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<div>Hi Lan,</div>
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<div>The PyNDN2 default constructor for Face makes a Unix socket connection to the local NFD. Therefore, if NFD crashes, the next attempt by the consumer application to send an interest to NFD gets "socket.error: [Errno 32] Broken pipe".</div>
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<div>This might be easy to detect. However, the producer application makes a connection and waits for NFD to send an interest to the application. If NFD crashes, it simply never gets an interest or an error.</div>
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<div>- Jeff T</div>
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<span style="font-weight:bold">From: </span>"Lan Wang (lanwang)" <<a href="mailto:lanwang@memphis.edu">lanwang@memphis.edu</a>><br>
<span style="font-weight:bold">Date: </span>Wednesday, September 3, 2014 2:26 PM<br>
<span style="font-weight:bold">To: </span>Junxiao Shi <<a href="mailto:shijunxiao@email.arizona.edu">shijunxiao@email.arizona.edu</a>><br>
<span style="font-weight:bold">Cc: </span>"Ashlesh Gawande (agawande)" <<a href="mailto:agawande@memphis.edu">agawande@memphis.edu</a>>, Jeff Thompson <<a href="mailto:jefft0@remap.ucla.edu">jefft0@remap.ucla.edu</a>><br>
<span style="font-weight:bold">Subject: </span>Re: [Nfd-dev] Help needed with PyNDN2<br>
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Junxiao: Thank you for the explanation.
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<div>Jeff: what's the behavior of PyNDN2 when nfd crashes?</div>
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<div>Lan<br>
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<div>On Sep 2, 2014, at 7:25 PM, Junxiao Shi <<a href="mailto:shijunxiao@email.arizona.edu">shijunxiao@email.arizona.edu</a>> wrote:</div>
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<div dir="ltr">Hi Ashlesh
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<div>I don't known PyNDN so I cannot answer the specific question.</div>
<div>The answer below is applicable to library design in general.</div>
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<div>A client library can choose what to do when the forwarder has crashed.</div>
<div>In most cases, the library would crash or raise an error if the connection to the server is lost. This is common among popular client libraries such as libmysql.</div>
<div>ndn-cxx chooses to have this behavior, because ndn-cxx developers believe that an application is useless when the forwarder is down.<br>
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<div>This means, when NFD is closing, all ndn-cxx applications connected to NFD will crash as well.</div>
<div>An external monitoring script can detect this, and restart NFD, followed by applications.</div>
<div>This behavior is acceptable on servers for those applications that don't have much internal states.</div>
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<div>However, it's undesirable to let a user-facing application crash when NFD is closing.</div>
<div>The application can catch the error raised by the library, and periodically attempt to connect the Face until a new connection is established, and then register the prefixes.<br>
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<div>Alternatively, the library can choose to detect the problem, and attempt to recover. If a library chooses to do this, it should still inform the application through the event or callback system when forwarder becomes unavailable and available again.</div>
<div>In both cases, the application may need to recover network-visible states after forwarder recovery (eg. ChronoSync entity may need to rejoin the sync-group).</div>
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<div class="gmail_extra">Yours, Junxiao<br>
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<div class="gmail_quote">On Tue, Sep 2, 2014 at 5:08 PM, Ashlesh Gawande (agawande)
<span dir="ltr"><<a href="mailto:agawande@memphis.edu" target="_blank">agawande@memphis.edu</a>></span> wrote:<br>
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Hi<br>
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I was working with PyNDN2 library which uses NFD.<br>
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In PyNDN2 Is there a way to check if NFD has gone down (or should there be a way to that in the library) ?<br>
If so how can it be done?<br>
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(I have added the conversation I had with Jeff T)<br>
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Thanks<br>
Ashlesh<br>
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