Best Practices Using Pollen API Web Services

The Google Maps Platform web services are a collection of HTTP interfaces to Google services providing geographic data for your maps applications.

This guide describes some common practices useful for setting up your web service requests and processing service responses. Refer to the developer’s guide for full documentation of the Pollen API.

What is a web service?

Google Maps Platform web services are an interface for requesting Maps API data from external services and using the data within your Maps applications. These services are designed to be used in conjunction with a map, as per the License Restrictions in the Google Maps Platform Terms of Service.

The Maps APIs web services use HTTP(S) requests to specific URLs, passing URL parameters and/or JSON-format POST data as arguments to the services. Generally, these services return data in the response body as JSON for parsing and/or processing by your application.

The following example shows the URL of a REST GET request to the forecast:lookup method:

https://pollen.googleapis.com/v1/forecast:lookup?&key=API_KEY

Note: All Pollen API applications require authentication. Get more information on authentication credentials.

SSL/TLS Access

HTTPS is required for all Google Maps Platform requests that use API keys or contain user data. Requests made over HTTP that contain sensitive data may be rejected.

Building a valid URL

You may think that a "valid" URL is self-evident, but that's not quite the case. A URL entered within an address bar in a browser, for example, may contain special characters (e.g. "上海+中國"); the browser needs to internally translate those characters into a different encoding before transmission. By the same token, any code that generates or accepts UTF-8 input might treat URLs with UTF-8 characters as "valid", but would also need to translate those characters before sending them out to a web server. This process is called URL-encoding or percent-encoding.

Special characters

We need to translate special characters because all URLs need to conform to the syntax specified by the Uniform Resource Identifier (URI) specification. In effect, this means that URLs must contain only a special subset of ASCII characters: the familiar alphanumeric symbols, and some reserved characters for use as control characters within URLs. This table summarizes these characters:

Summary of Valid URL Characters

Set	characters	URL usage
Alphanumeric	a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 0 1 2 3 4 5 6 7 8 9	Text strings, scheme usage (`http`), port (`8080`), etc.
Unreserved	- _ . ~	Text strings
Reserved	! * ' ( ) ; : @ & = + $ , / ? % # [ ]	Control characters and/or Text Strings

When building a valid URL, you must ensure that it contains only those characters shown in the table. Conforming a URL to use this set of characters generally leads to two issues, one of omission and one of substitution:

Characters that you wish to handle exist outside of the above set. For example, characters in foreign languages such as 上海+中國 need to be encoded using the above characters. By popular convention, spaces (which are not allowed within URLs) are often represented using the plus '+' character as well.
Characters exist within the above set as reserved characters, but need to be used literally. For example, ? is used within URLs to indicate the beginning of the query string; if you wish to use the string "? and the Mysterions," you'd need to encode the '?' character.

All characters to be URL-encoded are encoded using a '%' character and a two-character hex value corresponding to their UTF-8 character. For example, 上海+中國 in UTF-8 would be URL-encoded as %E4%B8%8A%E6%B5%B7%2B%E4%B8%AD%E5%9C%8B. The string ? and the Mysterians would be URL-encoded as %3F+and+the+Mysterians or %3F%20and%20the%20Mysterians.

Common characters that need encoding

Some common characters that must be encoded are:

Unsafe character	Encoded value
Space	`%20`
"	`%22`
<	`%3C`
>	`%3E`
#	`%23`
%	`%25`
\|	`%7C`

Converting a URL that you receive from user input is sometimes tricky. For example, a user may enter an address as "5th&Main St." Generally, you should construct your URL from its parts, treating any user input as literal characters.

Additionally, URLs are limited to 16384 characters for all Google Maps Platform web services and static web APIs. For most services, this character limit will seldom be approached. However, note that certain services have several parameters that may result in long URLs.

Polite Use of Google APIs

Poorly designed API clients can place more load than necessary on both the Internet and Google's servers. This section contains some best practices for clients of the APIs. Following these best practices can help you avoid your application being blocked for inadvertent abuse of the APIs.

