[ckan-discuss] interface versions

Thu May 6 09:55:08 BST 2010

John,

This is a good overview on the benefits of versioning an API and how
it might be managed - many thanks. (I suggest it is also a benefit for
a client to know which CKAN instance it can work with.)

Now perhaps you could make some specific suggestions of what we should
do in relation to CKAN?
e.g.
* Suggest some example URLs with the version in.
* Do we have a minor/major version number scheme? Can you define the
difference? (e.g. adding a new key to the package dict or a REST
register/entity is a minor change, changing a value format is major)
* Looking at our existing tests, are they sufficient to test the
current version?
* How do we tell if clients are clinging onto new versions? Are you
suggesting clients register that they are still using an API version
(the reference counting thing)? If so, what is the mechanism?
* How do clients know that new versions of the API might be available
and how do we push them to upgrade?

David

On 5 May 2010 13:23, John Bywater <john.bywater at appropriatesoftware.net> wrote:
> I've been thinking about how supporting different versions of CKAN's
> interfaces could work....
>
> MOTIVATION
> The need to support multiple versions of CKAN's interfaces arises because of
> the need to decouple the development cycles of CKAN from the development
> cycles of CKAN clients.
>
> In other words, without support for multiple interface versions, either the
> development of CKAN's interfaces must be restricted to those changes which
> preserve backwards compatibility, or all CKAN clients must be required to
> synchronise upgrades with upgrades of CKAN.
>
> With support for multiple interface versions, incompatible changes can be
> introduced by introducing a new version of the interface. Current clients
> can continue to use the old interface. New clients can be written against
> the new interface. Current clients can be upgraded to work with the new
> interface.
>
> OVERVIEW
> A "service oriented" approach is being taken. This approach involves
> authoring a series of Specifications of Service (SoS). Each SoS defines its
> services as a set of interfaces. Each interface is defined by a set of
> operations.
>
> A migration path through a non-backwards compatible change to an interface
> would involve authoring three SoSs: for the service with the old interface
> version; for the service with both the old and the new interface version;
> and for the service with only the new interface version.
>
> Then, over time, agreed customer expectations will follow these SoSs
> stepwise: a customer will expect to be able to use the old interface; then
> later, the same customer will expect to be able to use both interfaces;
> later still, a customer will expect to be able to use only the new interface
> (hopefully because the customer has determined that the old interface is
> redundant for her purposes).
>
> A Service Level Agreement (SLA) can be used to record the association of a
> particular customer with a particular SoS and a particular duration of time.
>
> TERMS OF REFERENCE
> Service Level Agreement (SLA) references a Specifications of Service (SoS),
> a customer, a provider, and time period.
>
> A Specification of Service (SoS) has a Service Specification and may
> have many Software Unit Specifications
>
> A Service Specification has Services, Software Services, Web
> Services, Service Interfaces, and Interface Operations.
>
>  - A Service may require other Services.
>
>  - A Software Service is a type of Service.
>
>  - A Software Service is defined by its Interfaces.
>
>  - An Interface may require many Interfaces.
>
>  - An Interface may offer many Operations.
>
>  - An Interface may require many Software Units.
>
>  - Web Service is a type of Software Service.
>
> Software Unit Specification has Software Units such that:
>
>  - A Software Unit may offer Interfaces.
>
> EXAMPLE SoS EVOLUTION
> For illustrative purposes, there are three example Specification of
> Service documents below showing the REST API developing through a series
> of major versions.
>
> The first SoS (vA) shows the current state of CKAN.
>
> The second SoS (vB) adds a new operation to the REST interface.
> Therefore clients of vA are compatible with the new REST interface, but
> new clients are not compatible with the old interface. [NB An interface
> without versions would be perfectly adequate for client migration in
> this case.]
>
> The third SoS changes a data format, therefore clients of vB are not
> compatible with the new REST interface. Again, new clients are not
> compatible with the old interface. [NB An interface without versions
