3. Concepts

This section presents some concepts that are helpful to understand when using the zhmcclient package.

3.1. Topology

The following figure shows the topology of Python applications using the zhmcclient package with an HMC and the CPCs managed by that HMC:

+----------------------------------------+  +--------------------+
|                  Node 1                |  |       Node 2       |
|                                        |  |                    |
|  +----------------+  +--------------+  |  |  +--------------+  |
|  |  Python app 1  |  | Python app 2 |  |  |  | Python app 3 |  |
|  +----------------+  +--------------+  |  |  +--------------+  |
|  |   zhmcclient   |  |  zhmcclient  |  |  |  |  zhmcclient  |  |
|  | S      S   NR  |  |    S  NR     |  |  |  |     S        |  |
|  +-v------v---^---+  +----v--^------+  |  |  +-----v--------+  |
+----|------|---|-----------|--|---------+  +--------|-----------+
     |      |   |           |  |                     |
 REST|  REST|   |JMS    REST|  |JMS              REST|
     |      |   |           |  |                     |
|                                                                |
|                             HMC                                |
|                                                                |
|                      ... resources ...                         |
|                                                                |
              |                                    |
              |                                    |
+-------------+------------+         +-------------+-------------+
|                          |         |                           |
|           CPC 1          |         |          CPC 2            |
|                          |         |                           |
|     ... resources ...    |         |    ... resources ...      |
|                          |         |                           |
+--------------------------+         +---------------------------+

The Python applications can be for example the zhmc CLI (provided in the zhmccli project), your own Python scripts using the zhmcclient API, or long-lived services that perform some function. In any case, each Python application in the figure runs in the runtime of exactly one Python process.

In that Python process, exactly one instance of the zhmcclient Python package is loaded. Performing HMC operations on a particular HMC requires a Session object (shown as S in the figure). Receiving notifications from a particular HMC requires a NotificationReceiver object (shown as NR in the figure).

For example, Python app 1 in the figure has two sessions and one notification receiver. For simplicity, the two sessions go to the same HMC in this example, but they could also go to different HMCs. Similarly, a Python app could receive notifications from more than one HMC.

3.2. Multi-threading considerations

The zhmcclient package supports the use of multi-threading in Python processes, but each Session, Client, and NotificationReceiver object can be used by only one thread at a time. However, this is not verified or enforced by the zhmcclient package, so ensuring this is a responsibility of the user of the zhmcclient package.

If your Python app is multi-threaded, it is recommended that each thread with a need to perform HMC operations has its own Session object and its own Client object, and that each thread with a need to receive HMC notifications has its own NotificationReceiver object. These different objects can very well target the same HMC.

3.3. Resource model concepts

The zhmcclient package provides a resource model at its API that represents exactly the resource model described in the HMC API book. Some of these resources are located on the HMC (for example HMC users), and some on the CPCs managed by the HMC (for example the CPC itself, or partitions on the CPC).

The entry points for a user of the zhmcclient API are two objects that need to be created by the user:

  • a Session object. A session object represents a REST session with exactly one HMC and handles all aspects of the session, such as the credentials for automatic logon and re-logon, the retry and timeout configuration, or the logging configuration.
  • a Client object. A client object is the top of the resource tree and is initialized with a Session object (if connecting to a real HMC) or with a FakedSession object (in unit tests that work against a mocked HMC). Despite its classname, a client object really represents the HMC (real or mocked).

A session that is logged on is always in the context of the HMC userid that was used for the session. That HMC userid determines what the Python application using that session object can see and what it is allowed to do. See Setting up the HMC for a list of access rights that are needed in order to see all resources and to perform all tasks supported by the zhmcclient package. The HMC API book details for each HMC operation which access rights are needed in order to perform the operation.

A client object is the top of the resource tree exposed by an HMC. Resources located on the HMC (e.g. HMC userids) are direct or indirect children of the client object. The CPCs managed by the HMC are direct children of the client object, and the resources located on each CPC are direct or indirect children of the Cpc object representing the CPC. There is a strict parent-child relationship in the resource model, so that the resource model is a strict tree without any shared children.

For each actual managed resource on the HMC or its managed CPCs, the zhmcclient package may provide more than one Python object representing that resource. For example, the child resources of a resource can be listed by using the list() method. Each time that method is invoked, it returns a new list of Python objects representing the state of the child resources at the time the call was made.

This is an important principle in the design of the zhmcclient API: Whenever a Python object representing a resource (i.e. objects of subclasses of BaseResource) is returned to the caller of the zhmcclient API, its state represents the state of the actual managed resource at the time the call was made, but the state of the Python resource object is not automatically being updated when the state of the actual managed resource changes.

As a consequence, there are multiple Python resource objects for the same actual managed resource.

