Wind Tunnel

Performance testing for Holochain, modelled as load tests. The name is a reference to aerodynamics testing and is a good way to refer to this project but the language does not extend to the code.

Navigating the Project

The Wind Tunnel Framework

The wind-tunnel framework, which is found in ./framework, is a collection of crates that implement the core logic for running load tests and collecting results. This code is not specific to Holochain and should stay that way. It provides extension points for hooking in Holochain specific behaviour.

It is divided into the following crates:

• wind_tunnel_instruments: Tools for collecting and reporting metrics.
• wind_tunnel_instruments_derive: Procedural macros to generate code that helps integrate the wind_tunnel_instruments crate.
• wind_tunnel_runner: The main logic for running load tests. This is a library that is designed to be embedded inside a test binary.

Holochain Bindings for Wind Tunnel

The bindings, found in the ./bindings, customise the wind-tunnel framework to Holochain. They are what you would be consuming when using wind-tunnel to test Holochain.

The bindings contains the following crates:

• holochain_client_instrumented: A wrapper around the holochain_client that uses instruments and instruments_derive to instrument the client. It exports an API that is nearly identical to the holochain_client crate, except that when constructing client connections you need to provide a reporter which it can write results to.
• holochain_wind_tunnel_runner: This is a wrapper around the wind_tunnel_runner crate that provides Holochain specific code to be used with the wind_tunnel_runner. The wind_tunnel_runner is re-exported, so you should just use this crate as your runner when creating tests for Holochain.

Scenarios

The scenarios, found in ./scenarios, are what describe the performance testing scenarios to be run against Holochain. Each scenario is a binary that uses the holochain_wind_tunnel_runner as a library. When it is run it will have all the capabilities that wind-tunnel provides.

There is more information about how to create scenarios in a separate section.

Creating hApps for use in scenarios

[!NOTE] This section is optional, you can skip it if you are working outside this repository or choose your own hApp packaging strategy.

When a scenario is run, it may install a hApp from any place or using any method that Holochain supports. While working in this repository, there are some helpers to make this easier.

The zomes directory contains Rust projects that are intended to be built into zomes. Please check the directory structure and naming of convention for existing zomes when adding new ones. In particular:

• Each zome should be in its own directory with a name that describes its purpose.
• Each zome should keep its coordinator and integrity zomes separate as Rust projects in coordinator and integrity directories.
• Each zome should reference the shared zome build script as build = "../../wasm_build.rs"
• The library that gets produced by the zome should be consistently named in the [lib] section as <zome_name>_(coordinator|integrity).

When you want to use one or more zomes in a scenario, you should package them into a hApp for that scenario. To achieve this your scenario needs to do three things:

1. Reference the custom build script which will package the zomes into a hApp for you as build = "../scenario_build.rs"
2. Add custom sections to the Cargo.toml to describe the hApps you need in your scenario. There is an example at the end of this section.
3. Reference the installed app from your scenario using the provided macro scenario_happ_path!("<hApp name>"). This produces a std::path::Path that can be passed to Holochain when asking it to install a hApp from the file system.

Adding a hApp to your scenario using the Cargo.toml:

[package.metadata.required-dna] # This can either be a single DNA or you can specify this multiple times as a list using [[package.metadata.required-dna]] 
name = "return_single_value" # The name to give the DNA that gets built
zomes = ["return_single_value"] # The name(s) of the zomes to include in the DNA, which must match the directory name in `./zomes`

[package.metadata.required-happ] # This can either be a single hApp or you can specify this multiple times as a list using [[package.metadata.required-happ]]
name = "return_single_value" # The name to give the hApp that gets built
dnas = ["return_single_value"] # The name(s) of the DNA to include in the hApp, which must match the name(s) given above.

If you need to debug this step, you can run cargo build -p <your-scenario-crate> and check the dnas and happs directories.

The Wind Tunnel Methodology

The Wind Tunnel framework is designed as a load testing tool. This means that the framework is designed to apply user-defined load to a system and measure the system's response to that load. At a high-level there are two modes of operation. Either you run the scenario and the system on the same machine and write the scenario to apply as much load as possible. Or you run the system in a production-like environment and write the scenario to be distributed across many machines. The Wind Tunnel framework does not distinguish between these two modes of operation and will always behave the same way. It is up to you to write scenarios that are appropriate for each mode of operation.

Load is applied to the system by agents. An agent is a single thread of execution that repeatedly applies the same behaviour to the system. This is in the form of a function which is run repeatedly by Wind Tunnel. There are either many agents running in a single scenario to maximise load from a single machine, or many scenarios running in parallel that each have a single agent. There is nothing stopping you from distributing the scenario and also running multiple agents but these are the suggested layouts to design scenarios around.

