Issue 3516: following question regarding modifications to CORBA core Jira Issue FT-3
Issue 3747: define the State as typedef any State Jira Issue FT-4
Issue 3778: Issue with 'factory' Jira Issue FT-5
Issue 3856: Propose Remove use of Filter Jira Issue FT-6
Issue 3908: Encoding of Service Contexts in Fault Tolerant CORBA specification missing Jira Issue FT-7
Issue 3910: typedef issue Jira Issue FT-8
Issue 3920: FT-FTF Issue: Request more powerful property management Jira Issue FT-1
Issue 3921: FT-FTF Issue: Intelligent factory selection Jira Issue FT-2
Issue 3976: Harmful deprecation of LOCATE_FORWARD_PERM for Faut Tolerant CORBA Jira Issue FT-9
Issue 4066: term "method" used wrongly Jira Issue FT-10
Issue 4109: On page 27-9 of the FT CORBA spec, under "Application-Controlled Membership Jira Issue FT-11
Issue 3516: following question regarding modifications to CORBA core (ft-ftf)
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Source: Oracle (Dr. Anita Jindal, nobody)
Nature: Clarification
Severity:
Summary:
Basically, the Failure OBJ_ADAPTER is considered a failover condition in the document that was sent out. In most cases OBJ_ADAPTER exception may be thrown when there is an internal ORB error. In case of an internal ORB error, the retry on the TAG_ALTERNATE_IIOP_ADDRESS may still yield the same exception. This may be inefficient. Do you see situations where doing a failover on this particular exception is useful.
The Fault Tolerant CORBA specification defines the State used by the get_state(), set_state(), get_update(), set_update() methods, as typedef sequence<octet> State Those methods must be implemented by the application programmers. They will find their task easier if we define the State as: typedef any State
The Fault Tolerant CORBA specification contains the following struct. struct FactoryInfo { GenericFactory factory; Location the_location; Criteria the_criteria; }; This causes a problem for the IDL compilers of some vendors, because "factory" is a keyword in CORBA V2.3. See CORBA V2.3, page 3-8, Lexical Conventions, June 1999. We need to change "factory" in this struct to "fact", "fctry", "generic_factory", or whatever. What is your preference? Motivation: The Notifier will be easier to replicate if it is a single object. At present, all Filters created by the Notifier must also be replicated. Furthermore, there is no requirement that a Filter be destroyed by the client that created it (once it is done using it), raising a garbage collection issue. FOr a connected consumer, if the consumer no longer exists the Notifier can discard the connection. There is no analagous test for Filters. The Notifier interface is already a collapsed version of multiple CosNotification APIs to get rid of the channel/admin/proxy objects in favor of one object, so I am just proposing we carry through on that approach. �ne proposal: First, remove method create_subscription_filter. Second, change the 2 connect_foo_fault_consumer methods (connect_structured_fault_consumer + connect_sequence_fault_consumer) to take just a consumer and a grammar: ConsumerId connect_foo_fault_consumer (in CosNotifyComm::FooPushConsumer, in string constraint_grammar) raises (�nvalidGrammar, AlreadyConnected) One or more event forwarding constraints is associated with each connected consumer, with the default being a constraint that matches all events. The ConsumerID returned from a call can be passed into methods that modify these constraints. When a consumer is disconnected, the associated constraints are discarded. Third add methods for manipulating constraints associated with a ConsumeID: string constraint_grammar(in ConsumerID) void add_constraints(in ConsumerID, ...) void remove_constraints(in ConsumerID, ...) void remove_all_constraints(in ConsumerID) void modify_constraints(in ConsumerID, ...) ConstraintExpSeq get_constraints(in ConsumerID) where ... means the normal arguments that are in the corresponding methods in the Filter spec. The above methods correspond to the Filter methdos that are required in the current version of the spec, except I left out 2 of them, match_structured and destroy. I do not think we need to support match_structured -- only the Notifier needs to be able to do matching. destroy is not needed because there is no filter object to be destroyed. (disconnect is sufficient.) ALTERNATE PROPOSAL A simpler scheme is to associate a single constraint with each consumer. This is not very restrictive, especially when you consider that there is currently only one event type in use in the FT spec. The default would still be a constraint that matched all events. In this case the only method needed to modify this constraint is: void replace_constraint(in ConsumerID, in EventTypeSeq event_types, in string constraint_expression) Further, if we are willing to stick to the default constraint grammar, no grammar needs to be specified, which simplifies connect_foo_consumer -- not only by removing the constraint_grammar argument but also by removing the InvalidGrammar exception, which comes from CosNotifyFilter. I believe one could simplify things enough to get rid of any dependencies on CosNotifyFilter. It is not clear how important this is, but I thought I should mention the possibility.
