DynamoDB_Geo
DynamoDB
This is an attempt at storing and querying geohash data in DynamoDB.
We store objects in DynamoDB, when we query, we look for a customizable number of stored items by zooming out exactly one level.
We return the objects in an attempt at the closest items first.
Items are searched as the following.
Given the lat, long we calculate a hash - assume 9x0qz. We go up one level to 9x0q and calculate all neighbouring hashes.
9x0p 9x0r 9x0x
9x0n 9x0q 9x0w => ["9x0q", "9x0r", "9x0x", "9x0w", "9x0t", "9x0m", "9x0j", "9x0n", "9x0p"]
9x0j 9x0m 9x0t
From this, we ask DynamoDB starting from our own cell, going north, and wrapping clockwise for all objects in these cells up to a configurable number of objects.
We then calculate the neighbours of our more local hash 9x0qz, and sort the results of the larger hashes using similar rules.
9x0rn 9x0rp 9x0x0
9x0qy 9x0qz 9x0wb => ["9x0qz", "9x0rp", "9x0x0", "9x0wb", "9x0w8", "9x0qx", "9x0qw", "9x0qy", "9x0rn"]
9x0qw 9x0qx 9x0w8
Using the same pattern as before (Middle -> North -> Clockwise), we sort the returned stores giving priority to the localization.
Real talk
I know this isn't the most wonderful way to do this, I am still trying to think of something better. Currently it uses up to 8 queries to DynamoDB, I'd like to cut that down to a single query.
There is a similar library written in Java and JS, but it uses Google S2 for the Geohashing, which has properties that allow them to do the zoom-out technique with a single query, however Google S2 does not exist as a C library (or Ruby library for that matter). The other alternative is Uber H3, however it has the same issues of not being C, or Ruby.
If we were able to use a unique but deterministic way to calcuate the range key based off of the hash key that would allow us to query every single larger cell in a single batch query. I am currently thinking of more clever ways to do this - obviously I am open to suggestions.
Objects
DynamodbManager
Attributes
table_name:
Sets the DynamoDB Table Name
hash_key:
Sets the name of the primary hash attribute
range_key:
Sets the name of the sort/range attribute
geohash_key:
Sets the name of the localized hash attribute
geojson_key:
Sets the name of the metadata attribute
hash_key_length:
Sets size of the outer hash length
local_area_size:
Sets the size of the inner hash length
max_item_return:
Sets the max number of items to return
Methods
Initialization
Description: Creates a new DynamodbManager object. Inputs are variables to your AWS account.
If access_key_id and secret_access_key are provided they are used.
If not provided, it falls back to ENV variables, then secret credential storage (profile name).
All arguments are keyword arguments
Input: region => String
table_name => String
access_key_id => String
secret_access_key => String
profile_name => 'default'
Output: Aws::DynamoDB::Client
#new => Object
Building and describing a DynamoDB Table
Description: Shows the current configured table, or creates a table to configured as requested
Input:
Output: Aws::DynamoDB::Types::DescribeTableOutput
#table => Object
Creating a new item
Description: Inserts a new item
Input: Store
Output: Aws::DynamoDB::Types::PutItemOutput
#put_store => Object
Querying stores
Description: Look for stores dependent on the input Lat, Long (as described above)
Input: Store
Output: Array[Store]
#get_stores => Array[Store]
Store
Methods
Initialization
Description: Initialization
Input: Hash => {
latitude # Required
longitude # Required
address
city
state
zip
area_code
phone
name
geohash # Calculated based on lat,long if not provided
}
Output: Store
#new => Store
DynamodbGeo
This only exists as a quick and easy way to create a DynamodbManager. The only method is .new and it passes all arguments along to DynamodbManager and returns an instance of DynamodbManager
Geohashing
Includes a Geohash implimentation written in C
https://github.com/simplegeo/libgeohash
Methods
Encoding a Geohash
Description: Takes in a latitude, longitude, and precision, and returns a geohash as a string
Input: latitude => Number
longitude => Number
precision => Number
Output: Geohash => String
Geohash.encode(lat, long, precision) => String
Decoding a Geohash
Description: Takes in a hash, and returns a Geocoord
Input: Geohash => String
Output: Geocoord => Object
Geohash.decode(hash) => Geocoord
Viewing all my neighbours
Description: Takes in a Geohash and shows the surrounding 8 neighbours
Physical representation:
NW N NE 7 0 1
W X E => 6 X 2
SW S SE 5 4 3
Flat representation:
N NE E SE S SW W NW
0 1 2 3 4 5 6 7
Input: Geohash => String
Output: Array[Geocoord]
Geohash.neighbours(hash) => Array[Geohash]
Viewing one of my neighbours
Description: Takes in a Geohash and shows the neighbour in the cardinal direction
NORTH = 0
EAST = 1
SOUTH = 2
WEST = 3
Input: Geohash => String
Direction => Integer
Output: Geohash
Geohash.neighbour(hash) => Geohash
Get width and height about a specific precision
Description: Takes in a precision value, returns the height and width of the box
Input: precision => Number
Output: Dimension => Object
Geohash.dimensions(precision) => Dimension
