Clobber is an alternative to so-called “object prevalence”, and in particular to cl-prevalence.
Clobber is both simpler, more flexible, and more robust than systems based on object prevalence.
Clobber is simpler because we do not take any snapshots at all. Other systems typically use a combination of transaction logs and snapshots. Clobber uses only a transaction log which is always read into an empty system.
Clobber is more flexible because the system itself does not define the format of a transaction. Client code can save transactions as Lisp lists, as instances of standard-object, or anything else that can be serialized. It is also more flexible because transactions can contain any object that can be serialized, including model objects. With this method, client code does not need to manipulate “manually created pointers” such as social security numbers, account numbers, membership numbers, etc. whenever it needs to execute a transaction. Instead it can use the model objects themselves such as people, accounts, automobiles and whatnot.
Clobber is more robust because serialization of instances of (subclasses
of) standard-object is not accomplished based on slots. Clobber considers slots to be
implementation details. In other object prevalence systems, whenever
the model evolves, the serialization might no longer be valid. In
contrast, Clobber serializes instances of standard-object as a list of
pairs, each one consisting of an initarg and a value. These pairs can
be handled by client code in any way it sees fit. They can be handled
by an :initarg, by initialize-instance, or they can be ignored. The
downside of the Clobber method is that client code must specify these
pairs in the form of an initarg and the name of an accessor function
to be called to obtain the value used for the initarg. This
inconvenience is however relatively minor, especially considering the
additional robustness it buys in terms of less sensitivity to changes
in the model classes.
At the heart of Clobber is a mechanism for serializing objects that
preserves object identity, much like the reader macros #= and ##,
except that Clobber detects sharing within the entire transaction log,
not only within a single transaction. This mechanism is what makes it
possible for client code to put any old object in a transaction, while
making sure that sharing is preserved.
Two examples are included - see files demo.lisp and demo2.lisp
To run demo.lisp -
(ql:quickload :clobber)
(in-package :clobber-demo)
(start "/path/to/new/file")
(do-things-1)
;; inspect the file to see the transaction logClobber is in the public domain in countries where it is possible to place works in the public domain explicitly. In other countries, we will distribute Clobber according to a license that lets the user do whatever he or she pleases with the code.
Send comments to robert.strandh@gmail.com
A manual might be written one day.
- If your application objects are instances of (subclasses of)
standard-object, useclobber:define-save-infoto tell Clobber how to serialize them. - Determine the data structure your application will use to represent each transaction, and how the application will restore state from each transaction.
e.g. a transaction could be a list whose
caris a function and whosecdris a list of arguments to the function. State can be restored from such a transaction through(lambda (txn) (apply (first txn) (rest txn))) - Use
clobber:open-transaction-logto create a transaction log, usually stored in a special variable. - Call
clobber:log-transactionwhenever you update your application state. You may wish to define a helper function or macro for this. - Close the transaction log using
clobber:close-transaction-log.