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Dependency-driven multi-step resource deallocation

https://blueprints.launchpad.net/murano/+spec/dependency-driven-resource-deallocation

Murano components may allocate various kinds of resources: virtual machines, networks, volumes etc. When these components get removed from the deployment appropriate resources have to be deallocated. Current implementation of this process has some significant limitations and flaws.

This specification aims to address these issues and provide a design for the better resource deallocation / garbage collection system for MuranoPL.

Problem description

In Murano the deallocation of resources is managed by a garbage collection system (GC). Its present implementation is based on the execution of special methods called .destroy() which may be defined in each MuranoPL class and are intended to contain the custom code to deallocate resources allocated by the objects of those classes. These methods get executed when their objects leave object graph, however the exact order of these executions is currently undefined.

There are two different scenarios when the objects may leave the object graph thus causing their .destroy methods to be called:

"Offline" changes of the Object Graph, i.e. the changes introduced in the serialized version of the object graph via the API. These changes are detected by comparing the incoming object graph (the one passed from the API for deployment) with the "snapshot" of current environment made after the previous deployment has been completed. If some object exists in the "snapshot" but is missing in the input graph it is considered to be removed. Such objects are deserialized from the snapshot and their .destroy methods are called in the order from deepest nested objects towards topmost ones.
"Runtime" changes. Some objects may be removed from the object graph during deployment: they may be unreferenced or assigned to Runtime properties or local variable only. As a result after the deployment completes these objects are not serialized neither into the output version of the object graph nor into its "snapshot". The next deployments will know nothing about their existence so the objects will be lost forever. To recover the resources allocated by such unreferenced objects murano analyzes its ObjectStore after the execution is complete. Each object which is present in the store but is not present in the output version of the object graph is considered to be "orphan" and thus its .destroy method is called. The order of these calls is currently undefined: the objects get destroyed based on their position in the object store, which is hardly predictable.

Such a design is insufficient for production grade applications which often require the following scenarios:

If some object is going to be deleted another object (either owning or just referencing it) may need to execute some actions before or after the object is deleted.
When a group of nested or interconnected objects is about to be deleted the order in which their destructors should be executed may be different in different cases.
Sometimes the actions being executed during the destruction of an object may depend on the fact whether some other object is about to be deleted or not.

Example

Consider an Application which consists of a Network component and several VM components. All the components are owned by the Application but there are no ownership relationships between them. When the Application is going to be deleted (i.e. its whole subgraph leaves the environment) all of its components are about to be removed as well, and so their ``.destroy`` methods will be called. Since Network does not own the VM's the order of these calls is undefined. Due to various implementation details it may be impossible to remove the Network before the VMs which are connected to it (e.g. in case when the VM has a mandatory requirement to be always connected to at least one network). In this case the ``.destroy`` of a Network component should be always called after all the VMs have been destroyed.

Proposed change

Several improvements have to be made in Murano engine to address the problems described above.

General concepts

Destruction dependencies

There should be a way to establish directional destruction dependencies between Murano objects. If object Foo establishes such a dependency on the object Bar then:

Foo will be notified when Bar is about to be destroyed. These notifications are covered in details in "Multi-step destruction" section below.
If both Foo and Bar are going to be destroyed in the same garbage collection execution, Bar will be destroyed before Foo.

These dependencies are not related to object ownership relationship or property-based cross-references: the owner may have a destruction dependency on its nested object or vise versa; the objects referencing each other may have some destruction dependency established. Even entirely unrelated objects may have a destruction dependency between them.

Since the destruction dependencies are directional there is a theoretical possibility of a circular dependency to exist. In case if two or more objects form such a circle they will still be notified about pending destruction of their dependencies, however the order of this notifications - and the destruction itself - is undefined in this case.

For now it's proposed to use destruction dependencies only for runtime garbage collection. Probably, it will be implemented when we find a solution for issue mentioned above in Known issues.

