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## Read Exceptions

Several exceptions can occur when you attempt a read operation in the store. The first of these is `ConsistencyException`. This exception indicates that the operation cannot be completed because the consistency policy cannot be met. For more information, see Consistency Guarantees.

The second exception is `RequestTimeoutException`. This means that the operation could not be completed within the amount of time provided by the store's timeout property. This probably indicates a store that is attempting to service too many read requests all at once. Remember that your data is partitioned across the shards in your store, with the partitioning occurring based on your shard keys. If you designed your keys such that a large number of read requests are occurring against a single key, you could see request timeouts even if some of the shards in your store are idle.

A request timeout could also be indicative of a network problem that is causing the network to be slow or even completely unresponsive.

To handle a `RequestTimeoutException`, you could simply log the error and move on, or you could pause for a short period of time and then retry the operation. You could also retry the operation, but use a longer timeout value.

You can also receive an `IllegalArgumentException`, which will be thrown if a `Row` that you are writing to the store does not have a primary key or is otherwise invalid.

You can also receive a general `FaultException`, which indicates that some exception occurred which is neither a problem with consistency nor a problem with the request timeout. Your only recourse here is to either log the error and move along, or retry the operation.

You can also receive a `MetadataNotFoundException`, which indicates that a client's metadata may be out of sync. It extends `FaultException` and can be caught by applications to trigger the need for a refresh of their metadata, and in particular, Table handles obtained via TableAPI.getTable().

Finally, if you are using a secure store that requires authentication, you can receive `AuthenticationFailureException` or `AuthenticationRequiredException` if you do not provide the proper authentication credentials. When using a secure store, you can also see `UnauthorizedException`, which means you are attempting an operation for which the authenticated user does not have the proper permissions.