Couchbase Server的数据操作

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This sets the document ‘message’ to ‘Hello World!’.

The expiry is an optional value, although different client libraries expose this in different ways. In PHP it’s an optional argument which can be added to the end of our method call. For example, to add a ten second expiry:

$cb->set('message', 'Hello World!', 10);

You can test the success of the operation by checking the return value in PHP:

if ($cb->set("spoon", "Hello World!", 10)) {
  echo "Message stored!";
}

In languages that support exceptions (Python, Ruby, .NET and Java) the exception system provides the information about whether individual operations succeed or note.

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     add(docid, docdata [, expiry])

    The add() function works different to set(). With add() the operation will fail if the document ID you specify already exists. For example, in the code below, the first operation will complete successfully, the second will fail:

    $cb->add('message', 'Hello World!');
    $cb->add('message', 'I pushed the button, but nothing happened!');

    The add() function is useful when you are storing data into the database for the first time. For example, consider storing user data using the email address as the document ID. Using set() would overwrite an old user record. Using add() would fail the registration and indicate the user should use the password recovery system.

    The add() function is also atomic, so multiple writers can be adding data to the cluster at the same time, and it’s safe to use within a multi-threaded environment.

Regardless of how you store the data, the actual format of the information you are storing is also important.

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Retrieving Data

To retrieve a document from the database, you need only supply the document ID of the document that you want to retrieve. The retrieve operations are designed so that the get() operation will return an error if the corresponding document ID does not exist.

There is only one main function, get(). For example:

$message = $cb->get('message')

If the value comes back as undefined in PHP, then the specified document ID did not exist.

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Retrieving in Bulk

There are times when you will have multiple different documents to collect. For example, if you’ve made use of Views, then you might have identified a number of documents that you want to retrieve. If you are using links within a document to other documents, you might want to load all of them at the same time.

As a rule, loading multiple documents in bulk is always faster and more efficient than loading the documents individually, largely because the latency in the requires and response has been removed by streaming the response of multiple documents.

Different libraries implement and expose this functionality in different ways, but the result is generally the same; either an array of the documents that you requested, or if supported by the language, a hash of the document IDs and corresponding document data. For example, in PHP the function is:

$ret = $cb->getMulti(array('recipe1','recipe2'));

This returns a PHP associative array, with each key/value pair as the document ID and document data. The document data is undefined if the requested document could not be retrieved.

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Updating Data

Updating data in Couchbase Server should be performed carefully. You can only update entire documents, and because of the basic structure—updating document data against a document ID—the format is similar to that used when initially storing a document. In fact, you can use the set() function mentioned earlier. The problem is that you may not want to update a document that doesn’t already exist. For this, you can use the replace() method. This only updates the document data if the specified document ID already exists. For example:

1. $cb->replace("welcome-message", "Hello World!");

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The above will fail until we use either add() or set() to create the document ID.

Generally the process for updating information is to load the record, update the information in it, and then use replace() or set() to save it back again:

1. $message = $cb->get('message');
   
2. $message = $message . " How are you today?";
   
3. $cb->replace('message',$message);

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When updating JSON-based documents, the operation is the same; you must load the existing document before updating a field and saving it back:

1. $record        = array('name' => 'MC Brown', 'company' => 'Couchbase');
   
2. $cb->set('user', json_encode($record));
   
3. 
   
4. $newrecord = json_decode($cb->get('user'), true);
   
5. $newrecord["nickname"] = "MC";
   
6. 
   
7. $cb->replace('user', json_encode($newrecord));

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Concurrent Updates

In any highly concurrent environment, particularly many modern websites, you must make sure that you do not try and update the same information from two or more clients simultaneously. This is even more important in an environment such as Couchbase Server where updates occur very quickly, and where you are storing larger documents that may contain a lot of compound information.

For example, consider the following scenario:

    Client A gets the value for the document “Martin”.

    Client B gets the value for the document “Martin”.

    Client A adds information to the document value and updates it.

    Client B adds information to the document value and updates it.

In the above sequence, the update by Client B will overwrite the information in the database, removing the data that Client A added.

To provide a solution to this, you can use the compare and swap (cas()) function, which uses an additional check value to ensure that the version of the document retrieved is the same as the one currently stored on the server.

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The result is a change to the above sequence:

1    Client A gets the value for the document “Martin” and the CAS ID.

2    Client B gets the value for the document “Martin” and the CAS ID.

3    Client A adds information to the document value and updates it, using the CAS ID as a check. The document is updated.

4    Client B adds information to the document value and tries to update it using the CAS ID. The operation fails, because the cached CAS ID on client B is now different from the CAS ID on the server after the update by client A.

CAS therefore supports an additional level of checking and verifies that the information you are updating matches the copy of the information you originally retrieved. CAS enforces what Couchbase calls optimistic locking, that is, we hope that we are the only client performing an update that has the right CAS value, and that all clients always use a CAS function to do updates.