Das2 servers typically provide data relevant to space plasma and magnetospheric physics research. To retrieve data, an HTTP GET request is posted to a das2 server by a client program and a self-describing stream of data values covering the requested time range, at the requested time resolution, is provided in the response body. This software, das2-pyserver provides a caching middleware layer between server-side das2 readers, which stream data at full resolution to standard out, and remote client programs such as Autoplot, SPEDAS and SDDAS, or custom programs written in Java (das2java), Python (das2py), IDL (das2pro, das2dlm ), or C (das2C ).

When a request for data is received, das2-pyserver inspects the HTTP GET URL and checks to see if its local cache contains the required data, at the desired time resolution or better. If the request is already cached, an HTTP response body is generated from cache blocks. If not, the associated reader program and data reducer listed in the DSDF (Data Source Description File) are invoked on the server and the standard output stream from the "reader_prog | reducer_prog" pipeline is delivered as the response body.

Das2-pyserver itself is released under the GPL, but we have intentionally separated the core server software from the data readers so that programs written in any programming language, released under almost any license, can plug into the system.

Compilation and installation of das2-pyserver has only been tested in Linux environments and depends on the following tools:

1. Python >= 3.4
2. Apache2, any remotely recent version
3. Redis, known to work with version 3.2 or higher
4. redis-py, known to work with version 2.10 or higher
5. das2C, latest version recommended

Since das2C provides small binaries needed by das2-pyserver, and since there are no pre-built das2C packages, installation instructions for both das2C and das2-pyserver are included below. In these instructions the '$' character is used at the beginning of a line to indicate commands that you'll need to run in a bourne compatible shell (bash, ksh, etc.).

Example prerequisite package installation commands are provided below for CentOS 7 ...

$ sudo yum install gcc git                               
$ sudo yum install expat-devel fftw-devel openssl-devel             
$ sudo yum install python3 python3-numpy python3-devel 
$ sudo pip3 install redis

... and Debian 9:

$ sudo apt-get install gcc git                           
$ sudo apt-get install libexpat-dev libfftw3-dev libssl-dev          
$ sudo apt-get install python3-dev python3-distutils python3-numpy
$ sudo apt-get install redis-server                                 
$ sudo apt-get install python3-redis

All sources are now on github.com

$ git clone https://github.com/das-developers/das2C.git
$ git clone https://github.com/das-developers/das2py.git
$ git clone https://github.com/das-developers/das2-pyserver.git

Decide where your das2-pyserver code and configuration information will reside. In the example below I've selected /var/www/das2srv but you can choose any location you like. These environment variables will be used through out the setup, so leaving your terminal window open though the testing stage will save time.

$ export PREFIX=/var/www/das2srv     # Adjust to taste
$ export N_ARCH=/                    # since das2 servers are typically machine bound
$ export PYVER=3.6                   # minimum 3.4
$ export SERVER_ID=solar_orbiter_2   # for example.  ID should not contain whitespace

Test your PYVER setting by making sure the following command brings up a python interpreter:

$ python$PYVER

Build and install commands can run without sudo if the install directory is created manually. The following will make the install directory and set it's ownership to the current account. We will lock it down after install.

$ sudo mkdir $PREFIX
$ sudo chown $LOGNAME $PREFIX

The following sequence will build, test, and install libdas2.3 and das2py if you have all prerequisite libraries installed:

$ cd das2C
$ make
$ make test     # Contacts remote services, okay if those tests fail
$ make install
$ cd ../

$ cd das2py
$ make 
$ make test     # Also contacts remote servers, okay if those tests fail
$ make install
$ cd ../

Now build and install the python module and example configuration files. Set --install-lib and --prefix as indicated, unless you want to hand edit das2server.conf after installation. There is no need to run build before this step.

$ cd ../das2-pyserver
$ python${PYVER} setup.py install --prefix=${PREFIX} --install-lib=${PREFIX}/lib/python${PYVER}

You can add the argument --no-examples to avoid installing the example datasets if these are not desired.

