Rest – How to design a RESTful API with good performance

You're adding in a layer of abstraction that is confusing

Your API starts off very clean and simple. A HTTP POST creates a new Deposit resource with the given parameters. Then you go off the rails by introducing the idea of "actions" that are an implementation detail rather than a core part of the API.

As an alternative consider this HTTP conversation...

POST /card/{card-id}/account/{account-id}/Deposit

AmountToDeposit=100, different parameters...

201 CREATED

Location=/card/123/account/456/Deposit/789

Now you want to undo this operation (technically this should not be allowed in a balanced accounting system but what the hey):

DELETE /card/123/account/456/Deposit/789

204 NO CONTENT

The API consumer knows that they are dealing with a Deposit resource and is able to determine what operations are permitted on it (usually through OPTIONS in HTTP).

Although the implementation of the delete operation is conducted through "actions" today there is no guarantee that when you migrate this system from, say, C# to Haskell and maintain the front end that the secondary concept of an "action" would continue to add value, whereas the primary concept of Deposit certainly does.

Edit to cover an alternative to DELETE and Deposit

In order to avoid a delete operation, but still effectively remove the Deposit you should do the following (using a generic Transaction to allow for Deposit and Withdrawal):

POST /card/{card-id}/account/{account-id}/Transaction

Amount=-100, different parameters...

201 CREATED

Location=/card/123/account/456/Transation/790

A new Transaction resource is created which has exactly the opposite amount (-100). This has the effect of balancing the account back to 0, negating the original Transaction.

You might consider creating a "utility" endpoint like

POST /card/{card-id}/account/{account-id}/Transaction/789/Undo <- BAD!

to get the same effect. However, this breaks the semantics of a URI as being an identifier by introducing a verb. You are better off sticking to nouns in identifiers and keeping operations constrained to the HTTP verbs. That way you can easily create a permalink from the identifier and use it for GETs and so on.

Edited to address question updates, previous answer removed

Looking over your changes to your question I think I understand the problem you are facing a bit more. As there is no field that is an identifier on your resources (just a link) you have no way to refer to that specific resource within your GUI (i.e. a link to a page describing a specific pet).

The first thing to determine is if a pet ever makes sense without an owner. If we can have a pet without any owner then I would say we need some sort of unique property on the pet that we can use to refer to it. I do not believe this would violate not exposing the ID directly as the actual resource ID would still be tucked away in a link that the REST client wouldn't parse. With that in mind our pet resource may look like:

<Entity type="Pet">
    <Link rel="self" href="http://example.com/pets/1" />
    <Link rel="owner" href="http://example.com/people/1" />
    <UniqueName>Spot</UniqueName>
</Entity>

We can now update the name of that pet from Spot to Fido without having to mess with any actually resource IDs throughout the application. Likewise we can refer to that pet in our GUI with something like:

http://example.com/GUI/pets/Spot

If the pet does not make any sense without an owner (or pets are not allowed in the system without an owner) then we can use the owner as part of the "identity" of the pet in the system:

http://example.com/GUI/owners/John/pets/1 (first pet in the list for John)

One small note, if both Pets and People can exist separate of each-other I would not make the entry point for the API the "People" resource. Instead I would create a more generic resource that would contain a link to People and Pets. It could return a resource that looks like:

<Entity type="ResourceList">
    <Link rel="people" href="http://example.com/api/people" />
    <Link rel="pets" href="http://example.com/api/pets" />
</Entity>

So by only knowing the first entry point into the API and not processing any of the URLs to figure out system identifiers we can do something like this:

User logs into the application. The REST client accesses the entire list of people resources available which may look like:

<Entity type="Person">
    <Link rel="self" href="http://example.com/api/people/1" />
    <Pets>
        <Link rel="pet" href="http://example.com/api/pets/1" />
        <Link rel="pet" href="http://example.com/api/pets/2" />
    </Pets>
    <UniqueName>John</UniqueName>
</Entity>
<Entity type="Person">
    <Link rel="self" href="http://example.com/api/people/2" />
    <Pets>
        <Link rel="pet" href="http://example.com/api/pets/3" />
    </Pets>
    <UniqueName>Jane</UniqueName>
</Entity>

The GUI would loop through each resource and print out a list item for each person using the UniqueName as the "id":

<a href="http://example.com/gui/people/1">John</a>
<a href="http://example.com/gui/people/2">Jane</a>

While doing this it could also process each link that it finds with a rel of "pet" and get the pet resource such as:

<Entity type="Pet">
    <Link rel="self" href="http://example.com/api/pets/1" />
    <Link rel="owner" href="http://example.com/api/people/1" />
    <UniqueName>Spot</UniqueName>
</Entity>

Using this it can print a link such as:

<!-- Assumes that a pet can exist without an owner -->
<a href="http://example.com/gui/pets/Spot">Spot</a>

or

<!-- Assumes that a pet MUST have an owner -->
<a href="http://example.com/gui/people/John/pets/Spot">Spot</a>

If we go with the first link and assume that our entry resource has a link with a relation of "pets" the control flow would go something like this in the GUI:

1. Page is opened and the pet Spot is requested.
2. Load the list of resources from the API entry point.
3. Load the resource that is related with the term "pets".
4. Look through each resource from the "pets" response and find one that matches Spot.
5. Display the information for spot.

Using the second link would be a similar chain of events with the exception being that People is the entry point to the API and we would first get a list of all people in the system, find the one that matches, then find all pets that belong to that person (using the rel tag again) and find the one that is named Spot so we can display the specific information related to it.