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From Adam Kocoloski <kocol...@apache.org>
Subject Re: continuous replication API discussion
Date Wed, 12 Aug 2009 04:03:08 GMT
On Aug 11, 2009, at 5:57 PM, Chris Anderson wrote:

>>> 4. How can we configure replications that survive a server restart?
>>
>> I think I'd like to see a replication config database, and a constant
>> replicator config process that reads the config info at server  
>> start and
>> starts the continuous replications.  It monitors the config db for  
>> changes
>> and as new settings are written spawns, restarts or shuts down  
>> replication
>> tasks.
>
> I think this is a great idea. The Futon interface for configuring this
> will probably take a lot of the mystery out of replication for our
> users.

Sure, we were just using a [replication] block in an .ini file for  
this purpose internally for a while now, but a DB is obviously much  
more flexible.  couch_rep_sup can easily monitor the config DB for  
changes and take the appropriate action(s).

The next question I have is whether there ought to be any  
correspondence between the doc in the configuration DB for a  
particular replication and the _local doc in the DB itself that we use  
to record replication history.  Some thoughts:

* we don't expose a view of _local docs, so currently users just have  
to guess the right ID to see the history.

* we could automatically add a field to the config document that  
identifies the corresponding _local doc ID.  We'd have to use  
something like the new _update handler to validate this field on every  
update, as a change in authentication information means a change in  
the corresponding _local doc ID.

* replication history can get updated very frequently, so for users'  
sanity we probably don't want to actually save the history in the  
configuration document.

A thought -- what if one-off POSTs to /_replicate automatically saved  
a config document as well, but with a "continuous":false field?  Then  
it would be easy to view all the replications that ever originated on  
the server, and admins could make certain ones continuous just by  
flipping a bit.

And most importantly, what do we call this DB? :-P  Couch has always  
used _underscored reserved names, but now we've got a "users" DB.  Best,

Adam


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