# Neptune openCypher exceptions
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When working with openCypher on Amazon Neptune, a variety of exceptions may occur. Below are common exceptions you may receive, either from the HTTPS endpoint or from the Bolt driver (all exceptions from the Bolt driver are reported as Server State Exceptions). Where the remedy is to retry the request, see [Exception Handling and Retries](transactions-exceptions.md) for guidance on developing an effective retry strategy.


| HTTP code | Error message | Retriable? | Remedy | 
| --- | --- | --- | --- | 
| 400 | *(syntax error, propagated directly from the openCypher parser)* | No | Correct query syntax, then retry. | 
| 500 | `Operation terminated (out of memory)` | Yes | Rework the query to add additional filtering criteria to reduce required memory | 
| 500 | Operation terminated (deadline exceeded) | Yes | Increase the query timeout in the DB cluster parameter group, or retry the request. | 
| 500 | Operation terminated (cancelled by user) | Yes | Retry the request. | 
| 500 | Database reset is in progress. Please retry the query after the cluster is available. | Yes | Retry when the reset is completed. | 
| 500 | Operation failed due to conflicting concurrent operations (please retry). Transactions are currently rolling back. | Yes | Retry the request. | 
| 400 | *(operation name)* operation/feature unsupported Exception | No | The specified operation is not supported. | 
| 400 | openCypher update attempted on a read-only replica | No | Change the target end point to the writer end point. | 
| 400 | MalformedQueryException (Neptune does not show the internal parser state) | No | Correct query syntax and retry. | 
| 400 | Cannot delete node, because it still has relationships. To delete this node, you must first delete its relationships. | No | Instead of using `MATCH (n) DELETE n` use `MATCH(n) DETACH DELETE(n)` | 
| 400 | Invalid operation: attempting to remove the last label of a node. A node must have at least one label. | No | Neptune requires all nodes to have at least one label, and if nodes are created without an explicit label, a default label `vertex` is assigned. Change the query and/or application logic so as not to delete the last label. A singleton label of a node can be updated by setting a new label and then removing the old label. | 
| 500 | Max number of request have breached, ConfiguredQueueCapacity=\$1\$1 for connId = \$1\$1 | Yes | Currently only 8,192 concurrent requests can be processed, regardless of the stack and protocol. | 
| 500 | Max connection limit breached. | Yes | Only 1000 concurrent Bolt connections per instance are allowed (for HTTP there is no limit). | 
| 400 | Expected a [one of: Node, Relationship or Path] and got a Literal | No | Check that you are passing the correct argument(s), correct query syntax, and retry. | 
| 400 | Property value must be a simple literal. Or: Expected Map for Set properties but didn't find one. | No | A SET clause only accepts simple literals, not composite types. | 
| 400 | Entity found passed for deletion is not found | No | Check that the entity you are trying to delete exists in the database.  | 
| 400 | User does not have access to the database. | No | Check the policy on the IAM role being used. | 
| 400 | There is no token passed as part of the request | No | A properly signed token must be passed as part of the query request on an IAM enabled cluster. | 
| 400 | Error message is propagated. | No | Contact AWS Support with the Request Id. | 
| 500 | Operation terminated (internal error) | Yes | Contact AWS Support with the Request Id. |