All Python resource objects provided by the zhmcclient package can be asked to update their state to match the current state of the actual managed resource, via the pull_full_properties() method. Alternatively, a new Python resource object with the current state of the actual managed resource can be retrieved using the find() method using filters on name or object ID so that only the desired single resource is returned. See Filtering for details.

With the exception of the Client object, Python resource objects are never created by the user of the zhmcclient package. Instead, they are always returned back to the user. Most of the time, resource objects are returned from methods such as list(), find() or findall(). They are methods on a manager object that handles the set of child resources of a particular type within a parent resource. For example, the Client object has a cpcs instance attribute of type CpcManager which handles the CPCs managed by the HMC. Invoking list() returns the CPCs managed by the HMC as Cpc resource objects. Each Cpc object has again instance attributes for its child resources, for example its partitions instance attribute of type PartitionManager handles the set of partitions of that CPC (but not the partitions of other CPCs managed by this HMC).

See Reference: Resources for a description of the resource model supported by the zhmcclient package.

3.4. Error handling

Errors are returned to the user by raising exceptions. All exception classes defined in the zhmcclient package are derived from zhmcclient.Error.

Exceptions may be raised that are not derived from Error. In all cases where this is possible, this is very likely caused by programming errors of the user (incorrect type passed in, invalid value passed in, etc.).

Some HTTP status code / reason code combinations returned from the HMC are silently handled by the zhmcclient package:

  • GET, POST, or DELETE with status 403 and reason 5: This combination means that the HMC session token has expired. It is handled by re-logon, creating a new session token, and retrying the original HMC operation.
  • POST with status 202: This status code means that the operation is being performed asynchronously. There are two cases for that:
    • If there is a response body, an asynchronous job has been started on the HMC that performs the actual operation. If wait_for_completion is True in the method that invoked the HMC operation, the method waits for completion of the job (via polling with GET on the job URI), gathering success or failure from the job results. In case of success, the job results are returned from the method. In case of failure, an HTTPError is raised based upon the error information in the job results.
    • If there is no response body, the operation is performed asynchronously on the HMC, but there is no job resource that can be used to poll for completion status. This is used only for operations such as restarting the HMC.

The other HTTP status / reason code combinations are forwarded to the user by means of raising HTTPError. That exception class is modeled after the error information described in section “Error response bodies” of the HMC API book.

The exception classes defined in the zhmcclient package are described in section Exceptions.

3.5. Filtering

The resource lookup methods on manager objects support the concept of resource filtering. This concept allows narrowing the set of returned resources based upon the matching of filter arguments.

The methods that support resource filtering, are:

A resource is included in the result only if it matches all filter arguments (i.e. this is a logical AND between the filter arguments).

A filter argument specifies a property name and a match value.

Any resource property may be specified in a filter argument. The zhmcclient implementation handles them in an optimized way: Properties that can be filtered on the HMC are actually filtered there (this varies by resource type), and the remaining properties are filtered on the client side.

For the findall() and find() methods, an additional optimization is implemented: If the “name” property is specified as the only filter argument, an optimized lookup is performed that uses a name-to-URI cache in this manager object.

The match value specifies how the corresponding resource property matches:

  • For resource properties of type String (as per the resource’s data model in the HMC API), the match value is interpreted as a regular expression that must match the actual resource property value. The regular expression syntax used is the same as that used by the Java programming language, as specified for the java.util.regex.Pattern class (see http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html).
  • For resource properties of type String Enum, the match value is interpreted as an exact string that must be equal to the actual resource property value.
  • For resource properties of other types, the match value is interpreted as an exact value that must be equal to the actual resource property value.
  • If the match value is a list or a tuple, a resource matches if any item in the list or tuple matches (i.e. this is a logical OR between the list items).

If a property that is specified in filter arguments does not exist on all resources that are subject to be searched, those resources that do not have the property are treated as non-matching. An example for this situation is the “card-location” property of the Adapter resource which does not exist for Hipersocket adapters.


  • This example uses the findall() method to return those OSA adapters in cage ‘1234’ of a given CPC, whose state is ‘stand-by’, ‘reserved’, or ‘unknown’:

    filter_args = {
        'adapter-family': 'osa',
        'card-location': '1234-.*',
        'state': ['stand-by', 'reserved', 'unknown'],
    osa_adapters = cpc.adapters.findall(**filter_args)

    The returned resource objects will have only a minimal set of properties.

  • This example uses the list() method to return the same set of OSA adapters as the previous example, but the returned resource objects have the full set of properties:

    osa_adapters = cpc.adapters.list(full_properties=True,
  • This example uses the find() method to return the adapter with a given adapter name:

    adapter1 = cpc.adapters.find(name='OSA-1')

    The returned resource object will have only a minimal set of properties.

  • This example uses the find() method to return the adapter with a given object ID:

    oid = '12345-abc...-def-67890'
    adapter1 = cpc.adapters.find(**{'object-id':oid})

    The returned resource object will have only a minimal set of properties.