Optional and responsive locking in collaborative graphics editing systems

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Optional and responsive locking in collaborative
graphics editing systems

• By

David Chen D.Chen_at_cit.gu.edu.au
Chengzheng Sun C.Sun_at_cit.gu.edu.au
School of Computing and Information
Technology Griffith University Brisbane,
Queensland 4111 Australia
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Outline

• Object graphics collaborative editing systems
• Locking
• The GRACE system
• Locking in GRACE
• Responsive operation generation
• Intention preservation
• Lock ownership
• Conclusion

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Real-time object graphics collaborative editing
systems

• These systems allow multiple users to edit the
  same document at the same time from different
  sites.
• An object graphics document is a collection of
  one or more graphical objects such as line,
  rectangle and circle, etc.
• A graphical object consists of attributes, such
  as type, position, size and color etc.
• Editing operations, such as create, move and fill
  etc, are used to modify the document.
• What is the use for locking?

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Locking in existing systems

• First used to maintain consistency in database
  systems.
• It is also used to maintain consistency in many
  object graphics collaborative editing systems,
  e.g. Ensemble, GroupDraw, GroupGraphics and
  GroupKit.
• How does locking work
• Before an operation can be generated to edit an
  object, a lock on that object has to be obtained.
• Locking prevents the generation of concurrent
  operations on the same object.
• Locking is compulsory.
• Disadvantages of locking
• Pessimistic locking slows down the response time.
• Optimistic locking does not preserve the
  generation effects of all operations. The system
  decides which effect the users would see based on
  unrelated information.

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The GRACE system

• GRACE (GRAphics Collaborative Editing) system.
• GRACE has a distributed replicated architecture.
• Each site maintains a copy of the shared
  document.
• When an operation is generated it is first
  applied to the local copy, then sent to all
  remote sites.
• The intention preservation property of GRACE
  ensures the generation effects of all operations
  are preserved.
• Object replication is used to maintain intention
  preservation.

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• The dependency relations are defined based on
  Lamports happen before relation
• Ob is dependent on Oa, iff Oa happened before Ob
  denoted by Oa Ob. E.g

Oa
Oa
Oa
Ob
Oc
Ob
Ob

• Oa and Ob are independent, iff neither Oa Ob,
  nor Ob Oa, denoted by Oa Ob. E.g

Oa
Ob
Intention of an operation The intention of an
operation O is the editing effect which can be
achieved by applying O on the document state from
which O was generated. Intention preservation
property For any operation O, the effects of
executing O at all sites are the same as the
intention of O, and the effect of executing O
does not change the effects of independent
operations.
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• Object replication scheme
• Definitions
• Independent operations targeting the same object
  and changing the same attribute to different
  values are called conflicting operations.
• Operations not conflicting are compatible.
• Replication effect
• For two conflicting operations Oa and Ob,
  targeting object G, replicates of G, Ga and Gb
  are made for the application of Oa and Ob
  respectively.

Move right
Move left
replica
replica

• For two independent compatible operations Oa and
  Ob targeting the same object, there must be at
  least one object/replica, which contains the
  effects of both Oa and Ob.

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Locking in GRACE

• The role of locking in GRACE
• Locking is used to reduce the generation of
  conflicting operations which cause object
  replication.
• Locking is not used to preserve intentions of
  operations, therefore, locking an object before
  applying operations is optional.
• What can be locked?
• Graphical objects.
• Regions.
• A region is an area in the drawing space.
• Locking a region can be used to obtain a private
  working space.
• A lockable item is either an object or a region.
• A locking operation specifies either one or more
  objects or a region to be locked.

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• What does a user get for locking an item?
• If a user locks an object G then the system
  guarantees s/he the editing right to G.
• If a user locks a region R then the system
  guarantees s/he the editing right to R and all
  objects within R.
• What if the user did not lock an item before
  editing?
• That users editing request will be rejected iff
  that item is locked by someone else.

To lock or not to lock,
that is the question.
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Responsive operation generation

• How locking operations can be generated
• Implicitly - generated by the system.
• Explicitly - generated by the users.
• With the introduction of locking, user generated
  editing/locking requests need to be validated.
• A users request is valid if the target item on
  the local document is unlocked or s/he owns the
  lock.
• Otherwise it is invalid, and is rejected right
  away and the user informed.
• This validation process relies only on the
  current local document state, therefore fast
  response time is ensured.
• For any each valid request, an operation is
  generated.

