Cast issue on int64 after variable interpolation in docker-compose.yml

[tiry]

I am using compose variable interpolation because I would like to make part of the compose template configurable.

This works for some parameters, however, there seems to be an issue with parameters that expect an integer value (tested against 1.5.2 and 1.6 dev).

Typically, if I want to change the cpu-shares parameter I end up with an error :

ERROR: json: cannot unmarshal string into Go value of type int64

Looking at what happens in interpolation.py and checking the content of the interpolation result:

• when the cpu-shares is "hard coded" the resulting dict is

  { ... 'cpu_shares': 1024 ... }

• when the cpu-shares is set to use a variable using cpu_shares: ${CPU_SHARES} the resulting dict is

  { ... 'cpu_shares': '1024' ... }

This looks to me like the root cause of the problem.

I am not sure what the best approach would be:

• should I try to add a tentative cast after the Template(string).substitute ?
• should I add some post processing in config.py or in validate_against_fields_schema ?

[dnephin]

I don't know of a good way to handle problem at this time, but I think it's something that does need to be fixed. We have the same issue with boolean fields.

[tiry]

Just trying to discover the code and see how this could be fixed.

Using Json Schema info

In service_schema_v1.json the cpu_shares is defined as

"cpu_shares": {"type": ["number", "string"]},

I am not sure why "string" is accepted, based on what I can see in the docker run doc it should not be the case (but I may very well have missed something).

I am asking because, if we had a single type in the schema, it could probably be used to do some automatic cast after the interpolation :

• get the type of the field from the schema
• do the cast from String as needed

The conversion could also be done as an error handler for the jsonschema validator, but I like the idea that the "automatic cast" is only applied for "interpolated variables".

Add conversion hint in the source template

An other option would be to add some hints in the templatised config file (ex cpu_shares: int:${CPU_SHARES}), but this does not seems like a clean approach (+ it is redundant info).