed DB Sync

A failed state means the transaction confirmed on-chain but the database update failed after multiple retries.

Usually no action needed:

The Gateway has a self-healing reconciler that detects and repairs inconsistencies when related data is accessed. Your next API call that reads this data will trigger the repair.

If time-sensitive:

// Force a read to trigger reconciliation
await fetch(`${API}/course/${courseId}/commitments`, { headers });
// The reconciler will notice the on-chain state and sync it

Network Errors

For transient network issues, implement retry with exponential backoff:

async function fetchWithRetry(
  url: string,
  options: RequestInit,
  maxRetries = 3
) {
  for (let i = 0; i < maxRetries; i++) {
    try {
      const res = await fetch(url, options);
      if (res.ok || res.status < 500) return res;
    } catch (e) {
      if (i === maxRetries - 1) throw e;
    }

    // Exponential backoff: 1s, 2s, 4s
    await new Promise(r => setTimeout(r, 1000 * Math.pow(2, i)));
  }
}

Wallet Errors

Common wallet-related issues:

Error	Cause	Resolution
User rejected	User cancelled signing	Prompt to try again
Insufficient funds	Not enough ADA	Show required amount
TX too large	Too many inputs/outputs	Contact support
Collateral not set	dApp collateral not configured	Prompt user to set collateral in wallet

try {
  const signedTx = await wallet.signTx(unsignedTx);
} catch (e) {
  if (e.message?.includes("user declined")) {
    // User cancelled — don't retry automatically
    return { error: "cancelled", retry: false };
  }
  if (e.message?.includes("insufficient")) {
    const required = parseRequiredAda(e.message);
    return { error: "insufficient_funds", required };
  }
  throw e; // Unknown error
}

Idempotency

Andamio's state machine is designed for safe retries:

Registration is idempotent: Registering the same tx_hash twice returns 409 Conflict but doesn't create duplicate records
DB syncs are idempotent: If a sync is interrupted and retried, it produces the same result
Draft upserts: Creating a commitment draft that already exists updates it rather than duplicating

This means if you're unsure whether an operation completed, it's safe to retry.

Debugging

Check Transaction Status

curl -H "X-API-Key: $API_KEY" \
  https://preprod.api.andamio.io/api/v2/tx/status/$TX_HASH

View Pending Transactions

curl -H "X-API-Key: $API_KEY" \
     -H "Authorization: Bearer $JWT" \
  https://preprod.api.andamio.io/api/v2/tx/pending

Check On-Chain State

For preprod:

curl https://preprod.andamioscan.io/api/tx/$TX_HASH

For mainnet:

curl https://andamioscan.io/api/tx/$TX_HASH

Best Practices

Always check before rebuilding expired TXs — Avoid duplicate on-chain actions
Implement SSE for real-time updates — Better UX than polling
Store tx_hash immediately after submit — Enables recovery if registration fails
Use draft-first flow — Provides instant feedback and safer recovery
Log state transitions — Helps debug issues in production

Next Steps

API Integration — Complete transaction flow
Transaction State Machine — State transition details
API Reference — Endpoint documentation