Unit tests for cmdlib / LogicalUnit’s







This is a design document describing unit tests for the cmdlib module. Other modules are deliberately omitted, as LU’s contain the most complex logic and are only sparingly tested.

Current state and shortcomings

The current test coverage of the cmdlib module is at only ~14%. Given the complexity of the code this is clearly too little.

The reasons for this low coverage are numerous. There are organisational reasons, like no strict requirements for unit tests for each feature. But there are also design and technical reasons, which this design document wants to address. First, it’s not clear which parts of LU’s should be tested by unit tests, i.e. the test boundaries are not clearly defined. And secondly, it’s too hard to actually write unit tests for LU’s. There exists no good framework or set of tools to write easy to understand and concise tests.

Proposed changes

This design document consists of two parts. Initially, the test boundaries for cmdlib are laid out, and considerations about writing unit tests are given. Then the test framework is described, together with a rough overview of the individual parts and how they are meant to be used.

Test boundaries

For the cmdlib module, every LogicalUnit is seen as a unit for testing. Unit tests for LU’s may only execute the LU but make sure that no side effect (like filesystem access, network access or the like) takes place. Smaller test units (like individual methods) are sensible and will be supported by the test framework. However, they are not the main scope of this document.

LU’s require the following environment to be provided by the test code in order to be executed:

An input opcode

LU’s get all the user provided input and parameters from the opcode.

The command processor

Used to get the execution context id and to output logging messages. It also drives the execution of LU’s by calling the appropriate methods in the right order.

The Ganeti context

Provides node-management methods and contains

  • The configuration. This gives access to the cluster configuration.

  • The Ganeti Lock Manager. Manages locks during the execution.

The RPC runner

Used to communicate with node daemons on other nodes and to perform operations on them.

The IAllocator runner

Calls the IAllocator with a given request.

All of those components have to be replaced/adapted by the test framework.

The goal of unit tests at the LU level is to exercise every possible code path in the LU at least once. Shared methods which are used by multiple LU’s should be made testable by themselves and explicit unit tests should be written for them.

Ultimately, the code coverage for the cmdlib module should be higher than 90%. As Python is a dynamic language, a portion of those tests only exists to exercise the code without actually asserting for anything in the test. They merely make sure that no type errors exist and that potential typos etc. are caught at unit test time.

Test framework

The test framework will it make possible to write short and concise tests for LU’s. In the simplest case, only an opcode has to be provided by the test. The framework will then use default values, like an almost empty configuration with only the master node and no instances.

All aspects of the test environment will be configurable by individual tests.

MCPU mocking

The MCPU drives the execution of LU’s. It has to perform its usual sequence of actions, but additionally it has to provide easy access to the log output of LU’s. It will contain utility assertion methods on the output.

The mock will be a sub-class of mcpu.Processor which overrides portions of it in order to support the additional functionality. The advantage of being a sub-class of the original processor is the automatic compatibility with the code running in real clusters.

Configuration mocking

Per default, the mocked configuration will contain only the master node, no instances and default parameters. However, convenience methods for the following use cases will be provided:

  • “Shortcut” methods to add objects to the configuration.

  • Helper methods to quickly create standard nodes/instances/etc.

  • Pre-populated default configurations for standard use-cases (i.e. cluster with three nodes, five instances, etc.).

  • Convenience assertion methods for checking the configuration.

Lock mocking

Initially, the mocked lock manager always grants all locks. It performs the following tasks:

  • It keeps track of requested/released locks.

  • Provides utility assertion methods for checking locks (current and already released ones).

In the future, this component might be extended to prevent locks from being granted. This could eventually be used to test optimistic locking.

RPC mocking

No actual RPC can be made during unit tests. Therefore, those calls have to be replaced and their results mocked. As this will entail a large portion of work when writing tests, mocking RPC’s will be made as easy as possible. This entails:

  • Easy construction of RPC results.

  • Easy mocking of RPC calls (also multiple ones of the same type during one LU execution).

  • Asserting for RPC calls (including arguments, affected nodes, etc.).

IAllocator mocking

Calls (also multiple ones during the execution of a LU) to the IAllocator interface have to be mocked. The framework will provide, similarly to the RPC mocking, provide means to specify the mocked result and to assert on the IAllocator requests.

Future work

With unit tests for cmdlib in place, further unit testing for other modules can and should be added. The test boundaries therefore should be aligned with the boundaries from cmdlib.

The mocked locking module can be extended to allow testing of optimistic locking in LU’s. In this case, on all requested locks are actually granted to the LU, so it has to adapt for this situation correctly.

A higher test coverage for LU’s will increase confidence in our code and tests. Refactorings will be easier to make as more problems are caught during tests.

After a baseline of unit tests is established for cmdlib, efficient testing guidelines could be put in place. For example, new code could be required to not lower the test coverage in cmdlib. Additionally, every bug fix could be required to include a test which triggered the bug before the fix is created.