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Renders and serves map tiles using apache
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mod_tile ======== A program to efficiently render and serve map tiles for www.openstreetmap.org map using Apache and Mapnik. Requirements ============ OSM map data imported into PostgreSQL using osm2pgsql Mapnik renderer along with the OSM.xml file and map symbols, world_boundaries shapefiles. Apache with development headers for APR module development. 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highest priority is for on the fly rendering of tiles not yet in the tile cache, two priority levels for re-rendering out of date tiles on the fly and two background batch rendering queues.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 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path to the hashed layout 3) Prioritizes rendering requests depending on the available resources on the server and how out of date they are. 4) Use tile storage other than a plain posix file system.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. Mod_tile / renderd 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. For example, many sea tiles are 103 bytes long. In the old scheme a meta tile of blank sea tiles would take 64 inodes of 4kB each, a total of 256kB. In this optimized scheme it needs a single file of about 7kB. The metatiles are then stored in the following directory structure: /[base_dir]/[TileSetName]/[Z]/[xxxxyyyy]/[xxxxyyyy]/[xxxxyyyy]/[xxxxyyyy]/[xxxxyyyy].png 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. Compiling ========= mod_tile and renderd utilize a number of third party, some of which it depends on and some provide optional features and are compiled in if the respective libraries are installed. Once the dependencies are installed you can compile and install mod_tile / renderd the usual way: ./autogen.sh ./configure make sudo make install sudo make install-mod_tile Setup ===== Create a new apache config file to load the module, e.g. /etc/httpd/conf.d/mod_tile.conf See the sample mod_tile.conf for details Edit /etc/renderd.conf to indicate the location of your mapnik style sheet and the uri you wish to use to access it. You may configure up to 10 (by default) mapnik style sheets - simply give each section a unique name and enter the uri and style sheet path. Make sure the /var/lib/mod_tile directory is writable by the user running the renderd process and create a file an empty file planet-import-complete in this folder. Run the rendering daemon 'renderd' Restart Aapche Note: SELinux will prevent the mod_tile code from opening the unix-socket to the render daemon so must be disabled. Try loading a tile in your browser, e.g. http://localhost/osm_tiles/0/0/0.png The render daemon should have produce a message like: Got incoming connection, fd 7, number 1 Render fd(7) xml(Default), z(0), x(0), y(0) The disk should start thrashing as Mapnik tries to pull in data for the first time. After a few seconds you'll probably see a 404 error. Wait for the disk activity to cease and then reload the tile. With a bit of luck you should see a tile of the world in your browser window. If this fails to happen check the http error log. You can increase the level of debugging using the LogLevel apache directive. If no log messages are shown check that you are accessing the correct virtual host - the new version of mod_tile is only installed on a single host by default. To install on multiple hosts either use ServerAlias or use the LoadTileConfigFile in each virtual host. 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.