Overview
一個cassandra 集群是由一些commodity server組成的ring-like的去中心化網(wǎng)絡(luò)。一個client連接cluster是通過連接這個ring中的任意一個node。然后使用CQL interface。這個client連接的node就叫做coordinator。
Cassandra是一個分布式的系統(tǒng)，它依賴于data partitioning去將數(shù)據(jù)分布到cluster的node上。 為了防止single-point-of-failure， 它需要employ data replication去存儲數(shù)據(jù)副本(replicas)。 這樣才會有high availability。
我們會設(shè)置一個參數(shù)，表示每一次成功的讀寫至少需要多少的replicas acknowledge。也可以說，當(dāng)一定數(shù)量的讀寫被relicas執(zhí)行之后，Coordinator 才會告訴client本次讀寫成功
一個operation在data propagated 給所有node的之前就可以標(biāo)記為成功了. 這個對performance是非常重要的。
Nodes in the Cassandra cluster rely on the Gossip Protocol to exchange information with each other.
This protocol allows nodes to obtain state information about other nodes by exchanging information a node has about itself and other nodes. A particular node does not directly exchange information with every other node in the cluster;
node通過流言協(xié)議就可以知道其它node的情況，所以不需要直接和所有其它node交換信息。
Cassandra Query Language
With the Cassandra server running, open a new terminal window and access the Cassandra Query Language shell by typing:
$ cqlsh
A keyspace is a container for our application data. You could think of it as an analogue to schema of a RDBMS. The keyspace requires that the replication strategy and replication factor be specified — the number of nodes data must be distributed as replicas to.
CREATE KEYSPACE test01
WITH REPLICATION = {
'class': 'SimpleStrategy',
'replication_factor': 1
};
$ DESCRIBE KEYSPACES
system_schema system_auth system system_distributed system_traces test01
swithc key space
USE test01;
CREATE TABLE countries (
id INT PRIMARY KEY,
official_name TEXT,
capital_city TEXT
);
INSERT INTO countries (id, official_name, capital_city) VALUES (1, 'Islamic Republic of Afghanistan', 'Kabul');
SELECT * FROM countries WHERE id = 1;
