[OSRM-talk] State of the Art - Dynamic Routing

Matthias Loeks matthias at loeks.net
Thu Oct 15 15:38:15 UTC 2015


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?


[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 <matthias at 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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