In general a scenario consists of setup and teardown hooks, and an agent behaviour to apply load to the system. There are global setup and teardown hooks that run once per scenario run. There are also agent setup and teardown hooks that run once per agent during a scenario run. There are then one or more agent behaviours. For simple tests you just define a single behaviour and all agents will behave the same way. For more complex tests you can define multiple behaviours and assign each agent to one of them. This allows more complex test scenarios to be described where different agents take different actions and may interact with each other. For example, you might have some agents creating data and other agents just reading the data.

Wind Tunnel is not responsible for capturing information about your system. It can store the information that you collect and do some basic analysis on it. Alternatively, it can push metrics to InfluxDB. But it is up to you to collect the information that you need and to analyse it in detail. For example, the Wind Tunnel bindings for Holochain capture API response times on the app and admin interfaces and automatically reports this to Wind Tunnel but if you need to measure other things then you will need to write your own code to do that.

Stress Testing a Single Instance of Your Test System

In this first mode of operation you want to run the scenario and the system on the same machine. You should write the scenario to apply as much load as possible to the system. That means keeping your agent behaviour hook as fast as possible. Preferably by doing as much setup as possible in the agent setup hook and then just doing simple actions in the agent behaviour hook.

This kind of test is good for finding the limits of the system in a controlled environment. It can reveal things like high memory usage, response times degrading over time and other bottlenecks in performance.

It may be useful to distribute this type of test. However, if it is written to maximise load then it only makes sense to distribute it if the target system is also distributed in some way. With Holochain, for example, this wouldn't make sense because although Holochain is distributed, it is not distributed in the sense of scaling performance for a single app.

Distributed Load Testing of Your System

In this second mode of operation you can still design and run the scenario on a single machine but that is just for development and the value comes from running it in a distributed way. The intended configuration is to have many copies of the scenario binary distributed across many machines. Each scenario binary will be configured to run a single agent. All the scenarios are configured to point at the same test system. When testing Holochain, for example, Holochain is distributed first then a scenario binary is placed on each node with Holochain and points at the local interface for Holochain.

Rather than looking to stress test the system in this mode, you are looking to measure the system's response to a realistic load. This is not understood by Wind Tunnel but you are permitted to block the agent behaviour hook to slow down the load the Wind Tunnel runner will apply. This allows you to be quite creative when designing your load pattern. For example, you could define a common agent behaviour function then create multiple agent behaviour hooks within your scenario that are use the common function at different rates. This would simulate varied behaviour by different agents.

Writing Scenarios for Holochain

[!NOTE] Writing scenarios requires some knowledge of wind-tunnel's methodology. That is assumed knowledge for this section!

Writing a Wind Tunnel scenario is relatively straight forward. The complexity is mostly in the measurement and analysis of the system once the scenario is running. To begin, you need a Rust project that with a single binary target.

cargo new --bin --edition 2021 my_scenario

You will probably need more dependencies at some point, but the minimum to get started are the holochain_wind_tunnel_runner and holochain_types crates.

cargo add holochain_wind_tunnel_runner
cargo add holochain_types

If this scenario is being written inside this repository then there are some extra setup steps. Please see the project layout docs.

Add the following imports to the top of your main.rs:

use holochain_types::prelude::ExternIO;
use holochain_wind_tunnel_runner::prelude::*;
use holochain_wind_tunnel_runner::scenario_happ_path;

Then replace your main function with the following:

fn main() -> WindTunnelResult<()> {
    let builder = ScenarioDefinitionBuilder::<HolochainRunnerContext, HolochainAgentContext>::new_with_init(
        env!("CARGO_PKG_NAME"),
    )
    .with_default_duration_s(60)
    .use_agent_behaviour(agent_behaviour);

    run(builder)?;

    Ok(())
}

This is the basic structure of a Wind Tunnel scenario. The ScenarioDefinitionBuilder is used to define the scenario. It includes a CLI which will allow you to override some of the defaults that are set in your code. Using the builder you can configure your hooks which are just Rust functions that take a context and return a WindTunnelResult.

The run function is then called with the builder. At that point the Wind Tunnel runner takes over and configures then runs your scenario.

