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OnEarth Time Service

What it does

This is a service that allows for the querying of time period and default date information per layer, as well as date snapping.

As part of OnEarth 2.0, it's used by mod_wmts_wrapper to complete layer requests that involve a time dimension.

The service takes queries via URL query parameters and returns a JSON response.

How it works

time_service is a Python script that's intended to be run by the mod_wsgi Apache module.

The script can provide different output based on the URL query parameters provided.

/{endpoint} -- provides a list of all layers, their default times, and available periods.

/{endpoint}?layer={layer_name} -- provides the default time and period list for the specified layer.

{endpoint}?limit={number_of_periods} or /{endpoint}?layer={layer_name}&limit={number_of_periods} -- limits the number of periods returned to the specified amount. A positive number n will return the first n periods, and a negative n will return the last n periods.

{endpoint}?skip={number_of_periods} or /{endpoint}?layer={layer_name}&skip={number_of_periods} -- skips a specified number of periods. When used with limit, the a skip of s and a limit of n would return periods of indices s + 1 through s + n. When a negative limit is specified, using skip will have its usual effect in the opposite direction, with the most recent s periods being skipped.

{endpoint}?periods_start=YYYY-MM-DD&periods_end=YYYY-MM-DD or {endpoint}?periods_start=YYYY-MM-DDThh:hh:ssZ&periods_end=YYYY-MM-DDThh:hh:ssZ -- only returns periods that come after periods_start and before periods_end. If periods_start or periods_end falls within a period, that period will be truncated to fit within these bounds. Use of one, both, or neither of these parameters is supported for any request that returns periods. Default times for each layer will be moved to fall within the specified range.

/{endpoint}?layer={layer_name}&datetime=YYYY-MM-DD or /{endpoint}?layer={layer_name}&datetime=YYYY-MM-DDThh:hh:ssZ -- provides the filename and snap date for the specified layer. If used with periods_start and/or periods_end, only periods falling within those bounds will be considered when determining the snap date.

The database can be split into multiple parts so as to separate layer information by projection, endpoint, etc. Up to 5 additional keys can be specified in the URL request, i.e.: /{endpoint}?layer={layer_name}&datetime=YYYY-MM-DD&key1=epsg4326&key2=best

time_service currently uses a Redis database for queries, but other handlers can be easily added.

Dependencies

  • Apache 2.4
  • mod_wsgi
  • Python 3
  • Redis
  • Redis-py

Installation

  1. Make sure mod_wsgi is installed and is being loaded by your Apache configuration.

  2. Install Redis. For CentOS 7, use yum install redis. Make sure the database is running.

  3. Install redis-py with pip

  4. Within the onearth repo, navigate to src/modules/time_service

  5. Install the OnEarth Python package with pip

pip install -e .

Configuration

Set up the Redis database with layer information

time_service is set to read values from a Redis database in the following format, for each layer configured:

layer:[layer_name]:default -- A string with the current default date for the layer specified by [layer_name]. Should be in either YYYY-MM-DD or YYYY-MM-DDThh:mm:ssZ format.

layer:[layer_name]:periods -- A set of strings in the following format: start_date/end_date/P[interval_number][interval_size]. For example, 2012-01-01/2016-01-01/P1Y.

Example

For testing, here's a fast way to set up a Redis database for testing.

  1. Enter the Redis CLI: redis-cli
  2. Add a default date: SET layer:test_layer:default "2015-06-01"
  3. Add some periods: ZADD layer:test_layer:periods 0 "2012-01-01/2013-01-01/P1M" 0 "2005-06-01/2005-12-01/P10D"

Creating levels for projection, endpoint, etc.

If you'd like to create a bit more separation in the database, you can add additional keys before layer. For example, you could put the layer information in the following location: geographic:best:layer:test_layer:default"2015-06-01"

These additional keys can be specified in the request parameters: time_service?key1=epsg4326&key2=best. They are processed in order. Up to 5 additional keys are allowed.

Database Handlers

Redis

  • handler_type -- set to "redis".
  • host -- sets the hostname for the Redis database you'll be using. Should be in quotes.
  • port (optional) -- sets the port number for your Redis database. Defaults to 6379.

Filename Format Handling

The time service returns a filename formatted with the snapped date. This allows you to customize the naming conventions used by the imagery files. A handful of different output formatters are available.

To specify the formatter, set the filename_format value (see example above) to one of the following values.

  • basic Outputs filenames in this format: [layer_name]-[date] where the snapped date is in the format %Y%j%H%M%S.

  • strftime Outputs filenames in this format: [layer_name][date], where [date] is the snapped date formatted using a strftime-compatible template string. For more information, see (http://man7.org/linux/man-pages/man3/strftime.3.html). To set the format string, set the format_str value in the options table as in this example:

    local filenameFormatHandler = {filename_format="strftime", options={format_str="%Y%j"}}

  • hash Outputs filenames in this format: [hash]-[layer_name]-[date], where hash is the first 4 characters of of the MD5 hash of the string [layer_name]-[date] The date format is %Y%j%H%M%S.

If no filename format handler is specified, the basic handler will be used.

Create the Apache configuration

In your Apache configuration, you need to set up an endpoint that the service will run under. This can exist under a <Directory> or <Location> block.

You'll need to make sure the following directives are in place (for more information, consult the mod_wsgi documentation):

Add paths you would like to map to your time service like

WSGIScriptAlias /time_service/time /var/www/html/time_service/wsgi_app_time_service.py