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Subject: Re: Large lists of data
From: J Chris Anderson <jchris@apache.org>
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Date: Sat, 24 Jul 2010 13:51:38 -0700
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To: user@couchdb.apache.org


On Jul 24, 2010, at 1:36 PM, John Logsdon wrote:

> In SQL it would be something like:
>=20
> Select * from entries e
> Where e.list-id=3D1234=20
> and key in ('0123456789', '012345678', '01234567', '0123456', =
'012345', '01234')
>=20

oh well in that case (you know all the keys in advance) you can do a =
multi-key query.

we tunnel a semantic GET over an http POST, so you post

{
"keys" : [array of keys]
}

to the view.

For some reason I thought you were looking for a special kind of range =
query.

Chris

>=20
> On 24 Jul 2010, at 21:20, J Chris Anderson <jchris@couch.io> wrote:
>=20
>>=20
>> On Jul 24, 2010, at 1:13 PM, John wrote:
>>=20
>>> I'm not sure if that will give me what I want exactly.
>>>=20
>>> I want to search for these exact numbers only nothing in-between
>>>=20
>>>>> 0123456789
>>>>> 012345678
>>>>> 01234567
>>>>> 0123456
>>>>> 012345
>>>>> 01234
>>>=20
>>> Therefore the most documents it could possibly return is 6 i.e. even =
though startKey is "01234" and endkey is "0123456789" the number =
"012346" is not a valid match.=20
>>>=20
>>=20
>> statykey "01234" endkey "012346" with inclusive_end =3D false should =
work, except it will bring in eg "0123457"
>>=20
>> If "0123457" is not OK then I'm flummoxed as to how to describe your =
key requirements in a simple way, regardless of technology.
>>=20
>>> Hope that makes sense!
>>>=20
>>> John
>>>=20
>>> On 24 Jul 2010, at 20:49, J Chris Anderson wrote:
>>>=20
>>>>=20
>>>> On Jul 24, 2010, at 12:35 PM, John wrote:
>>>>=20
>>>>> Hi
>>>>>=20
>>>>> Thanks to you both for the answers so far. Indeed my setup is far =
more complex than I have exposed to date but I'm making it into bite =
sized chunks around the Use Cases that I think are the more challenging =
for me.
>>>>>=20
>>>>> Although your answers were useful they don't quite hit the mark =
and that's probably because I didn't explain my problem well enough to =
start with!
>>>>>=20
>>>>> The database will contain entries from multiple lists (many =
thousands perhaps) so the _id will never be unique on a telephone =
number. Perhaps this might work though:
>>>>>=20
>>>>> GET /database/<list _id>#0123456789
>>>>>=20
>>>>> or I could just keep the _id as a uuid and move this problem (find =
by list id and number) to the view.
>>>>>=20
>>>>=20
>>>> for now I'd say just go with uuids and you can have a view by =
telephone number for direct (or starts_with) lookups.
>>>>=20
>>>>> The view by list wont work for me. I need to be able to query the =
view with something like:
>>>>>=20
>>>>> GET  =
/database/_design/portability/_view/NP?key=3D0123456789&list=3D<_id of =
list>
>>>>>=20
>>>>> In fact in some cases the problem is more complex than this as I =
need to search for "widest match":
>>>>>=20
>>>>> GET  =
/database/_design/portability/_view/NP?key=3D0123456789&list=3D<_id of =
list>&min_width=3D5
>>>>>=20
>>>>> which would return the widest match in:
>>>>>=20
>>>>> 0123456789
>>>>> 012345678
>>>>> 01234567
>>>>> 0123456
>>>>> 012345
>>>>> 01234
>>>>>=20
>>>>>=20
>>>>> I even have another use case where I need to do a STARTS_WITH e.g. =
provide a key of 01234 and return true if there are any numbers that =
start 01234.
>>>>>=20
>>>>=20
>>>> this is easy. have a view like:
>>>>=20
>>>> function(doc) {
>>>> emit([doc.list_id, doc.number], null)=20
>>>> }
>>>>=20
>>>> Then you can query with=20
>>>>=20
>>>> ?startkey=3D["mylist", "012"]&endkey=3D["mylist", "013"]
>>>>=20
>>>> to get everything with a prefix of "012" in the "mylist" list. you =