Before you can run this, you'll need to provide the agent behaviour hook. Add the following to your main.rs:

fn agent_behaviour(
    ctx: &mut AgentContext<HolochainRunnerContext, HolochainAgentContext>,
) -> HookResult {
    println!("Hello from, {}", ctx.agent_id());
    std::thread::sleep(std::time::Duration::from_secs(1));
    Ok(())
}

This is just an example hook and you will want to replace it once you have got your scenario running. Note the AgentContext that is provided to the hook. This is created per-agent and gives you access to the agent's ID and the runner's context. Both the agent and the runner context are used for sharing configuration between the runner and your hooks, and state between your hooks.

Your scenario should now be runnable. Try running it with

cargo run -- --duration 10

You should see the print messages from the agent behaviour hook. If so, you are ready to start writing your scenario. To get started, you are recommended to take a look at documentation for the holochain_wind_tunnel_runner crate. This has common code to use in your your scenarios and example of how to use them. This will help you get started much more quickly than starting from scratch. There is also a tips section below which you might find helpful as you run into questions.

Tips for Writing Scenarios

Run async code in your agent behaviour

The behaviour hooks are synchronous but the Holochain client is asynchronous. The ability to run async code in your hooks is exposed through the AgentContext and RunnerContext.

fn agent_behaviour(ctx: &mut AgentContext<HolochainRunnerContext, HolochainAgentContext>) -> HookResult {
    ctx.runner_context().executor().execute_in_place(async {
        // Do something async here
    })?;

    Ok(())
}

Record custom metrics

This is useful for scenarios that need to measure things that don't happen through the instrumented client that is talking to the system under test.

fn agent_behaviour(ctx: &mut AgentContext<HolochainRunnerContext, HolochainAgentContext>) -> HookResult {
    let metric = ReportMetric::new("my_custom_metric")
        .with_field("value", 1);
    ctx.runner_context().reporter().clone().add_custom(metric);
    
    Ok(())
}

The metric will appear in InfluxDB as wt.custom.my_custom_metric with a field value set to 1.

Running scenarios locally

When developing your scenarios you can disable anything that requires running infrastructure, other than the target system. However, once you are ready to run your scenario to get results you will need a few extra steps.

Running InfluxDB

InfluxDB is used to store the metrics that Wind Tunnel collects. You can run it locally from inside a Nix shell launched with nix develop:

influxd

This terminal will then be occupied running InfluxDB. Start another terminal where you can configure the database and create a user, again from inside the Nix shell:

configure_influx

This will do a one-time setup for InfluxDB and also configure your shell environment to use it. Next time you start a new terminal you will need to run use_influx instead.

You can now navigate to the InfluxDB dashboard and log in with windtunnel/windtunnel. The variables and dashboards you need will already be set up, so you can now run your scenario and the metrics will be pushed to InfluxDB.

Running Telegraf

This is used for pushing system metrics to InfluxDB. This is not required locally but if you would like to run it then you can do so from inside the Nix shell:

use_influx
start_telegraf

Running Holochain

For a zero-config and quick way to run Holochain, you can use the following command:

hc s clean && echo "1234" | hc s --piped create && echo "1234" | hc s --piped -f 8888 run

For more advanced scenarios or for distributed tests, this is not appropriate!

Running scenarios

Each scenario is expected to provide a README.md with at least:

• A description of the scenario and what it is testing for.
• A suggested command or commands to run the scenario, with justification for the configuration used.

For example, see the zome_call_single_value scenario.

As well as the command you use to run the scenario, you will need to select an appropriate reporter. Try running the scenario with the --help flag to see the available options. For local development, the default in-memory reporter will do. If you have influx running then you can use the influx-client option. If you have both Influx and Telegraf running then you can use the influx-file option.

Developer guide (for wind-tunnel)

There is a Nix environment provided, and it is recommended that you use its shell for development:

nix develop

Start a sandbox for testing:

hc s clean && echo "1234" | hc s --piped create && echo "1234" | hc s --piped -f 8888 run

It is recommended to stop and start this sandbox conductor between test runs because getting Holochain back to a clean through its API is not yet implemented.

You can then start a second terminal and run one of the scenarios in the scenarios directory:

RUST_LOG=info cargo run -p zome_call_single_value -- --duration 60 -c ws://localhost:8888

Published crates

Framework crates:

• Core functionality for use by other Wind Tunnel crates - wind_tunnel_core
• Instruments for measuring performance with Wind Tunnel - wind_tunnel_instruments
• Derive macros for the wind_tunnel_instruments crate - wind_tunnel_instruments_derive
• The Wind Tunnel runner - wind_tunnel_runner

Bindings crates for Holochain:

• An instrumented wrapper around the holochain_client - holochain_client_instrumented
• Customises the wind_tunnel_runner for Holochain testing - holochain_wind_tunnel_runner