Support standalone Redis (non-cluster) about ered HOT 8 OPEN

@zuiderkwast I suppose the client always knows whether it's connecting to a standalone or a cluster. Your suggestion sounds great to me, in any case!

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@zuiderkwast is it possible to connect to a standalone node, with PR #41 ?

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Even without #41, it's possible to connect to a single node using the ered_client module. #41 does make things a little more clear though with functions like ered:connect_client for connecting to a single instance.

It's not possible to use a standalone instance as if it were a cluster though, with the same functions, but actually I don't think we want to do this automatically. Cluster and standalone deployments are very different. Standalone mode allows SELECT and cross-slot commands, for example, which are forbidden in cluster mode.

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@zuiderkwast Thanks a lot for your useful information! Currently we're using eredis in VerneMQ so we cannot connect to a Redis cluster yet. And ered's stated goal is to replace both previous libraries, isn't it. In any case, I'll try to do some testing based on your suggestions.

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Yes, ered implements the functionality for single instance and cluster, but the API has a preference for cluster. This was done before I was the maintainer of this project. Now, I've been trying to make the API look nice for single instance too.

An idea: We use a tuple like {cluster, Pid} or {standalone, Pid} as the ered instance reference, returned by ered:connect functions. Then we can use the same functions like ered:command/N for both of them and it can dispatch to the right module based on the tuple. That's future work, if you like the idea. (ssl:connect and gen_tcp:connect return tuples like this, so it's not that strange.)

I've been busy with other work though.

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I've now written my plugin using ered:start_link/2 as main entry point. As expected, this only works for a cluster, not a standalone (standalone gets the unmapped_slots error). A question, though: with the idea proposed above, will ered be able to figure out by itself whether it connects to a standalone or a cluster?

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A question, though: with the idea proposed above, will ered be able to figure out by itself whether it connects to a standalone or a cluster?

It would be possible to auto-detect, but I think it's better that users connect explicitly to cluster or standalone. I'm thinking ered:connect(Host, Port, Opts) for standalone and ered:connect_cluster(AddressesAndPorts, Opts) for cluster. After that, ered:command(EredRef, Command) and similar can be used for both kinds of instances.

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Sounds great. That interface is perfect at least for me, as the client (the plugin) always knows whether it connects to a cluster or not.
Let me know if I can test (or do anything helpful). No worry & no hurry for anyone :)
I think it'll be a cool library to use and like the unified approach in it.

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