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From Chris Anderson <jch...@apache.org>
Subject Re: CouchDB Cluster/Partition GSoC
Date Wed, 01 Apr 2009 15:58:55 GMT
On Wed, Apr 1, 2009 at 8:37 AM, Adam Kocoloski <kocolosk@apache.org> wrote:
> On Apr 1, 2009, at 11:03 AM, Chris Anderson wrote:
>
>>>>  2) What about _all_docs and seq-num?
>>>>
>>>> I presume _all_docs gets merged like any other view.  _all_docs_by_seq
>>>> is a
>>>> different story.  In the current code the sequence number is incremented
>>>> by
>>>> one for every update.  If we want to preserve that behavior in
>>>> partitioned
>>>> databases we need some sort of consensus algorithm or master server to
>>>> choose the next sequence number.  It could easily turn into a bottleneck
>>>> or
>>>> single point-of-failure if we're not careful.
>>>>
>>>> The alternatives are to a) abandon the current format for update
>>>> sequences
>>>> in favor of vector clocks or something more opaque, or b) have
>>>> _all_docs_by_seq be strictly a node-local query.  I'd prefer the former,
>>>> as
>>>> I think it will be useful for e.g. external indexers to treat the
>>>> partitioned database just like a single server one.  If we do the
>>>> latter, I
>>>> think it means that either the external indexers have to be installed on
>>>> every node, or at least they have to be aware of all the partitions.
>>>
>>>
>>> If at all possible I think we should have the entire partition group
>>> appear
>>> as a single server from the outside. One thing that comes to mind here is
>>> a
>>> question about sequence numbers. Vector clocks only guarantee a partial
>>> ordering, but I'm under the impression that currently seq numbers have a
>>> strict ordering.
>>>
>>> Database sequence numbers are used in replication and in determining
>>> whether
>>> views need refreshing. Anything else I'm missing? Currently it seems
>>> there
>>> is no tracking of which updates actually change a view index (comment on
>>> line 588 of couch_httpd_view.erl on trunk). Improving this would be a
>>> nice
>>> win. See my answer to number (3).
>>>
>>> The easy way to manage seq numbers is to let one node be the write master
>>> for any cluster. (The root node of any partition group could actually be
>>> a
>>> cluster, but if writes always go through a master the master can maintain
>>> the global sequence number for the partition group).
>>
>> The problem with this approach is that the main use-case for
>> partitioning is when your incoming writes exceed the capacity of a
>> single node. By partitioning the key-space, you can get more
>> write-throughput.
>
> I think Randall was saying requests just have to originate at the master
> node.  That master node could do nothing more than assign a sequence number,
> choose a node, and proxy the request down the tree for the heavy lifting.  I
> bet we could get pretty good throughput, but I still worry about this
> approach for availability reasons.

Yes, I agree. I think vector clocks are a good compromise. I hadn't
considered that since index updates are idempotent, we can allow a
little slop in the global clock. This makes everything much easier.

>
>> I'm not sure that an update-seq per node is such a bad thing, as it
>> will require any external indexers to be deployed in a 1-to-1
>> relationship to nodes, which automatically balances the load for the
>> indexer as well. With a merged seq-id, users would be encouraged to
>> partition CouchDB without bothering to partition indexers. Maybe this
>> is acceptable in some cases, but not in the general case.
>
> So, the vector clock approach still has a per-node update sequence for each
> node's local clock, it just does the best job possible of globally ordering
> those per-node sequences.  We could easily offer local update sequences as
> well via some query string parameter.  I understand the desire to encourage
> partitioned indexers, but I believe that won't always be possible.  Bottom
> line, I think we should support global indexing of a partitioned DB.
>

I think you're right. As long as partitioned indexers are possible, I
have nothing against making global indexers as well.

>
> I'd like to hear more about how we implement redundancy and handle node
> failures in the tree structure.  In a pure consistent hashing ring, whether
> globally connected (Dynamo) or not (Chord), there are clear procedures for
> dealing with node failures, usually involving storing copies of the data at
> adjacent nodes along the ring.  Do we have an analogue of that in the tree?
>  I'm especially worried about what happens when inner nodes go down.
>

I like to think of partitioning and redundancy as orthogonal. If each
node has a hot-failover "twin", then parent nodes can track for all of
their children, the children's twins as well, and swap them out in
case of unavailability.

I'm not so hot on the Chord / Dynamo style of storing parts of
partitions on other partitions. Even just saying that is confusing.

Because physical nodes need not map directly to logical nodes, we just
need to be sure that each node's twin lives on a different physical
node (which it can share with other logical nodes).

The end result is that we can have N duplicates of the entire tree,
and even load-balance across them. It'd be a small optimization to
allow you to read from both twins and write to just one.

Chris

-- 
Chris Anderson
http://jchrisa.net
http://couch.io

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