In 2.1 ESD #3, message-based synchronization is removed, and you must transition message-based synchronization applications to replication-based synchronization applications. However, if you do not want to transition to replication-based synchronization at this time, you can continue using your message-based applications built with SDK version 2.1 ESD #2 or earlier.
For information on transitioning message-based synchronization applications to replication-based synchronization applications, see the Release Bulletin > Migrating Your Artifacts > Transition of iOS Native Custom Applications from MBS to RBS > Transitioning Existing Object API Applications to SDK Version 2.1 ESD #3.
For information about how you can use an older version of the SDK to maintain your message-based applications, see the Installation Guide for Sybase Mobile SDK > Getting Started > Backward Compatibility.