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What is the role of middleware in a distributed system?
To enhance the distribution transparency that is missing in network operat-ing systems. In other words, middleware aims at improving the single-systemview that a distributed system should have.
Explain what is meant by (distribution) transparency, and give examples ofdierent types of transparency.
Distribution transparency is the phenomenon by which distribution aspectsin a system are hidden from users and applications. Examples include ac-cess transparency, location transparency, migration transparency, reloca-tion transparency, replication transparency, concurrency transparency, fail-ure transparency, and persistence transparency.
Why is it sometimes so hard to hide the occurrence and recovery from fail-ures in a distributed system?
It is generally impossible to detect whether a server is actually down, or thatit is simply slow in responding. Consequently, a system may have to reportthat a service is not available, although, in fact, the server is just slow.
Describe precisely what is meant by a scalable system. Scalability can beachieved by applying dierent techniques. What are these techniques?
A system is scalable with respect to either its number of components, geo-graphical size, or number and size of administrative domains, if it can growin one or more of these dimensions without an unacceptable loss of per-formance. Scaling can be achieved through distribution, replication, andcaching.