# Publish and subscribe

KxSystems/kdb-tick contains functionality to allow processes to publish data and subscribe to it. It is worth highlighting how the publish-and-subscribe code can be used by any process on a standalone basis. The pubsub functionality is supplied in the u.q script of kdb+tick.

To give the ability to publish data to any process, a few things need to be done:

• load u.q
• declare the tables to be published in the top level namespace. Each table must contain a column called sym, which acts as the single key field to which subscribers subscribe
• initialise by calling .u.init[]
• publish data by calling .u.pub[table name; table data]

The list of tables that can be published and the processes currently subscribed are held in .u.w. When a client process closes a connection, it is removed from .u.w.

Subscriber processes must open a connection to the publisher and call .u.sub[tablename;list_of_symbols_to_subscribe_to].

.u.sub can be called synchronously or asynchronously. If .u.sub is called synchronously, the table schema is returned to the client. If the table being subscribed to is a keyed table, then the current value for each subscribed sym is returned, assuming it is stored. Otherwise, an empty schema definition is returned. Specifying  for either parameter of .u.sub means all – all tables, all syms, or all tables and all syms.

If a subscriber calls .u.sub again, the current subscription will be overwritten either for all tables (if a wildcard is used) or the specified table. To add to a subscription (e.g. add more syms to a current subscription) the subscriber can call .u.add.

The example scripts below can be downloaded from GitHub. Each script should be run from the OS command prompt e.g.

$q publisher.q$ q subscriber.q


## Publisher

The code below will generate some random data and publish it periodically on a timer.

\d .testdata

// set the port
@[system;"p 6812";{-2"Failed to set port to 6812: ",x,
". Please ensure no other processes are running on that port",
" or change the port in both the publisher and subscriber scripts.";
exit 1}]

// create some test data to be published
// this could also be read from a csv file (for example)
griddata:([]sym:2000?100?3; capacity:2000?100f; flowrate:2000?3000i)

// utility functions to get the next set of data to publish
// get the next chunk of data, return to start once data set is exhausted
counts:.testdata.meterdata.testdata.griddata!0 0
getdata:{[table;n]
res:time xcols update time:.z.p from (counts[table];n) sublist value table;
counts[table]+:n;
if[count[value table]<=counts[table]; counts[table]:0];
res}
getmeter:getdata[.testdata.meterdata]
getgrid:getdata[.testdata.griddata]

\d .

// the tables to be published - all must be in the top level namespace
// tables to be published require a sym column, which can be of any type
// apart from that, they can be anything you like
meter:([]time:timestamp$(); sym:long$(); reading:int$()) grid:([]time:timestamp$(); sym:symbol$(); capacity:float$(); flowrate:int$()) // load in u.q from tick upath:"tick/u.q" @[system;"l ",upath;{-2"Failed to load u.q from ",x," : ",y, ". Please make sure u.q is accessible.", " kdb+tick can be downloaded from https://github.com/KxSystems/kdb-tick"; exit 2}[upath]] // initialise pubsub // all tables in the top level namespace (.) become publish-able // tables that can be published can be seen in .u.w .u.init[] // functions to publish data // .u.pub takes the table name and table data // there is no checking to ensure that the table being published matches // the table schema defined at the top level // that is left up to the programmer! publishmeter:{.u.pub[meter; .testdata.getmeter[x]]} publishgrid:{.u.pub[grid; .testdata.getgrid[x]]} // create timer function to randomly publish // between 1 and 10 meter records, and between 1 and 5 grid records .z.ts:{publishmeter[1+rand 10]; publishgrid[1+rand 5]} /- fire timer every 1 second \t 1000  ## Subscriber // define upd function // this is the function invoked when the publisher pushes data to it upd:{[tabname;tabdata] show tabname; show tabdata} // open a handle to the publisher h:@[hopen;::6812;{-2"Failed to open connection to publisher on port 6812: ", x,". Please ensure publisher is running"; exit 1}] // subscribe to the required data // .u.sub[tablename; list of instruments] //  is wildcard for all h(.u.sub;;) \ Could also do (for example) Subscribe to 10 syms of meter data: h(.u.sub;meter;long$til 10)

h(.u.add;meter;20 21 22)

The subscriber will receive data from the publisher and output it to the screen. You can modify the subscription request and the upd` function of the subscriber as required. You can run multiple subscribers at once.