Curl Access for CaosDB
======================

As the API that is used to communicate with the CaosDB server is XML
over HTTP also simple HTTP clients, such as
`cURL <https://curl.haxx.se/>`__ can be used. cURL is an old and
established command line program for transferring files over networks
that implements various protocols including HTTP/HTTPS. It is installed
by default on many Linux distributions and can therefore be very useful
for testing and debugging of CaosDB.

This small manual also gives some practical insights about the CaosDB
protocol itself.

Doing a simple retrieve
-----------------------

So, let’s start right away with a few basic examples.

Let’s do a query on our demo instance:

.. code:: bash

   curl "https://demo.indiscale.com/Entity/?query=FIND%20Experiment"

By default cURL sends an HTTP GET request which is needed for doing
queries for CaosDB. CaosDB requires sending them to ``/Entity``. The
query itself is specified after the HTTP query string ``?query=``.
``%20`` is specific to URL encoding and corresponds to a space (see
https://en.wikipedia.org/wiki/Percent-encoding for details). So the
actual query we were doing was ``FIND Experiment`` which should return
all entities with name or with parent ``Experiment``.

The response should look like:

.. code:: xml

   <?xml version="1.0" encoding="UTF-8"?>
   <?xml-stylesheet type="text/xsl" href="https://demo.indiscale.com/webinterface/webcaosdb.xsl" ?>
   <Response srid="b56e3d1a442460c46dde924a54e8afba" timestamp="1561453363382" baseuri="https://demo.indiscale.com" count="3">
     <UserInfo>
       <Roles>
         <Role>anonymous</Role>
       </Roles>
     </UserInfo>
     <Query string="FIND Experiment" results="3">
       <ParseTree>(cq FIND (entity Experiment) &lt;EOF&gt;)</ParseTree>
       <Role />
       <Entity>Experiment</Entity>
       <TransactionBenchmark since="Tue Jun 25 11:02:43 CEST 2019" />
     </Query>
     <RecordType id="212" name="Experiment">
       <Permissions />
     </RecordType>
     <RecordType id="215" name="UnicornExperiment">
       <Permissions />
       <Parent id="212" name="Experiment" />
     </RecordType>
     <Record id="230">
       <Permissions />
       <Parent id="215" name="UnicornExperiment" />
       <Property id="216" name="date" datatype="DATETIME" importance="FIX">
         2025-10-11
         <Permissions />
       </Property>
       <Property id="217" name="species" datatype="TEXT" importance="FIX">
         Unicorn
         <Permissions />
       </Property>
       <Property id="214" name="Conductor" datatype="TEXT" importance="FIX">
         Anton Beta
         <Permissions />
       </Property>
       <Property id="221" name="Photo" datatype="Photo" importance="FIX">
         226
         <Permissions />
       </Property>
       <Property id="220" name="LabNotes" datatype="LabNotes" importance="FIX">
         227
         <Permissions />
       </Property>
       <Property id="218" name="UnicornVideo" datatype="UnicornVideo" importance="FIX">
         228
         <Permissions />
       </Property>
       <Property id="219" name="UnicornECG" datatype="UnicornECG" importance="FIX">
         229
         <Permissions />
       </Property>
     </Record>
   </Response>

We can see, that in fact two ``RecordTypes`` and one ``Record`` are
returned. Furthermore the response contains some additional information:

-  Attributes in the ``Response`` tag:

   -  ``srid="b56e3d1a442460c46dde924a54e8afba"`` A unique identifier
      for this request.
   -  ``timestamp="1561453363382"`` The `UNIX
      timestamp <https://en.wikipedia.org/wiki/Unix_time>`__ for this
      request.
   -  ``baseuri="https://demo.indiscale.com"`` The base URI of the
      instance of CaosDB that performed the request.
   -  ``count="3"`` The number of results (2 ``RecordTypes`` and 1
      ``Record``)

-  Information about the user in ``UserInfo``. In this case we have not
   logged in, so we are anonymous.
-  Detailled information about the query. This includes for example the
   parse tree and can be used for debugging and testing. Depending on
   the settings of the server instance, this tag includes more or less
   detail, for example a more detailled transaction benchmark.

More details about the retrieve
-------------------------------

The cURL statement used in the previous section made use of a lot of
default settings for cURL. Let’s have a closer look behind the options.
(I assigned the above URL to a shell variable, to make the statement
more readable.)

