Developer’s Guide

The following guide is intended for those interested in the inner workings of nodepool and its various processes.


If you send a SIGUSR2 to one of the daemon processes, Nodepool will dump a stack trace for each running thread into its debug log. It is written under the log bucket nodepool.stack_dump. This is useful for tracking down deadlock or otherwise slow threads.

Nodepool Builder

The following is the overall diagram for the nodepool-builder process and its most important pieces:

                     |    ZooKeeper    |
                       ^      |
                bld    |      | watch
+------------+  req    |      | trigger
|   client   +---------+      |           +--------------------+
+------------+                |           | NodepoolBuilderApp |
                              |           +---+----------------+
                              |               |
                              |               | start/stop
                              |               |
                      +-------v-------+       |
                      |               <-------+
            +--------->   NodePool-   <----------+
            |     +---+   Builder     +---+      |
            |     |   |               |   |      |
            |     |   +---------------+   |      |
            |     |                       |      |
      done  |     | start           start |      | done
            |     | bld             upld  |      |
            |     |                       |      |
            |     |                       |      |
        +---------v---+               +---v----------+
        | BuildWorker |               | UploadWorker |
        +-+-------------+             +-+--------------+
          | BuildWorker |               | UploadWorker |
          +-+-------------+             +-+--------------+
            | BuildWorker |               | UploadWorker |
            +-------------+               +--------------+


class nodepool.driver.Driver

The Driver interface

This is the main entrypoint for a Driver. A single instance of this will be created for each driver in the system and will persist for the lifetime of the process.

The class or instance attribute name must be provided as a string.

abstract getProvider(provider_config)

Return a Provider instance


provider_config (dict) – A ProviderConfig instance

abstract getProviderConfig(provider)

Return a ProviderConfig instance


provider (dict) – The parsed provider configuration


Called before loading configuration to reset any global state

class nodepool.driver.Provider(*args, **kw)

The Provider interface

Drivers implement this interface to supply Providers. Each “provider” in the nodepool configuration corresponds to an instance of a class which implements this interface.

If the configuration is changed, old provider instances will be stopped and new ones created as necessary.

The class or instance attribute name must be provided as a string.

abstract cleanupLeakedResources()

Clean up any leaked resources

This is called periodically to give the provider a chance to clean up any resources which make have leaked.

abstract cleanupNode(node_id)

Cleanup a node after use

The driver may delete the node or return it to the pool. This may be called after the node was used, or as part of cleanup from an aborted launch attempt.


node_id (str) – The id of the node


Return a list of labels which this provider has determined are in a permanent error state (for example, the cloud indicates that the configuration is invalid).

abstract getRequestHandler(poolworker, request)

Return a NodeRequestHandler for the supplied request

abstract idle()

Idle the provider

This is called before stop(). Providers should use this as a signal to idle themselves and stop performing any actions that may interfere with a new version of this provider starting up.

abstract join()

Wait for provider to finish

On shutdown, this is called after stop() and should return when the provider has completed all tasks. This may not be called on reconfiguration (so drivers should not rely on this always being called after stop).

abstract labelReady(name)

Determine if a label is ready in this provider

If the pre-requisites for this label are ready, return true. For example, if the label requires an image that is not present, this should return False. This method should not examine inventory or quota. In other words, it should return True if a request for the label would be expected to succeed with no resource contention, but False if is not possible to satisfy a request for the label.


name (str) – The name of the label


True if the label is ready in this provider, False otherwise.

abstract start(zk_conn)

Start this provider


zk_conn (ZooKeeper) – A ZooKeeper connection object.

This is called after each configuration change to allow the driver to perform initialization tasks and start background threads. The ZooKeeper connection object is provided if the Provider needs to interact with it.


Starts a background process to delete a node

This should return a NodeDeleter to implement and track the deletion of a node.


node (Node) – A locked Node object representing the instance to delete.


A NodeDeleter instance.

abstract stop()

Stop this provider

Before shutdown or reconfiguration, this is called to signal to the driver that it will no longer be used. It should not begin any new tasks, but may allow currently running tasks to continue.

abstract waitForNodeCleanup(node_id)

Wait for a node to be cleaned up

When called, this will be called after cleanupNode().

This method should return after the node has been deleted or returned to the pool.


node_id (str) – The id of the node

class nodepool.driver.ProviderNotifications

Notification interface for Provider objects.

This groups all notification messages bound for the Provider. The Provider class inherits from this by default. A Provider overrides the methods here if they want to handle the notification.


Called after the ZooKeeper object for a node is deleted.


node (Node) – Object describing the node just deleted.

class nodepool.driver.NodeRequestHandler(pw, request)

Class to process a single nodeset request.

The PoolWorker thread will instantiate a class of this type for each node request that it pulls from ZooKeeper.

Subclasses are required to implement the launch method.

abstract property alive_thread_count

Return the number of active node launching threads in use by this request handler.

This is used to limit request handling threads for a provider.

This is an approximate, top-end number for alive threads, since some threads obviously may have finished by the time we finish the calculation.


A count (integer) of active threads.


