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Re: [DESIGN] Proposed structure fol LedgerSMB web services


On 07/27/2015 11:54 AM, Erik Huelsmann wrote:

On Mon, Jul 27, 2015 at 7:03 PM, John Locke <..hidden..> wrote:
Hi, Erik,

Nice start. Some quick comments (not a lot of time...):

On 07/27/2015 05:51 AM, Erik Huelsmann wrote:


The user of the api should run an OPTIONS request on the base URL (/api) to discover version, options and features of the API.

This is starting to sound like a WSDL? I think that could be a huge benefit to do, as long as it's not required...

What I was talking about was this discussed in this blog post: http://zacstewart.com/2012/04/14/http-options-method.html

Very cool! Following some links from there led me here: http://www.restdoc.org/

As to what you mean by "required", I don't know. If you mean "required to read before using the API", then, no, it's not required. If you mean "required when implementing a new service", then I think the answer should be "yes, it's required". Every service requires documentation and if this is the only documentation, then I'm quite happy to accept the service.

Agreed, sounds good.

And if you're creating documentation anyway...


All URLs in the document are assumed to be relative to some base URL. E.g. assuming LedgerSMB to be hosted under the following URL: https://example.com/path/to/ledgersmb/ , the URL /api in this document in fact means https://example.com/path/to/ledgersmb/api .

The proposed URL structure is (as can be found in many existing web service schemas):

 (a) /api/<version>/<resource>/[?query parameters]
 (b) /api/<version>/<resource>/id
 (c) /api/<version>/<resource>/id?perform=<action>

The above is mostly inspired on the PayPal API which - I think - drives a system much like ours in the sense that their system manages workflow producing transactions.

In our case, I think the "id" specifier in the resource may be multiple path segments long; e.g. for currency rates: /api/v1/exchangerate/EUR/1/2015-12-12 where "EUR/1/2015-12-12" is the identifier for the: currency identifier, rate type and (start)date of the rate.

Form (a) will be used for creating (POST) and listing (GET) resources instances. Dojo proposes to use the 'Range:' HTTP header to limit results in the request. I think that makes more sense than to use query parameters for it.

For development/debugging, I really like having an API observe query parameters in addition to Range headers. I would suggest we support both, and pick one to win...

Hmm. I'm not really in favor of having duplicate functionality. People seem to be using cURL to test their services; it should be pretty easy to add the Range header (or others, for that matter) to a cURL request. What method do you use?

Ha. For a while I was using a home-grown Dojo single-page app for testing out APIs, have played around with quite a bit, but it's been a while since I've done a major API project. I've seen some decent browser extensions for some of these kinds of things...

The other thing I'm thinking of here is for more light-weight, reporting types of uses. I'm not sure how much control you can get over headers when doing a cross-domain request from a browser -- I'm thinking a lightweight JS app that might want to grab the last 10 sales invoices for a dashboard, or something like that -- with an iframe, for example, you can't necessarily set browser headers but you can easily add a GET parameter.

Not a big deal these days, there's so many decent tools for doing it right with a toolkit that we may not need the "lightweight" GET-only approach, but I do think there may be scenarios where it might prove useful...

Form (b) will be used for retrieving (GET) an individual resource instances.
Should we add support for PUT here?

Well, the entire API doesn't list PUT, because PUT was mapped to "U" of CRUD. And the design explicitly delegates updates of resources to RPC calls. The reasoning behind that is: old code looks the way it does because it *tries* to derive based on a (partial) "before" state and an "after" state what the user might have done in the web application. Most of the time it guesses correctly, but there are lots of cases where it simply can't tell the user's intent from the combined states. I don't want to propagate that mess to another level of teh application when I'm able to eliminate it somewhere (by building a different UI).
Form (c) will be used for (POST) modifying state of individual resource instances by executing <action> on the specified resource.


(Note that the API doesn't attach meaning to the HTTP request types PUT and PATCH (which the PayPal API *does* do)) -- I could see value in supporting a PATCH request for resources which require secondary approval and have not yet been approved (this is where PayPal uses it too).

 Retrieves an object or collection of objects, potentially restricted by query parameters or HTTP headers.

 Creates an object or collectino of objects when executed on a resource URL; when executed on a resource-instance URL, a required ?perform=<action> query string is to be added to the URL to specify which state transition is to be executed.

