VV> I'm using sbcl, so it is good to know that elephant with Berkeley db
VV> does not have such problems.
VV> But is is surprising to know that such a well established and mature
VV> lisp implementation as Allegro, cannot thread properly, while much
VV> younger and not so mature sbcl works better than expensive commercial
VV> solutions.
i think they just have different quality standards.
VV> Anyway, I would like to hear the accounts of real life usage of
VV> elephant in multi-user environment.
in elephant we have several different backends:
* BDB one is primary one, APIs were modelled according to capabilities of
BDB
* CLSQL can work with any SQL database supported by CLSQL (that's good),
but has some significant "sub-optimalities"
* Postmodern can work with PostgreSQL only (uses "postmodern" library,
hence the name) and has less sub-optimalities than CLSQL
postmodern one was specially developed to work with multiple threads,
multiple processes and on multiple machines,
so it might be worth testing it if "multi-user" is essential for your
application.
possible benefits :
* PostgreSQL uses MVCC model rather than locking. so it might allow more
concurrency
* PostgreSQL handles deadlocks automatically (iirc you need separate
deadlock-detection process with BDB)
* you can have separate server with PostgreSQL and connect to it from
different machines via network
* it hash optional client-side cache that is automatically synchronized
among different processes
* this backend was designed and tested to work in concurrent environment
with multiple processes, threads etc
but it has it's negative sides too:
* inter-process communications have significant overhead, upto 10x, if
we'll compare to in-process BDB
(but it's offseted by cache to some point -- we assume that under
constant load most data gets cached, so
only index queries and writes need to communicate with PostgreSQL
backend)
* some features work differently from BDB backend -- or, in other words,
they were not implemented correctly because there's no straightforward way
to do this in SQL backend.
most significant difference is sorting -- with db-postmodern you get
correct ordering in index only if all keys are either integers or strings.
* it's, um, less stable than BDB one, kinda still under development.
as for real-life usage, we are using elephant with postmodern backend as a
database for a web site, which is assumed to be multi-user one, and it sort
of works.
but OTOH our load, database size and complexity are relatively small, so i
can't say it was tested in really harsh environment.
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