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Posted to server-dev@james.apache.org by Ioan Eugen Stan <st...@gmail.com> on 2011/05/24 00:01:33 UTC

mailbox over HDFS/HBase

Hello,


I had some discussions with Eric about what will be the best way to
implement the mailbox over HDFS and we agreed that it's better to
inform the list about the situation.

The project idea that I applied for is to implement James mailbox
storage over Hadoop HDFS and one of the first steps was to find the
best way to interact with Hadoop. So I just did that. I have spent the
last week or so trying to figure out the best way to implement the
mailbox over Hadoop. I found the training videos from Cloudera to be
very helpful [2].
I also wrote on the Hadoop mailing list to ask them for an opinion
(before watching the videos) . You can read the discussion here [1].

I have come to the conclusion that there is no easy way to implement
the mailbox directly over HDFS, and my opinion is to use HBase, either
directly or over Gora. I will support my statement with some of the
things I found out.

First, about email :
- emails are essentially immutable. Once created they do not modify.
- meta information is read/write (like the status - read/unread).
maybe other stuff, I still have to get up to date.
- you can delete an email, but other than that you can't modify it.
- users usually access the last 50-100 emails (my observation)

About HDFS:

- is designed to work well with large data with the order of magnitude
of GB and beyond. It has a block size > 64 MB. This enables less disk
seeks when reading a file, because the file is less fragmented. It
uses bulk reads and writes enables to HDFS to perform better: all the
data is in one place, and you have a small number of open file
handlers, which means less over-heed.
- does not provide random file alteration. HDFS only supports APPEND
information at the end of an existing file. If you need to modify a
file, the only way to do it is to create a new file with the
modifications.


HBase:

- is a NoSQL implementation over Hadoop.
- provides the user a way to store information and access it very
easily based on some keys.
- provides a way to modify the files by keeping a log, similar to the
way journal file systems work: it appends all the modifications to a
log file. When certain conditions are met the log file is merged back
into the „database”.


My conclusions:

Because emails are small and require that a part of them needs to be
changed, storing them in a filesystem that was designed for large
files, which does not provide a way to modify these files is not a
sensible thing to do.

I see a couple of choices:

1. we use HBase
2. we keep the meta information in a separate database, outside
Hadoop, but things will not scale very well.
3. we design it on top of HDFS, but essentially we (I) will end up
solving the same problems that HBase solved

The most easy and straight forward solution is to use HBase, There is
a paper [3] that shows some results with an email store based on
Cassandra, so it is proven to work.
I am thinking of using Gora and avoiding to use HBase API directly.
This will ensure that James could use any NoSQL storage that Gora can
access. What keeps me back is that Gora does not seem to be very
active and it's also incubating so I may run into things not easy to
get out of.


What do you think?


[1] http://comments.gmane.org/gmane.comp.jakarta.lucene.hadoop.user/26022
[2] http://vimeo.com/search/videos/search:cloudera/st/48b36a32
[3] http://ewh.ieee.org/r6/scv/computer/nfic/2009/IBM-Jun-Rao.pdf
-- 
Ioan-Eugen Stan

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Re: mailbox over HDFS/HBase

Posted by Robert Burrell Donkin <ro...@gmail.com>.

On Tue, May 24, 2011 at 8:07 PM, Ioan Eugen Stan <st...@gmail.com> wrote:
>  (my observation)
>>
>> Kind of.. you often see an IMAP client todo some "big" FETCH on the first
>> connect to see if there are changes in the mailbox. Like
>>
>> a FETCH 1:* (FLAGS)
>>
>> This will hopefully get improved when Apache James IMAP supports the
>> CONDSTORE[a] and QRESYNC[b] extensions. But thats on my todo list ;)
>> Unfortunally this will need to change the API of the current mailbox release
>> (0.2), but thats not something you should care about atm. Just use
>> the 0.2 release for your development
>
> So I guess I should read the IMAP RFC to see how data is going to be
> accessed in order to make the data model just right.

The way clients use IMAP isn't deduceable from the RFCs. So don't
spend too much time trying to analyse the RFC...

Robert

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Re: mailbox over HDFS/HBase

Posted by Ioan Eugen Stan <st...@gmail.com>.

 (my observation)
>
> Kind of.. you often see an IMAP client todo some "big" FETCH on the first
> connect to see if there are changes in the mailbox. Like
>
> a FETCH 1:* (FLAGS)
>
> This will hopefully get improved when Apache James IMAP supports the
> CONDSTORE[a] and QRESYNC[b] extensions. But thats on my todo list ;)
> Unfortunally this will need to change the API of the current mailbox release
> (0.2), but thats not something you should care about atm. Just use
> the 0.2 release for your development

So I guess I should read the IMAP RFC to see how data is going to be
accessed in order to make the data model just right.

>
> Hope it helps,
> Norman
>
> [a] http://tools.ietf.org/html/rfc4551
> [b] http://tools.ietf.org/search/rfc5162
> [c] http://cr.yp.to/proto/maildir.html
> [d] http://www.courier-mta.org/imap/README.maildirquota.html
> [e] http://www.hazelcast.com/
>

Oh goodie, more reading . I hope this doesn't form a trend :D.

-- 
Ioan-Eugen Stan

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