Patrick,
thanks again for your explanations and opinions about dynamic updates to
the graph.
Being aware of the complex scenario that dynamic updates introduce, I'd
still like to think about the great features that it would allow for!
Implementing per-request dynamics to some extent would enable use cases
like traffic-based routing, disaster routing (avoiding locked down
roads), truck routing ... and much more.
Anything of these topics on the OSRM roadmap? ;-)
Of course, the "how" (to implement) is crucial here... Due to my lack of
knowledge about both CH and C++, I cannot offer help unfortunately.
I thought it has to be possible somehow, since GraphHopper offers this
traffic data integration thing [1], which might show the way how to do this?
Best,
Matthias
--
[1] - https://github.com/karussell/graphhopper-traffic-data-integration
On 14.10.2015 13:05, Patrick Niklaus wrote:
certain edges in the contracted graph should have to be ignored
If that set of 'dynamic edges' is known in advance you could use a
technique that does not contract nodes adjacent to that edges. This
would mean for those edges you could update the weights without
re-contraction. On the pre-processing side adding support for this is
quite trivial, essentially it is a variation of partial contraction.
However adding an interface for
updating the graph would be new. The main problem there is that you
either add some sort of "override set" to the query graph, or have a
copy for each graph for each thread.
The first implementation will incur high penalties on query time (you
would need an additional check every time you read the edge weight),
the second approach would have a high memory usage.
Currently we don't plan to implement this. But if anyone likes to give
it a try, I will of course help were I can.
Best,
Patrick
On Wed, Oct 14, 2015 at 12:18 PM, Matthias Loeks <matth...@loeks.net> wrote:
HI Patrick,
many thanks for your extensive answer and your interesting insights into the
possibilities of achieving dynamic routing with CH.
While partial graph contraction may be an option for adding traffic data
e.g. every 15 minutes, I'm afraid that it is still not an option if each
individual request has to deal with e.g. different avoid areas.
Each request would then need a differently contracted/pre-processed graph...
(impossible to pre-process on the fly)
Do you think there is any possibility to add some sort of "dynamic layer" on
top of the contracted graph? Based on the information in this layer, certain
edges in the contracted graph should have to be ignored by the routing
algorithm.
Is such a thing possible and are there any plans to incorporate this (or
similar concepts) into OSRM? Or is this just contrary to the CH approach and
only solveable with a usual (slow) Dijkstra?
Thanks a lot for your help!
Cheers,
Matthias
On 09.10.2015 15:37, Patrick Niklaus wrote:
If you want to ingest dynamic data like traffic information into the
routing, the main objective is to reduce pre-processing times so that
the data will not be stale before you can actually serve requests from
it.
There are several ways you can achieve this:
1. Don't do any pre-processing.
In that case you just use a normal Dijkstra based search.
2. Do pre-processing but don't update it on traffic updates.
For example if you use something ALT-based you can calculate the
heuristic using the average value and still yield good performance.
3. Re-run pre-processing and make it fast enough for your given update
cycle.
The primary knobs you can turn there are:
- reduce the size of your dataset
- reduce the quality of the pre-processing
We have been working on supporting 3 in OSRM with CH. We added a
parameter to now contract the graph completely but only partially.
This as dire consequences for query times however, depending on which
quality factor you pick. If you contract the graph only 95% you will
half your pre-processing time and increase the runtime 100x depending
on your dataset size. Features like alternative searches, distance
tables and similar will not work with this approach since it is much
too slow.
You can try partial contraction with `4.8.1` by using the `-k`
parameter like `-k 0.95` will contract the graph only to 95%.
Supporting real time traffic updates while still supporting
continental sized networks is not exactly trivial, even more so if you
support advanced features like turn restrictions. Consider the fact
that just reading/writing such a graph from/to disk might take longer
than your usual update cycle.
We are working on making it easier to support this for smaller
datasets though (like countries). Of course CH is really not suited
that well for this task, but it enables you to use the same platform
and process until CH can be replaced with alternative approaches.
Best,
Patrick
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