13.6.7 of the CORBA 2.3 specification states: "The context data for a particular service will be encoded as specified for its service-specific OMG IDL definition, and that encoded representation will be encapsulated in the context_data member of IOP::ServiceContext. (See Section 15.3.3, Encapsulation, on page 15-13)." The descriptions of service contexts in the FT spec are missing an explicit statement of the encoding of the service context data. Proposed Resolution: Add the following sentence in all appropriate sections: "When encoded in a request or reply message header, the <code>context_data</code> component of the <code>ServiceContext</code> struct will contain a CDR encapsulation of the xxxxxx struct."
One additional issue I have is that the ReplicationStyleValue, MembershipStyleValue, ConsistencyStyleValue, FaultMonitoringStyleValue, FaultMonitoringGranularityValue are typedefed to long, whereas the InitialNumberReplicasValue and MinimumNumberReplicasValue are typedefed to unsigned short. It might be more appropriate to typedef all of these to unsigned short.
There are 3 problems w.r.t. property management which I will list together since a solution could/should address all of them. I will send a proposed solution in another message. --------------------------------------------------------------- ** Problem A ** For sequence-valued properties, there should be a way to add or remove one or more elements from a sequence value without having to resort to using more than one method call. Notes: Currently, one has to 'get' the current sequence value, modify it, and then 'set' the sequence value. This results in a nasty race condition: it is not safe to have independent threads of control doing get-set combinations. A simple solution is to have new methods for element update. A complex solution is to allow any number of updates, including property set, element addition, element removal, for default and type and dynamic, all to be grouped and sent to the manager in one property_update request. For example, one might want to group the removal of a property P from type T and the addition of property P as a default property. --------------------------------------------------------------- ** Problem B ** The property management interface has insufficient power. Notes: One can query or update over a single type or a single object group, but not over a set of types or a set of object groups. Further, one cannot: * get a list of types that have + at least 1 property defined; + specific propert[y|ies] defined; + at least 1 factory at specific location[s]; [ or modify properties for the specified types ] * get a list of existing object groups that have + specific propert[y|ies] defined; + specific type[s]; + an active replica at specific location[s]; [ or modify properties for the specified object groups ] * get a list of active replicas that have + specific type[s]; + specific location[s]; [ or modify properties for the specified replicas ] * get a list of locations that have + at least 1 property defined; + specific propert[y|ies] defined; + at least 1 active replica; + an active replica for specific object group[s]; + an active replica of specific type[s]; [ or modify properties for the specified locations] * other query/update cases that should be supported? --------------------------------------------------------------- ** Problem C ** The property management interface does not sufficiently distinguish between high-level FT QoS properties used to manage entire object groups and low-level object construction properties used to select factories and create individual replicas. Notes: High-level QoS properties change infrequently, and never differ across replicas. Low-level construction properties change more frequently as factories are created/destroyed/lost, and they do differ across locations/replicas (different factories, different criteria). In each case, one must distinguish between properties for an existing object group and properties to be used for future object groups. Even for replica construction properties, one should be able to assign a different set of locations/factories to be used for new replica creation for existing object group[s] and for future object groups. Currently, low-level properties are buried in a single value that is stored with a single property (FactoryInfos), either for a specific type or for a specific object group. This makes it very hard to do lookup or modification of these properties by location or by the pair type x location or object group x location. To replace an Info for a single location one must replace the entire Infos sequence. Even with the ability to add/remove a member of a sequence, to replace either the factory or the criteria within a given Info one would have to remove the current Info and replace it with a new Info, where the Info would need to contain a copy of the part(s) that are not to be modified together with the modified part. BTW I am leaning towards splitting the PropertyManager into a GroupQoSManager and a FactoryManager, but other approaches are possible. One argument for the split is that it seems to make sense for a FactoryManager to monitor the liveness of registered factories and to provide logic for selecting an appropriate factory and associated criteria for construction of a new replica for a given group or type. In contrast, it does not make sense for a generic property manager to do monitoring (or to know anything about the values stored in properties).