Multi-step destruction

Instead of just iterating through all the objects going to be destructed and calling their .destroy methods Murano should perform a multi-step garbage collection according to the following algorithm:

Detect all the objects going to be destroyed using either comparison of current object graph with its snapshot or the reference detection (see "Offline" and "Online" changes in "Problem description" section)
Sort the list of detected objects using the following comparator: for any two objects A and B in the list:
```
IF (A has-a-destruction-dependency-on B)
    AND (NOT B has-a-destruction-dependency-on A)
THEN A>B
ELSE IF (A owns B) THEN A>B
ELSE A==B
```
where has-a-destruction-dependency-on means that the left operand object has a destruction dependency (probably transitive) on right operand object, owns means that the left operand object owns (probably transitively) the right operand object.

Note

The destruction order of objects which are considered to be equal by the algorithm above is undefined. Even more, future implementations may destroy such objects in parallel.
For each object in the list:

3.1. Notify all the objects having a destruction dependency on it that the

target object will be destroyed.

3.2. Call the .destroy() method of the object if it is present.

3.3. Change the object's status to "Destroyed" (see below).

Destroyed objects

When an object is being processed by a garbage collector, it means that there are no live references to it from the objects of the environment. However there may be cases when the code which handles either the pre-destroy notification (p. 3.1 above) or the actual .destroy method re-establishes the references to the object being destructed, and thus the object remains in the object graph after the GC is completed. Since the resources may be deallocated at this time the regular usage of the object is not possible, however if it is assigned to a property of some another object in the graph it may not always be possible to just nullify that property since it may cause a contract violation.

To resolve such collisions it is proposed to explicitly mark such destroyed objects as "destroyed". MuranoPL executor will not allow to execute any methods on such objects, however their properties remain accessible (i.e. readable) so any runtime information associated with them may be recovered. Destroyed objects will be serialized with the rest of object graph but the json-representation of the object will have a special flag in their class header (the "?" section) to indicate their special status. When deserialized from json such objects will retain their "destroyed" status, so the method execution will still be impossible even in subsequent deployments.

When the destroyed objects are unreferenced from the object graph they go away without additional actions: garbage collector ignores them since their resources have already been released.

Garbage collection executions

The multi-step object destruction described above should take place in three different scenarios:

(currently existing) Before the deployment, destroys objects which were present in the object graph after the previous deployment was finished but were not found in the incoming object graph of a new deployment, i.e. the ones explicitly removed using the API.
(currently existing) After the deployment, destroys the objects existing in the Object Store but not being a part of the persistable object graph of current environment, i.e. having no references to them from the persistable (In, Out, InOut) properties of the environment or its transitive children).
(proposed) During the deployment, explicitly initiated from MuranoPL code. Destroys objects which are not part of the complete object graph, i.e. having no references to them from any properties of the environment (including runtime and private properties) AND not being referenced by local variables in any frame of all the the green threads of current deployment.

To implement p.3 above a new algorithm is needed. It should analyze all the active contexts of all the running greenthreads of the current deployment and retrieve all the data variables from that context, traversing through the parent contexts as well. All the objects of MuranoObject type collected this way should be added to the "queue of active roots" to be used for further processing. For each object of this queue the algorithm should save the id of the object into the "result set" and then find objects which are reachable from the current one (i.e. the objects of MuranoObject type contained in properties of any kind). For each such object the algorithm should check whether its id is already present in the "result set". If not, the object is added to the end of "queue of active roots". The algorithm runs till it processes all the objects of the queue. The "result set" which the algorithm gets at the end of this process contains the ids of alive objects. All other objects of the object store should be considered as candidates for garbage collection.

There should be one additional check made before doing the actual destruction: some of the objects may have no valid access paths from MuranoPL objects, but could be referenced from some python-back objects. This happens when an Object is passed to some method backed with pythonic code. In this case the executor creates an object of type MuranoObjectInterface - a wrapper to simplify the work with a murano object from python. This object contains a reference to the actual MuranoPL object within. If appropriate python object is alive, then its corresponding MuranoPL object should not be garbage collected even if there are no references to it from the active roots of MuranoPL. To keep track of such situations the Garbage Collector should contain a special dictionary mapping ids of the objects to the weak proxies pointing to the MuranoObjectInterface objects passed to pythonic code. For every garbage collection candidate the algorithm should check if there is a map entry for this object and the weak proxy at that entry is alive. If this is true, then the object is excluded from the list of GC candidates.

