Re: ZEO as a kind of memcache

On Mon, Aug 7, 2017 at 6:22 PM, Jim Fulton <[email protected]> wrote:

>
> It kind of works. I haven't tested it much yet. A problem I already found
>> is that when I restart the ZEO server the Zope instances complain with this
>> message: ClientStorageError: [('127.0.0.1', 3100)] Client has seen newer
>> transactions than server!
>>
>
> This is because the client has a replica of a database that doesn't exist
> anymore.
>
> If you truly want this to be memory only, I think you'd want to find a way
> to signal to the clients that they should all discard their caches.  Or, as
> Sean suggested, store your results persistently.
>

I thought the "drop-cache-rather-verify" option would do that, but it
didn't work as expected.


>
>
>> I'd like your feedback:
>>
>> - Is this a very crazy idea? Why?
>>
>
> I don't think it's a crazy.
>
>
>> - What are the cons, comparing to memcache?
>>
>
> IDK but a potential major benefit is that it's easy to keep the cache
> current, because it's essentially a replica.
>
>
>> - Should I use other storage type, like mappingstorage?
>>
>
> IDK if anyone is maintaining tempstorage any more.  But you may want to
> have something more persistent than an in-memory storage.
>

Why would I want that? I want to store computed things, that can be
discarded and recalculated any time.


> You might be able to get around the client cache issue by creating a
> slightly customized mapping storage that initializes itself with a new
> transaction on startup.  There's a dance between the clients, server, and
> storage such that when you restarted the server, the clients would discard
> their caches.
>
> See https://github.com/zopefoundation/ZEO/blob/
> master/src/ZEO/interfaces.py#L101
> and https://github.com/zopefoundation/ZEO/blob/
> master/src/ZEO/StorageServer.py#L769
>

Will look into it if I decide to go that road. Thanks.

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