CTIS 490 DISTRIBUTED SYSTEMS

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CTIS 490DISTRIBUTED SYSTEMS

• WEEK 10
• RECOVERY
• REPLICA MANAGEMENT

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RECOVERY

• Another issue of fault tolerance is the recovery
  from an error.
• The idea of error recovery is to replace an
  erroneous state with an error-free state.
• The most widely used recovery method is the
  backward recovery.
• Backward recovery brings the system from its
  present erroneous state back into a previously
  correct state. To do so, it will be necessary to
  record the systems state from time to time and
  to restore such a recorded state when things go
  wrong.
• Each time the systems present state is recorded,
  a checkpoint is said to be made.

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CHECKPOINTING

• In distributed systems, a consistent global state
  is also called a distributed snapshot.
• In a distributed snapshot, if a process P has
  recorded the receipt of a message, then there
  should be a process Q that has recorded the
  sending of that message.

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INDEPENDENT CHECKPOINTING

• Each process saves its state from time to time to
  a locally available stable storage (which is
  designed to survive anything except major
  disasters), and we have to construct a consistent
  global state from these local states.
• A recovery line corresponds to the most recent
  collection of checkpoints.
• The distributed nature of checkpointing may make
  it difficult to find a recovery line. To discover
  a recovery line requires that each process is
  rolled back to its most recently saved state.
• If these local states jointly do not form a
  distributed snapshot, further rolling back is
  necessary.
• This process of a cascaded rollback leads to
  domino effect.

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INDEPENDENT CHECKPOINTING

• The state saved by P2 indicates the receipt of a
  message m, but no other process can be identified
  as its sender. So, P2 needs to be rolled back to
  an earlier state.
• P1 has recorded the receipt of message m, but
  there is no recorded event of this message being
  sent.
• In this example, the recovery line is the initial
  state of the system.

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COORDINATED CHECKPOINTING

• In coordinating checkpointing, all processes
  synchronize to jointly write their state to local
  stable storage. The main advantage is that the
  saved state is globally consistent.
• A coordinator first multicasts a
  CHECKPOINT_REQUEST message to all processes. When
  a process receives such a message, it takes a
  local checkpoint, queues any subsequent messages,
  and acknowledges that it has taken a checkpoint.
• When the coordinator has received an
  acknowledgment from all processes, it multicasts
  a CHECKPOINT_DONE message to allow the blocked
  processes to continue.

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REPLICA MANAGEMENT

• Replica management involves two issues where to
  place replicas and which mechanisms to use for
  keeping them consistent.
• Placing replica servers concerned with finding
  the best locations to place a server that can
  host part of a data store.
• Placing content finding the best servers for
  placing content.

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REPLICA-SERVER PLACEMENT

• Replica management involves two issues where to
  place replicas and which mechanisms to use for
  keeping them consistent.
• Analysis of client and network properties are
  useful to come to informed decisions.
• One approach is to consider the topology of the
  Internet as formed by the Autonomous Systems
  (AS).
• An AS can best be viewed as a network in which
  the nodes all run the same routing protocol and
  which is managed by single organization,
  typically Internet Service Provider (ISP).

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CONTENT REPLICATION PLACEMENT

• There are three types of replicas.

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PERMANENT REPLICAS

• Permanent replicas can be considered as the
  initial set of replicas that constitute a data
  store.
• For example, distribution of a Web site generally
  comes in two forms
• First, files that constitute a site are
  replicated across number of servers at a single
  location. Whenever a request comes in, it is
  forwarded to one of the servers, for instance
  using a round-robin method.
• Second, mirroring can bed used. In this case, a
  Web site is copied to a limited number of
  servers, called mirror sites which are
  geographically spread across the Internet.
  Clients choose one of the sites offered to them.

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SERVER-INITIATED REPLICAS

• Server-initiated replicas are used to enhance
  performance by placing temporary replicas
  (dynamically placing) to handle sudden burst of
  requests.
• Used mainly by the Web hosting services.
• Each server keeps track of access counts per file
  and where access requests come from.
• Given a client C, each server can determine which
  of the servers in the Web hosting service is
  closest to C (Such information can be obtained
  from routing database).
• If client C1 and C2 share the same closest server
  P, all access requests for file F jointly
  registered.
• When the number of requests for a specific file F
  drops below a certain threshold, that file can be
  removed from the server.

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SERVER-INITIATED REPLICAS

• Server-initiated replicas are generally used for
  placing read-only copies.

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CLIENT-INITIATED REPLICAS

• Client-initiated replicas are more commonly known
  as client caches. A cache is a local storage
  facility that is used by a client to temporarily
  store a copy of data.
• Managing cache is left to the client. However,
  client can rely on server to inform when cache
  has become stale.
• When most operations involve only reading data,
  performance can be improved by letting the client
  store requested data in nearby cache. Such a
  cache can be located on the clients machine or
  on a separate machine in the same LAN.
• Whenever requested data can be fetched from the
  local cached, a cache hit is said to have occured.

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CONTENT DISTRIBUTION

• There are three ways to propagate the updated
  content to the replica servers
• Propagate only a notification of an update
  Other copies are informed that an update has
  taken place, and the data they contain is no
  longer valid. The main advantage here is that use
  of little network bandwidth, and works best when
  there are many update operations compared to read
  operations, that is read-to-write ratio is
  relatively small.
• Transfer data from one copy to another It is
  useful when read-to-write ratio is relatively
  high. In that case, the probability that an
  update will be effective in the sense that the
  modified data will be read before the next update
  takes place is high.

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CONTENT DISTRIBUTION

• Propagate the update operation from one copy to
  another Tell each replica which update
  operation it should perform (sending only
  parameter values that those operations need).
  This approach, also referred as active
  replication assumes that each replica is
  represented by a process capable of actively
  keeping its associated data up to date.

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PULL versus PUSH PROTOCOLS

• Yet another design issue is whether updates are
  pulled or pushed.
• In a push-based approach, also referred as
  server-based protocols, updates are propagated to
  other replicas without those replicas asking for
  the updates. Push-based approach is used when
  replicas need to maintain a high degree of
  consistency i.e. replicas need to be kept
  identical. The server needs to keep track of all
  client caches.
• In pull-based approach, a server or client
  requests another server to send it any updates it
  has at that moment. This approach, also called
  client-based protocols are often used by client
  caches, for example by Web caches.