Active and Passive Replication in Distributed Systems

In the distributed systems research area replication is mainly used to provide fault tolerance. The entity being replicated is a process. Two replication strategies have been used in distributed systems: Active and Passive replication.

active-passive-replication

In active replication each client request is processed by all the servers. Active Replication was first introduced by Leslie Lamport under the name state machine replication. This requires that the process hosted by the servers is deterministic. Deterministic means that, given the same initial state and a request sequence, all processes will produce the same response sequence and end up in the same final state. In order to make all the servers receive the same sequence of operations, an atomic broadcast protocol must be used. An atomic broadcast protocol guarantees that either all the servers receive a message or none, plus that they all receive messages in the same order. The big disadvantage for active replication is that in practice most of the real world servers are non‚Äźdeterministic. Still active replication is the preferable choice when dealing with real time systems that require quick response even under the presence of faults or with systems that must handle byzantine faults.

In passive replication there is only one server (called primary) that processes client requests. After processing a request, the primary server updates the state on the other (backup) servers and sends back the response to the client. If the primary server fails, one of the backup servers takes its place. Passive replication may be used even for non‚Äźdeterministic processes. The disadvantage of passive replication compared to active is that in case of failure the response is delayed.

11 comments

  1. Paresh

    thnx 4 the help….

  2. aman

    thanks …………………good info…………..

  3. chanhduc

    A good introduction. So clear and easy to understand. Thanks.

  4. someone

    Thank you! Very well explained, short and concise! :)

  5. Anonymous

    Nice lesson for me…… Thank u…..

  6. Rajat Mishra

    very well explained in short…. Thanx

  7. Salinda

    thankzz..

  8. Prateek

    hello friend may you please the source of above daigram.

  9. Pingback: Service-level requirements: soddisfare i requisiti non funzionali di una architettura software - Francesco Ficetola

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