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    <p>The APN protocol appears to be quite awkward because it does not acknowledge successful receipt of a notification. Instead it just tells you which notifications it has successfully received when it encounters an error. The protocol is working on the assumption that you will generally send well formed notifications.</p> <p>I would suggest that you need some sort of expiring cache (a LinkedHashMap might work here) and you need to use the opaque identifier field in the notification as a globally unique, ordered value. A sequence number will work (but you'll need to persist if your client can be restarted).</p> <p>Every time you generate an APN</p> <ul> <li>set its identifier to the next sequence number</li> <li>send it</li> <li>place it in the LinkedHashMap with a string key of sequence number concatenated with the current time (eg String key = sequenceNumber + "-" + System.currentTimeMillis() )</li> </ul> <p>If you receive an error you need to reopen the connection and resend all the APNs in the map with a sequence number higher than the identifier reported in the error. This is relatively easy. Just iterate through the map removing any APN with a sequence number lower than that reported. Then resend the remain APNs in order, replacing them in the map with the current time (ie you remove an APN when you resend it, then re-insert into the map with the new current time).</p> <p>You'll need to periodically purge the map of old entries. You need to determine what is a reasonable length of time based on how long it takes the APN service to return an error if you send a malformed APN. I suspect it'll be a matter of seconds (if not much quicker). If, for example, you're sending 10 APNs / second, and you know that the APN server will definitely respond within 30 seconds, a 30 second expiry time, purging every second, might be appropriate. Just iterate along the map removing any elements which has a time section of it's key that is less than System.currentTimeMillis() - 30000 (for 30 second expiry time). You'll need to synchronize threads appropriately.</p> <p>I would catch any IOExceptions caused by writing and place the APN you were attempting to write in the map and resend.</p> <p>What you cannot cope with is a genuine network error whereby you do not know if the APN service received the notification (or a bunch of notifications). You'll have to make a decision based on what your service is as to whether you resend the affected APNs immediately, or after some time period, or not at all. If you send after a time period you'll want to give them new sequence numbers at the point you send them. This will allow you to send new APNs in the meantime.</p>
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