Ocean Navigator

Contents

• Overview
• Development
• Automate CLASS4 pickle generation

Overview

Ocean Navigator is a Data Visualization tool that enables users to discover and view 3D ocean model output quickly and easily.

The model outputs are stored as NetCDF4 files. Our file management is now handled by an SQLite3 process that incrementally scans the files for a dataset, and updates a corresponding table so that the Python layer can only open the exact files required to perform computations; as opposed to the THREDDS aggregation approach which serves all the files in a dataset as a single netcdf file. The THREDDS approach was unable to scale to the sheer size of the datasets we deal with.

The server-side component of the Ocean Navigator is written in Python 3, using the Flask web API. Conceptually, it is broken down into three components:

• Query Server

  This portion returns metadata about the selected dataset in JSON format. These queries include things like the list of variables in the dataset, the times covered, the list of depths for that dataset, etc.

  The other queries include things such as predefined areas (NAFO divisions, EBSAs, etc), and ocean drifter paths. The drifter paths are loaded from NetCDF files, but all the other queries are loaded from KML files.

• Plotting

  This portion generates an image plot, which could be a map with surface fields (or fields at a particular depth), a transect through a defined part of the ocean, depth profiles of one or more points, etc. We use the matplotlib python module to generate the plots.

  Because the model grid rarely lines up with the map projection, and profiles and transects don't necessarily fall on model grid points, we employ some regridding and interpolation to generate these plots. For example, for a map plot, we select all the model points that fall within the area, plus some extra around the edges and regrid to a 500x500 grid that is evenly spaced over the projection area. An added benefit of this regridding is that we can directly compare across models with different grids. This allows us to calculate anomalies on the fly by comparing the model to a climatology. In theory, this would also allow for computing derived outputs from variables in different datasets with different native grids.

• Tile Server

  This portion is really a special case of the plotting component. The tile server serves 256x256 pixel tiles at different resolutions and projections that can be used by the OpenLayers web mapping API. This portion doesn't use matplotlib, as the tiles don't have axis labels, titles, legends, etc. The same style of interpolation/regridding is done to generate the data for the images.

  The generated tiles are cached to disk after they are generated the first time, this allows the user request to bypass accessing the NetCDF files entirely on subsequent requests.

The user interface is written in Javascript using the React framework. This allows for a single-page, responsive application that offloads as much processing from the server onto the user's browser as possible. For example, if the user chooses to load points from a CSV file, the file is parsed in the browser and only necessary parts of the result are sent back to the server for plotting.

The main display uses the OpenLayers mapping API to allow the user to pan around the globe to find the area of interest. It also allows the user to pick an individual point to get more information about, draw a transect on the map, or draw a polygon to extract a map or statistics for an area.

Development

Local Installation

The instructions for performing a local installation of the Ocean Data Map Project are available at:https://github.com/DFO-Ocean-Navigator/Navigator-Installer/blob/master/README.md

• While altering Javascript code, it can be actively transpiled using:
  • cd oceannavigator/frontend
  • yarn run dev
• There's also a linter available: yarn run lint.
• For production use the command:
  • rm -r oceannavigator/frontend
  • cd oceannavigator/frontend
  • yarn run build

SQLite3 backend

Since we're now using a home-grown indexing solution, as such there is now no "server" to host the files through a URL (at the moment). You also need to install the dependencies for the netcdf indexing tool. Then, download a released binary for Linux systems here. You should go through the README for basic setup and usage details.

The workflow to import new datasets into the Navigator has also changed:

1. Run the indexing tool linked above.
2. Modify datasetconfig.json so that the url attribute points to the absolute path of the generated .sqlite3 database.
3. Restart web server.

Running the webserver for development

Assuming the above installation script succeeded, your PATH should be set to point towards ${HOME}/miniconda/3/amd64/bin, and the navigator conda environment has been activated.

• Debug server (single-threaded):
  • python ./bin/runserver.py
• Multi-threaded (via gUnicorn):
  • ./bin/runserver.sh

Running the webserver for production

Using the launch-web-service.sh script will automatically determine how many processors are available, determine the platform's IP address, what port above 5000 can be used, print out the IP and port information. The IP:PORT information can then be copied to a web browser to access the Ocean Navigator web service either locally or shared with others. This script will also copy all information bring written to stdout and place the information in the ${HOME}/launch-on-web-service.log file.

• Multi-threaded (via gUnicorn):* ./bin/launch-web-service.sh

Coding Style (Javascript)

Javascript is a dynamically-typed language so it's super important to have clear and concise code, that demonstrates it's exact purpose.

• Comment any code whose intention may not be self-evident (safer to have more comments than none at all).
• Use var, let, and const when identifying variables appropriately:

  • var: scoped to the nearest function block. Modern ES6/Javascript doesn't really use this anymore because it usually leads to scoping conflicts. However, var allows re-declaration of a variable.

  • let: new keyword introduced to ES6 standard which is scoped to the nearest block. It's very useful when using for() loops (and similar), so don't predefine loop variable:

    • Bad:

       	myfunc() { 		var i; 		... 		// Some code 		... 		for (i = 0; i < something; ++i) { 		} 	}

    • Good:

       	myFunc() { 		... 		// Some code 		... 		for (let i = 0; i < something; ++i) { 		} 	}

    Keep in mind that let does not allow re-declaration of a variable.

  • const: functionally identical to the let keyword, however disallows variable re-assignment. Just like const-correctness in C++, const is a great candidate for most variable declarations, as it immediately states that "I am not being changed". This leads to the next rule.

• Use const when declaring l-values with require(). Example:

   	const LOADING_IMAGE = require("../images/bar_loader.gif");

• Unless using for loops, DO NOT use single-letter variables! It's an extreme nuisance for other programmers to understand the intention of the code if functions are littered with variables like: s, t, etc. Slightly more verbose code that is extremely clear will result in a much lower risk for bugs.
  • Bad:

     	$.when(var_promise, time_promise).done(function(v, t) { 		// Some code 		... 	}

  • Good:

     	$.when(var_promise, time_promise).done(function(variable, time) { 		// Some code 		... 	}

• Try to avoid massive if chains. Obviously the most important thing is to get a feature/bugfix working. However if it results in a whole bunch of nested if statements, or if-for-if-else, etc., try to take that working result and incorporate perhaps a switch, or hashtable to make your solution cleaner, and more performant. If it's unavoidable, a well-placed comment would reduce the likelihood of a fellow developer trying to optimize it.

Coding Style (Python)

Coming soon...

Automate CLASS4 pickle generation

In order to generate the class4.pickle file daily. You should create a crontab entry for the user account hosting the Ocean Navigator instance. Use the command crontab -e to add the string @daily ${HOME}/Ocean-Data-Map-Project/bin/launch-pickle.sh. Then once a day at midnight the script launch-pickle.sh will index all the CLASS4 files.

Proper handling of the datasetconfig.json and oceannavigator.cfg configuration files

In order to provide a production ready and off-site configuration files. We have implemented a new configurations repository. When people clone the Ocean-Data-Map-Project repository they will need to perform an additional step of updating any defined submodules. The following command changes your working directory to your local Ocean-Data-Map-Project directory and then updates the submodules recursively.

• cd ${HOME}/Ocean-Data-Map-Project ; git submodule update --init --recursive