4 ms·

Prevayler sounds a lot like Redis, unless I’m missing something. I’m a big fan of Redis, but I was bitten many years ago when I tried to use it as a replacemen

Prevayler sounds a lot like Redis, unless I’m missing something. I’m a big fan of Redis, but I was bitten many years ago when I tried to use it as a replacement for an RDBMS. There were two reasons for this: 1) lack of development libraries and operational tools, and 2) lack of data integrity checks. 1 has changed these days, but 2 is still very much the case (and rightly so, IMHO). Perhaps Prevayler had this? But yep, it was fast at a time when our competition’s software was slow and clunky. It made a difference.

wpietri 7y ago
Not quite. Redis is still an external database. Prevayler was much simpler, just a library. With Prevayler, all data is kept in RAM, reachable from one root object, which Prevayler holds. All changes to the data model must be expressed as command objects. Each object is handed to Prevayler, which serializes it to a log and then executes it. Once in a while, you can snapshot the data out and start a new log. If there's a crash, you just load the latest snapshot and replay the log. You got exactly as much data integrity as you wrote into your objects and your commands. Which without much work could be quite a bit, because you get a lot of data integrity by not writing things. E.g., if a kind of object should never be deleted, you just don't write any deletion code. If, say, account balances should never be changed directly, but only as part of properly structured credits and debits, then you just write your code like that. Most databases, Redis I think included, are made for arbitrary operations on data. Developers add integrity and security later, hopefully. And that is often duplicative of the code base, so that one ends up having integrity checks both in the code and in the database. I should say that made a lot of sense for the era databases came out of. Databases were a huge step forward in the 1970s and 1980s. My dad was a developer in that era and it was a big relief not to have to get a bunch of programmers to all follow the same conventions for exactly which record was stored in exactly which spot on their precious and expensive disk drives. Not having to know the minutia of the hardware let a lot of people just get in there and build business reports. But if we were starting fresh today, I don't think we would do anything like a SQL database. Redis was definitely a step away from that era, and I look forward to many more.
- solarkraft 7y ago
  If I didn't need the log, would I need to use Prevayler at all? It sounds to me like a plain old "keep objects in RAM", which is what most programming languages already natively do.
  - wpietri 7y ago
    What Prevayler got you was the ACID guarantees: https://en.wikipedia.org/wiki/ACID https://en.wikipedia.org/wiki/ACID So if you didn't need persistence, you certainly didn't need it. Ditto if data integrity wasn't important. It would also have made distribution much easier. Since each mutation was already serialized to disk before execution, you could also send it over the wire to read-only replicas and hot spares for the master.

wpietri 7y ago

Not quite. Redis is still an external database. Prevayler was much simpler, just a library. With Prevayler, all data is kept in RAM, reachable from one root object, which Prevayler holds. All changes to the data model must be expressed as command objects. Each object is handed to Prevayler, which serializes it to a log and then executes it. Once in a while, you can snapshot the data out and start a new log. If there's a crash, you just load the latest snapshot and replay the log. You got exactly as much data integrity as you wrote into your objects and your commands. Which without much work could be quite a bit, because you get a lot of data integrity by not writing things. E.g., if a kind of object should never be deleted, you just don't write any deletion code. If, say, account balances should never be changed directly, but only as part of properly structured credits and debits, then you just write your code like that. Most databases, Redis I think included, are made for arbitrary operations on data. Developers add integrity and security later, hopefully. And that is often duplicative of the code base, so that one ends up having integrity checks both in the code and in the database. I should say that made a lot of sense for the era databases came out of. Databases were a huge step forward in the 1970s and 1980s. My dad was a developer in that era and it was a big relief not to have to get a bunch of programmers to all follow the same conventions for exactly which record was stored in exactly which spot on their precious and expensive disk drives. Not having to know the minutia of the hardware let a lot of people just get in there and build business reports. But if we were starting fresh today, I don't think we would do anything like a SQL database. Redis was definitely a step away from that era, and I look forward to many more.

solarkraft 7y ago
If I didn't need the log, would I need to use Prevayler at all? It sounds to me like a plain old "keep objects in RAM", which is what most programming languages already natively do.
- wpietri 7y ago
  What Prevayler got you was the ACID guarantees: https://en.wikipedia.org/wiki/ACID https://en.wikipedia.org/wiki/ACID So if you didn't need persistence, you certainly didn't need it. Ditto if data integrity wasn't important. It would also have made distribution much easier. Since each mutation was already serialized to disk before execution, you could also send it over the wire to read-only replicas and hot spares for the master.