Prevalence is a simple technique to provide ACID properties to an in-memory object model based on binary serialization and write-ahead logging. It works like this:
- Start with a snapshot. Serialize the object model and write it to a file.
- Create a journal file. For every call into the object model, serialize the call and its arguments.
- When the journal gets too big, you're shutting down, or it's otherwise convenient, perform a checkpoint: write a new snapshot and truncate the journal.
- To roll back or recover from a crash or power hit, load the last snapshot and re-execute all the calls recorded in the journal.
The precautions needed to make this work are:
- Don't let mutable object references escape or enter the prevalence layer. You need some sort of proxy or OID scheme, as if you were doing RPC. (This is such a common newbie mistake it's been nicknamed the 'baptism problem'.)
- All the logic reachable from a call must be completely deterministic, and must not perform business-logic-meaningful I/O or OS calls. Writing to a diagnostic log is probably OK, but getting the system time or launching an asynchronous delegate is generally not. This is so that the journal replays identically even if it's restored on a different machine or at a different time. (Most prevalence code provides an alternate time call to get the transaction timestamp.)
- Writer concurrency introduces ambiguity in journal interpretation, so it is prohibited.
Is it because ...
- people developed a bad taste for them after trying to use one on a project that wasn't well suited* to it?
- Klaus Wuestefeld's strident advocacy turned people off?
- people who like the imperative programming model dislike separating I/O from calculation, preferring instead to interleave computation with I/O and threading calls?
- prevalence layers are so conceptually simple and so intimately bound to the characteristics of the framework they inhabit that they're usually custom-rolled for the project, which makes them too alien/nonstandard/risky?
- it's just too hard to keep straight what you have to be careful not to do?
- newbies' heads just seem to explode when faced with something that isn't the same kind of two-tier database-driven app they learned to write in school? ;)
*The entire data set fits in RAM, you don't need writer concurrency, and you don't need to do ad-hoc queries, reporting, or export to a data warehouse. With apologies to SQLite, prevalence is an improvement on save-files, not a replacement for Oracle.
