Error response on data query

[sensor-freak]

I have an application that performs a data query against a FROST server. The application is the VirtualCityPublisher (v5.3.27) and is using the SensorThings Plugin delivered with that version. The FROST server is a self-hosted instance.

The plugin is supposed to display a diagram of sensor values. When the plugin is activated, the browsers developer console displays the following error messages:

https://sdm-frost.cloud.iai.kit.edu/FROST-Server/v1.1/Datastreams?%20%20%20%20$filter=ObservedProperty/name%20eq%20%27Air%20temperature%27%20and%20Thing/id%20eq%2014&%20%20%20%20$orderby=Observations/phenomenonTime%20desc&%20%20%20%20$top=1&%20%20%20%20$expand=Observations($top=1;$orderby=phenomenonTime%20desc)

Error: Failed fetching url https://sdm-frost.cloud.iai.kit.edu/FROST-Server/v1.1/Datastreams?
    $filter=ObservedProperty/name eq 'Air temperature' and Thing/id eq 14&
    $orderby=Observations/phenomenonTime desc&
    $top=1&
    $expand=Observations($top=1;$orderby=phenomenonTime desc) with status: 400

Note: the second block seems to be a decoded version of the previous block. It has been copied from the browser console.

I've tried to determine the cause for this error, but I'm not sure whether it is due to an invalid request, or due to a failure in the FROST server itself. According to some research in the web I believe that the problematic statement comes from the two arguments in the expand parameter - which might be valid syntax, or not.

My problem is now: whom should I blame?

• Is it an invalid request? Then I should go to VirtualCitySystems and ask for correction.
• Is it a failure of the FROST server? Then I might get help from here...

Can anybody shed some light on this?

[hylkevds]

That query works on our demo server, with a different Thing ID and ObservedProperty name of course:

https://airquality-frost.k8s.ilt-dmz.iosb.fraunhofer.de/v1.1/Datastreams?%20%20%20%20$filter=ObservedProperty/name%20eq%20%27PM10%27%20and%20Thing/id%20eq%201&%20%20%20%20$orderby=Observations/phenomenonTime%20desc&%20%20%20%20$top=1&%20%20%20%20$expand=Observations($top=1;$orderby=phenomenonTime%20desc)

What is the output of FROST when you execute that query?
What version of FROST are you using, and is there anything in the FROST logs?

That said, the query is a bit odd. Sorting Datastreams by the phenomenonTime of their Observations is ambiguous. It's more efficient to sort Datastreams based on the phenomenonTime/end of the Datastream itself, though that may not be supported by all implementations.

[sensor-freak]

I'm running the FROST server from dockerhub (fraunhoferiosb/frost-server:2.7.2 with postgis/postgis:14-3.5-alpine) in a Kubernetes cluster.

The logs in the FROST server when opening the SensorThings object are:

09:01:32.061 DEBUG         d.f.i.i.f.parser.path.PathParser - Parsing: /Things
09:01:32.061 DEBUG         d.f.i.i.f.parser.path.PathParser - Parsing: /Things
09:01:32.061 DEBUG       d.f.i.i.f.parser.query.QueryParser - Parsing: $top=1000&$filter=Datastreams/ObservedProperty/name eq 'Cloud Cover oder Cloud Cover Factor oder Cloudiness oder Cloudage' and st_within(Locations/location, geography'POLYGON ((-180 -90, 180 -90, 180 90, -180 90, -180 -90))')&$expand=Locations($top=1)
09:01:32.061 DEBUG         d.f.i.i.f.parser.path.PathParser - Parsing: /Things
09:01:32.062 DEBUG         d.f.i.i.f.parser.path.PathParser - Parsing: /Things
09:01:32.062 DEBUG       d.f.i.i.f.parser.query.QueryParser - Parsing: $top=1000&$filter=Datastreams/ObservedProperty/name eq 'Air temperature' and st_within(Locations/location, geography'POLYGON ((-180 -90, 180 -90, 180 90, -180 90, -180 -90))')&$expand=Locations($top=1)
09:01:32.062 DEBUG       d.f.i.i.f.parser.query.QueryParser - Parsing: $top=1000&$filter=Datastreams/ObservedProperty/name eq 'Relative humidity' and st_within(Locations/location, geography'POLYGON ((-180 -90, 180 -90, 180 90, -180 90, -180 -90))')&$expand=Locations($top=1)
09:01:32.062 DEBUG       d.f.i.i.f.parser.query.QueryParser - Parsing: $top=1000&$filter=Datastreams/ObservedProperty/name eq 'Global radiation, in' and st_within(Locations/location, geography'POLYGON ((-180 -90, 180 -90, 180 90, -180 90, -180 -90))')&$expand=Locations($top=1)
09:01:32.062 DEBUG  d.f.iosb.ilt.frostserver.util.WktParser - Parsing: POLYGON ((-180 -90, 180 -90, 180 90, -180 90, -180 -90))
09:01:32.062 DEBUG  d.f.iosb.ilt.frostserver.util.WktParser - Parsing: POLYGON ((-180 -90, 180 -90, 180 90, -180 90, -180 -90))
09:01:32.063 DEBUG  d.f.iosb.ilt.frostserver.util.WktParser - Parsing: POLYGON ((-180 -90, 180 -90, 180 90, -180 90, -180 -90))
09:01:32.063 DEBUG  d.f.iosb.ilt.frostserver.util.WktParser - Parsing: POLYGON ((-180 -90, 180 -90, 180 90, -180 90, -180 -90))
09:01:32.064 DEBUG         org.postgresql.jdbc.PgConnection -   setAutoCommit = false
09:01:32.064 DEBUG         org.postgresql.jdbc.PgConnection -   setAutoCommit = false
09:01:32.064 DEBUG         org.postgresql.jdbc.PgConnection -   setAutoCommit = false
09:01:32.064 DEBUG         org.postgresql.jdbc.PgConnection -   setAutoCommit = false
09:01:32.077 DEBUG         org.postgresql.jdbc.PgConnection - Rollback requested but no transaction in progress
09:01:32.077 DEBUG         org.postgresql.jdbc.PgConnection -   setAutoCommit = true
09:01:32.077 DEBUG         org.postgresql.jdbc.PgConnection - Rollback requested but no transaction in progress
09:01:32.077 DEBUG         org.postgresql.jdbc.PgConnection -   setAutoCommit = true
09:01:32.079 DEBUG         org.postgresql.jdbc.PgConnection - Rollback requested but no transaction in progress
09:01:32.079 DEBUG         org.postgresql.jdbc.PgConnection - Rollback requested but no transaction in progress
09:01:32.079 DEBUG         org.postgresql.jdbc.PgConnection -   setAutoCommit = true
09:01:32.079 DEBUG         org.postgresql.jdbc.PgConnection -   setAutoCommit = true
09:01:39.432 DEBUG         d.f.i.i.f.parser.path.PathParser - Parsing: /Datastreams
09:01:39.432 DEBUG       d.f.i.i.f.parser.query.QueryParser - Parsing:     $filter=ObservedProperty/name eq 'Air temperature' and Thing/id eq 14&    $orderby=Observations/phenomenonTime desc&    $top=1&    $expand=Observations($top=1&$orderby=phenomenonTime desc)
09:01:39.433 ERROR       d.f.i.i.f.parser.query.QueryParser - Exception parsing:     $filter=ObservedProperty/name eq 'Air temperature' and Thing/id eq 14&    $orderby=Observations/phenomenonTime desc&    $top=1&    $expand=Observations($top=1&$orderby=phenomenonTime desc)