Handler may implement this to verify a node can be re-used. The OpenStack handler uses this to verify the node az is correct.


Return a list of reasons to decline this request


Checks if a provider has enough quota to handle a list of nodes. This does not take our currently existing nodes into account.


node_types – list of node types to check


True if the node list fits into the provider, False otherwise


Checks if the predicted quota is enough for an additional node of type ntype.


ntype – node type for the quota check


True if there is enough quota, False otherwise

abstract imagesAvailable()

Handler needs to implement this to determines if the requested images in self.request.node_types are available for this provider.


True if it is available, False otherwise.

abstract launch(node)

Handler needs to implement this to launch the node.

abstract launchesComplete()

Handler needs to implement this to check if all nodes in self.nodeset have completed the launch sequence.

This method will be called periodically to check on launch progress.


True if all launches are complete (successfully or not), False otherwise.


Execute node request handling.

This code is designed to be re-entrant. Because we can’t always satisfy a request immediately (due to lack of provider resources), we need to be able to call run() repeatedly until the request can be fulfilled. The node set is saved and added to between calls.


Handler may implement this to store driver-specific metadata in the Node object before building the node. This data is normally dynamically calculated during runtime. The OpenStack handler uses this to set az, cloud and region.


Attempt unlocking all Nodes in the node set.


clear_allocation (bool) – If true, clears the node allocated_to attribute.

class nodepool.driver.NodeRequestHandlerNotifications

Notification interface for NodeRequestHandler objects.

This groups all notification messages bound for the NodeRequestHandler. The NodeRequestHandler class inherits from this by default. A request handler overrides the methods here if they want to handle the notification.


Handler may implement this to be notified when a node is re-used. The OpenStack handler uses this to set the choozen_az.

class nodepool.driver.ProviderConfig(provider)

The Provider config interface

The class or instance attribute name must be provided as a string.

abstract getSchema()

Return a voluptuous schema for config validation

abstract getSupportedLabels(pool_name=None)

Return a set of label names supported by this provider.


pool_name (str) – If provided, get labels for the given pool only.

abstract load(newconfig)

Update this config object from the supplied parsed config

abstract property manage_images

Return True if provider manages external images, False otherwise.

abstract property pools

Return a dict of ConfigPool-based objects, indexed by pool name.

Writing A New Provider Driver

Nodepool drivers are loaded from the nodepool/drivers directory. A driver is composed of three main objects:

  • A ProviderConfig to manage validation and loading of the provider.

  • A Provider to manage resource allocations.

  • A NodeRequestHandler to manage nodeset (collection of resource) allocations.

Those objects are referenced from the Driver main interface that needs to be implemented in the file of the driver directory.


The ProviderConfig is constructed with the driver object and the provider configuration dictionary.

The main procedures of the ProviderConfig are:

  • getSchema() exposes a voluptuous schema of the provider configuration.

  • load(config) parses the provider configuration. Note that the config argument is the global Nodepool.yaml configuration. Each provided labels need to be referenced back to the global config.labels dictionary so that the launcher service know which provider provide which labels.


The Provider is constructed with the ProviderConfig.

The main procedures of the Provider are:

  • cleanupNode(external_id) terminates a resource

  • listNodes() returns the list of existing resources. This procedure needs to map the nodepool_node_id with each resource. If the provider doesn’t support resource metadata, the driver needs to implement a storage facility to associate resource created by Nodepool with the internal nodepool_node_id. The launcher periodically look for non-existent node_id in listNodes() to delete any leaked resources.

  • getRequestHandler(pool, request) returns a NodeRequestHandler object to manage the creation of resources. The contract between the handler and the provider is free form. As a rule of thumb, the handler should be in charge of interfacing with Nodepool’s database while the provider should provides primitive to create resources. For example the Provider is likely to implement a createResource(pool, label) procedure that will be used by the handler.


The NodeRequestHandler is constructed with the assigned pool and the request object. Before the handler is used, the following attributes are set:

  • self.provider : the provider configuration.

  • self.pool : the pool configuration.

  • self.zk : the database client.

  • self.manager : the Provider object.

The main procedures of the NodeRequestHandler are:

  • launch(node) starts the creation of a new resource.

  • launchesComplete() returns True if all the node of the nodesets self attributes are READY.

An Handler may not have to launch each node of the nodesets as Nodepool will re-use existing nodes.

The launch procedure usually consists of the following operations:

  • Use the provider to create the resources associated with the node label. Once an external_id is obtained, it should be stored to the node.external_id.

  • Once the resource is created, READY should be stored to the node.state. Otherwise raise an exception to restart the launch attempt.

State Machine Drivers

To use this system, you will need to implement a few subclasses. First, create a ProviderConfig subclass as you would for any driver.

Then, subclass Instance to map remote instance data into a format the driver can understand.

Next, create two subclasses of StateMachine to implement creating and deleting instances.

Subclass Adapter to implement the main methods that interact with the cloud.

Finally, subclass StateMachineDriver to tie them all together.