Copy over the example configuration file:

$ cd $PREFIX/etc
$ cp das2server.conf.example das2server.conf

We are done with server software installation, lock down the install area (if desired). The cache subdirectory shoud be owned by the account that runs asynchronous data-reduction processing. The cache subdirectory should not be owned by the webserver account, or root, but any other account is fine.

$ sudo chown -R root:root $PREFIX
$ sudo chown $LOGNAME $PREFIX/cache         # i.e. any non-apache, non-root account

Apache configurations vary widely by Linux distribution and personal taste. The following procedure is provided as an example and has been tested on CentOS 7.

First determine which directory on your server maps to an Apache HTTPS CGI directory. To provide better URLs for your site add the line:

ScriptAlias /das/ "/var/www/cgi-das/"

directly under the line:

ScriptAlias /cgi-bin/ "/var/www/cgi-bin/"

inside the <IfModule alias_module> section of httpd.conf.

Of course the cgi-das directory will have to be created. and on CentOS anyway, the proper SELinux context applied:

sudo mkdir /var/www/cgi-das
# You should apply a proper SELinux context to this directory, though the author
# does not know how to do so.  Any contributions on this manner are welcome as
# SELinux does not report to the console and audit logs are ususally empty.

Then provide configuration information for your /var/www/cgi-das directory inside the /etc/httpd/conf.d/ssl.conf file. We're editing the ssl.conf instead of httpd.conf because das2 clients may transmit passwords.

<Directory "/var/www/cgi-das">
  Options ExecCGI FollowSymLinks

  # Make sure Authorization HTTP header is available to Das CGI scripts
  RewriteRule ^ - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
  RewriteEngine on

  AllowOverride None
  Allow from all
  Order allow,deny
</Directory>

By default, authorization headers are not made available to CGI scripts. The re-write rule above allows the Authorization header to be passed down to the das2_srvcgi_main script. This is needed to allow your server to support password protected datasets.

Now symlink the top level CGI scripts into your new CGI directory. Choose the name of the symlink carefully as it will be part of the public URL for your site:

$ cd /var/www/cgi-das
$ sudo ln -s $PREFIX/bin/das2_srvcgi_main server
$ sudo ln -s $PREFIX/bin/das2_srvcgi_logrdr log

The main server script needs to be able to find the main log reader script and vice versa. If you use something other than the default values above update the following config entries in your das2server.conf.

VIEW_LOG_URL = "log"
MAIN_SRV_URL = "server"

Set the permissions of the log directory so that Apache can write logging information:

$ chmod 0777 $PREFIX/log   # Or change the directory ownership

Finally, trigger a re-read of the Apache's configuration data:

$ sudo systemctl restart httpd.service
$ sudo systemctl status httpd.service

Test the server by pointing your web browser at:

https://localhost/das/server
https://localhost/das/log

If this works, try browsing your new server with Autoplot. To do so, copy the following URI into the Autoplot address bar and hit the green "Go" button:

vap+das2server:https://localhost/das/server

The CGI scripts and worker programs read thier configuration data from the file:

$PREFIX/etc/das2server.conf

Take time to customize a few items in your config file such as the site_name and the contact_email. You may also want change the file ${PREFIX}/static/logo.png or even the style sheet at ${PREFIX}/static/das2server.css to something a little nicer.

Das2-pyserver is a caching and web-transport layer for das2 readers. Readers are the programs that generate the initial full resolution data streams. The entire purpose of das2-pyserver and das2 clients is to leverage the output of your reader programs to produce efficient, interactive science data displays. Example readers are included in the $PREFIX/examples directory to assist you with the task of creating readers for your own data. These examples happen to be written in python, however there is no requirement to use python for your programs, in fact much more efficent compiled languages such as Java, D and C++ are more suitable for the task. Any language may be used so long as:

1. all data are written to standard output
2. all error messages are written to standard error

For further information on your das2-pyserver instance, including:

• reader programs
• authentication
• request caching
• automatic HAPI conversion
• federated catalog integration
• EPN-TAP integration

consult the users guide document das2_pyserver_ug.pdf included in the docs directory of the repository.

All code in the examples directory (not including the temporary pycdf subdirectory) is release under the UNLICENSE and may be used without restrictions of any kind.