Add reading & writing of flags

[aspett]

Adds reading and writing of flags to achieve compatibility with Dalli (ruby client). Fixes #7. Takes inspiration from #8 and expands on comments.

• Allows passing a flags option to Memcache.xxx functions that take options.
• Supports :serialised, :compressed flags (0x1, 0x2 respectively)
• Adds a encode_flags callback for Coders with a default passthrough impl.
• Adds a default :serialised flag to the JSON Coder.

There are some slightly breaking changes here, but the core API (Memcache module) is not affected.

1. Accessing Memcache.Connection.execute/4 and Memcache.Connection.execute_quiet/2 directly will give results back in a different format, potentially; {:ok, value :: any, flags :: keyword} instead of {:ok, value :: any}. I'm pretty sure it's impossible to add flag support without this change, unfortunately.
2. Coders should now use Memcache.Coder rather than @behaviour use Memcache.Coder and optionally implement encode_flags/2.

[aspett]

👀 Tests passed, but builds failed?

[ananthakumaran]

$ ! mix help format > /dev/null || mix format --check-formatted
** (Mix) mix format failed due to --check-formatted.
The following files were not formatted:
  * test/memcache/connection_test.exs
  * lib/memcache/receiver.ex
  * lib/memcache/connection.ex
  * lib/memcache.ex
The command "! mix help format > /dev/null || mix format --check-formatted" exited with 1.

[aspett]

Thanks @ananthakumaran. I didn't notice it was running that. Have mix format'd it with elixir 1.7.

[aspett]

Bump @ananthakumaran

[ananthakumaran]

sorry for the delay. I am kind of reluctant about merging patches that add the ability to read/write flags. There are two aspects, 1) I haven't given much thought to flags during the initial implementation which makes it hard to retrofit. 2) There aren't many use cases which require the flags. I don't want a situation where you finish the PR, but I decide not to merge because of the above reasons.

With that said, here are my thoughts.

I don't like to attach meaning to flag (aka bit 1 is serialized, bit 2 is compressed, etc). As the flag page indicates this is different across language bindings. What I would instead like is, just expose it via the coder.

encode(any, options :: Keyword.t()) :: iodata
encode(any, options :: Keyword.t()) :: {flag :: binary, iodata}

@callback decode(iodata, options :: Keyword.t()) :: any
@callback decode(iodata, flag :: binary, options :: Keyword.t()) :: any

basically, encode can return flag value and decode can accept flag,
we probably have to make some changes to support the old coder
interface. With this, a custom coder could be used to support dalli without changes anywhere else in the application.

I am torn about the changes in Memcache.Connection. I don't want to break the API, but I don't have any better suggestion either. If I had used a map/struct for the return value, it would have made things a lot easier.

[wizardone]

@ananthakumaran It seems that you are not eager to merge this PR as it is. Are you willing to reconsider it or would you like to see a different approach being taken here so we can eventually have the flags functionality in place?

[ananthakumaran]

Could you explain your use case? I am not very eager at this point, but that would change if more people want this

…

On Tue, Jun 4, 2019, 7:23 AM Stefan Slaveykov ***@***.***> wrote: @ananthakumaran <https://github.com/ananthakumaran> It seems that you are not eager to merge this PR as it is. Are you willing to reconsider it or would you like to see a different approach being taken here so we can eventually have the flags functionality in place? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#18?email_source=notifications&email_token=AABEN5WEBHK4ARMLAT4NUJTPYXDH5A5CNFSM4G37KPKKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGODW3FKDI#issuecomment-498488589>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AABEN5XYIEDJHDGZQ6ACFILPYXDH5ANCNFSM4G37KPKA> .

[aspett]

Closing due to no movement.