Please note that this part of the documentation is not yet finished. It will still be revised and updated.

Connection objects can be used to define connections between Scenario-Devices and Setup-Devices. This allows to specify the connections you need (Scenario-Devices) or which connections you have (Setup-Devices). So for example, if you have a scenario, that needs a serial connection between two devices, it doesn’t matter if your setup has both, a serial and an ethernet connection. However, the other way around it does matter. If your scenario needs both, a serial and an ethernet connection, but your setup only has a serial connection, it will not work.

Balder is a scenario based testsystem, for which it is necessary to determine if a Scenario matches with a Setup. For this, in addition to the feature matching, especially the Connection trees between the devices are important.

This section shows the basic functionality of connections and how you can use them in the Balder ecosystem.

Global connection-trees

Internally Balder knows exactly how the connections are arranged with each other. For this it refers to the global connection-tree. For example, this tree defines that a TcpV4Connection is based on an IPV4Connection. It also knows that a HttpConnection is based on an TcpV4Connection or an TcpV6Connection.

The global connection tree holds all this relationships.

You can add your own connection object to that tree or also define a complete new tree by your own. You can find more about this in the section Connection-Trees.


Every time you make a connection statement, you create a sub-connection tree. So for example, if you connect two devices with each other over an HttpConnection, this would be a sub-connection tree. A sub-connection-tree is always a part of the global-connection-tree. This allows to make a short definition, by jumping over some other connections.

For example:


This statement is the same like the following:


But why are these both statements the same? - the normal Balder global-connection-tree is defined like the following structure:

IPV4Connection             IPV6Connection
      |                          |
   TcpV4Connection    TcpV6Connection
                |        |

Balder will automatically resolve UNRESOLVED sub-trees according to its current active global-connection-tree.

OR/AND connection relations

You can combine connection objects with each other. This makes it possible that a connection is based on a connection or on another connection (OR). So for example, a HTTP connection can be based on TcpV4 OR TcpV6:

conn = HttpConnection.based_on(TcpV4Connection, TcpV6Connection)

You can specify OR dependencies simply by providing a list of Connection objects or as seen in our example above, the most functions provides multiple arguments. These are always OR relationships.

It is also possible that a connection requires multiple other connections (AND). For example a DnsConnection requires a UdpConnection AND a TcpConnection, because DNS uses UDP per default, but it uses TCP for requests that sends data that is to much for UDP.

So we can define an AND connection simply by using tuples:

conn = DnsConnection.based_on((UdpConnection, TcpConnection))

Limits of connection-relations

You can define AND/OR definitions in almost any possible variation, but there is one limit. It is not allowed to define AND connections inside other AND connections, so for example:

conn = SpecialConnection.based_on((AConnection, BConnection), CConnection)
conn = SpecialConnection.based_on((AConnection, BConnection, (CConnection, DConnection)))

The last definition is not allowed because we use an inner AND connection there. We can write the same logic more easier by refactoring the both AND relations:

# same like the NOT ALLOWED tree from above - BUT NOW IT IS ALLOWED
conn = SpecialConnection.based_on((AConnection, BConnection, CConnection, DConnection)))

This limitation makes it easier to read the logic.

Using the base connection object

You can use the base connection object for different use cases.

General connection

If you want to specify that you need a connection, but it doesn’t matter which connection type, you can use the Connection class.

conn = Connection()

This is the universal connection that describes a can-be-everything connection

A general connection does never have based-on elements!

Container connection

Sometimes you want to create a statement AConnection OR BConnection. This can easily defined with an container connection:

conn = Connection.based_on(AConnection, BConnection)

A container connection always has based-on elements.

Defining your own connection

Balder allows to define own connections. For that you have to provide a connections module somewhere in your project. Balder automatically looks into all existing modules with this name and loads all custom connections.

If you want to define your own connection class, you have to create a new class that inherits from the general Connection class:

# file `lib/`

import balder
import balder.connections as conns

class MyConnection(balder.Connection):

This sets and enables the connection. But till now, it is inserted without some parent or child dependencies.

Inserting into the tree

You can insert your connection also in the global connection tree. For this you have to insert it with the decorator @balder.insert_into_tree(..). This decorator allows you to define parents of the connection. These dependencies will be set globally for the whole Balder session. If you have a connection that is based on a TcpV4 connection, you can implement this easily:

# file `lib/`

import balder
import balder.connections as conns

class MyConnection(balder.Connection):


Note that we do not use inheritance to specify children connections. If you want to add a connection and insert it into the global connection tree, use the decorator @balder.insert_into_tree(..).


Note that you have to add the connection into a file or make it importable from a connections module (directory connections with file). It is only require that the module has the name connections, but it doesn’t matter where it is located inside your environment.

You are now able to use this connection. It is integrated in the project global connection tree.

Global connection tree

In Balder all connections are embedded in a so called global connection trees. This tree defines how the connections are arranged to each other.

The global connection tree

Balder provides an global connection tree. This tree is already specified for all integrated connections objects (see ConnectionsAPI). Per default Balder uses this pre-defined tree.


COMING SOON - We are working on a graphical tool to show this global connection tree.

Overwrite the default global tree

Per default, the @balder.insert_into_tree(..) decorator inserts the connection in the global connection tree. If you want to use another connection tree, you can specify the tree_name=".." argument in the @balder.insert_into_tree(..). This allows to specify an complete own connection tree by your own.


If you want to use your newly defined global tree, you have to set the property used_global_connection_tree in the BalderSettings object of your testenvironment to the same name!


If you define a global-connection-tree by your own, every pre-defined arrangement is not applicable anymore.

So let’s take a look at the following example:

# file ''

import balder

class Settings(balder.BalderSettings):
    used_global_connection_tree = 'my_project_one'
# file `lib/`
import balder
from balder import connections as conns

@balder.insert_into_tree(parents=[conns.TcpConnection], tree_name="my_project_one")
class MyTcpConnection():


Be careful with changing the standard connection tree. With that, there is no connection included in the tree anymore, so you have to define every connection by yourself. If you use standard Balder connections note that some BalderHub projects uses the original Balder connections.

If you want to change the tree dependencies for an existing tree, you can use the class method set_parents(..).

from balder import connections as conns

    parents=[(conns.UdpConnection, conns.TcpConnection)], tree_name="my_project_one")

What means CONTAINED-IN?


This section is still under development.