One question about TWKB is if there is any gain compared to just compressing geoJSON. That question is worth some investigation. In php for instance you can compress all data on the fly before sending it to the client. Then the browser will decompress the message and use it as normal. This works really good and fast. So, the question is if it is worth the effort to build a twkb-geometry.
I have three reasons why it is.
- twkb seems to actually be smaller than compressed geoJSON.
- Compressing takes cpu time and power
- If you take the geometry directly from the database, there will be a lot more data to send from the database to the web server.
To demonstrate this I have published a web page http://220.127.116.11/twkb_test.
It is a little bit messy so I will guide you.
I find the dev tools in Google Chrome nice to compare the sizes and timing of the compressed data.
First press the button “Get available Layers”. That will send a websocket message to the server to return what layers we can use. Then you should get a list in the list box “Choose layer”.
- Ok, now choose “Areal Types” (or whatever you want, but just to follow my numbers).
Now you have 4 buttons to choose from. See the upper most row on the picture above. The two buttons to the left will both give you twkb geometries. The websocket button to the left sends twkb “as is” uncompressed, and the second button sends the twkb geometries compressed through php. To the right you get the corresponding buttons for geoJSON.
Behind the twkb-buttons is a query against the database that looks like this:
SELECT ST_AsTWKB(geom,5) FROM prep.n2000_arealtyper;
and the geoJSON query looks like this:
SELECT ST_AsgeoJSON(geom,5) FROM prep.n2000_arealtyper;
So, we are querying the same data in both cases and asks for 5 decimals in both cases. The SRID of the table is 4326, so it is lat lon coordinates. that is why 5 decimals makes sense.
Comparing the web socket and php implementations in timing makes no sense. There is too many unknown in that. But it is interesting to compare the sizes. To see how much it gets compressed.
Then you see that the compressed version of areal types in geoJSON is 1.2 mb. If you test the uncompressed version of TWKB you will see that it is about 720 kb. geoJSON compresses from 3.6 mb so it gets compressed quite a lot to 1.2 mb. TWKB only compresses to 660 kb from 720 kb so geoJSON compresses much better. But anyway, uncompressed twkb is quite a lot smaller than compressed geoJSON.
The differences will vary depending on dataset and number of decimals, but it seems like TWKB gets smaller all over.
The two second arguments I had in the beginning, why twkb is a better choice is about the server load. I have no good way in investigating what takes time on the server, but how long we have to wait before we get any data from the server says something about the work the server needs to do. In the network tab in the chrome dev tools you can hold the mouse over the bar to the left showing the time spent on getting the data. Then you will get the timing divided in “waiting” and “receiving” like in the picture below.
The first thing you can note is that my connection is quite slow. Getting 1.2 mb takes about 7 seconds today. So, reducing size of content to send over internet is still important, even if you as a developer is sitting on a Gb line. What you also can see, and is more important is that you have to wait 1.37 seconds before you start to get anything back, no matter how fancy internet connection you have. If you do the same thing with twkb, by asking for compressed twkb data you will probably get a timing around 0.8 seconds. So there is a difference of at least 0.5 seconds.
in both cases all the data gets sent from the database to php before anything gets sent. So what we compare is how long it takes for twkb vs geoJSON to:
- Be read from disc and twkb vs geoJSON gets constructed in the database
- The data sent from the database to php
- Php builds the page
- It gets compressed
In all those steps 3.6 mb needs to be handled in the geoJSON case compared to 720 kb in the TWKB case. In the TWKB case the size gets already when the twkb geometry gets constructed.
Maybe 0.5 seconds doesn’t sounds that much. But this load is not only affecting me sitting waiting for a map showing up. It affects resources shared by anyone asking for a map from the same server.
You can play around with the different data sets and compare timing and sizes of twkb vs geojson and compressed vs uncompressed.