See the example provider for an example.

class nodepool.driver.statemachine.Instance

Represents a cloud instance

This class is used by the State Machine Driver classes to represent a standardized version of a remote cloud instance. Implement this class in your driver, override the load() method, and supply as many of the fields as possible.

The following attributes are required:

  • ready: bool (whether the instance is ready)

  • deleted: bool (whether the instance is in a deleted state)

  • external_id: str (the unique id of the instance)

  • interface_ip: str

  • metadata: dict

The following are optional:

  • public_ipv4: str

  • public_ipv6: str

  • private_ipv4: str

  • cloud: str

  • az: str

  • region: str

  • host_id: str

  • driver_data: any

  • slot: int

And the following are even more optional (as they are usually already set from the image configuration):

  • username: str

  • python_path: str

  • shell_type: str

  • connection_port: str

  • connection_type: str

  • host_keys: [str]

This is extremely optional, in fact, it’s difficult to imagine that it’s useful for anything other than the metastatic driver:

  • node_attributes: dict


Return quota information about this instance.


A QuotaInformation object.

class nodepool.driver.statemachine.StateMachine
class nodepool.driver.statemachine.Adapter(provider_config)

Cloud adapter for the State Machine Driver

This class will be instantiated once for each Nodepool provider. It may be discarded and replaced if the configuration changes.

You may establish a single long-lived connection to the cloud in the initializer if you wish.


provider_config (ProviderConfig) – A config object representing the provider.


Delete an image from the cloud


str (external_id) – The external id of the image to delete


Delete the supplied resource

The driver has identified a leaked resource and the adapter should delete it.


resource (Resource) – A Resource object previously returned by ‘listResources’.


Return a list of availability zones for this provider

One of these will be selected at random and supplied to the create state machine. If a request handler is building a node set from an existing ready node, then the AZ from that node will be used instead of the results of this method.


A list of availability zone names.

getConsoleLog(label, external_id)

Return the console log from the specified server

  • ConfigLabel (label) – The label config for the node

  • str (external_id) – The external id of the server

getCreateStateMachine(hostname, label, image_external_id, metadata, log)

Return a state machine suitable for creating an instance

This method should return a new state machine object initialized to create the described node.

  • hostname (str) – The hostname of the node.

  • label (ProviderLabel) – A config object representing the provider-label for the node.

  • image_external_id (str) – If provided, the external id of a previously uploaded image; if None, then the adapter should look up a cloud image based on the label.

  • dict (metadata) – A dictionary of metadata that must be stored on the instance in the cloud. The same data must be able to be returned later on Instance objects returned from listInstances.

  • Logger (log) – A logger instance for emitting annotated logs related to the request.


A StateMachine object.

getDeleteStateMachine(external_id, log)

Return a state machine suitable for deleting an instance

This method should return a new state machine object initialized to delete the described instance.

  • external_id (str) – The external_id of the instance, as supplied by a creation StateMachine or an Instance.

  • Logger (log) – A logger instance for emitting annotated logs related to the request.


Return information about the quota used for a label

The default implementation returns a simple QuotaInformation for one instance; override this to return more detailed information including cores and RAM.


label (ProviderLabel) – A config object describing a label for an instance.


A QuotaInformation object.


Return the quota limits for this provider

The default implementation returns a simple QuotaInformation with no limits. Override this to provide accurate information.


A QuotaInformation object.


Indicate whether a label is ready in the provided cloud

This is used by the launcher to determine whether it should consider a label to be in-service for a provider. If this returns False, the label will be ignored for this provider.

This does not need to consider whether a diskimage is ready; the launcher handles that itself. Instead, this can be used to determine whether a cloud-image is available.


label (ProviderLabel) – A config object describing a label for an instance.


A bool indicating whether the label is ready.


Return an iterator of instances accessible to this provider.

The yielded values should represent all instances accessible to this provider, not only those under the control of this adapter, but all visible instances in order to achive accurate quota calculation.


A generator of Instance objects.


Return a list of resources accessible to this provider.

The yielded values should represent all resources accessible to this provider, not only those under the control of this adapter, but all visible instances in order for the driver to identify leaked resources and instruct the adapter to remove them.


A generator of Resource objects.


Release any resources as this provider is being stopped

uploadImage(provider_image, image_name, filename, image_format=None, metadata=None, md5=None, sha256=None)

Upload the image to the cloud

  • ProviderImageConfig (provider_image) – The provider’s config for this image

  • str (sha256) – The name of the image

  • str – The path to the local file to be uploaded

  • str – The format of the image (e.g., “qcow”)

  • dict (metadata) – A dictionary of metadata that must be stored on the image in the cloud.

  • str – The md5 hash of the image file

  • str – The sha256 hash of the image file


The external id of the image in the cloud

class nodepool.driver.statemachine.StateMachineDriver

Entrypoint for a state machine driver


Instantiate an adapter


provider_config (ProviderConfig) – An instance of ProviderConfig previously returned by getProviderConfig().


An instance of Adapter


Return a Provider instance


provider_config (dict) – A ProviderConfig instance


Instantiate a config object


provider (dict) – A dictionary of YAML config describing the provider.


A ProviderConfig instance with the parsed data.