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From Adam Kocoloski <adam.kocolo...@gmail.com>
Subject Re: replication error
Date Sun, 01 Feb 2009 18:01:44 GMT
On Feb 1, 2009, at 11:52 AM, Sho Fukamachi wrote:

>
> On 02/02/2009, at 2:23 AM, Adam Kocoloski wrote:
>>
>> Hi Sho, are you getting req_timedout errors in the logs?  It seems  
>> a little weird to me that ibrowse starts the timer while it's still  
>> sending data to the server; perhaps there's an alternative we  
>> haven't noticed.
>
> Yeah. Like this:
>
> [<0.166.0>] retrying couch_rep HTTP post request due to {error,  
> req_timedout}: "http://localhost:2808/media/_bulk_docs"
>
> then bombs out:
>
> [error] [emulator] Error in process <0.166.0> with exit value:  
> {{badmatch,ok},[{couch_rep,update_docs,4}, 
> {couch_rep,save_docs_buffer,3}]}
>
> After the 10 retries it gives an error report but I assume you know  
> what it says .. if not I can post it.

No need, I understand what's going on.

> Anyway, it finishes eventually, just needs a lot of babysitting.
>
>> There's no way to change the request timeout or bulk docs size at  
>> runtime right now, but if you don't mind digging into the source  
>> yourself you change these as follows:
>>
>> 1) request timeout -- line 187 of couch_rep.erl looks like
>>
>> case ibrowse:send_req(Url, Headers, Action, Body, Options) of
>>
>> You can add a timeout in milliseconds as a sixth parameter to  
>> ibrowse:send_req.  The default is 30000.  I think the atom  
>> 'infinity' also works.
>
> OK, I tried this. Unfortunately I have no idea what I am doing in  
> Erlang so completely screwed it up. I made it this:
>
> case ibrowse:send_req(Url, Headers, Action, Body, Options, 120000)
>
> Compiles fine but now throws this error if I try to replicate:
>
> [error] [<0.50.0>] Uncaught error in HTTP request: {error, 
> {badmatch,undefined}}
>
> No doubt every Erlang programmer here wants to punch me for doing  
> something that dumb, but putting that aside for the moment .. any  
> hints? : )

That's odd.  I tried setting a 120 second timeout and didn't have any  
trouble.  Then again, I only ran the test suite; I didn't actually  
force a timeout to occur or anything.  Sorry, I don't have any hints  
at the moment.

>> 2) bulk_docs size -- The number "100" is mentioned three times in  
>> couch_rep:get_doc_info_list/2.  You can lower that to something  
>> that works better for you.
>
> Well, a change in 1 place seems better than in 3 places .. I'll  
> stick to the timeout for now.
>
> My feeling is that CouchDB should probably start reducing the bulk  
> docs size, or increasing the timeout, or both, automatically when it  
> hits a timeout error - or making them configurable in local.ini. As  
> discussed here before, people are using Couch to store largish  
> attachments, and this is an intended use, so this kind of thing will  
> definitely come up again. Or, of course, if the upcoming multipart  
> feature will solve all of this, then .. not, heh.

Multipart won't solve the problem where ibrowse throws a timeout error  
even while it's still sending data.  That seems like a pretty curious  
choice on ibrowse' part to me.  Maybe when I have some more free time  
I can look into the timeout algo and see if it can be tweaked so that  
it only starts after the request has been fully transmitted.  I think  
that would pretty much solve this problem.  Barring that, I agree that  
some sort of back-off algorithm that lengthens the timeout after each  
failed request is warranted.

There's also one more knob we can turn.  During replication we are  
checking the memory consumption of the process collecting docs to send  
to the target.  If it hits 10MB we send the bulk immediately,  
regardless of whether it's 1 doc, 10, or 99.  10MB may be much too  
high given a 30 second timeout window in which we have to transmit the  
data; 1MB is possibly a better fit for home broadband users.  If you  
want to fiddle with that knob instead of the ibrowse timeout you can  
try changing line 224 of couch_rep.erl so that instead of

couch_util:should_flush()

it would read (value is in bytes)

couch_util:should_flush(1000000)

I don't have a strong opinion at this point in time about how many of  
these parameters ought to be tunable in local.ini.  Best,

Adam

>
>
> Thanks a lot for the help ..
>
> Sho
>
>


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