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79 lines
2.6 KiB
HTML
79 lines
2.6 KiB
HTML
<!--{
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"Title": "JSON-RPC: a tale of interfaces"
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}-->
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<p>
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Here we present an example where Go's
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<a href="/doc/effective_go.html#interfaces_and_types">interfaces</a> made it
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easy to refactor some existing code to make it more flexible and extensible.
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Originally, the standard library's <a href="/pkg/net/rpc/">RPC package</a> used
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a custom wire format called <a href="/pkg/encoding/gob/">gob</a>. For a
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particular application, we wanted to use <a href="/pkg/encoding/json/">JSON</a>
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as an alternate wire format.
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</p>
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<p>
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We first defined a pair of interfaces to describe the functionality of the
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existing wire format, one for the client, and one for the server (depicted
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below).
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</p>
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<pre>
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type ServerCodec interface {
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ReadRequestHeader(*Request) error
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ReadRequestBody(interface{}) error
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WriteResponse(*Response, interface{}) error
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Close() error
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}
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</pre>
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<p>
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On the server side, we then changed two internal function signatures to accept
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the <code>ServerCodec</code> interface instead of our existing
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<code>gob.Encoder</code>. Here's one of them:
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</p>
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<pre>
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func sendResponse(sending *sync.Mutex, req *Request,
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reply interface{}, enc *gob.Encoder, errmsg string)
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</pre>
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<p>
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became
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</p>
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<pre>
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func sendResponse(sending *sync.Mutex, req *Request,
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reply interface{}, enc ServerCodec, errmsg string)
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</pre>
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<p>
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We then wrote a trivial <code>gobServerCodec</code> wrapper to reproduce the
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original functionality. From there it is simple to build a
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<code>jsonServerCodec</code>.
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</p>
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<p>
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After some similar changes to the client side, this was the full extent of the
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work we needed to do on the RPC package. This whole exercise took about 20
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minutes! After tidying up and testing the new code, the
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<a href="http://code.google.com/p/go/source/diff?spec=svn9daf796ebf1cae97b2fcf760a4ab682f1f063f29&r=9daf796ebf1cae97b2fcf760a4ab682f1f063f29&format=side&path=/src/pkg/rpc/server.go">final changeset</a>
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was submitted.
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</p>
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<p>
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In an inheritance-oriented language like Java or C++, the obvious path would be
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to generalize the RPC class, and create JsonRPC and GobRPC subclasses. However,
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this approach becomes tricky if you want to make a further generalization
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orthogonal to that hierarchy. (For example, if you were to implement an
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alternate RPC standard). In our Go package, we took a route that is both
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conceptually simpler and requires less code be written or changed.
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</p>
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<p>
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A vital quality for any codebase is maintainability. As needs change, it is
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essential to adapt your code easily and cleanly, lest it become unwieldy to work
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with. We believe Go's lightweight, composition-oriented type system provides a
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means of structuring code that scales.
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</p>
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