103 |
is signed with the appropriate key, and contains a timestamp |
is signed with the appropriate key, and contains a timestamp |
104 |
newer than that of the current version. |
newer than that of the current version. |
105 |
|
|
106 |
*Oceanstore* [XXXref] XXX |
In *Oceanstore* [XXXref], every document has a |
107 |
|
*primary replica*, a small set of servers that |
108 |
|
maintain the current version of the document. |
109 |
|
To make the connection between document and version, |
110 |
|
the primary replia signs *heartbeats*, tuples |
111 |
|
containing a document id, the hash of a version, |
112 |
|
a timestamp and a serial version number. |
113 |
|
To obtain the most current version, one sends a request |
114 |
|
to the primary replica. Many applications, however, |
115 |
|
are believed to be able to cope with loser consistency |
116 |
|
semantics; the NFS client for CFS only requests |
117 |
|
new heartbeats less than 30 seconds old. |
118 |
|
Oceanstore is unique among the versioned P2P systems |
119 |
|
in that it implements non-destructive updates, |
120 |
|
allowing previous versions of documents to be accessed |
121 |
|
(by using their old heartbeats). |
122 |
|
|
123 |
*Freenet* [XXXref] was originally planned to have an |
*Freenet* [XXXref] was originally planned to have an |
124 |
update mechanism where the network would remember |
update mechanism where the network would remember |
142 |
|
|
143 |
Finally, a simplistic approach to P2P versioning |
Finally, a simplistic approach to P2P versioning |
144 |
is to have a centralized server keep track of the current version |
is to have a centralized server keep track of the current version |
145 |
of a document, such as in the Content-Addressable Web |
of a document, such as in the *Content-Addressable Web* |
146 |
proposal [XXXref -> google it], in which clients download |
proposal [XXXref -> google it], in which clients download |
147 |
a file's hash through HTTP (using an HTTP URI), |
a file's hash through HTTP (using an HTTP URI), |
148 |
then download the file through, e.g., a file-sharing network. |
then download the file through, e.g., a file-sharing network. |
149 |
This provides some of the efficiency advantages of a P2P Web. |
This provides some of the efficiency advantages of a P2P Web. |
150 |
However, there is still a single point of failure; |
However, there is still a single point of failure, and |
151 |
links still break when pages move to a different directory |
links still break when pages move to a different directory |
152 |
or Web server; and when the original publisher loses interest |
or Web server. |
153 |
|
|
154 |
|
In a file-sharing system, a file remains accessible |
155 |
|
as long as there is a copy on any of the participating hosts. |
156 |
|
None of the above versioning systems have this property. |
157 |
|
In CFS, version information is stored in the DHT, and only |
158 |
|
as long as the publisher regularly requests it to be kept. |
159 |
|
In OceanStore, requesting a document's current version |
160 |
|
depends on querying its primary replica; when the primary |
161 |
|
replica ceases to function, the current version cannot be |
162 |
|
determined any longer (for some applications, this only happens |
163 |
|
30 seconds after the primary replica disappears). |
164 |
|
<<XXX what to say about freenet?>> |
165 |
|
In the Content-Addressable Web proposal, the situation |
166 |
|
is the same as on the current Web. |
167 |
|
|
168 |
|
Thus, in these systems, when the original publisher loses interest |
169 |
and stops publishing a page, it disappears, even if |
and stops publishing a page, it disappears, even if |
170 |
someone else would have kept a copy. |
someone else would have kept a copy. |
171 |
|
|
176 |
|
|
177 |
.. Standing on the shoulders of giants: Example of Web links rotting away |
.. Standing on the shoulders of giants: Example of Web links rotting away |
178 |
|
|
179 |
These are important concerns. |
This is an important concern. |
180 |
In 1997, NASA launched the Cassini-Huygens spacecraft |
In 1997, NASA launched the Cassini-Huygens spacecraft |
181 |
on a mission to Saturn. Before the launch, the mission |
on a mission to Saturn. Before the launch, the mission |
182 |
was widely criticized for its use of radioisotope |
was widely criticized for its use of radioisotope |