Probably the last two lines are the ones that are related to the problem. There seems to be an error with parsing the filter condition. But except for some space characters I cannot see anything that is obviously wrong.

In the postgres container are no entries in the logs.

[hylkevds]

$expand=Observations($top=1&$orderby=phenomenonTime desc)

In the expand the items must be separated with ; not with &.
So your client is not sending the request you think it is sending...

[sensor-freak]

Good point, that seems to be the cause of the exception...

... but why does my client behave so? And is it really the client? When I look at the message in the browser console (see first message above) there is a ; in the $expand parameter.

In the meantime I tried to reproduce the problem with some other tools, namely Bruno.

Here are the results (the URL is copied over from the URL field in the Bruno GUI):
This one works, the response contains a FROST response as expected:

https://airquality-frost.k8s.ilt-dmz.iosb.fraunhofer.de/v1.1/Datastreams?$filter=ObservedProperty/name eq 'PM10' and Thing/id eq 1&$orderby=Observations/phenomenonTime desc&$top=1&$expand=Observations($top=1;$orderby=phenomenonTime desc)

This one fails with the given error. Note that I've taken the URL above and replaced the host, the URL prefix, the property name and ID only. Especially the $expand parameter is unchanged!

https://sdm-frost.cloud.iai.kit.edu/FROST-Server/v1.1/Datastreams?$filter=ObservedProperty/name eq 'Air temperature' and Thing/id eq 14&$orderby=Observations/phenomenonTime desc&$top=1&$expand=Observations($top=1;$orderby=phenomenonTime desc)

When I encode the ; to %3B (in the $expand parameter), the request does work and the response of our FROST server instance is comparable to the response of the Fraunhofer FROST server:

https://sdm-frost.cloud.iai.kit.edu/FROST-Server/v1.1/Datastreams?     $filter=ObservedProperty/name eq 'Air temperature' and Thing/id eq 14&     $orderby=Observations/phenomenonTime desc&     $top=1&     $expand=Observations($top=1%3B$orderby=phenomenonTime desc)

So it seems that the FROST server taken from DockerHub behaves differently (compared to the instance running at Fraunhofer). To me it seems that the server itself (the one from DockerHub) is converting the ; to a & somewhere.

Can someone confirm this?

[sensor-freak]

There is a possible place that might be involved in the source code in this method:

FROST-Server/FROST-Server.Tests/src/test/java/de/fraunhofer/iosb/ilt/statests/util/EntityHelper2.java

Line 288 in 15c6a8a

public JsonNode getEntityJson(EntityType entityType, PkValue pk, NavigationProperty np, String filter, List<String> select, String expand, String orderby) {

There are some operations where the semicolon, the ampersand and the $expand parameter might be involved...

[hylkevds]

It's not the server itself, but probably a reverse proxy between you and the server.
You can look in the access logs of Tomcat to see exactly what URL reaches the server

[sensor-freak]

Ok, this seems to be the missing point... the access log (logs/localhost_access_log.2026-04-30.txt) shows the following:

10.42.212.186 - - [30/Apr/2026:13:55:02 +0000] "GET /FROST-Server/v1.1/Datastreams?$filter=ObservedProperty/name%20eq%20%27Air%20temperature%27%20and%20Thing/id%20eq%2014&$orderby=Observations/phenomenonTime%20desc&$top=1&$expand=Observations($top=1&$orderby=phenomenonTime%20desc) HTTP/1.1" 400 220

So if this is not caused somewhere inside tomcat, it may come from the K8s proxy/ingress/whatever... I have to check this...

However, thank you very much for your support so far!