On May 14, 2009, at 10:36 AM, Matt Goodall wrote:
2009/5/14 Adam Kocoloski <[email protected]>:
Hi Ben, welcome! At the moment, CouchDB does not have any capacity
for
intra-document replication checkpointing. And you're right, in the
specific
situation you describe Couch would have a difficult time making any
replication progress.
Given that replication over slow, unreliable links is absolutely a
CouchDB
design goal, I think we might eventually conjure up some more magic
to make
some sort of intra-document (or at least intra-attachment)
checkpointing
possible. I think it will be post-1.0, though. Best,
Adam
On May 14, 2009, at 7:12 AM, Ben Cohen wrote:
Hi all --
This is my first message to the list. I've been watching it for a
little
while now and so far everything I read about the design of couchdb
I like a
lot! Thanks so much for all the cool work!
One of the uses I'm planning for couchdb involves replicating a
database
across a slow, unreliable link which will never become anything
other than
slow and unreliable. I understand the replication is incremental
and
designed to 'pick up where it left off' in the case of replication
interruption. From the technical overview on the website:
The replication process is incremental. At the database level,
replication only examines documents updated since the last
replication. Then
for each updated document, only fields and blobs that have
changed are
replicated across the network. If replication fails at any step,
due to
network problems or crash for example, the next replication
restarts at the
same document where it left off.
Is this actually accurate? It suggests that documents are replicated
one-by-one and that replication can be interrupted at any point and
will continue from wherever it got to before the interruption.
Firstly, I believe the whole replication has to complete before any
updates are visible in the target database.
No, each update is seen on the target as it's written by the replicator.
If I restart the server in
charge of replication and then restart the replication it always seems
to start from the beginning. i.e. the Futon's "Processed source update
#xxx" status starts from 0 (when replicating an empty database).
It can start scanning from the beginning, but it will not copy again
documents it's already replicated.
The checkpointing work prevents it from scanning back from 0, but
there are failure scenarios where it might start from 0 anyway. Adam
has some ideas for a simple fix we can make this far less likely to
happen.
Secondly, if the network connection fails in the middle of replication
(closing an ssh tunnel is a good way to test this ;-)) then it seems
to retry a few (10) times before the replicator process terminates. If
the network connection becomes available again (restart the ssh
tunnel) the replicator doesn't seem to notice. Also, I just noticed
that Futon still lists the replication on its status page.
There is lot of work we can do here, right now replication is strictly
a batch operation. Eventually we will have permanent replications,
where replicators are always working in near realtime, and
indefinitely retrying when network connections are failing.
-Damien
If I'm correct, and I really hope I'm missing something, then
couchdb's replication is probably not currently suitable for
replicating anything but very small database differences over an
unstable connection. Does anyone have any real experience in this sort
of scenario?
- Matt
I've got a question about this process. Say you have a document
to be
replicated with a 1 megabyte attachment. A replication process
starts, half
the doc is transferred successfully and then the connection dies.
Assuming
no changes to the source doc, when the replication restarts will the
transfer start from the beginning of the document or will it pick up
somewhere within the doc?
For my use case I have a slow link that will periodically come
online for
a certain fixed amount of time and initiate a replication. If the
replication isn't incremental 'within' a single document, then a
document in
the database above a certain size will for me, never make it
across and I
would imagine cause the replication to never make forward
progress ...
Does couchdb's replication magic avoid the issue for me and
eventually
transfer the document across my link?
Thanks much,
Ben Cohen
