Overpass API User's Manual

Component Overview

This page gives an overview which files exist in Overpass related directories, which processes run during executing an Overpass API instance, and how the parts work together.

Running Processes

For an Overpass API endpoint that is accessible over HTTPS (or HTTP) you need two things:

The dispatcher is a permanently running daemon. It coordinates requests and updates and prevents requests from reading inconsistent data, i.e. basically all other processes are unable to start if the dispatcher is down. A sole exception is running osm3s_query with an extra --db-dir=$DIR parameter: in that case the osm3s_query is run against the database in the given directory under the assumption that no concurrent updates take place.

The dispatcher can be shut down by calling dispatcher --osm-base --terminate on the command line or by sending a SIGTERM to it. In both cases it will notify a running update_database to shutdown to ensure a proper shutdown of everything.

It is quite common that two dispatcher are running. The second instance needs to be started to coordinate the creation and upates of areas. The two can be discerned by that one runs with the argument --osm-base and the other with --areas. This dispatcher can be shut down by calling dispatcher --areas --terminate on the command line or by sending a SIGTERM to it.

The update process is performed by having the bash scripts fetch_osc.sh and apply_osc_to_db.sh run permanently. An alternative update process is performed by the having the bash script fetch_osc_and_apply.sh run permanently. The former pair keeps the minute diffs while the latter single process discards the downloaded OSC files after successfully applying them. These processes call update_database to apply the downloaded files to the database. All of these processes can be safely shutdown by sending a SIGTERM to one of them. The process update_database get notified by apply_osc_to_db.sh resp. fetch_osc_and_apply.sh when those are shutdown, and in turn apply_osc_to_db.sh resp. fetch_osc_and_apply.sh watch whether update_database has been shutdown and follow that suit.

The permanent process for the area updates is rules_loop.sh or rules_delta_loop.sh. Both of them periodically start an worker process osm3s_query --rules, and that process does the actual area updates.

You may see a couple of usually short lived processes called interpreter. These are the worker processes called from the web server by CGI to run the actual requests.

Files in bin

The files in the bin directory are those that are designed to run permanently or are needed sometimes for auxiliary jobs. Only a subset is necessary for the standard running instances, many are there to keep up backwards compatibility.

The two files that are most essential are dispatcher and update_from_dir.

The dispatcher coordinates running requests and updates and is therefore inevitable in any typical system. So is the binary update_from_dir which is called from both of the provided update mechanisms.

The update process on the public instances keeps the downloaded minute diffs to allow for forensics if the system crashed. This update process is performed by having the bash scripts fetch_osc.sh and apply_osc_to_db.sh run permanently. An alternative update process is performed by the bash script fetch_osc_and_apply.sh. This update process discards the downloaded minute diffs once they have successfully been applied.

Both update mechanisms make every minute use of update_from_dir and use migrate_database once after startup to bring the database format to the version that is supported by the used software version for updates.

The binary osm3s_query is equivalent to the interpreter endpoint for the supported query language but designed for use at the command line.

The bash scripts rules_loop.sh and rules_delta_loop.sh perform the updates for the generated, relation based areas. They rely on osm3s_query. These are again alternatives to each other: while rules_loop.sh will always generate all areas from scratch, the script rules_delta_loop.sh regenerates only those areas of which the uderlying data has changed.

Finally, the bash script download_clone.sh shall be executed to download a copy of the database. As this is a very large download it is designed to run only once and afterwards keep the database up to date with minute diffs.

These are the files that are useful for a standard Overpass API instance. From here on follow a couple of files which offer features for special interest only or manual maintenance.

The binary update_database allows to update or to initially set up the database from a single OSM or OSC XML file. It is used to create a database from a planet file.

The script clone.sh is the server end of the clone mechanism and creates a consolidated copy of the database.

The binary translate_xapi is a helper for the XAPI compatibility layer. It translates the XAPI query language to the Overpass XML query language.

The three binaries bbox_brim_query, draw_route_svg, and sketch_route_svg are helpers for the line diagram feature, see below in the cgi-bin directory.

The three binaries escape_xml, tocgi, and uncgi are further helpers for both the XAPI compatibility layer as well as for the line diagram feature. They may be useful to perform what they are called in isolation, i.e. URL de- or encode for debugging purposes.

