Performance is threatened by the required
Figure 1. Single server compared to a replicated service
A) B)
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coordination tasks that the server set has to perform before replying to each client. Such
coordination has to do mainly with data consistency.
For instance, let us consider a very simple service that just stores a number in a database.
Clients retrieve or update that number invoking such a remote service through a network.
Without replication, the system would just be formed by a set of clients, the network, and a
single server. In this case, server software can easily be designed to serve requests sequentially,
storing and returning the number as requests arrive. Naturally, in case the server crashes, the
service would become unavailable. To enhance its availability, let us suppose that two servers
cooperate to serve such requests. In this scenario, if one client sends a request to store one
value, the servers have to cooperate to store the same value. Doing so, any of them will be
able to reply to subsequent client requests, even if the other server crashes.
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