Question

I have started to implement a blog pinging service in Java, using the Apache RPC Client libraries. However, I’m a little confused and I can’t seem to find a definitive specification for what a blog ping response should look like to check that it is successful.

I’ve looked at this, which appears to be an (official?) spec for a pingback.
http://www.hixie.ch/specs/pingback/pingback-1.0

However, this mentions that faultcodes will be returned, e.g.

http://xmlrpc-epi.sourceforge.net/specs/rfc.fault_codes.php

A number of RPC Servers such as Google Blog search, seem to return an ‘flerror’ and ‘message’ element in their XML response, which seems more akin to this:

http://xmlrpc.scripting.com/weblogsCom.html

Whats going on here? I realise pingback was something that the web kind of hacked together, and it became a standard – but I’m confused as to what to code against, or indeed trust in a response. Can I trust the below? and will it work for all blog ping servers?

public  boolean ping( String urlToPing, String title, String url, String urlChanges, String urlRSS ) throws MalformedURLException, XmlRpcException
{
    XmlRpcClientConfigImpl config = new XmlRpcClientConfigImpl();
    config.setServerURL( new URL( urlToPing ) );

    XmlRpcClient client = new XmlRpcClient();
    client.setConfig( config );

    Object[] params = new Object[] { title, url, urlChanges, urlRSS };
    HashMap result = ( HashMap )client.execute( "weblogUpdates.extendedPing", params );

    try
    {
        errors.put( url, Boolean.parseBoolean( result.get( "flerror" ).toString() ) );
    }
    catch( Exception e )
    {
        log.error( "RPC Problem Parsing response to Boolean trying: " + result.get( "flerror" ) );
    }

    return Boolean.parseBoolean( result.get( "flerror").toString()) ;
}

Answer 1

Can I trust the below? and will it work for all blog ping servers?

The curt answer is no. Different server implementations will have bugs or misinterpret the spec, so you can’t write code that will work for all blog ping servers. The best you can do is to be liberal in what you accept, and try to deal with non standard/buggy servers as best you can.

The pingback spec says,

If the pingback request is successful, then the return value MUST be a
single string, containing as much information as the server deems
useful. This string is only expected to be used for debugging
purposes.

If the result is unsuccessful, then the server MUST respond with an
RPC fault value. The fault code should be either one of the codes
listed above, or the generic fault code zero if the server cannot
determine the correct fault code.

So a client expecting the server to comply to the spec would do something like,

try {
     client.execute( "weblogUpdates.extendedPing", params );
} catch(XmlRpcException e) {
    //check the code of the rpc exception as shown below,
    //log the error, or perhaps rethrow it?
    return false;
} 

If the server is following the pingback spec, it should return one of the following fault codes,

0
A generic fault code. Servers MAY use this error code instead of any of the others if they do not have a way of determining the correct fault code.
0×0010 (16)
The source URI does not exist.
0×0011 (17)
The source URI does not contain a link to the target URI, and so cannot be used as a source.
0×0020 (32)
The specified target URI does not exist. This MUST only be used when the target definitely does not exist, rather than when the target may exist but is not recognised. See the next error.
0×0021 (33)
The specified target URI cannot be used as a target. It either doesn't exist, or it is not a pingback-enabled resource. For example, on a blog, typically only permalinks are pingback-enabled, and trying to pingback the home page, or a set of posts, will fail with this error.
0×0030 (48)
The pingback has already been registered.
0×0031 (49)
Access denied.
0×0032 (50)

As you mentioned, several pingback servers return an error code, so you have to check for that as well with code like,

try {
    Object rpcRVal = client.execute( "weblogUpdates.extendedPing", params );
    if(rpcRVal instanceof Map) {
        Object flError = ((Map) rpcRVal ).get("flerror");
        if(flError != null && flError instanceof Boolean) {
            return ((Boolean) flError).booleanValue());        
        }
    }
    return true;
} catch(XmlRpcException e) ...