mod_tile and renderd

This software contains two main pieces:

1. mod_tile: An Apache 2 module to deliver map tiles.
2. renderd: A daemon that renders map tiles using mapnik.

Together they efficiently render and serve raster map tiles for example to use within a slippy map. The two consist of the classic raster tile stack from OpenStreetMap.org.

As an alternative to renderd its drop-in replacement Tirex can be used in combination with mod_tile.

Dependencies

• GNU/Linux Operating System (works best on Debian or Ubuntu)
• Apache 2 HTTP webserver
• Mapnik
• Cairo 2D graphics library
• Curl library (SSL variant)
• Iniparser library
• GLib library
• Memcached library (optional)
• RADOS library (optional)

Installation

Starting from the following operation systems and their versions:

• Debian 11 (Bullseye)
• Ubuntu 21.04 (Hirsute Hippo)

the software and all dependencies can be installed simply with:

$ apt install libapache2-mod-tile renderd

These packages for Debian and Ubuntu are being maintained by the Debian GIS Team in the respective repository.

Compilation

You may want to compile this software yourself. Either for developing on it or when using it on an operating system this is not being packaged for.

We prepared instructions for you on how to build the software on the following distributions:

• CentOS 7
• Fedora 34
• Ubuntu 20.04 (this should work as well for Debian 10)
• Debian 12

Configuration

After you either installed the software packages or compiled the software yourself, you can continue with the configuration. For your convenience example configuration files are distributed with the software packages and located in the etc directory of this repository.

A very basic example-map and data can be found in the utils/example-map directory. For a simple test copy it over to /var/www/example-map.

Copy the configuration files to their place, too:

$ cp etc/renderd/renderd.conf /etc/renderd.conf
$ cp etc/apache2/renderd.conf /etc/apache2/conf-available/renderd.conf
$ cp etc/apache2/renderd-example-map.conf /etc/apache2/conf-available/renderd-example-map.conf

Enable the configuration:

$ sudo a2enmod tile
$ sudo a2enconf renderd
$ sudo a2enconf renderd-example-map

Restart apache2:

$ sudo a2enmod tile
$ sudo a2enconf renderd

And run the rendering daemon

$ renderd -f

Make sure the /var/cache/renderd/tiles directory is writable by the user running the renderd process.

Try loading a tile in your browser, e.g.

http://localhost/renderd-example/tiles/0/0/0.png

You may edit /etc/renderd.conf to indicate the location of different mapnik style sheets (up to ten) and the endpoints you wish to use to access it.

It is recommended to checkout switch2osm for nice tutorials on how to set up a full tile server like on OpenStreetMap.org, using this software together with a PostgreSQL database and data from OpenStreetMap.

Details about renderd: Tile rendering

The rendering is implemented in a multithreaded process called renderd which opens either a unix or tcp socket and listens for requests to render tiles. It uses Mapnik to render tiles using the rendering rules defined in the configuration file /etc/renderd.conf. Its configuration also allows to specify the number of rendering threads.

The render daemon implements a queuing mechanism with multiple priority levels to provide an as up-to-date viewing experience given the available rendering resources. The highest priority is for on the fly rendering of tiles not yet in the tile cache, two priority levels for re-rendering out of date tiles on the fly and two background batch rendering queues. The on the fly rendering queues are limited to a short 32 metatile size to minimize latency. The size of the main background queue is determined at compile time, see: render_config.h

Details about mod_tile: Tile serving

An Apache module called mod_tile enhances the regular Apache file serving mechanisms to provide:

1. When tiles have expired it requests the rendering daemon to render (or re-render) the tile.
2. Remapping of the file path to the hashed layout.
3. Prioritizes rendering requests depending on the available resources on the server and how out of date they are.
4. Use tile storage other than a plain posix file system. e.g it can store tiles in a ceph object store, or proxy them from another tile server.
5. Tile expiry. It estimates when the tile is next likely to be rendered and adds the appropriate HTTP cache expiry headers. This is a configurable heuristic.

To avoid problems with directories becoming too large and to avoid too many tiny files. They store the rendered tiles in "meta tiles" in a special hashed directory structure. These combine 8x8 actual tiles into a single metatile file. This is a more efficient use of disk space and inodes.

The metatiles are then stored in the following directory structure: /[base_dir]/[TileSetName]/[Z]/[xxxxyyyy]/[xxxxyyyy]/[xxxxyyyy]/[xxxxyyyy]/[xxxxyyyy].meta

Where base_dir is a configurable base path for all tiles. TileSetName is the name of the style sheet rendered. Z is the zoom level. [xxxxyyyy] is an 8 bit number, with the first 4 bits taken from the x coordinate and the second 4 bits taken from the y coordinate. This attempts to cluster 16x16 square of tiles together into a single sub directory for more efficient access patterns.

Apache serves the files as if they were present under /[TileSetName]/Z/X/Y.png with the path being converted automatically.

Notes about performance

mod_tile is designed for high performance tile serving. If the underlying disk system allows it, it can easily provide > 10k tiles/s on a single serve.

Rendering performance is mostly dependent on mapnik and postgis performance, however renderd tries to make sure it uses underlying hardware as efficiently as possible and scales well on multi core systems. renderd also provides built-in features to scale to multi server rendering set-ups.

Copyright and copyleft

Copyright (c) 2007 - 2022 by mod_tile contributors (see AUTHORS)

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.

See the COPYING for the full license text.