tilestrata

A pluggable map tile server.

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TileStrata
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TileStrata is a pluggable "slippy map" tile server that emphasizes code-as-configuration. The primary goal is painless extendability. It's clean, highly tested, performant, and integrates seamlessly with an elastic load balancer designed specifically for tile serving: TileStrata Balancer. Also, there's a built-in profiler and dashboard for debugging render times (read more).
$ npm install tilestrata --save

Table of Contents

- Basics - Express.js / Connect Integration - Metatile-Aware Load Balancing - Rebuilding the Tile Cache - Health Checks - Profiling / Debugging Performance

Introduction

TileStrata consists of five main actors, usually implemented as plugins:
  • "provider" – Generates a new tile (e.g mapnik)
  • "cache" – Persists a tile for later requests (e.g. filesystem)
  • "transform" – Takes a raw tile and transforms it (e.g. image scaling / compression)
  • "request hook" – Called at the very beginning of a tile request.
  • "response hook" – Called right before a tile is served to the client.

List of Plugins

Configuration

const tilestrata = require('tilestrata');
const disk = require('tilestrata-disk');
const sharp = require('tilestrata-sharp');
const mapnik = require('tilestrata-mapnik');
const dependency = require('tilestrata-dependency');
const strata = tilestrata();

// define layers
strata.layer('basemap')
    .route('tile@2x.png')
        .use(disk.cache({dir: '/var/lib/tiles/basemap'}))
        .use(mapnik({
            pathname: '/path/to/map.xml',
            tileSize: 512,
            scale: 2
        }))
    .route('tile.png')
        .use(disk.cache({dir: '/var/lib/tiles/basemap'}))
        .use(dependency('basemap', 'tile@2x.png'))
        .use(sharp(function(image, sharp) {
            return image.resize(256);
        }));

// start accepting requests
strata.listen(8080);

Once configured and started, tiles can be accessed via:
/:layer/:z/:x:/:y/:filename

Routing Without Filenames

As of 2.1.0, if you desire a routing scheme that's closer to other tile servers (where there's no filename) like outlined in #21, use the following format when registering routes:
.route('*.png') // /layer/0/0/0.png
.route('*@2x.png') // /layer/0/0/0@2x.png

Integrate with Express.js / Connect

TileStrata comes with middleware for Express that makes serving tiles from an existing application really simple, eliminating the need to call listen on strata.
const tilestrata = require('tilestrata');
const strata = tilestrata();
strata.layer('basemap') /* ... */
strata.layer('contours') /* ... */

app.use(tilestrata.middleware({
    server: strata,
    prefix: '/maps'
}));

Usage Notes

Fault-Tolerant Initialization

By default, TileStrata will error when initializing if any of the layer handlers fail to initialize. If you would like to ignore errors so that other layers are booted up and available, use the skipFailures option:
var strata = tilestrata({ skipFailures: true });

Metatile-Aware Load Balancing & Layer Sharding

TileStrata >= 2.0.0 supports integration with TileStrata Balancer, an elastic load balancer designed specifically for the nuances of tile serving – particularly metatiles. Generic load balancers have no knowledge of metatiles and thus will naively split tile requests out to multiple servers which leads to redundant rendering (slow and a waste of computing power).
As an added bonus, the balancer does not assume all servers in the pool have the same layers available. The balancer keeps track of the layers provided on each node so it knows where to route. In sum, the overview:
  • Fully elastic w/minimal setup
  • Consistent routing (improves local cache hits)
  • Metatile-aware (prevents redundant rendering)
  • Layer-aware (allows heterogeneous distribution of layers in the cluster)

View TileStrata Balancer Documentation →
Note: One could use cookie-persistence with traditional load balancers, but this forces users onto a single machine (not optimal).

Rebuilding the Tile Cache

If you update your map styles or data, you'll probably want to update your tiles. Rather than dump all of them at once and bring your tile server to a crawl, progressively rebuild the cache by requesting tiles with the X-TileStrata-SkipCache header. TileMantle makes this process easy:
npm install -g tilemantle
tilemantle http://myhost.com/mylayer/{z}/{x}/{y}/t.png \
    -p 44.9457507,-109.5939822 -b 30mi -z 10-14 \
    -H "X-TileStrata-SkipCache:mylayer/t.png"

For the sake of the tilestrata-dependency plugin, the value of the header is expected to be in the format:
X-TileStrata-SkipCache:*
X-TileStrata-SkipCache:[layer]/[file],[layer]/[file],...