This issue was determined to be out-of-scope of the Fault Tolerant CORBA Finalization Task Force.
An FT-FTF (Fault Tolerance Finalization Task Force) Issue: GOAL: Introduce intelligent factory selection. On a single machine (perhaps an N-way multiprocessor, but even for a uniprocessor) one might want to have N factories, corresponding to N processes that will have replicas created in them. Ideally, only one replica for a given group should be started for the entire machine. Similarly, if one has several subnets, one might have factories on all machines, but ideally only one replica should be started per subnet, if appropriate factories are available. If the only factories available for a given type happen to be on the same subnet or same machine, then it should be possible to specify either that it is OK to go ahead with replicas on the same subnet or same machine or it is not OK. Alternatively, I might want all replicas to be on the same subnet, if possible, to reduce coordination costs, while still wanting a different hosts requirement. How to extend the specification to enable this feature? One proposal is to take advantage of the fact that location names are structured. While any structuring is allowed, we could declare that if you want to use an intelligent factory selection policy you must use names that capture the hierarchical nature of fault domains. E.g., for my scenario I could use names that capture subnet/host/processor distinctions: sA.h1.p1, sA.h1.p2, sA.h2.p1, sA.h2.p2, ... sA.hJ.p1, sA.hJ.p2 sB.h1.p1, sB.h1.p2, sB.h2.p1, sB.h2.p2, ... sB.hK.p1, sB.hK.p2 I believe there should be a LocationHints property for types or groups that is distinct from the issue of how many actual locations have available factories, where hints are like location names but can have wildcards. Thus, I could specify sA.*.* and sB.*.* as LocationHints for type T to indicate that I prefer replicas for type T to be started on machines on subnets sA and sB. Note that this is very different from giving a list of specific locations. (I certainly do not want to specify which processor number to use!) While the set of available factories might change frequently, the hints should be relatively stable. Assume that as factories are created at specific locations (such as a new factory F1 at location sA.h3.p1) they could be registered with a FactoryManager. This manager knows all the location names that have factories registered for a given group or object type. One algorithm to select a location, given a set of existing replica locations and possibly some location hints, is to choose a location name that matches one of the hints and has the greatest difference from the existing names, where a difference in the i'th part of a name dominates a difference in the j'th part of the name. Alternative algorithms are possible, e.g., one might prefer to keep replica groups in the same subnet but on different machines, which corresponds to a rule that says equality of the first part of the name is the primary determinant, while for positions 2 and on, use the greatest difference rule above. We could have a QoS property called FactorySelectionPolicy which is a string and have some predefined algorithms (+ algorithm names). Vendors could define additional algorithms. An alternative to having a fixed number of predefined algorithms is to introduce a means of describing a whole class of algorithms. Here is one approach. For a given part, one of 5 requirements holds: . NC : no constraint . EB : equality best, inequality allowed . ER : equality required . DB : difference best, equality allowed . DR : difference required A policy string is a sequence of <requirement> specs separated by dots ("."). Each requirement applies to the part at the given location, while the final <requirement> applies to the part at its location and all subsequent locations. E.g., the spec ER.DB.DR requires equality for part 1, prefers difference for part 2 (but not required), and requires difference for all remaining parts (3, 4, ... ). DR/ER constraints have higher priority than DB/EB constraints (all DR/ER constraints must be met). When there are optional constraints, a solution that satisfies an earlier optional constraint has priority over a solution that satisfies a later optional constraint. This is true regardless of how many optional constraints can be satisfied, e.g., satisfying the first optional constraint but not the second or third has priority over satisfying both the second and third optional constraint but not the first. The reverse ordering (favoring later optional constraints over earlier ones) can be selected by adding a less-than ("<") sign at the end of the policy string. For solutions that satisfy the same earliest (or latest in the case of "<") optional constraint, solutions that satisfy more optional constraints have priority over solutions that satisfy fewer optional constraints. This rule can be overridden by adding "MIN:" as a prefix to the policy string (indicating that the minimal number of optional constraints should be met --- i.e., at least one optional constraint should be met, if possible, but beyond this, solutions that satisfy the fewest additional optional constraints are favored). The resulting location selection policy implicitly includes a final global constraint: the locations chosen for a given group must be unique. N.B. When locations have a different number of parts, EB and DB requirement are ignored for missing part locations, while