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There are two major aspects to transaction processing, atomicity and automatic recovery. These are related yet different aspects of transaction processing, and not all products supply both. c-treeDB provides a set of functions and file modes that cover both aspects of transaction processing.
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Often, when updating a table, you perform several functions in a group. For instance, when creating an invoice, you update several tables: the account balance in the customer file, the invoice file, an invoice detail file, inventory records, and others. It is important that all of these actions take place to keep the files synchronized. If some of the actions take place, but not all, your files may be out of sync, and it can be difficult to correct the problem later. If one action cannot take place, it would be best to not let any take place. We call this atomicity. The c-treeDB API provides functions that provide this feature. You can mark a set of operations so that none will take place unless they can all take place. The API goes beyond this, allowing you to create "savepoints" where you can partially back out a group of operations, and "roll back" transactions to a given point, so that you can restore your data back to a state that it was in sometime in the past.
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Once you establish full transaction processing by creating tables using the CTCREATE_TRNLOG mode, you can take advantage of the automatic recovery feature. Atomicity will generally prevent problems of files being partially updated. However, there are still situations where a system crash can cause data to be lost. Once you have signaled the end of a transaction, there is still a "window of vulnerability" while the application is actually committing the transaction updates to disk. In addition, for speed considerations some systems buffer the data files and indexes, so that updates may not be flushed to disk immediately. If the system crashes, and one of these problems exists, the recovery logic detects it. If you set up the system for automatic file recovery, the recovery logic automatically resets the table back to the last, most complete, state that it can. If any transaction sets have not been completed, or "committed", they will not affect the table.
Error During Automatic Recovery
An error 14, FCRP_ERR, indicates that FairCom Server detected that files appear corrupt at open. This occurs if files have been updated but not properly closed. They were not processed by automatic recovery so they are in an unknown (inconsistent) state.
If your transaction logs are corrupted, preventing automatic recovery from occurring, you can either:
or
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Only tables created with the create modes CTCREATE_PREIMG and CTCREATE_TRNLOG will participate in a transaction. This means that the ctdbBegin(), ctdbCommit(), and ctdbAbort() functions provide transaction atomicity when used with records from these two types of tables. This also means that these three functions do NOT provide atomicity when used with records from other types of tables, such as CTCREATE_NORMAL tables. Record operations (and locks) on NORMAL tables are handled as if there was no transaction running. For CTCREATE_PREIMG and CTCREATE_TRNLOG tables to provide the promised protections, all operations that add, modify, and delete records in these tables must be done inside a transaction (between calls to ctdbBegin() and ctdbCommit() / ctdbAbort()). This is enforced by c-treeDB; attempting to modify records outside of a transaction for these tables will generate an error.
Tables created with CTCREATE_PREIMG mode will participate in a transaction, and will benefit from transaction atomicity, but will not have automatic recovery, because the transaction log files needed for automatic recovery will not be generated. This saves disk space and gives speed gains, at the cost of more difficult recovery in case of a crash.
Tables created with CTCREATE_TRNLOG have all the positive attributes of CTCREATE_PREIMG but will also generate the transaction logs necessary for automatic recovery.