In advanced use cases, it might be necessary for tiles to not be returned by the server until the cache is actually written (particularly when order matters due to dependencies). To achieve this, use:
X-TileStrata-CacheWait:1

Health Checks

TileStrata includes a /health endpoint that will return a 200 OK if it can accept connections. The response will always be JSON. By setting the "healthy" option to a function that accepts a callback you can take it a step further and control the status and data that it returns.
// not healthy
const strata = tilestrata({
    healthy: function(callback) {
        callback(new Error('CPU is too high'), {loadavg: 3});
    }
});

// healthy
const strata = tilestrata({
    healthy: function(callback) {
        callback(null, {loadavg: 1});
    }
});

Profiling / Debugging Performance

Unless the TILESTRATA_NOPROFILE environment variable is set, TileStrata keeps track of basic latency and size information (min, max, avg) for all steps in the tile serving flow for the lifetime of the process. Data is kept for every plugin on every route of every layer and is broken down by zoom level. To access it, visit: /profile in your browser. If this information needs to be kept private, you can set the TILESTRATA_PASSWORD environment variable to a password that TileStrata will prompt for (username is ignored). The page will have tables like the one below:
t.pngz1z2z3z4z5z6z7z8z9
reqhook#0errors000000000
dursamples112111111
durmax454443465250586181
durmin454442465250586181
duravg454442.5465250586181
cache#0.geterrors000000000
dursamples112111111
durmax454544584762466053
durmin454544584762466053
duravg454544584762466053
hits000000000
misses112111111
provider#0errors000000000
dursamples112111111
durmax344396122119108115103129
durmin344364122119108115103129
duravg344380122119108115103129
sizesamples112111111
sizemax500B501B576B578B504B540B501B776B736B
sizemin500B501B565B578B504B540B501B776B736B
sizeavg500B501B571B578B504B540B501B776B736B
transform#0errors000000000
dursamples112111111
durmax323335614957535069
durmin323334614957535069
duravg323334.5614957535069
reshook#0errors000000000
dursamples112111111
durmax454345636355486068
durmin454344636355486068
duravg454344.5636355486068
cache#0.seterrors000000000
dursamples112111111
durmax121313142723262927
durmin121310142723262927
duravg121311.5142723262927

API Reference

TileServer

server.listen(port, hostname, callback)
Starts accepting requests on the specified port. The arguments to this method are exactly identical to node's http.Server listen() method. It returns the http.Server instance.
server.close(callback)
Stops listening for incoming requests on the port. If TileStrata Balancer is configured, it will also proactively notify it so that the node is removed from the pool.
server.layer(name, opts)
Registers a new layer with the given name and returns its TileLayer instance. If the layer already exists, the existing instance will be returned. Whatever name is used will be the first part of the url that can be used to fetch tiles: /:layer/.... The following options can be provided:
- bbox: A bounding box (GeoJSON "bbox" format) that defines the valid extent of the layer. Any requests for tiles outside of this region will result in a 404 Not Found. This option can also be set to an array of bounding boxes for rare cases when a layer is noncontinuous. - minZoom: The minimum z to return tiles for. Anything lesser will return a 404 Not Found. - maxZoom: The maximum z to return tiles for. Anything greater will return a 404 Not Found.
server.getTile(layer, filename, x, y, z, callback)
Attempts to retrieve a tile from the specified layer (string). The callback will be invoked with three arguments: err, buffer, and headers.
server.uptime()
Returns an object containing "duration" and "start" (both in milliseconds). If the server hasn't started, the result will be null.
server.version
The version of TileStrata (useful to plugins, mainly).

TileLayer

layer.route(filename, options)
Registers a route and returns a TileRequestHandler instance to be configured. Setting filename to something like "*.ext" or "*@2x.ext" will omit the filename from the request and make the tiles available at /{z}/{x}/{y}.ext and /{z}/{x}/{y}@2x.ext, respectively (see #21).
The available options are:
- cacheFetchMode: Defines how cache fetching happens when multiple caches are configured. The mode can be "sequential" or "race". If set to "race", TileStrata will fetch from all caches simultaneously and return the first that wins.

