JOSM as local editor?

If you are working on a single file, edit it in any text editor, changing (or adding) upload='false' in the <osm> tag. This way JOSM won't ask you to upload the file, and instead will give a warning if you try to.

I am using JOSM to edit a local .osm file for features on a map that should not be added to the Openstreetmap database.

Pretty easy to open local files, edit them and save them using the "Open" and "Save" or "Save As" options under the file menu.

However JOSM warns me every time I quit the program that I have unsaved changes (even though I've saved them) and its default action is to upload them. I just make sure I click on the "Exit Now" button.

Then when using mapnik to render my customized map I simply specify the local .osm file as the data source for some of the layers. Works out pretty well for things like adding a UTM grid, local nicknames for locations, etc. to my map image.

JOSM knows that you have saved your file locally. You have the choice on both: you can exit with only a save action. But it's true that we should be able to specify that tou are wroking on a local database of your choice, or that you should be able to submit the data to more databases (each one with ots own status) You could do that by using a dummy database API url, such as "local:" followed by nothing (JOSM would ignore completely the database API.), or "file:" followed by nothing (JOSM in that case would allow you to choose a file destination to save a new copy; ideally it should support "svn:" or "git:" to submit a new version in a local version repository). This would require developing an API adapter. Another possibility is simply to install you own local API server, and specify it as the URL. Note however that object ID's are only significant within a single database (you may use another API URL, but this only works because these APIs are connected to the same target database).

We still lack the possibility to manage multiple ID's to perform synchronization between databases. The only mechanism avaliable is the use of "negative" ID's which means that we still don't know the remote ID as it does not exist remotely.

Sync'ing different databases (with their own spaces for their own IDs) is something that JOSM still does not perform: this requires manual preparation and there's no automated tool to link objects from one database to another. and select those that we will correlate. All we can do is to have separate layer worksheets, each one connectd to a single database (with negative IDs for objects still missing in the associated database).

But the UI still does not allow selecting the target database API to associate with a layer worksheet. But ideally all objects loaded in JOSM should be able to store one ID per database, and each database adapter should specidy if it supports ID's from other databases (in OSM, you could associate a custom "ref:<db>=<id>" attribute to objects, and should be able to specify those to discard/ignore on output to the API. These foreign keys should not be retrieved by default, unless the user explicitly requests them. They should be stored in tables separated from other attributes (tag/value)

The OSM API should also facilitate the retrieval of objects using their foreign IDs. In my opinion this should not even be standard attributes/tags but managed in a separate index table as "foreign keys". We could also have adatation profiles indicating how to convert tags from one database to tags for the other database with a set of mapping rules for each adaptation.

The OSM database could store these adapter rules as new kind of objects.

An example of use would be the creation of separzte stores for working collaboratively on some areas or on some kind of objects and occasionally syn them with another target database. This could allow things like perliminary QA checks, collaborative work to prepare merges before imports.

May be someday the API will evolve into a system like Git, where each user has its own store for the data he is working with, and with othe people working in synchronizations (the synchronizations could then become bidirectional)

As far as I know to edit map you have to agree to the license which says that all edits you do, you have to share/upload. And osm and tools are not for making maps for your use only. I hope you're aware of it. Maybe I'm missing the point, I'm still new to all this. If I do, don't mind this post.