Open Stack

On 06/12/2012 04:24 AM, Gabriel Hurley wrote:
> Mark,
>
> Apparently you must have missed my lightning talk at the Essex summit... ;-) (http://gabrielhurley.github.com/slides/openstack/apis_like_orms/index.html)
>
> Filtering, pagination, and many other API features are *critical* for a rich dashboard experience. If you want to talk specifics, the entire Horizon team would be happy to have a long chat with you.
Yes and no.  The reality is that it is a  trade off. Server side, you 
pay by doing a network round trip, and having to determine ahead of time 
the mechanisms for sorting, caching, paging, etc.

The real problem is that the parsing of the results can blow out the 
stack space in the browser.

>
> That said, we have also considered the case you propose where you effectively "request everything and handle it on the client-side"... however, I see that as a tremendously lazy solution. On the service-provider end you have access to powerful database methods that can do these operations in fractions of the time the client-side can (especially with good indexes, etc.). And if you've ever worked in mobile applications you'll know that minimizing data across the wire is crucial. The only argument I've heard in favor of that is basically "it's easier for us not to add API features".

At the expense of loading your Database.  Serverside paging and 
filtering both require one of two things:  caching or additional 
Database queries, and both increase your server footprint.  For small 
datasets, or for limited queries,  this is not a problem,  but for 
scalability you want to limit the work you do on the server.

For Keystone using the LDAP backend,  caching and pagination are 
extremely expensive, and not something I would like to support.  an LDAP 
query is not guaranteed to come back in any particular order, so you 
can't just do the SQL trick of executing the query at offset + window 
size.  You have to do the equivalent of a Cursor,  and this places 
serious load on the LDAP server,  the Keystone server, and possibly 
impacts other apps dependand on LDAP.

>
> To speak on the specific feature of pagination, the problem of 'corruption' by simultaneous writers is no excuse for not implementing it. You think Google, Facebook, Flickr, etc. etc. etc. don't have this problem? If you consume their feeds you'll notice you can fetch offset-based pagination with ease. You'd never expect to see a navigation element at the bottom of Google search results that said "take me to results starting with the letter m".

There is a major difference.  We are working with data that has to be 
ACID.  Google, Facebook and flickr do not.  Before you migrate a VM,  
you need to know if the host meets the criteria for the VM. If it 
changes between when you check and when you reserve the space for the 
VM,  you have just over committed.  "Get it right eventually" does not 
work for management apps.

>
> None of this is a case of "someone might use it". The Horizon team has been loudly asking for these features for 8+ months now. And not just from Keystone but from all the projects. I have a list a mile long of API features we need to really deliver a compelling experience. I was just adding some items to it today, in fact.
>
> The rest of your points I have no strong feelings on and generally agree, but when it comes to API features... I feel *very* strongly.

Note that I am not saying "don't do pagination" as I agree, it is 
essential for good user experience.  What I am stating is that we need 
to be smart about the techniques and technologies we choose, as there is 
always an upside and a downside.

>
> All the best,
>
>      - Gabriel
>
>
>> -----Original Message-----
>> From: openstack-bounces+gabriel.hurley=nebula.com at lists.launchpad.net
>> [mailto:openstack-
>> bounces+gabriel.hurley=nebula.com at lists.launchpad.net] On Behalf Of
>> Mark Nottingham
>> Sent: Monday, June 11, 2012 10:27 PM
>> To: Joseph Heck
>> Cc: openstack at lists.launchpad.net (openstack at lists.launchpad.net)
>> Subject: Re: [Openstack] [keystone] v3 API draft (update and questions to
>> the community)
>>
>> On 11/06/2012, at 6:58 AM, Joseph Heck wrote:
>>
>>> First - what's the current thought of support for PATCH vs PUT in updating
>> REST resources? Are there any issues with clients being able to use a PATCH
>> verb? It's not something I'm super familiar with, so I'm looking for feedback
>> from the community here. Ideally, I'd like to support the semantics of the
>> PATCH HTTP verb, and possibly just assert no support for the PUT verb to be
>> clear about intended functionality. Is that going to throw anyone for a loop?
>>
>> I answered a question about PATCH before; don't want to repeat myself, but
>> it should be workable. Happy to chat more about it if you have specific
>> questions.
>>
>>> Second - filtering/searching for resources. The draft includes a section
>> labelled "Query By Name", which is probably mis-labelled, as it's intended to
>> cover the general idea of passing in query parameters to general listing
>> resource endpoints to filter the result set. The API endpoints across all the
>> resources are defined as plurals, with the idea that specificity comes later in
>> the URI (for referencing a single resource), or that we could add on these
>> query parameters to restrict/filter by resource type.
>>
>> I'm in the middle of doing some log analysis and other research about how
>> the APIs are used at Rackspace. It's too early to share results (although I do
>> intend to, in some form, because the idea is to inform future API design), but
>> one of the things that's very noticeable is how (extremely!) little pagination
>> and filtering seem to be used in anger.
>>
>> In fact, if you take a look at the libraries, you'll find that they often don't use
>> or even support filtering or pagination; e.g., libcloud doesn't, AFAICT.
>>
>> So, it's worth having a think about what the use cases actually are; both
>> filtering and pagination are usually ways to save one or more of:
>>    a) client-side work
>>    b) server-side work
>>    c) bandwidth / latency
>>
>> One interesting exercise would be to estimate the largest number of users
>> (or whatever else you'd be listing) that a reasonable deployment would put
>> in a single response, triple it, do a dummy serialisation in JSON, and then gzip
>> it, so that you can estimate the size, see how long it takes to parse on the
>> client, etc.
>>
>> >From what I've seen (in OpenStack as well as in other APIs that have
>> nothing to do with Cloud), API designers tend to overestimate the utility of
>> pagination and especially filtering ("somebody might use it"), but users just
>> ignore them, doing all of the work on the client side, except in extreme
>> circumstances (e.g., VERY large responses / very high latency).
>>
>> Unless you have strong use cases for them, I'd be inclined to drop them; they
>> increase implementation, QA, and documentation complexity, as well as
>> making the API harder to understand. YMMV, of course :)
>>
>> The other issue with pagination is that a relative paged approach (like you're
>> taking) means that readers' views of the complete set of items can be
>> corrupted by simultaneous writers. While in some instances this is just an
>> annoying UI bug (missing or duplicated entries on different pages, lower
>> cache hit rates), in some circumstances it can be more serious (clients not
>> understanding the true state of the system, and making bad decisions as a
>> result).
>>
>> Again, a source of bugs and complexity (we came up with one approach to
>> this with archived feeds in RFC5005, but it's pretty heavyweight, especially
>> for use cases like this).
>>
>> Hope this helps,
>>
>> P.S. the X-Subject-Token stuff is breaking HTTP; you need to either put the
>> token (or a facsimile for it) in the URL, or put Vary: Subject-Token in EVERY
>> response those resources generate. The former is preferred; this is over TLS,
>> right? Sorry I didn't see that earlier.
>>
>> P.P.S If it's not too late, drop the X- from that header!
>> <http://tools.ietf.org/html/draft-ietf-appsawg-xdash-05>
>>
>>
>> --
>> Mark Nottingham   http://www.mnot.net/
>>
>>
>>
>>
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>
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