GemStone/S has a configuration STN_TRAN_FULL_LOGGING that, if set to TRUE, causes all transactions to be recorded to the redo-log. In this mode a backup plus transaction logs can be used to recover if the extents are lost. If the configuration is set to FALSE, small transactions are still logged but large transactions cause a “checkpoint” in which all data is written to the extents before the transaction completes. In this mode recovery from a crash is possible if all the extents and the most recent transaction log are available (older logs are typically deleted automatically in this mode). The logs themselves are not sufficient to allow recovery from a backup in partial logging mode, so partial logging mode is inappropriate for production (but handy sometimes for development since old logs are typically deleted automatically).

In some applications or situations it would be nice to segregate transactions into those that should be logged for crash recovery and those that can be easily recreated if needed (and avoid the overhead of time and space in writing to the log). For example, it is common to do a bulk-load of new data before it is needed by the application. If the system crashes, we’d prefer to recover the live data quickly and reload the new data later. With Oracle one can specify a table as being NOLOGGING and then do a series of “Direct-Load INSERT” commands that bypass the rollback/replay log. A key limitation with this capability, however, is that it does not support referential integrity. Likewise, with SQL Server one can do a BULK INSERT that will, under some circumstances, have minimal logging. By default, this will leave the constraint on the table marked as not-trusted.

For a relational database, a foreign key is a value like any other (typically an integer or string) and if a JOIN fails to find a match then the matching data is NULL or the row is ignored. In GemStone/S, an object reference is guaranteed to be valid and there is no easy way to handle data that lacks referential integrity. Thus, if an object is visible in the database (e.g., from a bulk load), then any session can, in a transaction, create a reference to it. If the reference is in the transaction (redo) log, then the object must itself be in the transaction log. Therefore, because in GemStone/S referential integrity is required, not optional, the relational approach to avoiding the transaction log is not safe.

The relational solutions described above for reducing transaction log activity allow for a bulk load of data that can, eventually (following a full back, recreation of indexes, and reapplication of any constraints) be equivalent to all other data. This fits an application upgrade (bulk load) use case quite well.

Another possible use case is short-lived data that is to be shared by multiple sessions but will not be saved over the long-term. One example is transient data where a summary is to be kept but the raw data discarded after the analysis. Another example is information about logged-in sessions with in-progress activity (such as Seaside session state) that could be safely lost in a system crash (presumably the users would need to start over anyway). This use-case does not present the same referential integrity concerns and GemStone/S is developing a feature that is intended to address this use-case: a SymbolDictionary in Globals named #’NotTranloggedGlobals’. The idea is that any object referenced from this root would not be recorded in the transaction log and should not be referenced from any other path. At present this is still a work-in-progress and customers are advised to not use it.

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