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Fault Tolerant — Open Issues

  • Acronym: FT
  • Issues Count: 2
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FT-FTF Issue: Intelligent factory selection

  • Key: FT-2
  • Legacy Issue Number: 3921
  • Status: open  
  • Source: AT&T ( Robert Gruber)
  • Summary:

    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.

  • Reported: FT 1.0b1 — Thu, 28 Sep 2000 04:00 GMT
  • Updated: Fri, 6 Mar 2015 20:58 GMT

FT-FTF Issue: Request more powerful property management

  • Key: FT-1
  • Legacy Issue Number: 3920
  • Status: open  
  • Source: AT&T ( Robert Gruber)
  • Summary:

    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).

  • Reported: FT 1.0b1 — Thu, 28 Sep 2000 04:00 GMT
  • Updated: Fri, 6 Mar 2015 20:58 GMT