The resulting list of GC candidates is then destroyed as described in "Multi-step destruction" section above.

Known issues

Murano is using Objects and ObjectsCopy objects to transfer data between deployments. When destruction dependencies will be implemented handler will make changes (if any) to objects in ObjectsCopy. So, this changes aren't applied during next deployment.

It's proposed to change the way of object model generation, with updating new model objects if they have been changed after last deployment. However, solving this issued is not an aim of this specification, so we can skip details.

Code changes

GC class

A new python-backed Murano class called GC should be added to the core library. It should have the following static methods:

collect() - initiates garbage collection of unreferenced objects of current deployment (see p.3 in "Garbage collection executions" section above).
isDestroyed(object) - checks if the object was already destroyed during some GC session and thus its methods cannot be called.
subscribe(target, handler=null) - establishes a destruction dependency from the caller to the object passed as target. Method may be called several times, in this case only a single destruction dependency will be established, however the same amount of calls of unsubscribe will be required to remove it.

handler argument is optional. If passed it should be the name of an instance methods defined by the caller class to handle notification of target's destruction (see "Multi-step destruction" section above: this handlers is executed for p. 3.1)

The following arguments will be passed to the handler method
- sender - an instance of GC class describing the current garbage collection session.
- object - a target object which is going to be destroyed. It is not recommended to persist the reference to this object anywhere. This will not prevent the object from being garbage collected but the object will be moved to the "destroyed" state which is almost always bad. The option to do so is considered to be advanced feature which should not be done unless it is absolutely necessary.
unsubscribe(target) - removes the destruction dependency from the caller to the object passed as target. Method may be called several times without any side-effects. If subscribe was called more than once the same (or more) amount of calls to unsubscribe is needed to remove the dependency.

An instance of GC class will be created during the garbage collection session to encapsulate runtime information about this session. It defines a single method which may be of use during a GC session:

isDoomed(object) - checks if the object is marked for destruction during this GC session.

Pythonic back-end of the GC class should be able to establish destruction dependencies by storing the back-refs to the dependent object in attributes of the python's instance of MuranoObject representing the dependency.

Note

This is the opposite of how the destruction dependencies are stored when the model is serialized: in serialized form that is the dependent object who owns the reference to the dependency object. In runtime it is the dependency object who owns the reference to dependent object.

When the garbage collection is needed the class will be instantiated and a list of objects-to-delete created based on the current state of object graph, object store and the execution context. Garbage collections will use this object for all the steps of the workflow.

Alternatives

Application developers may try to implement their own event-based notification logic to notify about pending and completed object destructions. However it will solve only part of the problem: notifications will work properly, but they will not affect the order in which the objects are destroyed, so the workflows will be too complicated. Also this alternative will not have the advanced features proposed in this spec, such as ability to check if some object is going to be destroyed.

Data model impact

None

REST API impact

None

Versioning impact

The proposed change is completely backwards compatible: without explicit destruction dependencies objects will be collected based on their ownership relationships, i.e. as it is done in the current implementation.

The packages containing classes which explicitly call the methods of GC should have package format of at least 1.4 to prevent their execution on older versions of Murano which do not have this feature.

Other end user impact

None

Deployer impact

None

Developer impact

Developers will get the new MuranoPL-based API to manage resource deallocation lifecycle. If they do not want to use it they don't need to do anything.

