A9_DistSystPrinc_Ch578_FA09

A9_DistSystPrinc_Ch578_FA09 - Distributed System Principles...

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1 Distributed System Principles Naming: 5.1 Fault Tolerance: 8.1
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2 Naming Names are associated to entities (files, computers, Web pages, etc.) Entities (1) have a location and (2) can be operated on. Name Resolution : the process of associating a name with the entity/object it represents. Naming systems prescribe the rules for doing this.
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3 Names Types of names Addresses Identifiers Human friendly Representation of names Human friendly format Machine readable – generally random bit strings
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4 Addresses as Names To operate on an entity in a distributed system, we need an access point. Access points are physical entities named by an address. Compare to telephones, mailboxes Objects may have multiple access points Replicated servers represent a logical entity (the service) but have many access points (the various machines hosting the service)
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5 Addresses as Names Entities may change access points over time A server moves to a different host machine, with a different address, but is still the same service. New entities may take over the vacated access point and its address. Better: a location-independent name for an entity E should be independent of the addresses of the access points offered by E.
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6 Identifiers as Names Identifiers are names that are unique and location independent. Properties of identifiers: An identifier refers to at most one entity Each entity has at most one identifier An identifier always refers to the same entity; it is never reused. Human comparison? An entity’s address may change, but its identifier cannot change.
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7 Human-Friendly Names Human-friendly names are designed to be used by humans instead of a computer They usually contain contextual information; e.g., file names or DNS names. Do not usually contain information that is useful to a computer
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8 Representation Addresses and identifiers are usually represented as bit strings (a pure name) rather than in human readable form. Unstructured or flat names. Human-friendly names are more likely to be character strings (have semantics)
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9 Name Resolution The central naming issue: how can other forms of names (human-friendly, identifiers) be resolved to addresses? Naming systems maintain name-to- address bindings In a distributed system a centralized directory of name-address pairs is not practical.
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10 Naming Systems Flat Naming Unstructured; e.g., a random bit string Structured Naming Human-readable, consist of parts; e.g., file names or Internet host naming Attribute-Based Naming An exception to the rule that named objects must be unique Entities have attributes; request an object by specifying the attribute values of interest.
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11 3.2 Flat Naming Addresses and identifiers are usually pure names (bit strings – often random) Identifiers are location independent
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A9_DistSystPrinc_Ch578_FA09 - Distributed System Principles...

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