[Tile-serving] [osm2pgsql-dev/osm2pgsql] Cache-Invalidierung (Expire) Equal Earth projection (EPSG:8857) (Discussion #2327)

[Astrid]

I'm currently taking my first steps with **osm2pgsql**, **Themepark**, and the **Equal Earth projection** (EPSG:8857). In my Lua script, I set `themepark:set_option('srid', 8857)`, and at first glance, it seems to be working correctly. The `WITH_PROJ` option is enabled (as described [[here](https://github.com/osm2pgsql-dev/osm2pgsql?tab=readme-ov-file#using-the-proj-library)](https://github.com/osm2pgsql-dev/osm2pgsql?tab=readme-ov-file#using-the-proj-library)).

For importing data, I'm using the following command:

```
osm2pgsql --output flex --style LUAFILE -d DBNAME DATAFILENAME
```

However, I’m still unclear on how **tile expiration** works—specifically, how data tiles are invalidated over time or after changes. I've been looking at this script for guidance: [[expire.lua](https://github.com/pnorman/spirit/blob/main/expire.lua)](https://github.com/pnorman/spirit/blob/main/expire.lua).

While using it, I encountered the following error:

```
themepark.lua:378: Error in 'define_table': Expire only allowed for geometry columns in Web Mercator projection.
```

>From what I can tell, this error is triggered by the following code in osm2pgsql:
[https://github.com/osm2pgsql-dev/osm2pgsql/blob/210fde0f3911edfb1f94bcd42960327beefac7c9/src/flex-lua-table.cpp#L211](https://github.com/osm2pgsql-dev/osm2pgsql/blob/210fde0f3911edfb1f94bcd42960327beefac7c9/src/flex-lua-table.cpp#L211)

**Does this mean tile expiration is strictly limited to the Web Mercator projection, or is there a way to enable it for other projections like Equal Earth (EPSG:8857)?**


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