One of the third-party clients that we have tested with Service Bus is Proton-C, a client that is part of the Apache Qpid project. Proton-C has gone through five versions since we started working with it, and each one has some differences from previous versions which are important for users to know about. The samples for using Proton-C to access Service Bus queues and topics are posted on GitHub and incorporate this information in #ifdefs and comments, but here is a summary. These remarks apply specifically to the “Messenger” API provided by Proton-C.
In general, we recommend using the latest version of Proton-C, which is 0.8 as of this writing. Each new version incorporates important bug fixes and valuable new features.
We advise setting the outgoing window via pn_messenger_set_outgoing_window() in all versions.
PeekLock mode versus ReceiveAndDelete mode
Service Bus supports retrieving messages in two modes, which we call PeekLock and ReceiveAndDelete. In PeekLock mode, the broker retains the message until the client explicitly accepts it. If the client explicitly rejects the message, or does not specify a disposition before the peek lock times out, then the message becomes available again to consumers. This mode provides at-least-once reliability, at the expense of more network traffic and slower throughput. In ReceiveAndDelete mode, the broker disposes of the message immediately after sending it to the client. This mode can provide higher throughput if an application can tolerate potential message loss on a crash.
In Proton-C versions 0.5 and later, setting the incoming window via pn_messenger_set_incoming_window() is required in order to interoperate with Service Bus in PeekLock mode. Not setting the incoming window leaves an application in what one might call Browse mode: messages are peeked and locked, but the client has no way of completing them, with the result that the locks expire and the same messages are delivered again and again. With Proton-C versions 0.5, 0.6, and 0.7, setting the incoming window is sufficient to enter PeekLock mode, and there is no way to use ReceiveAndDelete mode. In version 0.8, setting the incoming window leaves the application in ReceiveAndDelete mode, and additional API calls are necessary to enter PeekLock mode. See the version-specific notes for 0.8 for more details.
Proton-C version 0.4 has the concept of “accept modes” instead; see the version-specific notes for more details.
Service Bus entity URLs
The general form of the URL that Proton-C uses to address a Service Bus entity is amqps://user:email@example.com/entitypath. This form remains the same across all versions with one important exception. The key is a Base64-encoded version of an ACS or SAS key, and as such can contain certain characters which are reserved in URLs, so one would normally URL-encode it. Despite that, for versions 0.6 and earlier, the key MUST NOT be URL-encoded, even if the key contains the ‘/’ character. For versions 0.7 and later, Proton-C’s URL parser changed and the key MUST be URL-encoded.
This is the oldest version that we have tested, and the most different from the other versions.
pn_messenger_send() always sends all messages which have been queued for sending by pn_messenger_put(). Also, pn_messenger_send() does not support blocking sends. The only way to get the final status of a send is to keep polling the tracker using pn_messenger_status() until it returns a final status (such as PN_STATUS_ACCEPTED or PN_STATUS_REJECTED) or the application has waited long enough to give up and assume the send failed.
pn_messenger_error() returns an error string directly.
This version has the concept of “accept modes”. To enter PeekLock mode, call pn_messenger_set_accept_mode() to set the mode PN_ACCEPT_MODE_MANUAL.
Significant API changes:
pn_messenger_send() now takes a number of messages to send, and supports blocking sends. When blocking, the call will not return until it has a final state for the message(s) sent.
pn_messenger_error() returns type pn_error_t which contains the error string and other info.
Introduces the concept of the incoming window and the API pn_messenger_set_incoming_window().
Added new message states which allow for better error reporting: PN_STATUS_RELEASED, PN_STATUS_ABORTED, and PN_STATUS_SETTLED, which are all final states. PN_STATUS_ABORTED is of particular interest because it indicates a send attempt which failed before reaching the network level (for example, the client couldn’t even connect to the broker).
Starting with this version, the key in the URL must be URL-encoded.
This version introduces the new APIs pn_messenger_set_snd_settle_mode() and pn_messenger_set_rcv_settle_mode(). In order to enter PeekLock mode, it is necessary to set the modes to PN_SND_UNSETTLED and PN_RCV_SECOND, respectively, prior to setting the size of the incoming window.