Murano-dashboard / Horizon impact

None

Implementation

Assignee(s)

Primary assignee:: ativelkov
Other contributors:: starodubcevna

Work Items

Implement a system to define and use destruction dependencies in runtime.
Introduce changes to MuranoObject class to keep track of "destroyed" object status.
Modify the serializer / deserializer to properly persist the value of the "destroyed object" flag.
Modify the code which instantiates yaql contexts for MuranoPL so all the created contexts are tracked by the execution session.
Implement sorting algorithms to arrange objects-to-be-destroyed based on criteria defined in p.2 of "Multi-step destruction" section above.
Modify an algorithm to collect alive object roots from the runtime and private properties and local variables of executing threads.
Implement multi-step destruction workflow.
Implement GC class to bind all the above.
Create test-runner-based tests to cover all the test scenarios.
Document the new features.

Dependencies

The development of this feature will enable Application Development Framework [1] to address resource deallocation problems during application uninstall.

Testing

Tests should be written for test-runner to cover various scenarios of resource deallocation.

Runtime garbage collection

There should be test cases covering that:

objects assigned to persistent (Input, Output, InputOutput) properties (both locally-declared and inherited) of objects reachable from the current roots are NOT garbage collected;
objects assigned to transient (Runtime and undeclared) properties (both locally-declared and inherited) of objects reachable from the current roots are NOT garbage collected; target properties should be both locally-declared and inherited;
objects assigned to static properties of various classes are NOT garbage collected;
objects passed to python-backed objects and unreferenced in MuranoPL are NOT garbage collected unless their MuranoObjectInterface proxies are unreferenced / GC'ed in python;
objects assigned to local variables of the current execution frame (i.e. variables of the current method and all the caller methods in call stack) including method arguments are NOT garbage collected;
single unreferenced objects ARE garbage collected;
graphs of interconnected objects having no references from non-collected objects ARE garbage collected;
objects passed to python-backed objects and unreferenced in both MuranoPL and python ARE garbage collected;
garbage collector correctly processes stack-frame objects from green-threads other than the one it is executed from;

Destruction dependency resolution order

There should be test cases covering that:

if some child object has a destruction dependency on its parent, the parent gets destroyed before the child;
if some parent object has a destruction dependency on its child, the child gets destroyed before the parent;
if some objects not being the part of some ownership hierarchy have some destruction dependency, the dependency-object is destroyed before the dependent one;
if some objects have circular destruction dependency they are all destroyed (the order is not enforced by the test);

Destruction events

Given the base scenario of object A having a destruction dependency on object B and B being GC'ed, there should be tests covering that:

the right order of events occurs (A gets warned about possible B's destruction -> A is notified about inevitable B's destruction -> B is destroyed -> A is notified that B was destroyed);
A may prevent B's destruction by establishing a reference on B in the warning handler;
A may cancel GC in both warning and pre-destroy notification handlers;
A may establish more then 1 destruction dependency on B and still be notified just once;
A may remove the destruction dependency and not get notified on B's destruction;
If A established N destruction dependencies and then removed them M times, (N>M) then notifications are still delivered;
If A established N destruction dependencies and then removed them M times, (N<=M) then notifications are not delivered;
B may establish a destruction dependency on itself thus subscribing to appropriate notifications;
phase property of GC instance is correct in appropriate event handlers;
isDoomed and isDestroyed methods return appropriate values when called by A for B in appropriate event handlers.

Documentation Impact

Developers documentation should be updated to describe the new GC class and its static and instance methods, as well as the design guidelines for application developers to follow to utilize the new capability.

References

[1] https://github.com/openstack/murano-specs/blob/master/specs/newton/approved/application-development-framework.rst

22 KiB Raw Blame History

Dependency-driven multi-step resource deallocation

Problem description

Proposed change

General concepts

Destruction dependencies

Multi-step destruction

Destroyed objects

Garbage collection executions

Known issues

Code changes

GC class

Alternatives

Data model impact

REST API impact

Versioning impact

Other end user impact

Deployer impact

Developer impact

Murano-dashboard / Horizon impact

Implementation

Assignee(s)

Work Items

Dependencies

Testing

Runtime garbage collection

Destruction dependency resolution order

Destruction events

Documentation Impact

References

22 KiB

Raw Blame History