Error handling

Every non-2xx response from the daemon is mapped to a typed error category. This is the complete reference: the error categories, the HTTP-status mapping, the daemon's error envelope, and recovery patterns.

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The error model

Mojo can only raise the built-in Error type (exception subclassing does not exist), so the client models its error categories as Error constructors that match the other MongrelDB language clients. Each error message is prefixed with the category name and embeds the HTTP status, the server's structured code, and the offending op index:

ConflictError: constraint violation (status=409, code=UNIQUE_VIOLATION, op_index=0)

Category	Meaning	Typical cause
MongrelDBError	Base category for all client errors	(matches any client failure)
AuthError	HTTP 401 or 403	Missing/bad credentials against an auth-enabled daemon
NotFoundError	HTTP 404	Missing table, schema, or resource
ConflictError	HTTP 409	Unique, foreign-key, check, or trigger violation at commit
QueryError	HTTP 400 or 5xx, plus network	Malformed request, server failure, transport error

The daemon's error envelope

{
  "status": "aborted",
  "error": {
    "code": "UNIQUE_VIOLATION",
    "message": "duplicate key in column 1",
    "op_index": 0
  }
}

Common code values: UNIQUE_VIOLATION, FK_VIOLATION, CHECK_VIOLATION, NOT_FOUND.

HTTP status -> error category mapping

HTTP status	Category	Notes
401, 403	AuthError	Bad/missing credentials
404	NotFoundError	Resource not found
409	ConflictError	Constraint violation at commit
400	QueryError	Malformed request / bad query
5xx	QueryError	Daemon-side failure
2xx	(no error)	Success

Network and encoding problems are also mapped to QueryError.

Discriminating errors

Catch Error and match the category prefix in the message:

try:
    _ = db.schema_for("missing_table")
except e:
    var msg = String(e)
    if msg.contains("NotFoundError"):
        print("table does not exist")
    elif msg.contains("AuthError"):
        print("bad credentials")
    else:
        print("server error or malformed request: " + msg)

By details - status, code, and op index

The HTTP status, the server's structured code (when reported), and the offending op_index within a batch are embedded in the message:

try:
    _ = txn.commit()
except e:
    print(String(e))
    # ConflictError: constraint violation (status=409, code=UNIQUE_VIOLATION, op_index=0)

Recovery patterns

Auth failure - do not retry blindly

try:
    _ = db.schema()
except e:
    if String(e).contains("AuthError"):
        print("credentials rejected; refresh token")

Not found - fall back, do not crash

try:
    _ = db.schema_for(table_name)
except e:
    if String(e).contains("NotFoundError"):
        pass  # table missing - treat as empty

Transient failure - retry with an idempotency key

QueryError covers transport and 5xx failures. With an idempotency key, retrying a transaction is safe (see transactions.md).

Quick reference

# Category prefixes in the error message:
#   AuthError      401/403
#   NotFoundError  404
#   ConflictError  409
#   QueryError     400/5xx/network
#   MongrelDBError base category

Next steps

• transactions.md - constraint handling and retries in context
• auth.md - credential management