cassandra 存储二进制data

Posted 2020-09-03 将者，智、信、仁、勇、严也。

Blob type 

The Cassandra blob data type represents a constant hexadecimal number defined as 0[xX](hex)+ where hex is a hexadecimal character, such as [0-9a-fA-F]. For example, 0xcafe. The maximum theoretical size for a blob is 2 GB. The practical limit on blob size, however, is less than 1 MB. A blob type is suitable for storing a small image or short string.

Blob conversion functions 

These functions convert the native types into binary data (blob):

• typeAsBlob(value)
• blobAsType(value)

For every native, nonblob data type supported by CQL, the typeAsBlob function takes a argument of that data type and returns it as a blob. Conversely, the blobAsType function takes a 64-bit blob argument and converts it to a value of the specified data type, if possible.

This example shows how to use bigintAsBlob:

CREATE TABLE bios ( user_name varchar PRIMARY KEY, 
   bio blob
 );

INSERT INTO bios (user_name, bio) VALUES (‘fred‘, bigintAsBlob(3));
 
SELECT * FROM bios;
 
 user_name | bio
-----------+--------------------
      fred | 0x0000000000000003
      

This example shows how to use blobAsBigInt.

ALTER TABLE bios ADD id bigint;

INSERT INTO bios (user_name, id) VALUES (‘fred‘, blobAsBigint(0x0000000000000003));

SELECT * FROM bios;

 user_name | bio                | id
-----------+--------------------+----
      fred | 0x0000000000000003 |  3

官方说了，见 https://datastax.github.io/python-driver/getting_started.html?highlight=blob ：

Type Conversions

For non-prepared statements, Python types are cast to CQL literals in the following way:

Python Type	CQL Literal Type
None	NULL
bool	boolean
float	float double
int long	int bigint varint smallint tinyint counter
decimal.Decimal	decimal
str unicode	ascii varchar text
buffer bytearray	blob

摘自：https://github.com/datastax/python-driver/blob/master/tests/integration/standard/test_types.py

    def test_can_insert_blob_type_as_bytearray(self):
        """
        Tests that blob type in Cassandra maps to bytearray in Python
        """
        s = self.session

        s.execute("CREATE TABLE blobbytes (a ascii PRIMARY KEY, b blob)")

        params = [‘key1‘, bytearray(b‘blob1‘)]
        s.execute("INSERT INTO blobbytes (a, b) VALUES (%s, %s)", params)

        results = s.execute("SELECT * FROM blobbytes")[0]
        for expected, actual in zip(params, results):
            self.assertEqual(expected, actual)

 

最后的可以工作的python代码：

from cassandra.cluster import Cluster

cluster = Cluster(["10.178.209.161"])
session = cluster.connect(‘my_keyspace‘)

s = session
s.execute("CREATE TABLE blobbytes (a ascii PRIMARY KEY, b blob)")
params = [‘key1‘, bytearray(b‘blob1‘)]
s.execute("INSERT INTO blobbytes (a, b) VALUES (%s, %s)", params)
results = s.execute("SELECT * FROM blobbytes")[0]
for expected, actual in zip(params, results):
     print (expected, actual)