lunedì 10 luglio 2017

Multiuser concurrent write access

UCanAccess uses the Jackcess API to manipulate the Access database. Jackcess performs read and write operations directly on the Access database file – .accdb or .mdb – without using the Access Database Engine. When a Jackcess process makes modifications to the database file it does so in isolation; it has no way of controlling (or even knowing) what other processes, including other Jackcess processes, might be doing to the file.

UCanAccess does support concurrent read/write operations from multiple threads of the same JVM (Java Virtual Machine) process. So, for example, multiple concurrent users are able to update the database if they connect to a Java web application or web service that uses UCanAccess to perform the updates from a single JVM process.

However, UCanAccess cannot support separate concurrent processes making simultaneous changes to the Access database. This includes:

  • Concurrent updates by several users of a stand-alone UCanAccess application that connects directly to the same Access database from different machines,
  • A UCanAccess application directly updating the database while another UCanAccess application on the same machine updates the same database from a different JVM process, e.g., a background process or web application that writes to the database,
  • A UCanAccess application updating the database while other processes perform updates with applications that use the Access Database Engine, e.g., Microsoft Access itself, or applications that use Microsoft’s own data-access technologies (Access ODBC, Access OLEDB, or ACE/Jet DAO).

UCanAccess does support multiple concurrent processes where:

  • all processes read the database but do not write to it, or
  • only one process writes to the database while all other processes just read it,

although in the latter case

  1. There could be a significant performance penalty because if UCanAccess detects that the Access database has been changed by another process then it must re-load the entire database to ensure that it has the most current information, and
  2. If a UCanAccess application updates a database while other users have the same database open in Microsoft Access itself then the Access users may have to close and re-open the database before they can see the changes made by the UCanAccess application.

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