Operation generation
Local execution
Remote execution
User request
Local validation
Rejection
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Intention Preservation

• Locking introduces new intention preservation
  problems
• How to preserve the intention of an editing
  operation which targets an object locked by an
  independent locking operation?
• How to preserve the intentions of independent
  locking operations targeting the same object?
• Solution allow these independent operations to
  be applied to the same object.

Move(G)
Lock(G)
Lock(G)
Lock(G)
An unstable period of a lock is used to inform
lock owners that the locked item may be edited or
locked other by independent operations. An
unstable period of a lock generated by locking
operation L, is the period starting when L is
applied on the item until all operations
independent of L targeting that item have been
applied.
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• If there are two or more independent locking
  operations targeting the same item, then all
  users who generated these locking operations will
  own the lock on that item. This item is locked by
  a shared lock.
• All owners of an item locked by shared lock can
  edit that item.
• After the unsafe period, if only one user owns
  the lock, then that user will have exclusive
  access to that item. This item is locked by an
  exclusive lock.
• After the unstable period, only the lock owner(s)
  can editing that item.
• Notations
• Let Owner(I) denotes the a set of users who owns
  the lock on item I.
• Let Owner(I) denotes the number of users who
  owns the lock on I.
• A lock on an item I is exclusive if Owner(I)
  1.
• A lock on an item I is shared if Owner(I) 1.

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Shared lock ownership

• What should be the lock ownership for independent
  locking operations those target item is the same
  or overlaps?
• Let L1 and L2 be two independent locking
  operations generated by users U1 and U2.
• If L1 and L2 are object locking operations
  targeting the same graphical object G, then
  Owner(G) U1, U2.
• If L1 and L2 are region locking operations with
  overlapping region P and non-overlapping regions
  R1 and R2 for L1 and L2 then Owner(P) U1, U2,
  Owner(R1) U1 and Owner(R2) U2.

R1
P
R2
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• If L1 is an object lock targeting object G and L2
  is a region lock targeting region R, where G is
  within R, then what should be the lock ownership
  for G and R?
• G is inside R, therefore, Owner(R) should also be
  able to edit G, therefore, Owner(G) U1, U2.
• U1 does not own a region lock on R, therefore,
  Owner(R) U2.

R

• The resulting effect of G
• U1 can edit G, but cannot move G within R.
• U2 can edit G and can move G within R.
• Both U1 and U2 can move G outside of R.
• If G is moved outside of R, then U2 loses the
  ownership on G, i.e. Owner(G) U1.

G
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Reduce ownership for replicates

• For any graphical object G with lock that is
  shared or in the unstable period, conflicts may
  occur on G. What should be the lock ownership for
  replicates of G?

U1, U2

• Consider the following situation
• Let O1 and O2 be the conflicting operations
  targeting G, such that O1 and O2 are generated by
  U1 and U2 where U1, U2 ? Owner(G).
• According to replication scheme, replicates G1
  and G2 will be made for O1 and O2 respectively.
• Shared lock can be assigned to both replicas such
  that Owner(G1) Owner(G2) U1, U2.
• Or exclusive locks can be assigned such that
  Owner(G1) U1 and Owner(G2) U2.

replication
U1, U2
U1, U2
or
U1, U2
replication
U1
U2
It is better to have exclusive locks than shared
locks since conflict cannot occur on objects with
an exclusive lock.
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• Consider U3 ?Owner(G) generated operation O3 to
  edit G.
• Let O3 be independent and compatible with both O1
  and O2. O3 will be applied to both replicates G1
  and G2.
• If U3 is to own only one replicas lock, then
  that replica cannot be consistently chosen at all
  sites until all independent operations have
  arrived.
• U3 should own the lock on all objects O3 is
  applied to.

U1, U2, U3
replication
U1, U3
U2, U3

• Lock ownership for replicated objects
• Let S be a set of independent operations all
  targeting the locked object G, such that there is
  at least a pair of conflicting operations in S.
  For any user U such that U?Owner(G), then after
  replication
• If U generated an operation O such that O?S, then
  for any replica G' of G which O is applied to,
  U?Owner(G').
• If U did not generated any operation in S, then U
  will own the locks for all replicates of G.

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Summary and Future work

• An optional and responsive locking scheme to
  reduce conflicts being generated was presented.
• Highlights
• The introduction of region locking.
• The lock ownership for independent overlapping
  region and object locks.
• The introduction of unsafe period for applying
  independent operations.
• Taking advantage of object replication to reduce
  lock ownership.
• Future work
• A good user interface design is needed to display
  locked items and to allow users to generate
  locking requests.
• A new challenge is to provide instantaneous
  exclusive locking for both region and object
  locks.

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The End