Design Question: Client Capabilities Based On Server Version

[Tony Barbieri]

Hello!

I am pretty new to nodejs and I'm writing a client api module that will provide CRUD operations by calling json-rpc methods on an outside server.  The server provides it's currently running version via a json-rpc request and based on that the client api may or may not have additional functionality.

My question is more of a design question than anything else.  I'm not sure the best way to get the server's version and cache it within an asynchronous environment.  I am currently using the http module to make the requests and the q module to use promises.  Below is some pseudo code to illustrate basically what I have.

function Client(url, opts) {
    var info = {
        'url': url,
        'server_version': null
    };
};

 

Client.prototype.server_version = function (cb) {
    if (!this.info.server_version){
        this.request("info").then(function(result) {
            this.info.server_version = result.version;
        }).nodeify(cb);
    } else {
        cb(this.info.server_version);
    };
};

 

Client.prototype.read = function(params, cb) {
    this.server_version(function(err, result) {
        if (result > [3, 3, 0]) {
            ...
        };
        else {
            ...
        }
    });
    return this.request(params).nodeify(cb);
};

 

Client.prototype.request = function(method) {
    defer = Q.defer();
    ...
    return defer.promise;
};

I guess my question is this the "right" way to deal with figuring out the server version to make logical decisions at various points in the code?  I see a few issues with the way I am currently doing it, if the server_version is requested multiple times before it's been cached there will be unnecessary additional calls made to the server.  It would be nice to have a better way to attach callbacks to the original promise that's made when the first request is sent out.  Also using always using a callback and nested functions to check the server version feels a bit inelegant.  I'm not sure if this is the way it needs to be done.  Is this a situation where I would want to use yield?  If so do I need to run the unstable 11.* version of Node to take advantage of that?

I apologize in advance if this question is vague.  I am trying to understand how to do something asynchronous that I would normally do synchronously, ie: halting code flow the first time a call to check the server_version is made and being able to reference "this.info.server_version" afterwards versus always having to use a callback to do the check.

Any and all advice is much appreciated.

Best,

Tony

[Daniel Rinehart]

Have your module expose a create method that makes the initial request to get the version and have that return an object with all of the logic tied to the specific server version to the callback. That way you can avoid needing to check the version in all the subsequent calls.

-- Daniel R. <dan...@neophi.com> [http://danielr.neophi.com/]

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[Tony Barbieri]

Thanks for the reply Daniel.

Would the create method accept a callback?  Could I and should I perform the initial http request to find the server version synchronously?

Thanks again,

Tony

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--
-tony

[Daniel Rinehart]

I would have the create method accept a callback since doing synchronous HTTP with node is tricky and discouraged. While you are asynchronously creating this object (which may indicate a different method name should be used) you are ultimately returning an object that exposes an API appropriate for the server version without then having to do a bunch of conditional code in each exposed method.

-- Daniel R. <dan...@neophi.com> [http://danielr.neophi.com/]

To view this discussion on the web visit https://groups.google.com/d/msgid/nodejs/CAJhmvsThAuvjemEUuub_BHBJ5xakr6mX1hHJ50VGVwD-cBmvYg%40mail.gmail.com.