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version 1.2, 2002/02/15 00:13:30
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version 1.3, 2002/02/15 00:28:33
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| <h1>Layout</h1> | <h1>Layout</h1> |
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| All files belonging to the instance repository are stored under the | All files belonging to the instance repository are stored under the |
| repository/instances directory. For each class two files are maintained: |
repository/instances directory. For each class, two files are maintained: |
| and index file (with a ".idx" extension) and an instance file which bears |
an index file (with a ".idx" extension) and an instance file which bears |
| the name of the class whose instances it contains. For example, suppose | the name of the class whose instances it contains. For example, suppose |
| there is a class called "Zebra". Then two files are used to manage its | there is a class called "Zebra". Then two files are used to manage its |
| instances: | instances: |
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| modified or deleted since the last reorganization of the instance file. | modified or deleted since the last reorganization of the instance file. |
| When this number--called a dirty count--reaches a configurable limit, the | When this number--called a dirty count--reaches a configurable limit, the |
| index file and instance file are reorganized to reclaim unused space; | index file and instance file are reorganized to reclaim unused space; |
| unused space is left by delete and modify operations (discussed later). |
unused space (or gaps) is left by delete and modify operations (discussed |
| The dirty count is expressed as eight hex digits (the number had to have a |
later). The dirty count is expressed as eight hex digits (the number had to |
| fixed size so that it could be updated in place without having to rewrite |
have a fixed size so that it could be updated in place without having to |
| the index file). |
rewrite the index file). |
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| Each subsequent line of the index file corresponds to an instance contained | Each subsequent line of the index file corresponds to an instance contained |
| in the instance file. Each line has the following fields: | in the instance file. Each line has the following fields: |
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| hash code of the key and look for entries with the same hash code. | hash code of the key and look for entries with the same hash code. |
| It is still necessary to compare the keys when the hash codes are | It is still necessary to compare the keys when the hash codes are |
| the same (since collisions are possible), but only when they are the | the same (since collisions are possible), but only when they are the |
| same which is statistically rare. |
same which is rare and hence this scheme saves many comparisons. |
| </li> | </li> |
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| <li> | <li> |
| offset - offset within the instance file where the instance begins. |
offset - offset within the instance file to where the instance begins. |
| </li> | </li> |
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| <li> | <li> |
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| This key is in standard form which we define as follows: all property | This key is in standard form which we define as follows: all property |
| names are shifted to lower case and the key-bindings are sorted by | names are shifted to lower case and the key-bindings are sorted by |
| property names (in this way it suffices to compare key expressions to | property names (in this way it suffices to compare key expressions to |
| determine if two compound keys refer to the same instance. |
determine if two compound keys refer to the same instance). |
| </li> | </li> |
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| </ul> | </ul> |
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| 0 38B42754 4293 1431 Y.key1=1005,key2="Hello World 5" | 0 38B42754 4293 1431 Y.key1=1005,key2="Hello World 5" |
| </pre> | </pre> |
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| First notice that the dirty count is equal to two and that two entries are |
Notice that the dirty count is equal to two and that two entries are marked |
| marked as deleted (these quantities must be equal). This indicates that the |
as deleted (these quantities must be equal). This indicates that the instance |
| instance file has two instances which are no longer used. The space used by |
file has two instances which are no longer used. The space used by these |
| these instances will be reclaimed during reorganization. |
instances will be reclaimed during reorganization. |
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| The layout of the instance file is trivial. Instances are always appended to | The layout of the instance file is trivial. Instances are always appended to |
| to the instance file. The instances are kept end-to-end in the file. |
the the instance file. The instances are kept end-to-end in the file. |
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| <h1>Operations</h1> | <h1>Operations</h1> |
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| create, modify, and delete. This section describes how these operations affect | create, modify, and delete. This section describes how these operations affect |
| the index and instance file. Note that the process described below for | the index and instance file. Note that the process described below for |
| performing the three operations actually contains extra steps described in | performing the three operations actually contains extra steps described in |
| thd "Recovery" section. |
the "Recovery" section. |
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| Creation. During creation, the instance is appended to the instance file and | Creation. During creation, the instance is appended to the instance file and |
| an entry is appended to the index file. If an instance with the same key is | an entry is appended to the index file. If an instance with the same key is |
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| Deletion. To delete an instance, the corresponding entry in the index file is | Deletion. To delete an instance, the corresponding entry in the index file is |
| marked as deleted (by changing the first column from '0' to '1'). And then the | marked as deleted (by changing the first column from '0' to '1'). And then the |
| dirty count is incremented and updated. If the dirty count has reached the | dirty count is incremented and updated. If the dirty count has reached the |
| configured threshold, then the index and instance files are reogranized (see |
configured threshold, the index and instance files are reogranized (see |
| the section entitled "Reorganization" for an explanation of how this is | the section entitled "Reorganization" for an explanation of how this is |
| done). | done). |
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instances.txt
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