can mess around with the endkey_inclusive (or is it inclusive_endkey) =3D =
true / false to not get the exact number "013" in your result set.
>>>>=20
>>>> from this technique you can see how you could do starts-with =
against just phone numbers also, with a view like
>>>>=20
>>>> function(doc_ {
>>>> emit(doc.number, null)
>>>> }
>>>>=20
>>>> Note I have telephone numbers as strings in this example as a =
regular number 012 is the same as 12.
>>>>=20
>>>>> This is a typical telecom problem and it would be good to document =
a Design Pattern for this Use Case. In fact there's a discussion for =
another day on how/where we could document this patterns and get peer =
reviews on them.
>>>>>=20
>>>>> Thanks again
>>>>>=20
>>>>> John
>>>>>=20
>>>>> On 24 Jul 2010, at 19:15, J Chris Anderson wrote:
>>>>>=20
>>>>>>=20
>>>>>> On Jul 24, 2010, at 7:41 AM, mickael.bailly@free.fr wrote:
>>>>>>=20
>>>>>>> Hello,
>>>>>>>=20
>>>>>>> 1/ it's a little hard to answer this question, your setup is =
certainly a little more complex than what you expose in your email :-) =
However thousands of documents are gracefuly handled by CouchDB.
>>>>>>>=20
>>>>>>> 2/ At first sight your documents will look like :
>>>>>>> { "_id": 0123456789 , "list": "mylist", "type": "NP", =
"status":"portedIn", "operatorId":1234 }
>>>>>>>=20
>>>>>>> That way you can query your document by phone number :
>>>>>>>=20
>>>>>>> GET /database/0123456789
>>>>>>>=20
>>>>>>> and have all documents belonging to the list "mylist" by =
creating a view that emits the "list" field :
>>>>>>>=20
>>>>>>> function (doc) {
>>>>>>> if ( doc.list  && doc.type =3D=3D "NP" ) {
>>>>>>> emit (doc.list,null);
>>>>>>> }
>>>>>>> }
>>>>>>>=20
>>>>>>> and fetching them with something like :
>>>>>>>=20
>>>>>>> GET =
/database/_design/portability/_view/NP?key=3D"mylist"&include_docs=3Dtrue
>>>>>>>=20
>>>>>>> 3/ When updating a document : the document is of course =
immediately available. However the view index won't be updated. In =
CouchDB view indexes are rebuilt on view query (not on document update). =
When you'll query CouchDB "give me all the documents of the view NP", =
Couch will take all documents that have changed (added, updated, =
deleted) since the last time you asked Couch for the view, and will =
update indexes accordingly. You have the option of fetching the view =
without rebuilding the index, with the "stale" parameter, but in this =
case, of course, you won't see the changes. During the rebuilt of the =
index, subsequent view queries are queued until the index is up to date.
>>>>>>>=20
>>>>>>> 4/ I setup CouchDB to parse network logs. A view took something =
like 25 minuts for 100 millions documents, on a Dell PowerEdge 2950 Xen =
Virtual Machine with two dedicated processors and 4gigs ram. Numbers can =
heavily vary according to the complexity of the view, so it's always =
hard (and dangerous) to give numbers. Moreover my indexes were not only =
numbers, but also strings.
>>>>>>>=20
>>>>>>=20
>>>>>> this is a good response. I'd only follow up to say that there are =
some techniques you can use to further tune view-generation performance. =
one: keysize and entropy can make a big difference. the view by list, as =
above, looks pretty good on that front.
>>>>>>=20
>>>>>> CouchDB can also be configured to store view indexes on a =
separate disk from the database file, which can reduce IO contention if =
you are at the edge of what your hardware can do.
>>>>>>=20
>>>>>> Also, there is the option to query views with stale=3Dok, which =
will return a query based on the latest snapshot, with low latency, so =
clients aren't blocked waiting for generation to complete. then you can =
use a cron-job with a regular view query and limit=3D1 to keep the index =
up to date. so clients always see a fairly recent snapshot, with low =
latency.
>>>>>>=20
>>>>>>>=20
