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HI Patrick,<br>
<br>
many thanks for your extensive answer and your interesting insights
into the possibilities of achieving dynamic routing with CH.<br>
<br>
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. differentÂ
avoid areas.<br>
Each request would then need a differently contracted/pre-processed
graph... (impossible to pre-process on the fly)<br>
<br>
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.<br>
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?<br>
<br>
Thanks a lot for your help!<br>
<br>
Cheers,<br>
Matthias<br>
<br>
<div class="moz-cite-prefix">On 09.10.2015 15:37, Patrick Niklaus
wrote:<br>
</div>
<blockquote
cite="mid:CAANKLwjc=rP06cHvC1SstGiqKqa4v6bqvc7kd9HwpnJgD6e1jw@mail.gmail.com"
type="cite">
<pre wrap=""><meta http-equiv="content-type" content="text/html; charset=utf-8">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
</pre>
</blockquote>
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