Each POST request in the API carries a payload where the consuming service should support at least one of the following formats (as indicated by the OPTIONS response)

 (i) application/json
 (ii) application/xml
 (iii) application/form-data
 (iv) application/x-www-form-urlencoded

The API itself should be responsible for doing this conversion -- and should allow the consuming client to send whichever of these it wants. The API can then convert to a Perl data object of some kind to pass off to the internal code.

Ok. You're saying there's *always* going to exist a mapping from application/form-data to application/json? I mean, I can imagine that a mapping like that for non-nested structures, but what about nested objects in arrays? I mean, in the new multi currency branch we have form fields named debit_1 and debit_fx_1; how do those map to a JSON/_javascript_ object?

Don't get me wrong; I'd like to delegate this to the request consumer too.

I think we just define a convention, and describe it. Perhaps make it simply mirror the Json structure with _ separated parts? e.g. debit_1_value=234&debit_1_fx=222&debit_2_value=444&debit_2_fx=400 maps to json as: (intentionally swapping the index to the 2nd position)

... I mean we already do this for form posts now, we need to convert it to some sort of data object internally anyway, why not build a library that does this for us, regardless of what format it receives in the request? Might need to change some of the current form field names...

This and the previous note does bring up something missing here: response format. Like the Range header, there's the "Accept" header the client can send, and I've also found it useful for very quick browser debugging to allow overriding that with a GET parameter.

So we should discuss the formats we support for the response:


... and of course how we handle these. Json, XML, CSV are pretty straightforward (hey, are there any industry-specific XML formats we should leverage/offer?) -- for nested data in CSV I've typically seen Json used...
For those last 3, clearly there's a need for templates for each kind of object...

If we've done a good job on the API, we should be able to plug in request formatters and response formatters easily -- so we could add text/yaml by writing a new plugin for both response and request handling...

Ah, yes, and that's exactly why I think we need to support a GET parameter in addition to Range: header -- then you can simply generate a URL to get a CSV or HTML report of the most recent 10 payments from client X.

Is PUT to be added to this list? I would expect PUT to update values of an existing object, and needs to contain all new values for the object. Obviously since we're doing financial transactions, this probably can only modify drafts and not anything posted (in a financial sense). But for drafts, reconciliation, batches, etc. this seems useful.

POST or PATCH can be used for modifying just a field on an object, or handling things like payments on an object?

Ah. Good point. POST(with an rpc endpoint) would be for adding a payment to an open item. PATCH would be to change the values of an existing object which is still editable.

Ok. That all sounds fine to me...


When an api call affects multiple resources and the API call returns an error *none* of the affected resources are to be affected.

Do we want the API to support a transaction, allow a bunch of operations to get batched with atomicity? e.g. failure after a series of web service calls rolls back the whole batch, if there are no errors entire batch gets committed?

If we can support that, that seems like another big win...

Hmm. My initial reaction is that we can support most of this by attaching transactions to an (unapproved) batch and then offering the user the option to either approve or remove the entire batch. Would that work for the use-case you envision?

Yes, that's exactly what I'm envisioning. Although a different "batch" mechanism than our current batches -- something specific to the API that can do all sorts of data changes and then approve in one go...

Actually, thinking about it, I can see how to put it all into one transaction. However, if that works, it depends on what you expect on subsequent calls within the same batch. Do you expect any queries to return the new values while they have not been fully committed to the database? Or do you just expect to send loads of modifications? Do you expect to be returned new IDs?

Good questions, and this gets beyond my experience -- I haven't actually done that much transactional programming to know the best practices here...

I would think we would expect subsequent calls to have the new values, and I do know that Dojo stores have supported "placeholder" ids that can be replaced with permanent ones after the data is committed, so I would tend to think that pattern should work, a "placeholder" that is returned while the batch is "open", and when the batch is "approved" a set of replacements get returned so the client can update with final IDs.

Should we be considering UUIDs here?

My basic idea was to batch up all RPC calls and delay them until the final "COMMIT" comes in and executing all the batched commands inside a single database transaction.

Hmm. Goes against REST, but then we are talking about financial systems, practically the definition of transactional logic. It feels like we are reinventing SOAP!

I'm thinking about the scenarios here, and the one that comes to mind is "shipping" some products on a sales order. We use this all the time --skipping the shipping screen, we just put in a value in the "ship" box and "Create Invoice." The current LSMB adjusts the sales order line items/totals, and commits that, and then takes you to a create invoice form that is completely open, unsaved, and in my opinion really should be in a transaction -- the sales order qtys shouldn't get updated until the invoice is posted (or at least saved as a draft).

That's a scenario I think the current app should do in a transaction, and doesn't.

