--- a/doc/manual.tex	Tue Apr 15 19:12:49 2014 -0400
+++ b/doc/manual.tex	Thu Apr 17 17:41:24 2014 -0400
@@ -2459,10 +2459,10 @@
   \item \begin{verbatim}
 typedef void (*uw_callback)(void *);
 typedef void (*uw_callback_with_retry)(void *, int will_retry);
-void uw_register_transactional(uw_context, void *data, uw_callback commit,
-                               uw_callback rollback, uw_callback_with_retry free);
+int uw_register_transactional(uw_context, void *data, uw_callback commit,
+                              uw_callback rollback, uw_callback_with_retry free);
   \end{verbatim}
-  All side effects in Ur/Web programs need to be compatible with transactions, such that any set of actions can be undone at any time.  Thus, you should not perform actions with non-local side effects directly; instead, register handlers to be called when the current transaction is committed or rolled back.  The arguments here give an arbitary piece of data to be passed to callbacks, a function to call on commit, a function to call on rollback, and a function to call afterward in either case to clean up any allocated resources.  A rollback handler may be called after the associated commit handler has already been called, if some later part of the commit process fails.  A free handler is told whether the runtime system expects to retry the current page request after rollback finishes.
+  All side effects in Ur/Web programs need to be compatible with transactions, such that any set of actions can be undone at any time.  Thus, you should not perform actions with non-local side effects directly; instead, register handlers to be called when the current transaction is committed or rolled back.  The arguments here give an arbitary piece of data to be passed to callbacks, a function to call on commit, a function to call on rollback, and a function to call afterward in either case to clean up any allocated resources.  A rollback handler may be called after the associated commit handler has already been called, if some later part of the commit process fails.  A free handler is told whether the runtime system expects to retry the current page request after rollback finishes.  The return value of \texttt{uw\_register\_transactional()} is 0 on success and nonzero on failure (where failure currently only happens when exceeding configured limits on number of transactionals).
 
   Any of the callbacks may be \texttt{NULL}.  To accommodate some stubbornly non-transactional real-world actions like sending an e-mail message, Ur/Web treats \texttt{NULL} \texttt{rollback} callbacks specially.  When a transaction commits, all \texttt{commit} actions that have non-\texttt{NULL} rollback actions are tried before any \texttt{commit} actions that have \texttt{NULL} rollback actions.  Furthermore, an SQL \texttt{COMMIT} is also attempted in between the two phases, so the nicely transactional actions have a chance to influence whether data are committed to the database, while \texttt{NULL}-rollback actions only get run in the first place after committing data.  The reason for all this is that it is \emph{expected} that concurrency interactions will cause database commits to fail in benign ways that call for transaction restart.  A truly non-undoable action should only be run after we are sure the database transaction will commit.
 

--- a/include/urweb/urweb_cpp.h	Tue Apr 15 19:12:49 2014 -0400
+++ b/include/urweb/urweb_cpp.h	Thu Apr 17 17:41:24 2014 -0400
@@ -280,7 +280,7 @@
 uw_Basis_int uw_Basis_datetimeDayOfWeek(struct uw_context *, uw_Basis_time);
 extern const uw_Basis_time uw_Basis_minTime;
 
-void uw_register_transactional(struct uw_context *, void *data, uw_callback commit, uw_callback rollback, uw_callback_with_retry free);
+int uw_register_transactional(struct uw_context *, void *data, uw_callback commit, uw_callback rollback, uw_callback_with_retry free);
 
 void uw_check_heap(struct uw_context *, size_t extra);
 char *uw_heap_front(struct uw_context *);

--- a/src/c/urweb.c	Tue Apr 15 19:12:49 2014 -0400
+++ b/src/c/urweb.c	Thu Apr 17 17:41:24 2014 -0400
@@ -3469,11 +3469,12 @@
 
 size_t uw_transactionals_max = SIZE_MAX;
 
-void uw_register_transactional(uw_context ctx, void *data, uw_callback commit, uw_callback rollback,
+int uw_register_transactional(uw_context ctx, void *data, uw_callback commit, uw_callback rollback,
                                uw_callback_with_retry free) {
   if (ctx->used_transactionals >= ctx->n_transactionals) {
     if (ctx->used_transactionals+1 > uw_transactionals_max)
-      uw_error(ctx, FATAL, "Exceeded limit on number of transactionals");
+      // Exceeded limit on number of transactionals.
+      return -1;
     ctx->transactionals = realloc(ctx->transactionals, sizeof(transactional) * (ctx->used_transactionals+1));
     ++ctx->n_transactionals;
   }
@@ -3482,6 +3483,8 @@
   ctx->transactionals[ctx->used_transactionals].commit = commit;
   ctx->transactionals[ctx->used_transactionals].rollback = rollback;
   ctx->transactionals[ctx->used_transactionals++].free = free;
+
+  return 0;
 }