The two scripts init_osm3s.sh and run_osm3s_minutely.sh are convenience scripts and have been used on the French public instance. The script reboot.sh has been used in the past to enable an automated restart after a reboot but is currently unmaintained.

Files in cgi-bin

The files in the cgi-bin subdirectory are exactly those that can be executed via HTTPS. Thus, they are called endpoints. None of these executables is intended to run for a long time, although interpreter may run quite long if there is a huge query for it. All others should be quick, otherwise there is a malfunction.

The endpoint interpreter is the standard query util. It has its name because it interprets the queries that are provided to it.

The endpoint timestamp informs whether the used database is current. It returns the timestamp of the latest applied diff.

The endpoint status informs a single client about its quota status. This includes some general information about the server as well to enable the client to detect outdated answers. It is a bash script.

The endpoint kill_my_queries is the only endpoint with alters the state of the server. It is intended to kill running queries of the same client. Many clients are not able to stop a query reliably over HTTPS, hence they can do so by this endpoint to relieve the server and save quota credits. It is a bash script.

All other endpoints are special interest or legacy fetures outside the scope of the core Overpass API. In particular, all of them are bash scripts. All of them can be safely removed in a private instance unless the specific functionality is desired.

The endpoint map is a compatibility layer for the export feature of the OpenStreetMap main website. It is a bash script to wrap the actual request for a given bounding box.

The endpoint convert helps to convert between the older XML based query language and the newer OverpassQL query language. It is a bash script to call osm3s_query with appropriate parameters to do the actual conversion.

The endpoints convert_xapi, xapi and xapi_meta are compatibility layers for a query language of the even older XAPI. No productive use of XAPI since about a decade is known, so this is now really a matter of legacy.

The endpoints augmented_diff, augmented_diff_status and augmented_state_by_date have been the first attempt to supply Augmented Diffs. These have been superseded by just using a proper query for it, because the interplay of minute diffs and timestamps of objects is non-trivial.

The endpoint template has been used in the past to manage server side templates for custom output formats. This had been a helper feature for the Permanent Id to control what is shown for disambiguation. There has not been any activity to diversify these disambiguation pages, so this feature got in the meantime unmaintained, while otherise the Permanent Id is fully operational.

The endpoints draw-line, sketch-line, sketch-options, and sketch-route allow to show public transit routes. Of these, sketch-line can be accessed via this form on the public instances. It calls osm3s_query and sketch_route_svg from the bin directory. The other three endpoints do so as well, but are rarely used.

The endpoint trigger_clone shows the directory you can start to download a clone from. This works only if the server is configured to offer clone, i.e. only for the dev instance.

Files in the Database Directory

During normal operations, more than 100 files are in the database directory. They can be grouped most easily by looking at their extensions:

We discuss the other file types further below.

The Update Mechanics

The update process is designed to ensure integrity to all processes that are reading during one or more update cycles.

This is ensured on the level of file blocks within the payload (the map and bin) files. During an update run changed blocks are rewritten as new blocks, and there are then two or more views of file blocks to form a complete index. The existing one is in the corresponding idx file while the newly constructed view is in the shadow file.

Further valid views can exist when the bookkeeping of the dispatcher indicates that one or more long running process has been based on an even older block list than the current idx file. The dispatcher releases a block for overwriting only if it knows that no reading process has any reference to it. These blocks are at the beginng of the update cycle written to the files ending in .bin.shadow or .map.shadow.

At the end of the update cycle the update process notifies the dispatcher of its completion. The dispatcher then sets a lock which prevents reading processes from starting, and then it moves the .idx.shadow files to the .idx files to become the new current idx list.

If the dispatcher finds its own lock file at startup then it concludes that moving the files has been failed and reperforms that before admitting reading processes. With that beheaviour, the update process is truly atomic.

The update process for areas is similar.

Some other update processes exist for special cases but are not discussed in detail here:

If a migration of the file format is necessary then that migration copies the payload to a new payload file with .next in its name and an idx file and finally moves that pair of files in place.

If the database is initialized from such a large source file that an in-memory sort is impossible then it creates temporary files with numbers in the file name that are finally merge-sorted. This allows for a faster import.

Payload vs Meta vs Museum vs Areas

The Overpass API database can be operated in three different levels of data depth.