Exponential Backoff

In rare cases something may go wrong serving your request; you may receive a 4XX or 5XX HTTP response code, or the TCP connection may simply fail somewhere between your client and Google's server. Often it is worthwhile re-trying the request as the followup request may succeed when the original failed. However, it is important not to simply loop repeatedly making requests to Google's servers. This looping behavior can overload the network between your client and Google causing problems for many parties.

A better approach is to retry with increasing delays between attempts. Usually the delay is increased by a multiplicative factor with each attempt, an approach known as Exponential Backoff.

For example, consider an application that wishes to make this request to the Time Zone API:

https://maps.googleapis.com/maps/api/timezone/json?location=39.6034810,-119.6822510&timestamp=1331161200&key=YOUR_API_KEY

The following Python example shows how to make the request with exponential backoff:

import json
import time
import urllib.error
import urllib.parse
import urllib.request

# The maps_key defined below isn't a valid Google Maps API key.
# You need to get your own API key.
# See https://developers.google.com/maps/documentation/timezone/get-api-key
API_KEY = "YOUR_KEY_HERE"
TIMEZONE_BASE_URL = "https://maps.googleapis.com/maps/api/timezone/json"


def timezone(lat, lng, timestamp):

    # Join the parts of the URL together into one string.
    params = urllib.parse.urlencode(
        {"location": f"{lat},{lng}", "timestamp": timestamp, "key": API_KEY,}
    )
    url = f"{TIMEZONE_BASE_URL}?{params}"

    current_delay = 0.1  # Set the initial retry delay to 100ms.
    max_delay = 5  # Set the maximum retry delay to 5 seconds.

    while True:
        try:
            # Get the API response.
            response = urllib.request.urlopen(url)
        except urllib.error.URLError:
            pass  # Fall through to the retry loop.
        else:
            # If we didn't get an IOError then parse the result.
            result = json.load(response)

            if result["status"] == "OK":
                return result["timeZoneId"]
            elif result["status"] != "UNKNOWN_ERROR":
                # Many API errors cannot be fixed by a retry, e.g. INVALID_REQUEST or
                # ZERO_RESULTS. There is no point retrying these requests.
                raise Exception(result["error_message"])

        if current_delay > max_delay:
            raise Exception("Too many retry attempts.")

        print("Waiting", current_delay, "seconds before retrying.")

        time.sleep(current_delay)
        current_delay *= 2  # Increase the delay each time we retry.


if __name__ == "__main__":
    tz = timezone(39.6034810, -119.6822510, 1331161200)
    print(f"Timezone: {tz}")

You should also be careful that there isn't retry code higher in the application call chain that leads to repeated requests in quick succession.

Synchronized Requests

Large numbers of synchronized requests to Google's APIs can look like a Distributed Denial of Service (DDoS) attack on Google's infrastructure, and be treated accordingly. To avoid this, you should make sure that API requests are not synchronized between clients.

For example, consider an application that displays the time in the current time zone. This application will probably set an alarm in the client operating system waking it up at the start of the minute so that the displayed time can be updated. The application should not make any API calls as part of the processing associated with that alarm.

Making API calls in response to a fixed alarm is bad as it results in the API calls being synchronized to the start of the minute, even between different devices, rather than being distributed evenly over time. A poorly designed application doing this will produce a spike of traffic at sixty times normal levels at the start of each minute.

Instead, one possible good design is to have a second alarm set to a randomly chosen time. When this second alarm fires the application calls any APIs it needs and stores the results. When the application wants to update its display at the start of the minute, it uses previously stored results rather than calling the API again. With this approach, API calls are spread evenly over time. Further, the API calls do not delay rendering when the display is being updated.

Aside from the start of the minute, other common synchronization times you should be careful not to target are at the start of an hour, and the start of each day at midnight.

Processing Responses

This section discusses how to extract these values dynamically from web service responses.

The Google Maps web services provide responses which are easy to understand, but not exactly user friendly. When performing a query, rather than display a set of data, you probably want to extract a few specific values. Generally, you will want to parse responses from the web service and extract only those values which interest you.

The parsing scheme you use depends on whether you are returning output in JSON. JSON responses, being already in the form of Javascript objects, may be processed within Javascript itself on the client.