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Hi @rocuinneagain , thanks for the detailed reply and some informative clarification!
But I still have some confusion…the crux is this sentence you wrote
2. The messages arrive to PubSub and .z.pg evaluates them. This mean upd/insert (pubsub.q line 8 will save the incoming data to quote/trade. PubSub now has some data cached.
I understand that .z.pg is a function under the .z namespace, my questions are:
- Can I say that it is like a callback function which will be called by q when some events (i.e. synchronous request in this case) happen?
- In the caller side (https://github.com/kxcontrib/websocket/blob/ad2f0b268afaee1fc5f4dda2fc2467440c7e2f0c/AppendixB/fh.q#L18-L19), since we called the handler this way: h(`upd;`quote;(n#.z.N;s;getbid'[s];getask'[s]));, does it mean that .z.pg will evaluate the parameters, i.e., `upd, `quote and (n#.z.N;s;getbid'[s];getask'[s]) and .z.pg will interpret these parameters as: “execute the function upd and pass two parameters (i.e., `quote and (n#.z.N;s;getbid'[s];getask'[s])) to it”?
- Regarding this statement: “This code is a basic demo so it may have some holes in it’s logic (like never clearing data in PubSub so eventually memory will run out)”, do you mean that if feedhandler constantly inserts new rows by calling h(`upd;`quote;(n#.z.N;s;getbid'[s];getask'[s]));, tables `quote and `trade will grow larger and larger and the memory will be filled by these two tables?
- Regarding this function (https://github.com/kxcontrib/websocket/blob/ad2f0b268afaee1fc5f4dda2fc2467440c7e2f0c/AppendixB/pubsub.q#L33-L36), due to brevity of kdb/q, I am still decoding what exactly it is doing. But after reading you last reply, my vague understanding is that subscribers registered their handlers in the table `subs. So the pub function is essentially enumerating the `subs table and call all subscribers’ handlers one by one. This series of function calls is what we say to “publish data” to subscribers. Is this understanding basically correct?Thanks!