networked priority queue
A priority queue. Clients attach through TCP. Available operations are push,
pop, subscribe, and unsubscribe.
The available abstractions are the queue and the packet. A queue is a set
of packet. A queue is created on demand by pushing to it.
A packet consists of a key and a payload.
Each packet has a time to live (TTL). Once that time has passed the packet is
removed from the queue.
There are two data types; byte sequences and integers. On the wire, integers are encoded as big endian unsigned.
The packet is a sequence of bytes laid out like this;
0 1 2 3 4
| key len | payload len | key ... | payload ...
Key length and payload length units are bytes.
push - push a packet to a queue. There can be more than one packet with the
same key. Packet metadata is a TTL (in milliseconds), and a priority (with
1 being the highest priority, and 0 being no priority).
The push is a byte sequence like this;
0 1 2 3 4 5 6
| 0x70 | queue name len| TTL | prio | packet... | queue name...
There is no return value.
pop - pop a packet from a queue. The packet will be the one with the highest
priority (priority 1 will be selected before priority 2). If there are
more than one such packet, it will be the newest one, (i.e. the last one
inserted). If there are other packets with the same key, they will also be
popped. A packet with priority 0 is not selectable, but will be returned as a
side effect if it has the same key as the selected packet.
The pop is a byte sequence like this;
0 1 2 3
| 0x50 | queue name len| queue name...
Return value is a byte sequence like this;
0 1 2
| number of pkts| pkt... | pkt...
If the queue is empty, the return value will be;
0 1 2
| 0x0 |
subscribe - subscribe to a queue. A packet will be sent as soon as one
becomes available. If there are more than one available packet, the highest
priority one will be selected as per pop. Redundant subscribe are ignored.
Note that a packet with priority 0 is never a candidate for selection; it can
only be delivered as a side effect of having the same key as a selected packet.
0 1 2 3
| 0x73 | queue name len| queue name...
There is no reply.
When subscribing, the client indicates that it is ready to receive a new
packet by sending the single byte 0x41.
unsubscribe - unsubscribe from a queue. Redundant unsubscribe are ignored.
0 1 2 3
| 0x75 | queue name len| queue name...
There is no reply.
$ rebar3 compile