Re: [openstack-dev] [charms] evolving the ha interface type

[Liam Young]

Hi James,
I like option 2 but I think there is a problem with it. I don't
think the hacluster charm sets any data down the relation with the
principle until it has first received data from the principle. As I
understand it option 2 would change this behaviour so that hacluster
immediately sets an api-version option for the principle to consume.
The only problem is the principle does not know whether to wait for
this api-version information or not. eg when the principle is deciding
whether to json encode its data it cannot differentiate between:

a) An old version of the hacluster charm which does not support
api-version or json
b) A new version of the hacluster charm which has not set the api-version yet.

Thanks
Liam

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Re: [openstack-dev] [charms] evolving the ha interface type

[Billy Olsen]

On 11/28/2017 01:50 PM, Dmitrii Shcherbakov wrote:

Hi James,

The side effect of using JSON is that we will lose type information:

In [0]: json.loads(json.dumps([{1: 2}]))
Out[0]: [{'1': 2}]

In [1]: ast.literal_eval(repr([{1: 2}]))
Out[1]: [{1: 2}]


I'm not sure this is really an issue for the hacluster interface. To my 
knowledge, all of the keys used for resources today are strings defining 
the pacemaker resource options (clones, groups, colocation options etc). 
The hacluster charm simply formats crm commands with the name/value 
pairs anyways [0]. It is true that type information is lost, but I don't 
think its an issue for this interface.


[0] - 
https://github.com/openstack/charm-hacluster/blob/master/hooks/hooks.py#L314




This is a hard requirement in the JSON spec:

https://tools.ietf.org/html/rfc7159#section-4
"An object structure is represented as a pair of curly brackets
surrounding zero or more name/value pairs (or members).  A name is
a string. "

I wonder if we could use some type-aware serialization format but I do
not have any suggestions without trade-offs because we need to keep in
mind that, in general, charms may not be python charms, may run on
hosts with different protocol parsing libraries etc.

If we only cared about python I would suggest using "pickle" but this
is not universal and not human-readable:

https://docs.python.org/3/library/pickle.html#comparison-with-json
"JSON, by default, can only represent a subset of the Python built-in
types, and no custom classes; pickle can represent an extremely large
number of Python types"

JSON has an advantage of being readable as opposed to a binary protocol
so we can explore what's available in relation data by doing relation-
get without additional tools. It is also ubiquitous so we don't have to
worry about spec changes, parsing bugs and other potential problems.

Thanks for bringing this topic up.

Dmitrii Shcherbakov

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