In the recommended base mode it contains only the geographical data, i.e. coordinates, element ids and refs, and the tags of the elements. It then consists of the files:

In the meta mode which you only may run if you have a legitimate interest, it contains also per element the changeset id, timestamp, newest version, and the user id and user name of the uploading user of that newwest version. It then encompasses the following additional files over the base mode:

In the attic mode which includes in addition to the meta and base mode data (and which also may only be run for a legitimate interest), the database stores in addition the full history of elements at any timestamp. It then encompasses the following additional files over the meta mode:

Indepently of the chosen mode it is possible to have areas enabled. In that case the following files are present in the database directory: areas.bin, area_blocks.bin, area_tags_global.bin, area_tags_local.bin, and for each one corresponding idx file.

Other Files in the Database Directory

See above for the files osm_base_shadow.lock and area_shadow.lock. These two are temporary files for running updates.

Also, the two files osm3s_areas and osm3s_osm_base are sockets managed by the dispatcher and deleted during its shutdown. A starting dispatcher detects by their presence that another dispatcher is already running and refuses to start. All other processes connect to the dispatcher by opening a socket on this file, i.e. refuse to start in the absence of these sockets.

The files area_version and osm_base_version contain the timestamp of the last update. This is the timestamp transmitted along the update and not the time at which the update completed. Similarly, the file replicate_id contains the number of the latest osc minute diff that has been applied.

The file base-url contains the URL of the used clone instance to help track back anomalies in the data.

The file server_name is what server_status shows as the name of the server to ensure that end users can discern different instances behind a load balancer.

The file osm_base_shadow.status is a temporary file created by the dispatcher when it is queried for the status.

The log file database.log is jointly written by the dispatcher as well as reading and writing processes. It contains information whether the communication between dispatcher and its clients is working. The log file apply_osc_to_db.log contains a log of which diffs have been attempted to be applied to the database and whether the update attempts have succeeded. The log file transactions.log contains the requests, client ids and resource consumption per request.

The rules subdirectory usually contains two files, one for rules_loop.sh and one for rules_delta_loop.sh. These files specify the rules according to which it is decided which relations are converted to areas.

The augmented_diffs and templates directories exist now only for historical reasons.

Other Files

There are some files outside the bin, cgi-bin, and database directory. These are non-essential in the sense of that an Overpass API instnce can be brought back on track without them but they are temporarily necessary and otherwise may by their content help to figure out what has happened.

If the twin scripts fetch_osc.sh and apply_osc_to_db.sh perform the updates then the fetch_osc.sh stores a partial replication of the minute diffs from remote in the diff directory. The diff directory can be found as the third argument to fetch_osc.sh. It contains the minute diffs itselfs as .osc.gz file and one corresponding .state.txt file per minute diff, all arranged in the three-level directory hierarchy known from upstream.

In addition, at the root of the diff directory the file state.txt is the newest state file found. The presence of this file upstream is the indicator whether the corresponding diffs files are actually valid. Finally, a fetch_osc.log contains a log of which file has when been downloaded.

The entire directory is neither needed nor created if the script fetch_osc_and_apply.sh is used instead. It fully relies on storing the diffs only temporarily.

There is a bunch of munin scripts in the munin/ subdirectory which you might want to move to /etc/munin/plugins/ directory and adapt the file paths to the database directory or directory of the executables (parent of bin/ and cgi-bin/) to get munin monitoring.

Other Objects

There are only few artifacts outside the file system that are relevant to the Overpass API.

In the standard configuration, two shared memory entries are maintained by the dispatcher. This is osm3s_osm_base for the OSM data dispatcher, and osm3s_areas for the area dispatcher. On Linux they can be found under /dev/shm, but other POSIX compliant operating systems may have them elsewhere. They are deleted on shutdown of the respective dispatcher, and if present convey to a starting dispatcher that another instance is already running, i.e. the dispatcher subsequently does not start. Reading and writing clients use these shared memories to find the location of the database directory.

This is impractical under some circumstances, e.g. on a Docker volume. Therefore, one can alternatively set the environment variable OVERPASS_SOCKET_DIR or OVERPASS_DB_DIR to let the client process instead look there for the socket. In that case the shared memory is not used at all.

next: Installation of an Own Instance