if one location has a part but another does not, this satisfies the DR requirement and fails the ER requirement. Some example selection policies: [1] NC No part is constrained. Due to the implicit global constraint, NC selects unique locations, but selection is otherwise random. [2] DR *Every* part must differ. This policy is not often used; it is more common to follow one or more DR constraints with some optional constraints or with NC, as in the next example. [3] DR.NC The first part must differ, while there are no constraints on the other parts. [4] DB A difference is best for each part, but not required for any given part. The result is a selection algorithm that attempts to find a difference in the earliest possible part. When several locations differ starting at the same earliest part, the algorithm favors selecting locations that differ in as many subsequent parts as possible. [5] MIN:DB Like DB, except when several locations differ starting at the same earliest part, the algorithm favors selecting locations that differ in as few subsequent parts as possible. [6] DB< Like DB, except the algorithm favors locations that differ in the latest possible part. [7] EB Equality is best for every part, but not required for any part. The result is a selection algorithm that attempts to find equality in the earliest possible part. When several locations are equal starting at the same earliest part, the algorithm favors selecting locations that are equal in as many subsequent parts as possible. [8] ER.DB Equality of the first part required, while differences in other parts are preferred but not required, with earlier optional differences dominating later ones. [9] EB.DB Equality of the first part is preferred, while differences in other parts are preferred but not required, with earlier optional differences dominating later ones (EB dominates DB and earlier DB differences dominate later ones). Consider the subnet.host.processor location naming scheme. + DR.NC would choose a different subnet for each replica and otherwise choose an arbitrary factory in each subnet. + EB.DB would choose the same subnet for all replicas, if possible, but if necessary would use different subnets. For locations in the same subnet, it would attempt to use different hosts and different processors, with higher priority given to using different hosts. + EB.EB.DB< would attempt to find locations that differ in the processor part but have the same host and subnet, where the processor difference has highest priority, host equality has next highest priority, and subnet equality has least priority. This would tend to cluster replicas as close together as possible, optimizing coordination cost while sacrificing some reliability. + MIN:DB< has the same effect as EB.EB.DB< : it specifies minimal DB matches (beyond 1 match) with priority given to later parts over earlier ones. MIN:DB< has the advantage that it works with locations of any length, while EB.EB.DB< is only useful for locations of length 3.This issue was determined to be out-of-scope of the Fault Tolerant CORBA Finalization Task Force.
Earlier this year, the interop FTF deprecated the LOCATE_FORWARD_PERM exception because of several reasons : - it was badly specified - it made the implementation of hash() difficult, and broke most of the existing ones. It turns out that the Fault Tolerance specification published a little earlier crucially requires a similar mechanism. In normal life, most applications can rely on plain LOCATE_FORWARD because there is no reason to expect the death of the originally pointed component. In the case of Fault Tolerant CORBA, this is entirely different: it is precisely when we issue a LOCATE_FORWARD_PERM that we know for sure that the original component is dead, and might never return. If all the backup profiles of an IOR enjoy the same death, all hope is gone. This means that without a mechanism similar to LOCATE_FORWARD_PERM, the Fault Tolerant CORBA spec cannot address the requirements of real fault-tolerant systems. This is why the Fault-Tolerant CORBA FTF would like to see LOCATE_FORWARD_PERM re-introduced in some way. Here are a few ideas that might help : Issue of scope: The scope of LOCATE_FORWARD_PERM is ORB lifetime. Issue of hash() : Let us be reminded that the Fault-Tolerant CORBA spec defines teh concept of an Interoperable Object Group Reference (IOGR). The IOGR contains a specific profile that contains a group identifier. - When an ORB receives and IOGR, it should compute the value of hash() based on the GroupID contained in the IOGR, and performs LOCATE_FORWARD_PERMs if requested. - When an ORB receives a normal IOR (i.e. an IOR lacking a group profile) it computes hash() in the customary way, and doesn't have to respond to LOCATE_FORWARD_PERMs.
Throughout the document, the authors use the term "method" several times where they should be talking about "operations" instead. This violates the general understanding of the OMG terminology, where IDL interfaces contain "operations", not "methods". The term "method" is usually reserved as a concept of oo programming languages. I recommend that for the next revision, the authors run a global search&replace and identify where they want to talk about methods and where of operations.
On page 27-9 of the FT CORBA spec, under "Application-Controlled Membership", "The application-controlled (MEMB_INF_CTRL) Membership Style" should be corrected to read "The application-controlled (MEMB_APP_CTRL) Membership Style"