Features

If your data fits in RAM, you can write your system as a pure Clojure function, without any database complexity. It will be much simpler, cleaner and orders of magnitude faster.

Prevayler takes care of persistence.

Usage

• Get enough RAM to hold all your data.
• Implement the business logic of your system as a pure event handling function. Keep any I/O logic (accessing some web service, for example) separate.
• Guarantee persistence by applying all events to you business system through Prevayler, like this:

(defn my-business [state event timestamp]            
  ...)                                   ; Your business logic as a pure function. Returns the new state with event applied.

(with-open [p1 (prevayler! {:business-fn my-business})]
  (assert (= @p1 {}))                    ; The default initial state is an empty map.
  (handle! p1 event1)                    ; Your events can be any Clojure value or Serializable object.
  (handle! p1 event2)
  (assert (= @p1 new-state)))            ; Your system state with the events applied.

(with-open [p2 (prevayler! my-business)] ; Next time you run,
  (assert (= @p2 new-state)))            ; the state is recovered, even if there was a system crash.

What it Does

Prevayler-clj implements the system prevalence pattern: it keeps a snapshot of your business state followed by a journal of events. On startup or crash recovery it reads the last state and reapplies all events since: your business state is restored to where it was.

Shortcomings

• RAM: Requires enough RAM to hold all the data in your business state.
• Start-up time: Entire state is read into RAM.

Files

Prevayler's default file name is journal4 but you can pass in your own file (see tests). Prevayler-clj will create and write to it like this:

journal4

Contains the state at the moment your system was last started, followed by all events since. Serialization is done using Nippy.

journal4.backup

On startup, the journal is renamed to journal4.backup and a new journal4 file is created. This new journal will only be consistent after the business state has been written to it so when journal.backup exists, it takes precedence over journal4.

journal4.backup-[timestamp]

After a new consistent journal is written, journal.backup is renamed with a timestamp appendix. You can keep these old versions elsewhere if you like. Prevayler-clj no longer uses them.

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"Simplicity is prerequisite for reliability." Dijkstra (1970)