Datomic makes particular use of two kinds of RuntimeException:
IllegalArgumentException indicates a program bug. For example, attempts to refer to an entity via a :db/ident that does not exist will trigger an IllegalArgumentException. Such failures should not be retried.
IllegalStateException indicates that an operation is not allowed given the current state of the system. For example, a transaction that violates a :db.unique constraint will throw an IllegalStateException.
For other kinds of failures, Datomic uses Java runtime exceptions. However, note that checked exceptions might still arise from underlying subsystems, so "catch-all" code should catch Throwable or Exception.
Exceptions in transaction functions occur on the transactor, but may be of interest to peers. Exceptions in transaction functions are reported as follows:
On the transactor, the exception stack trace is written to the log.
On the peer, an exception will be thrown when the transaction future is dereferenced. This peer exception will contain the following information:
- The peer exception will have the same string message as the original exception on the transactor.
- If the transactor exception is an instance of a class whose package name begins with "java", then the peer exception will be an instance of the same class. Otherwise, the exception will be a runtime exception.
- If the transactor exception implements clojure.lang.IExceptionInfo, the peer exception will also, and will contain a map of additional information.
These peer rules make it possible for transaction functions to report different failures in a way that peers can usefully distinguish between them.
Transaction functions that report different problems with different exception types should match Datomic's own use of IllegalArgumentException and IllegalStateException, if possible, to avoid proliferating cases that peer code might need to handle.
Note that exceptions may be wrapped to comply with Java conventions, so you may need to walk the .getCause chain to find the "interesting" exception. In particular, the futures returned by transact and transactAsync will wrap exceptions in an ExecutionException to comply with the contract of Future.get.
In Clojure, it is idiomatic to attach a map of additional information to exceptions, accessible via the ex-data function. Many exceptions thrown by Datomic support this, implementing the clojure.lang.IExceptionInfo interface.
Except where explicitly documented as a part of a function's contract, ex-data information is for diagnostic use only. It is not public API and is subject to change.