I am also thinking about how you do transactions in a database, that you generally have to start a transaction with a "BEGIN" and otherwise it's not in a transaction. I'm thinking we just model the API the same way, that it's not in a transaction unless it's explicitly called for.

I also think this entire transaction functionality can be deferred until a later version, as long as you're thinking about it with the current version so it's something that can be added later...


The API user logs in by creating a new session through the /api/<version>/session/ API. Each application login (including API logins) is attached to an application user. the webservice caller thereby identifies itself as an application user/employee. Currently, credentials will be provided through basic auth on the first *and* all following requests. Session replay attacks are prevented by sending cookies back and forth; just as they are now. Each request should provide the cookies created during the session; possibly updated by the response of the last request -- basic cookie management.
At the end of a session, the session is to be removed by issueing a DELETE request on the session resource instance.

Regardless of whether the response generated by the server is a failure or a success, the session cookies should be updated on each request. The client must respect cookie updates regardless of the type of response.

Hmm. What if the same client is running multiple, parallel transactions? How would we handle race conditions here? Is it possible for the same session to have multiple sequences?

Good point, but it seems to work for PHP, RoR, ... I'll look around and try to find how others solve it. Maybe by opening a second session?

I think the general approach is a token sent in the body, not a cookie. The browser will send all cookies in any session... You can probably go to some extra steps to isolate sessions with curl, but I mostly just use the "cookiejar" in curl that makes it act like a browser here...

IIRC, the Drupal Form API sets a "form-build-id" and a "form-token". The build-id essentially is the session/form sent to the browser, and the token is used to validate and detect replays.

Drupal then caches the entire form using the build-id as the cache key.



When obtaining a resource from the server, the serving webservice may include embedded in its response objects that it refers to; e.g. the server may decide to include address data included in a response to a query for a customer. The server isn't required to include more than just the key by which the resource can be queried out of the resource collection.

Nested resources in the URL space (such as the GitLab example with team members in a project [2]).
*** Nested resources like the GitLab example pollute the namespace, because there's a two way correspondence: users-in-project and projects-in-user. *** How to handle this in the way that creates the least complexity??? *** Presumably, we want things to be layered, building complex resources on simple ones; so it's problematic in the gitlab example to make the user aware of the projects... ***

We should support and default to "obvious" nested resources. e.g. line items on an invoice, payment lines, etc.

Do you mean that these nested resources should be made available at the URL level? Or simply *always* be embedded in the response object? Basically, I wasn't thinking of the journal lines as individual resources. I think the *journal* is the individual resource, with a number of lines "inside" it. Would it make more sense to you to make the individual lines into resources too? [I can see reasoning for that too, because it allows running queries on the journal-line resource and filter out all lines on e.g. a single account...]

Well... yes. I think this boils down to a question of "document database" or "relational database". Obviously, we're built on a relational database, and I've never truly warmed up to pure document/object storage, the "NoSQL" movement... At the same time, the structure of an invoice in LSMB is pretty well-defined, and doesn't vary much, so we can present the entire thing as a "document", even though the lines are themselves first-class objects.

Maybe this is just force of habit for me, and there may well not be any actual need for it, but I would think that pretty much anything that can be a line on a report or an invoice should be directly addressable. But maybe that's overkill?

I've built one very complex system from scratch, and with that one I just made each level of the hierarchy extend the base data object class, and so I did essentially get the basic CRUD APIs for this for free, once I mapped my API layer to the data object -- about the only thing that needed attention at each level were the fields available for index queries -- and then the nesting issues we're discussing. I guess I didn't think that much about whether we *needed* that level of access (though it certainly helped when debugging).
I do think we should plan to allow the client to request what data to nest, perhaps either a custom header or a parameter (or both)? This would be one area that needs to be self-documenting, what resources can be excluded/included/expanded in which requests, and what is included by default.
I like that. I'll think about how we can model this.

As I think about it, I really only see two levels here: expanded, or condensed. Expanded, for an invoice, the response would include the customer record, each line item detail, each payment line. Condensed, it would only contain references to these other records, which would have to be retrieved separately if they don't exist.

How much deeper is useful to go?

Would we ever want to load the product from the line item? Perhaps, and then need to look up a pricegroup for a customer for the product... not exactly sure how this is currently modeled. But that really seems as complicated as this system gets. Oh, I guess there's entity/eca/contact method.

Thanks for your response! With a few times going back-and-forth, we can probably have something we can start working with and build our experience in the context of *this* application.

Sounds good!

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