问题
I use a cloudant couchdb and I've noticed see that the "_changes" query on the database returns an "update_sequence" that is not a number, e.g.
"437985-g1AAAADveJzLYWBgYM..........".
What is more, the response is not stable: I get 3 different update_sequences if a query the db 3 times.
Is there any change in the known semantics of the "update_sequence", "since", etc. or what?
Regards, Vangelis
回答1:
Paraphrasing an answer that Robert has previously given:
The update sequence values are opaque. In CouchDB, they are currently integers but, in Cloudant, the value is an encoding of the sequence value for each shard of the database. CouchDB will likely adopt this in future as clustering support is added (via the BigCouch merge).
In both CouchDB and Cloudant, _changes will return a "seq" value with every row that is guaranteed to return newer updates if you pass it back as "since". In cases of failover, that might include changes you've already seen.
So, the correct way to read changes since a particular update sequence is this;
- call /dbname/_changes?since=
- read the entire response, applying the changes as you go
- Record the last_seq value as your new checkpoint seq value.
Do not interpret the two values, you can't compare them for equality. You can, if you need to, record any "seq" value as you go in step 2 as your current checkpoint seq value. The key thing you cannot do is compare them.
回答2:
It'll jump around, the representation is a packed base64 string representing the update_seq of the various replicas of each shard of your database. It can't be a simple integer because it's a snapshot of a distributed database.
As for CouchDB, treat the update_seq as opaque JSON and you'll be fine.
来源：https://stackoverflow.com/questions/20446673/update-sequence-changed-semantics-in-cloudant-db