Client-Server Caching

1
Client-Server Caching

• James Wann
• April 4, 2000

2
Client-Server Architecture

• A client requests data or locks from a
  particular server
• The server in turn responds with the requested
  items
• Otherwise known as a data shipping architecture

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Why Caching?

• Better utilizes the CPU and memory resources of
  clients
• Reduces reliance on the server
• Increases the scalability of the system

4
Disadvantages of Caching

• Increased network utilization
• Extra load on system
• Increased transaction abort rates, depending on
  algorithm

5
Test Workloads for Caching Algorithms

• HOTCOLD There is a high probability that pages
  in the hot set will be read. However, there is
  equal probability that pages in either the hot
  set or cold set will be written
• FEED There is one client that writes to pages
  in the hot set. The other clients have a high
  probability of reading from the hot set

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Test Workloads for Caching Algorithms (contd)

• UNIFORM All pages have equal probability of
  being either read or written
• HICON There is a high probability of conflicts
  in reading/writing

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Server-Based Two-Phase Locking

• Client transactions must obtain locks from the
  server before accessing a data item
• Easiest algorithm to implement
• Heavy messaging overhead
• Best for workloads with high data contention

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Optimistic Two-Phase Locking

• Each client has its own lock manager
• Upon commit, the client sends a message to the
  server stating which pages are updated
• The server sends an update message to all
  clients with copies of the pages
• The next action depends on algorithm

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Commit Phase
client
client
server
client
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Update Message Phase
client
client
server
client
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Non-Dynamic O2PL Algorithms

• O2PL-Invalidate (O2PL-I) invalidates the
  updated pages in the clients receiving the
  message
• O2PL-Propagate (O2PL-P) propagates the changed
  pages to the clients that already have the pages

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Dynamic O2PL Algorithms

• O2PL-Dynamic (O2PL-D) chooses between
  propagation and invalidation based on a certain
  criteria
• Criteria 1 The page is at the client where the
  message is being sent
• Criteria 2 The page was previously propagated
  to the client and it has since been reaccessed

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Dynamic O2PL Algorithms (contd)

• O2PL-New Dynamic (O2PL-ND) uses the same
  criteria as O2PL-D with one additional
  characteristic
• A structure called the invalidate window is used
  hold the last n invalidated pages
• Most recently invalidated pages are placed in
  front of the window

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Dynamic O2PL Algorithms (contd)

• If a page is accessed and its number is found in
  the invalidate window, then the entry is marked
  as being a mistaken invalidation
• Criteria 3 The page was found to be previously
  invalidated by mistake

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Evaluation of O2PL Algorithms (HOTCOLD)

• O2PL-I and O2PL-ND have a higher throughput than
  O2PL-P and O2PL-D
• This is due to the fact that propagated updates
  may not necessarily be accessed again (wasted
  propagations)
• O2PL-I, O2PL-D, and O2PL-ND have similar
  performance on a faster network

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Evaluation of O2PL Algorithms (FEED)

• O2PL-P, O2PL-D, and O2PL-ND have better
  throughput than O2PL-I
• This scenario benefits propagation (keeps hot
  pages in buffer)
• However, the performances are comparable in
  small buffers

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Evaluation of O2PL Algorithms (UNIFORM)

• O2PL-P and O2PL-D have far less throughput than
  the other algorithms
• Higher probability of wasted propagations

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Figures 1 through 6 in paper
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Callback Locking

• Allows caching of data pages and locks
• Clients obtain locks by making a request to the
  server
• If there is a lock conflict, clients with the
  locks are asked to release the locks
• The lock request is granted only when all the
  locks are released

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CB-Read

• Only read locks are cached
• When a write lock request is made, the server
  requests all clients with the specified page to
  release the page
• If all clients comply, then a write lock is
  granted
• All subsequent lock requests are blocked until
  the write lock is released

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CB-All

• Both locks are cached and write locks are not
  released at the end of a transaction
• A page copy at a certain client is designated as
  the exclusive copy
• Upon a read request from another client, then
  exclusive copy is received and the original
  client no longer has the exclusive copy
• This is called a downgrade request

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Evaluation of Callback Algorithms (HOTCOLD)

• O2PL-ND has better throughput than the Callback
  algorithms
• The Callback algorithms require more messages
  per transaction
• However, the throughput difference is not
  significant

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Evaluation of Callback Algorithms (FEED)

• O2PL-ND has better throughput than either
  Callback algorithms
• This is because the pages are usually already
  found in the clients in O2PL-ND and so no extra
  messages are needed
• Again, the performance difference is not
  significant

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Evaluation of Callback Algorithms (UNIFORM)

• All three algorithms have similar throughput

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Evaluation of Callback Algorithms (HICON)

• O2PL-ND performance suffers because of frequent
  aborts due to late deadlock-detection
• CB-Read has higher throughput than CB-All
  because of smaller messaging requirements

26
Figures 8 through 13 in paper
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Figures 14 through 15 in paper
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Conclusion

• O2PL-ND proves to be a more flexible algorithm
  than O2PL-D
• Invalidation is the default, rather than
  propagation
• Ideal for a small number of clients

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Conclusion (contd)

• CB-Read is a more adaptable algorithm than
  O2PL-ND and CB-All
• Detects deadlock earlier than O2PL-ND and avoids
  aborts for long transactions
• Has lower messaging overhead than CB-All
• Server-based 2PL works best with a large number
  of clients in a high-contention situation.
  Perhaps further research should be done in
  consideration of faster LANs (e.g. fast ethernet)?