Realtime database¶
kdb+tick is used to capture, store and analyze massive volumes of data in real time.
A standard setup might consist of
- a tickerplant to capture and log incoming data
- a historical database (HDB) to access all the data prior to the current day
- a real-time database (RDB) to store the current day’s data in memory and write it to the HDB at the end of day
As a minimum, it is recommended to have RAM of at least 4× expected data size, so for 5 GB data per day, the RDB machine should have at least 20 GB RAM. In practice, much larger RAM might be used.
Data feeds¶
Data feeds can be any market or other time-series data. A feedhandler converts the data stream into a format suitable for writing to kdb+. These are usually written in a compiled language, such as C, C++, Java or C#.
In the example described here, the data feed is generated at random by a kdb+ process.
Tickerplant¶
The data feed could be written directly to the RDB. More often, it is written to a kdb+ process called a tickerplant, which will:
- write all incoming records to a log file
- push all data to the RDB
- push all or subsets of the data to other processes
Other processes would subscribe to a tickerplant to receive new data, and each would specify what data should be sent: all or a selection.
Example¶
The demo scripts run a simple tickerplant/RDB configuration.
The layout is:
feed
|
tickerplant
/ / | \ \ \
rdb vwap hlcv tq last show
/\ /\ /\ /\ /\
... client applications ...Where
- feed
-
is a demo feedhandler that generates random trades and quotes and sends them to the tickerplant. In practice, this would be replaced by real feedhandlers.
- tickerplant
-
gets data from feed and pushes it to clients that have subscribed. Once the data is written, it is discarded.
- rdb
-
is an RDB (realtime database) than stores all intra-day data received in memory
- vwap, hlcv, tq, last, show
-
are RTE (realtime engines) that have subscribed to the tickerplant. Note that these databases can be queried by a client application.
| database | role |
|---|---|
| vwap | has volume-weighted averages for selected stocks |
| hlcv | has high, low, close, volume for selected stocks |
| tq | has a trade and quote table for selected stocks; each row is a trade joined with the most recent quote |
| last | has the last entries for each stock in the trade and quote tables |
| show | counts the updates, and displays the counts periodically |
Note that all the client processes load the same script file cx.q, with a parameter that selects the corresponding code for the process in that file. Alternatively, each process could load its own script file, but since the definitions tend to be very short, it is convenient to use a single script for all. More RTE examples:
KxSystems/kdb/tick/c.q
KxSystems/kdb/e/c.q
Running the demo¶
Download kdb+tick.
Download the demo scripts.
Copy sym.q (table schema) to <kdb-tick directory>/tick/sym.q
From the downloaded kdb-tick directory create a tickerplant:
q tick.q -p 5010q tick/r.q -p 5011From this directory run each of the following realtime engines in seperate terminal windows:
q cx.q hlcv -p 5014 -t 1000
q cx.q last -p 5015 -t 1000
q cx.q tq -p 5016 -t 1000
q cx.q vwap -p 5017 -t 1000
q cx.q showCreate a feedhandler simulator to start data flowing into the system:
q feed.q localhost:5010 -t 500Process examples¶
Set focus on the RDB, and view the trade table. Note that each time the table is viewed, it is updated with the latest data:
q)trade
sym | time price size stop cond ex
----| ------------------------------------
AAPL| 14:36:02.656 97.37 11 0 A N
AIG | 14:36:02.870 19.92 86 0 P O
AMD | 14:36:03.405 23.21 94 1 W N
...Set focus on the VWAP window, and view the vwap table. Note that price is actually price*size. This can be updated much more efficiently than storing actual prices and sizes.
q)vwap
sym | price size
----| -------------------
AAPL| 6.70234e+07 705352
AMD | 1.998351e+07 699901
DOW | 1.709416e+07 705367
...To get the correct weighted-average price:
q)select sym,price%size,size from vwap
sym price size
--------------------
AAPL 95.02686 706049
AMD 28.54816 700441
DOW 24.23159 705727
...kdb+tick modifications¶
The standard components of kdb+tick support various options. In the basic set-up outlined here, the tickerplant publishes all data immediately, and does not create a log file. Optional parameters of
q tick.q [schema] [destination directory] [-t N] -p 5010can be supplied. If the destination directory is set, then the schema must also be defined. To modify the supplied example to create a tickerplant log file and to publish data in 1-second batches rather than immediately, start the process with:
q tick.q sym ./hdb -t 1000 -p 5010Similarly the real-time database can be started with optional host:port:user:pass of the tickerplant and historic database to reload at end-of-day:
q tick/r.q [tickerplant host:port] [hdb host:port] -p 5011e.g.
q tick/r.q :5010 :5012 -p 5011Process communication¶
The qkdb+ processes communicate by sending a function with arguments using Interprocess communication.
For example, the tickerplant sends new data to the subscribers by calling the upd function with the table name and new data. In the last process, this is:
upd:{[t;x].[t;();,;select by sym from x]}]