Simulating A Networking Failure

Using Kurtosis you can control which services can communicate inside and across subnetworks. Kurtosis provides arbitrarily fine grained control over which services can communicate with each other. This can be useful to simulate network faults, partition tolerance or even partial network outages, including package drops.

This guide illustrates the concept of Kurtosis subnetworks by simluating a partial networking failure within a distributed system.

Step 1: Start all services in an enclave

The example system is composed of a Cassandra cluster with 3 nodes, 2 redundant services serving the main application and a single port proxy to load balance the requests.

MAIN_SUBNETWORK = "main_subnetwork"
SECONDARY_SUBNETWORK = "secondary_subnetwork"

def run(args):
    add_service(
        service_name="cassandra_node_1",
        config=ServiceConfig(
            ...
            subnetwork=MAIN_SUBNETWORK,
        ),
    )
    add_service(
        service_name="cassandra_node_2",
        config=ServiceConfig(
            ...
            subnetwork=MAIN_SUBNETWORK,
        ),
    )
    add_service(
        service_name="cassandra_node_3",
        config=ServiceConfig(
            ...
            subnetwork=MAIN_SUBNETWORK,
        ),
    )
    add_service(
        service_name="app_service_1",
        config=ServiceConfig(
            ...
            subnetwork=MAIN_SUBNETWORK,
        ),
    )
    add_service(
        service_name="app_service_2",
        config=ServiceConfig(
            ...
            subnetwork=MAIN_SUBNETWORK,
        ),
    )
    add_service(
        service_name="single_port_proxy",
        config=ServiceConfig(
            ...
            subnetwork=MAIN_SUBNETWORK,
        ),
    )

When all the services are added to the Kurtosis enclave, it will look something like:

                     Kurtosis enclave                  
  -----------------------------------------------------
 |                   main_subnetwork                   |
 |-----------------------------------------------------|
 |                                                     |
 |                  cassandra_node_1                   |
 |                  cassandra_node_2                   |
 |                  cassandra_node_3                   |
 |                    app_service_1                    |
 |                    app_service_2                    |
 |                  single_port_proxy                  |
 |                                                     |
  -----------------------------------------------------

At the start, all connections are allowed.

• all the Cassandra nodes can communicate with each other
• the two services can reach all Cassandra nodes
• the single port proxy is playing its role of dispatching requests to both app_service_1 and app_service_2

Setp 2: Add a second subnetwork by updating services

Subnetworks make it easy to simulate a network failure that completely isolates cassandra_node_2 and app_service_2 from the rest of the system.

First,cassandra_node_2 and app_service_2 need to be assigned to a distinct subnetwork, secondary_subnetwork. This can be done by updating those services.

This can be done by "updating" those services.

MAIN_SUBNETWORK = "main_subnetwork"
SECONDARY_SUBNETWORK = "secondary_subnetwork"

def run(args):
    # ...
    # all the services are added using the code snippet above
    # ...

    update_service(
        service_name="cassandra_node_2",
        config=UpdateServiceConfig(
            subnetwork=SECONDARY_SUBNETWORK,
        ),
    )
    update_service(
        service_name="app_service_2",
        config=UpdateServiceConfig(
            subnetwork=SECONDARY_SUBNETWORK,
        ),
    )

Kurtosis has the concept of a default connection between subnetworks. The default connection is the connection configuration that all pairs of subnetworks will inherit. At enclave creation, the default connection is set to allow communication between subnetworks. In this case, when the cassandra_node_2 and app_service_2 are assigned secondary_subnetwork, nothing changes. main_subnetwork and secondary_subnetwork can continue to communicate because the default connection is unblocked.

                     Kurtosis enclave                  
  -----------------------------------------------------
 |     main_subnetwork     |   secondary_subnetwork    |
 |-----------------------------------------------------|
 |                         .                           |
 |   cassandra_node_1      .                           |
 |                         .    cassandra_node_2       |
 |   cassandra_node_3      .                           |
 |    app_service_1        .                           |
 |                         .      app_service_2        |
 |   single_port_proxy     .                           |
 |                         .                           |
  -----------------------------------------------------

Setp 3: Configure the connection between the two subnetworks

The creation of main_subnetwork and secondary_subnetwork now makes it possible to reconfigure the connection between them. Communications between the two subnetworks can now be blocked partially or completely by overriding the default connection for those two specific subnetworks.

MAIN_SUBNETWORK = "main_subnetwork"
SECONDARY_SUBNETWORK = "secondary_subnetwork"

def run(args):
    # ...
    # all the services are added and `cassandra_node_2` and `app_service_2` are updated using the code snippet above
    # ...

    set_connection(
        (MAIN_SUBNETWORK, SECONDARY_SUBNETWORK), 
        config=ConnectionConfig(
            packet_loss_percentage=90.0, # 90% of the packet are dropped
        ),
    )

The result is the following: cassandra_node_2 and app_service_2 are partially unreachable and the resilience of the system can be tested.

                     Kurtosis enclave                  
  -----------------------------------------------------
 |     main_subnetwork     |   secondary_subnetwork    |
 |-------------------------|---------------------------|
 |                         |                           |
 |   cassandra_node_1      |                           |
 |                         |     cassandra_node_2      |
 |   cassandra_node_3      |                           |
 |    app_service_1        |                           |
 |                         |       app_service_2       |
 |   single_port_proxy     |                           |
 |                         |                           |
  -----------------------------------------------------