[SAGA-RG] SAGA and advert URIs

[Andre Merzky]

Hi again, 

Quoting [Bruno Harbulot] (Sep 08 2009):
> 
> Hi Andre,
> 
> Andre Merzky wrote:
> >Quoting [Bruno Harbulot] (Sep 07 2009):
> 
> >>While this can work at a small scale, there are a number of issues with 
> >>this approach.
> >>
> >>Firstly, if another adapter exists one day for another DBMS (for example 
> >>MySQL or Oracle), which one will be used? It's not uncommon to have 
> >>hosts that run both PostgreSQL and MySQL for example.
> >>It's a problem similar to letting 'any://' guess the protocol. Although 
> >>by luck 'ssh://host/file' and 'ftp://host/file' are likely to be the 
> >>same because the underlying file system structure is the same, a 
> >>PostgreSQL server and a MySQL server running on the same machine won't 
> >>have the same data at all.
> >
> >While this is true, this is considered to be a feature, not
> >a bug.  Along the same lines one could argue that the 'ftp'
> >schema for file access is not uniquely specifying the
> >adaptor to be used.  In fact, 'ftp://' could be accepted by
> >the gridftp adaptor, but the curl adaptor, and by a
> >(hypothetical) plain ftp adaptor.  Yes, one or the other may
> >fail to run the command - then the next in line will be
> >used.  Adaptor selection can be optimized, by configuration,
> >by heuristics, or otherwise - but that is an implementation
> >detail hidden from the application.
> 
> My concern (both for advert:// and any://) is more about the notion of 
> identifier, which the URI is. It makes perfect sense to be able to use a 
> number of adaptors for the same URI: this is indeed an implementation 
> detail that ought to be hidden from the application.
> However, letting the application developers and users use identifiers 
> that are ambiguous is certainly going to lead to some trouble further 
> down the line, more so if one day they have to talk to some other 
> application, which wouldn't be surprising in the grid world.

Yes, exchange of these URLs with other applications is not
straight forward (the other way around will work, usually
work though).

The API spec, however, provides something like
'url.translate (stub)' for exactly that reason.  The
intented use case is (using my earlier example):


 1: saga::url ftp ("ftp://host/etc/passwd");
 2: saga::url http = ftp.translate ("http://host"); 
 3: saga::url file = ftp.translate ("file://host"); 
  
Line 1  will do what you expect.  Line 2 will *throw* as it
is (in our example) not possible to find a http based URL
pointing to the same backend entity.  Line 3 will return the
url "file://host/var/pub/etc/passwd".  The same should work
for "any://".

Now, having that call in the API is nice and well, but at
the moment, I do not know if any implementation provides a
sensible mechanism to really translates URLs between
different backends.  Anyway: should the problem arise for
some user groups, it can be solved this way.


> >>Finally, SAGA is an API, but this makes SAGA enter the territory of 
> >>network protocols. If you addressed the issues above by specifying the 
> >>database structure and how to query it, you'd end up defining another 
> >>protocol, which would certainly duplicate the job of protocols that 
> >>already exist (there are a number of pub/sub protocols, for example one 
> >>could be using Atom).
> >
> >No, we do *not* define a protocol. We simply don't  We have
> >nowehere in our code a protocol definition.  Nor do we
> >actually talk on byte level on the connection.  We simply
> >use existing protocols like ftp, the postgres protocol, etc.
> 
> Well, you do hide the protocol, but it's there, and it's defined in a 
> fuzzy way. If you do a retrieve_object on 
> "advert://user:pass@host/path?dbname=mydb&dbtype=sqlite3", you imply a 
> mechanism for dereferencing that URI. The API will have to find what to 
> do with this URI and will have to make the connection to the appropriate 
> database, with the appropriate structure. That's where you're blurring 
> the line with network protocols.

Yes, we hide the protocol, on purpose.  But many
applications do that, really.  The URL scheme is *not*
semantically bound to the network protocol.  For example,
there is an official 'file' scheme listed in the first URL
RFC - 'file' however is not a protocol, but defines a name
space to be used for entity resolution.

Wikipedia says on the topic
(http://en.wikipedia.org/wiki/URI_scheme):

  URI schemes are sometimes erroneously referred to as
  "protocols", or specifically as URI protocols or URL
  protocols, since most were originally designed to be used
  with a particular protocol, and often have the same name.
  The http scheme, for instance, is generally used for
  interacting with Web resources using HyperText Transfer
  Protocol. Today, URIs with that scheme are also used for
  other purposes, such as RDF resource identifiers and XML
  namespaces, that are not related to the protocol.
  Furthermore, some URI schemes are not associated with any
  specific protocol (e.g. "file") and many others do not use
  the name of a protocol as their prefix (e.g. "news").

