Techniques for increasing the usefulness of transaction logs

A method of making a log of a stream of transactions that extends a previously-made log from a checkpoint in the previously-made log, the method being executed by a computer processor in a database management system, the checkpoint representing a point in the stream of transactions and providing access to saved state that is required only to include the state of all transactions that are active both at the point represented by the checkpoint and at a prior point in the previously-made log that corresponds to a prior point in the stream, the method comprising the steps of:

• making the extending log beginning at the prior point in the previous log;
• and
• when the checkpoint is reached, using the saved state to which the checkpoint gives access to continue making the extending log.

A method of determining where to make checkpoint from which a log of a transaction stream can be extended, the method being executed by a computer processor in a database management system, extension of a log from a given checkpoint requiring processing of information about the transaction stream beginning from a checkpoint prior to the given checkpoint and the method comprising the steps of;

• receiving an input from a user of a system in which the log is being made indicating a desired period of time between when production of the extended log begins and the extended log becomes available;
• and
• making checkpoints in the log such that a point at which a checkpoint is made in the log is determined at least in part by the desired period of time.

A method of creating a checkpoint in a logical log of a stream of transactions that is made using a physical log of the stream of transactions, the method being executed by a computer processor in a database management system, the checkpoint representing a point in the physical log and specifying saved state, the specified saved state being used to extend the logical log from the specified point, and the method comprising the steps of:

• selecting a point in the physical log;
• and
• in the logical log, making a checkpoint at a point in the logical log that corresponds to the selected point by saving the saved state, the state that is required to be saved being only the state of all transactions that were uncommitted at a prior point in the physical log and are still uncommitted at the point in the physical log represented by the checkpoint.

A method of making a logical log of a stream of transactions that extends a previously-made logical log from a checkpoint in the previously-made logical log, the method being executed by a computer processor in a database management system, the logical logs being made from a physical log of the stream of transactions, the checkpoint representing a point in the physical log, and the checkpoint providing access to saved state that is required to include only the state of all transactions that are uncommitted at both the point represented by the checkpoint and a prior point in the physical log, the method comprising the steps of:

• making the extending logical log beginning at the prior point in the physical log;
• and
• when the checkpoint is reached, using the saved state to which the checkpoint gives access to continue making the extending logical log from the physical log.

A method of determining where to make a checkpoint in a logical log of a stream of transactions, the method being executed by a computer processor in a database management system, the logical log being made from a physical log comprising blocks indicating changes made by transactions in the stream and certain of the blocks being unsafe for checkpointing, the method comprising the steps of:

• determining a first block in the physical log that corresponds to a point in the transaction stream that should be represented by a checkpoint;
• and
• if the first block is unsafe, finding a safe block, the checkpoint being made to represent a point in the transaction stream that corresponds to the safe block.