.. code:: bash

   URL="https://demo.indiscale.com/Entity/?query=FIND%20Experiment"

   curl -X GET -b cookie.txt -D head.txt $URL

This command specifies three more options:

-  ``-X GET`` Do a GET request. This can of course be replaced by POST,
   PUT, DELETE or any other HTTP operation.
-  ``-b cookie.txt`` This instructs cURL to use cookies from the file
   ``cookie.txt`` (which we don’t have yet, see below)
-  ``-D head.txt`` Tell cURL to store the received header in the file
   ``head.txt``.

Running this command will give us a similar response than in the
previous section, but additionally a file ``head.txt``:

::

   HTTP/1.1 200 OK
   Content-Type: text/xml; charset=UTF-8
   Date: Tue, 25 Jun 2019 09:17:23 GMT
   Accept-Ranges: bytes
   Server: Restlet-Framework/2.3.12
   Vary: Accept-Charset, Accept-Encoding, Accept-Language, Accept
   Transfer-Encoding: chunked

There is nothing special in that header. Most importantly the request
has lead to a response without HTTP error.

Logging in
----------

You might have asked yourself, what we need the cookie for. The simple
answer is: authentication. For many operations done on the CaosDB server
we have to log in first. The demo instance is configured to allow for
anonymous read access by default. But depending on the instance you are
accessing, even this might be disallowed.

You can log in to the server using cURL with the following command:

.. code:: bash

   URL="https://demo.indiscale.com/login"

   curl -X POST -c cookie.txt -D head.txt -H "Content-Type: application/x-www-form-urlencoded" -d username=salexan -d password=$PW $URL

So here we are doing a ``POST`` request instead of a ``GET``. We are
instructing cURL to use the ``cookie.txt`` file to **store** the cookies
it recieves. This is done using the ``-c`` option (instead of the ``-b``
option above). This time we are explicitely specifying the content type
of the content we are sending (using the ``POST`` request) with the
``-H`` option. The actual content sent are the two fields specified
using the ``-d`` options. This boils down to the two key-value-pairs
“username=” and “password=”.

This time we are sending the information to a different context
indentified by ``/login``.

If you don’t want to supply your passowrd in plain text you can for
example use a password manager (like I do), in my case
`pass <https://www.passwordstore.org/>`__ as follows to store your
password in the variable ``$PW``:

.. code:: bash

   PW=$(pass your/passowrd/identifier)

Custom Certificates
-------------------

If you are running your own CaosDB instance it might be necessary to use
a custom SSL certificate.

You can specify this using the option ``--cacert``, e.g.:

::

   --cacert "/path/to/certificate/root.cert.pem"

Uploading files
---------------

According to the specification, a file upload is a POST with multipart
form data. This can be achieved using CURL with the following simple
command line:

.. code:: bash

   curl -b cookie.txt \
        -F "FileRepresentation=<file.xml" \
        -F "testfile.bla=@testfile.bla" \
        $URL

Here I am using a previously stored cookie from cookie.txt.

There are two prerequisites to executing the above command: - The file
representation ``file.xml`` which is in this case stored in a file. -
The actual file to be uploaded with name ``testfile.bla``. The left hand
side of the assignment is very important as the identifier given here
(which in this case is ``testfile.bla``) will be used in the XML for
identifying the file that is described in the corresponding XML tag.

Let’s have a look at the contents of file.xml:

.. code:: xml

   <Post>
     <File upload="testfile.bla" destination="/test/testfile.bla" description="bla" 
   checksum="672f8ff4ae8530de295f9dd963724947841e6277edec3b21820b5e44d0a64baef90fb04e22048028453d715f79357acc5bd2d566fe6ede65f981ba3dda06bae4"
   size="3"/>
   </Post>

The attributes have the following meaning: - ``upload=testfile.bla`` The
filename given here is actually no filename, but an identifier to find
the multipart data chunk that contains the file. I called it
``testfile.bla`` for simplicity here. -
``destination="/test/testfile.bla"`` The destination path on the CaosDB
server file system.

Before looking at the other attributes let’s have a look at the file
``testfile.bla`` itself:

::

   ok

The file has size “3” which can be verified on linux using a:

.. code:: bash

   stat testfile.bla

It’s hashsum is important for checking the integrity after the transfer
to the server. It can be computed on linux using:

.. code:: bash

   sha512sum testfile.bla

These information has to be supplied as the remaining attributes to the
XML.