Further, the URL RFC says:

  "[...] a new URL scheme must include a definition of an
   *algorithm for accessing of resources* [...]" 
  (emphasis mine).

We certainly do have such an algorithm (even if it is ill
specified).


> >>In the case where identifiers are ambiguous and can point to 
> >>several distinct things, this sounds like a fundamental architectural 
> >>flaw (once it's released as it's the case for gsiftp URIs, it's almost 
> >>impossible to fix [*]).
> >
> >I can give you simplier examples.
> >
> >  http://host//etc/passwd
> >  ftp://host//etc/passwd
> >
> >will usually not refer to the same physical file, but, for
> >example, to
> >
> >  file://host//var/http_root/etc/passwd
> >  file://host//var/pub/etc/passwd
> >
> >and neither refers to the canonical
> >
> >  file://host//etc/passwd
> >
> >Yes, users need to be aware of that.
> 
> Well, that's not quite the same problem as gsiftp URIs. 
> "http://host/path/something" and "ftp://host/path/something" are 
> fundamentally disctinct URIs and therefore identify different resources 
> (which may or may not be files). Whether these resources may be aliases 
> for one another (e.g. via a redirection mechanism) is a different matter.
> 
> The problem with gsiftp:// URIs is that "gsiftp://host/path/something" 
> will refer to two distinct resources depending on whether you use 
> globus-url-copy or the CoG kit. This really is a pain when you want to 
> track data and simply refer to something independently of whether you're 
> using C or Java (or any other language for that matter). In most cases, 
> you have no way of knowing which implementation was talking about which 
> API, even if you try.

We will not attempt to fix specific GridFTP implementations.
That is left to either the adaptor, the backend, or the
application.


> That's the trap I'd like SAGA not to fall into, although at least SAGA 
> lets you specify a given protocol (disambiguating any:// can indeed be 
> done by being more specific), whereas gsiftp:// cannot be more specific.
> 
> >>[*] http://blog.distributedmatter.net/post/2006/12/08/gsiftp-URI-madness
> 
> 
> As a side-note, I've been using Restlet <http://www.restlet.org/> for a 
> while, and there's a couple of points that I had in mind and that may be 
> of interest.
> 
> Firstly, like SAGA, the Restlet tries to provide a uniform API for a 
> number of protocols, and provides a number of "connectors" that 
> implement those protocols (similar to SAGA adapters). The API is 
> modelled around the HTTP semantics 
> <http://wiki.restlet.org/docs_1.1/13-restlet/27-restlet/130-restlet.html>. 
> I think comparing the way the mappings have been done would be an 
> interesting exercise (and perhaps looking into the changes from Restlet 
> 1.0 and 2.0 correspond to similar steps in the evolution of SAGA).
> 
> 
> Secondly, I can't help notice the similarities between what SAGA aims 
> for and the mechanisms designed into HTTP, along with the way they've 
> been implemented in Restlet. For example, at an architectural level the 
> issues of guessing the protocol based on the any:// or advert:// 
> identifiers could be addressed by a proxy layer (not necessarily actual 
> network proxies, but a proxy layer in the API). The advertising system 
> could be done using PUT/GET and perhaps the Atom Publishing Protocol in 
> the back.
>
> From what I've seen from the SAGA Shell, it looks like it's trying to 
> provide a uniform interface, even for the sub-groups of classes in SAGA 
> (e.g. advert, file, job). It looks like there could be a further layer 
> of abstraction, providing a common interface between those types (and 
> you'd probably end up with something very similar to the HTTP verbs).

The goal of SAGA is not abstraction per se, i.e. is not to
find the smallest common semantic denominator for expressing
various backend operations.  The aim of SAGA is rather to
provide exactly those abstractions which seem to best match
the application level operations performed in distributed
applications.  One can discuss on what level those
abstractions live - but REST is certainly not amongst them.

URL management in itself is really just a very small part of
what SAGA tries to achieve, and was originally left out, doe
to ther can of worms it is bound to open.


> I'm not saying HTTP is ideal for what SAGA is trying to achieve, but it 
> looks like a number of mechanisms provided by the web architecture are 
> similar to what SAGA provides as an API.

Understood, and to some extend agreed.  REST etc certaily
try to come up with a syntactic and semantic framework for
remote service interaction.  I think its great for
implementing service interfaces/protocols, in most cases.
SAGA, however, targets a very different set of use cases,
and users.

Thanks, Andre.


-- 
Nothing is ever easy.