TileRequestHandler

handler.use(plugin)
Registers a plugin, which is either a provider, cache, transform, request hook, response hook, or combination of them. See the READMEs on the prebuilt plugins and/or the "Writing TileStrata Plugins" section below for more info.

TileRequest

A request contains these properties: x, y, z, layer (string), filename, method, headers, qs, and hasFilename.
If a tile request is in the filenameless format (see here), hasFilename will be false. To illustrate: if the request is to /layer/0/0/0@2x.png, filename will be set to t@2x.png (for compatibility with caches and plugins that expect a filename) and hasFilename will be false.
tile.clone()
Returns an identical copy of the tile request that's safe to mutate.

Writing TileStrata Plugins

All plugins allow optional init and destroy lifecycle methods that will be called at startup and teardown. The first argument will be the TileServer instance, and the second will be the callback.

Writing Request Hooks

A request hook implementation needs one method: reqhook. The hook's "req" will be a http.IncomingMessage and "res" will be the http.ServerResponse. This makes it possible to respond without even getting to the tile-serving logic (just don't call the callback).
module.exports = function(options) {
    return {
        name: 'myplugin',
        init: function(server, callback) {
            callback(err);
        },
        reqhook: function(server, tile, req, res, callback) {
            callback();
        },
        destroy: function(server, callback) {
            callback(err);
        }
    };
};

Writing Caches

A cache implementation needs two methods: get, set. If a cache fails (returns an error to the callback), the server will ignore the error and attempt to serve the tile from the registered provider.
module.exports = function(options) {
    return {
        name: 'myplugin',
        init: function(server, callback) {
            callback(err);
        },
        get: function(server, tile, callback) {
            callback(err, buffer, headers, /* refresh */);
        },
        set: function(server, tile, buffer, headers, callback) {
            callback(err);
        },
        destroy: function(server, callback) {
            callback(err);
        }
    };
};

A special behavior exists for when a cache returns a hit, but wants a new tile to be generated in the background. The use case: you have tile that's old enough it should be regenerated, but it's not old enough to warrant making the user wait for a new tile to be rendered. To accomplish this in a plugin, have get() return true as the fourth argument to the callback.
callback(null, buffer, headers, true);

Writing Providers

Providers are responsible for building tiles. A provider must define a serve method:
module.exports = function(options) {
    return {
        name: 'myplugin',
        init: function(server, callback) {
            callback(err);
        },
        serve: function(server, tile, callback) {
            callback(err, buffer, headers);
        },
        destroy: function(server, callback) {
            callback(err);
        }
    };
};

Writing Transforms

Transforms modify the result from a provider before it's served (and cached). A transform must define a transform method:
module.exports = function(options) {
    return {
        name: 'myplugin',
        init: function(server, callback) {
            callback(err);
        },
        transform: function(server, tile, buffer, headers, callback) {
            callback(err, buffer, headers);
        },
        destroy: function(server, callback) {
            callback(err);
        }
    };
};

Writing Response Hooks

A response hook implementation needs one method: reshook. The hook's "req" will be a http.IncomingMessage and "res" will be the http.ServerResponse. The "result" argument contains three properties: headers, buffer, and status — each of which can be modified to affect the final response.
module.exports = function(options) {
    return {
        name: 'myplugin',
        init: function(server, callback) {
            callback(err);
        },
        reshook: function(server, tile, req, res, result, callback) {
            callback();
        },
        destroy: function(server, callback) {
            callback(err);
        }
    };
};

Multi-Function Plugins

Sometimes a plugin must consist of multiple parts. For instance, a plugin tracking response times must register a request hook and response hook. To accommodate this, TileStrata supports arrays:
module.exports = function() {
    return [
        {name: 'myplugin', reqhook: function(...) { /* ... */ }},
        {name: 'myplugin', reshook: function(...) { /* ... */ }}
    ];
};

Misc Utilities

Concurrency Limiting

In some cases you'll want explicitly limit concurrency of a provider so that requests are queued if too many happen at once. To support this, we provide a wrapWithMaxConcurrency method that wraps providers. This is particularly useful with the tilestrata-proxy plugin.
const { wrapWithMaxConcurrency } = tilestrata.utils;

.use(wrapWithMaxConcurrency(provider(...), 5))

Contributing

Before submitting pull requests, please update the tests and make sure they all pass.
$ npm test

License

Copyright © 2014–2021 Natural Atlas, Inc. & Contributors
Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at: http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.