These examples are not intended to represent a complete application, but are more of the bare minimum code required to access various Cassandra features. As such, the examples are useful for cut and paste into your applications according to the Apache license below.
See the Reservation Service repository for a more well rounded example. Licensed under the Apache License, Version 2. You may obtain a copy of the License at. See the License for the specific language governing permissions and limitations under the License.
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Imagine what you could do if scalability wasn't a problem. One very powerful feature that Cassandra provides is the ability to expire data that is no longer needed. This expiration is very flexible and works at the level of individual column values.
The time to live or TTL is a value that Cassandra stores for each column value to indicate how long to keep the value. The TTL value defaults to null , meaning that data that is written will not expire. As you can see, the clock is already counting down our TTL, reflecting the several seconds it took to type the second command. Remember that TTL is stored on a per-column level. There is currently no mechanism for setting TTL at a row level directly. As with the timestamp, there is no way to obtain or set the TTL value of a primary key column, and the TTL can only be set for a column when we provide a value for the column.
CQL supports a flexible set of data types, including simple character and numeric types, collections, and user-defined types. These types are similar to standard types in Java and other languages:.
The smallint and tinyint types were added in the Cassandra 2. While enumerated types are common in many languages, there is no direct equivalent in CQL. A common practice is to store enumerated values as strings. For example, using the Enum. UTF-8 is the more recent and widely used text standard and supports internationalization, so we recommend using text over ascii when building tables for new data.
By default, cqlsh prints out control and other unprintable characters using a backslash escape. The identity of data elements such as rows and partitions is important in any data model in order to be able to access the data. Cassandra provides several types which prove quite useful in defining unique partition keys.
While we noted earlier that each column has a timestamp indicating when it was last modified, you can also use a timestamp as the value of a column itself. The time can be encoded as a bit signed integer, but it is typically much more useful to input a timestamp using one of several supported ISO date formats.
For example:. The best practice is to always provide time zones rather than relying on the operating system time zone configuration. Releases through Cassandra 2. The 2. As with timestamp , these types support ISO formats.
Although there are new java. The time type maps to a Java long representing the number of nanoseconds since midnight. A universally unique identifier UUID is a bit value in which the bits conform to one of several types, of which the most commonly used are known as Type 1 and Type 4. UUIDs are typically represented as dash-separated sequences of hex digits. The uuid type is often used as a surrogate key, either by itself or in combination with other values.
Because UUIDs are of a finite length, they are not absolutely guaranteed to be unique. However, most operating systems and programming languages provide utilities to generate IDs that provide adequate uniqueness, and cqlsh does as well.
This type is frequently used as a conflict-free timestamp. The availability of these convenience functions is one reason why timeuuid tends to be used more frequently than uuid. After you create a table, there is no way to modify the primary key, because this controls how data is distributed within the cluster, and even more importantly, how it is stored on disk. Now we have a more robust table design, which we can extend with even more columns as we learn about more types.
The cqlsh is case insensitive in accepting these values but outputs True or False. A binary large object blob is a colloquial computing term for an arbitrary array of bytes.
The CQL blob type is useful for storing media or other binary file types. Cassandra does not validate or examine the bytes in a blob. CQL represents the data as hexadecimal digits—for example, 0xab83cf0. If you want to encode arbitrary textual data into the blob you can use the textAsBlob function in order to specify values for entry.
However, the values are represented using the dotted decimal format in cqlsh output—for example, IPv6 addresses are represented as eight groups of four hexadecimal digits, separated by colons—for example, dbaa2e
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