>>>>>>> What you should be aware of is that CouchDB requires maintenance =
tasks to keep great performances, it's called "compact" and should be =
run on databases (to rebuilt the db file that is append-only) and on =
databases views (to rebuild the index file that is append-only). During =
the compact, database is still available but performances are degraded =
(from my personnal experience).
>>>>>>> Also, a new replication engine is in the pipe and should greatly =
improve the replication experience.
>>>>>>>=20
>>>>>>>=20
>>>>>>> Mickael
>>>>>>>=20
>>>>>>> ----- Mail Original -----
>>>>>>> De: "John" <john.logsdon@netdev.co.uk>
>>>>>>> =C0: user@couchdb.apache.org
>>>>>>> Envoy=E9: Samedi 24 Juillet 2010 11h37:56 GMT +01:00 Amsterdam / =
Berlin / Berne / Rome / Stockholm / Vienne
>>>>>>> Objet: Large lists of data
>>>>>>>=20
>>>>>>> Hi=20
>>>>>>>=20
>>>>>>> I'm currently evaluating couchdb as a candidate to replace the =
relational databases as used in our Telecom Applications.
>>>>>>> For most of our data I can see a good fit and we already expose =
our service provisioning as json over REST so we're well positioned for =
a migration.
>>>>>>> One area that concerns me though is whether this technology is =
suitable for our list data. An example of this is Mobile Number =
Portability where we have millions of rows of data representing ported =
numbers with some atrributes against each.
>>>>>>>=20
>>>>>>> We use the standard Relational approach to this and have an =
entries table that has a foreign key reference to a parent list.=20
>>>>>>>=20
>>>>>>> On our web services we do something like this:
>>>>>>>=20
>>>>>>> Create a List:
>>>>>>>=20
>>>>>>> PUT /cie-rest/provision/accounts/netdev/lists/mylist
>>>>>>> { "type": "NP"}
>>>>>>>=20
>>>>>>> To add a row to a list=20
>>>>>>> PUT =
/cie-rest/provision/accounts/netdev/lists/mylist/entries/0123456789
>>>>>>> { "status":"portedIn", "operatorId":1234}
>>>>>>>=20
>>>>>>> If we want to add a lot of rows we just POST a document to the =
list.
>>>>>>>=20
>>>>>>> The list data is used when processing calls and it requires a =
fast lookup on the entries table which is obviously indexed.
>>>>>>>=20
>>>>>>> Anyway, I'd be interested in getting some opinions on:
>>>>>>>=20
>>>>>>> 1) Is couchdb the *right* technology for this job? (I know it =
can do it!)
>>>>>>>=20
>>>>>>> 2) I presume that the relationship I currently have in my =
relational database would remain the same for couch i.e. The entry =
document would ref the list document but maybe there's a better way to =
do this?
>>>>>>>=20
>>>>>>> 3) Number portability requires 15 min, 1 hour and daily syncs =
with a central number portability database. This can result in bulk =
updates of thousands of numbers. I'm concerned with how long it takes to =
build a couchdb index and to incrementally update it when the number of =
changes is large (Adds/removes). =20
>>>>>>> What does this mean to the availability of the number? i.e. Is =
the entry in the db but its unavailable to the application as it's entry =
in the index hasnt been built yet?
>>>>>>>=20
>>>>>>> 4) Telephone numbers like btrees so the index building should be =
quite fast and efficient I would of thought but does someone have =
anything more concrete in terms of how long it would take typically? I =
think that the bottleneck is the disk i/o and therefore it may be vastly =
different between my laptop and one of our beefy production servers but =
again I'd be interested in other peoples experience.
>>>>>>>=20
>>>>>>> Bit of a long one so thanks if you've read it to this point! =
There's a lot to like with couchdb (esp the replication for our use =
case) so I'm hoping that what i've asked above is feasible!
>>>>>>>=20
>>>>>>> Thanks
>>>>>>>=20
>>>>>>> John
>>>>>>>=20
>>>>>>>=20
>>>>>>=20
>>>>>=20
>>>>=20
>>>=20
>>=20