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../../protocols/rpc.rst:65
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XDR also supports the definition of unions, structures, ... Additional details are provided in :rfc:`1832`.
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../../protocols/rpc.rst:67
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A second popular method to encode data is the JavaScript Object Notation (JSON). This syntax was initially defined to allow applications written in JavaScript to exchange data, but it has now wider usages. JSON :rfc:`4627` is a text-based representation. The simplest data type is the integer. It is represented as a sequence of digits in ASCII. Strings can also be encoding by using JSON. A JSON string always starts and ends with a quote character (`"`) as in the C language. As in the C language, some characters (like `"` or `\\`) must be escaped if they appear in a string. :rfc:`4627` describes this in details. Booleans are also supported by using the strings `false` and `true`. Like XDR, JSON supports more complex data types. A structure or object is defined as a comma separated list of elements enclosed in curly brackets. :rfc:`4627` provides the following example as an illustration.
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../../protocols/rpc.rst:86
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This object has one field named `Image`. It has five attributes. The first one, `Width`, is an integer set to 800. The third one is a string. The fourth attribute, `Thumbnail` is also an object composed of three different attributes, one string and two integers. JSON can also be used to encode arrays or lists. In this case, square brackets are used as delimiters. The snippet below shows an array which contains the prime integers that are smaller than ten.
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../../protocols/rpc.rst:94
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Compared with XDR, the main advantage of JSON is that the transfer syntax is easily readable by a human. However, this comes at the expense of a less compact encoding. Some data encoded in JSON will usually take more space than when it is encoded with XDR. More compact encoding schemes have been defined, see e.g. [BH2013]_ and the references therein.
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../../protocols/rpc.rst:98
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Reaching the callee
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../../protocols/rpc.rst:100
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The second sub-problem is how to reach the callee. A simple solution to this problem is to make sure that the callee listens on a specific port on the remote machine and then exchange information with this server process. This is the solution chosen for JSON-RPC [JSON-RPC2]_. JSON-RPC can be used over the connectionless or the connection-oriented transport service. A JSON-RPC request contains the following fields:
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../../protocols/rpc.rst:102
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`jsonrpc`: a string indicating the version of the protocol used. This is important to allow the protocol to evolve in the future.
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../../protocols/rpc.rst:103
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`method`: a string that contains the name of the procedure which is invoked
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../../protocols/rpc.rst:104
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`params`: a structure that contains the values of the parameters that are passed to the method
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../../protocols/rpc.rst:105
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`id`: an identifier chosen by the caller
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../../protocols/rpc.rst:107
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The JSON-RPC is encoded as a JSON object. For example, the example below shows an invocation of a method called `sum` with `1` and `3` as parameters.
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../../protocols/rpc.rst:113
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Upon reception of this JSON structure, the callee parses the object, locates the corresponding method and passes the parameters. This method returns a response which is also encoded as a JSON structure. This response contains the following fields:
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../../protocols/rpc.rst:115
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`jsonrpc`: a string indicating the version of the protocol used to encode the response
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../../protocols/rpc.rst:116
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`id`: the same identifier as the identifier chosen by the caller
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../../protocols/rpc.rst:117
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`result`: if the request succeeded, this member contains the result of the request (in our example, value `4`).
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../../protocols/rpc.rst:118
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`error`: if the method called does not exist or its execution causes an error, the `result` element will be replaced by an `error` element which contains the following members :
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../../protocols/rpc.rst:120
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`code`: a number that indicates the type of error. Several error codes are defined in [JSON-RPC2]_. For example, `-32700` indicates an error in parsing the request, `-32602` indicates invalid parameters and `-32601` indicates that the method could not be found on the server. Other error codes are listed in [JSON-RPC2]_.
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../../protocols/rpc.rst:121
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`message`: a string (limited to one sentence) that provides a short description of the error.
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../../protocols/rpc.rst:122
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`data`: an optional field that provides additional information about the error.
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../../protocols/rpc.rst:124
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Coming back to our example with the call for the `sum` procedure, it would return the following JSON structure.
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