> would fail to be adequate for client migration in this case.]
>
> The fourth SoS drops an old interface. [NB The interface can only be
> removed from the Software Unit when all users have agreed to stop using
> the old interface.]
>
> == CKAN Specification of Service (vA) ==
>
> Software Services:
>  - CKAN
>
> Interfaces:
>  - CKAN:
>    - CWUI vW
>    - CRST vX
>
> Operations:
>  - CRST vX:
>    - PackageRegisterGet, PackageEntityGet
>
> Software Units:
>  - CKAN vL (provides: CWUI vW, REST vX)
>
>
> == CKAN Specification of Service (vB) [added operation] ==
>
> Software Services:
>  - CKAN
>
> Interfaces:
>  - CKAN:
>    - CWUI vW
>    - CRST vX
>    - CRST vY
>
> Operations:
>  - CRST vX:
>    - PackageRegisterGet(PackageFormatP)
>    - PackageEntityGet(PackageFormatP)
>  - CRST vY
>    - PackageRegisterGet(PackageFormatP)
>    - PackageEntityGet(PackageFormatP)
>    - PackageEntityPut(PackageFormatP)
>
> Software Units:
>  - CKAN vM (provides: CWUI vW, REST vX, REST vY)
>
>
> == CKAN Specification of Service (vC) [changed message format] ==
>
> Software Services:
>  - CKAN
>
> Interfaces:
>  - CKAN:
>    - CWUI vW
>    - CRST vX
>    - CRST vY
>
> Operations:
>  - CRST vX:
>    - PackageRegisterGet(PackageFormatP)
>    - PackageEntityGet(PackageFormatP)
>  - CRST vY
>    - PackageRegisterGet(PackageFormatP)
>    - PackageEntityGet(PackageFormatP)
>    - PackageEntityPut(PackageFormatP)
>  - CRST vZ
>    - PackageRegisterGet(PackageFormatQ)
>    - PackageEntityGet(PackageFormatQ)
>    - PackageEntityPut(PackageFormatQ)
>
> Software Units:
>  - CKAN vN (provides: CWUI vW, REST vX, REST vY, REST vZ)
>
>
>
> == CKAN Specification of Service (vD) [original interface removed] ==
>
> Software Services:
>  - CKAN
>
> Interfaces:
>  - CKAN:
>    - CRST vX
>    - CRST vY
>
> Operations:
>  - CRST vY
>    - PackageRegisterGet(PackageFormatP)
>    - PackageEntityGet(PackageFormatP)
>    - PackageEntityPut(PackageFormatP)
>  - CRST vZ
>    - PackageRegisterGet(PackageFormatQ)
>    - PackageEntityGet(PackageFormatQ)
>    - PackageEntityPut(PackageFormatQ)
>
> Software Units:
>  - CKAN vN (provides: CWUI vW, REST vX, REST vY, REST vY)
>
>
> == CKAN Specification of Service (vE) [software unit updated] ==
>
> Software Services:
>  - CKAN
>
> Interfaces:
>  - CKAN:
>    - CRST vX
>    - CRST vY
>
> Operations:
>  - CRST vY
>    - PackageRegisterGet(PackageFormatP)
>    - PackageEntityGet(PackageFormatP)
>    - PackageEntityPut(PackageFormatP)
>  - CRST vZ
>    - PackageRegisterGet(PackageFormatQ)
>    - PackageEntityGet(PackageFormatQ)
>    - PackageEntityPut(PackageFormatQ)
>
> Software Units:
>  - CKAN vO (provides: CWUI vW, REST vY, REST vY)
>
>
> SERVICE ORIENTED MANAGEMENT IMPLICATIONS
> It follows that:
>
> 1. CKAN is required only to support interfaces that are included in an SoS
> that is referenced by at least one SLA.
>
> 2. SLAs referencing SoSs with old interfaces may not be issued after the new
> interface has been deployed.
>
> 3. After all SLAs referencing SoSs with an old interface have been
> terminated or have expired, the old interface may be removed from the
> software and would therefore no longer be available.
>
> 4. Care must be taken when issuing SLAs to balance the wishes of customers
> to have their services available indefinitely with the wishes of the service
> providers to continue to enhance their offering.
>
> 5. An effective service desk process is required to acknowledge and resolve
> in a transparent and responsive manner all issues raised by all customers.
>
> SOFTWARE ARCHITECTURAL IMPLICATIONS
> It also follows that:
>
> 1. Having one domain model support many interfaces requires good tests
> so that adapters can be added safely to the existing implementations of
> old interfaces, to permit advancements of the domain model.
>
> 2. Unwanted aspects of the model must not be removed until there are
> no longer any interfaces depending on those aspects.
>
> 3. Interfaces must be supported for a duration of time that is
> sufficient for service clients to be upgraded.
>
> 4. There must be close alignment between the agreed expectations of
> service capabilities and the implementation tests. This can be reliably
> achieved by driving both the agreed expectations and the tests from a
> version controlled SoS document that includes definitions of the interfaces
> (and the operations of the interfaces).
>
> 5. An interface can be removed from the Software Unit once no SLA exists
> that references a Service specified to include that interface.
>
> 6. Interface resource locations must include designators for both the name
> and the version of the interface.
>
>
>
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