What is Cassandra Back

• Cassandra is a ideal database used for social network. It combines Dynamo of Amazon with BitTable of Google, which is based on Column Family(列簇).
• Official Suggestion: The Apache Cassandra database is the right choice when you need scalability(可擴展性) and high availability without compromising performance.
• Some characteristics:
  • Dynamo:
    • Symmetric(對稱的) P2P framework
      • Without special node(solving the problem of SPOF(單點失效))
    • Distributed Management based on Gossip
    • Store data with distributed hash tables.
      • pluggable partition(可插拔分區)
      • pluggable topologies(可插拔拓撲)
      • pluggable store strategies(可插拔存放策略)
    • Configurable(eventual consistency)
  • BigTable:
    • Column Family Data Model
      • configurable, 2-levels maps, super column family
    • SSTable
      • Append-only commit log
      • Memtable(buffer and sort)
      • Unmodifiable SSTable files
    • Integrated Hadoop

Data Model

• Column: the minimum components of a data, and it's composed of Name, Value, and Timestamp.
• Notice: name and value should be byte[], which a string of any length.

{
    "name": "email",
    "value": "aleen42@vip.qq.com",
    "timestamp": 1455517240
}

• SuperColumn: the name is same as Column, while the value is a set of columns. In additionally, they don't have timestamps.

{
    "name": "addresses",
    "value": {
        "street": { "name": "street", "value": "xxx xxx xxx Road", "timestamp": 1455517515 },
        "city": { "name": "city", "value": "Foshan", "timestamp": 1455517515 },
        "zip": { "name": "zip", "value": "528031", "timestamp": 1455517515 }
    }
}

• Both SuperColumn Family and Column Family describe a set of SuperColumn and Column.
• Sorting:
  • Notice: in Cassandra, data is sorted by Column name by default.
  • Configurable:
    • Set CompareWith Attribute (Column Family)
    • Set CompareSubColumnsWith (SuperColumn Family)
  • Value for configuration:
    • BytesType
    • UTF8Type
    • LexicalUUIDType
    • TimeUUIDType
    • AsciiType
    • Column name

Partition Strategies

• In Cassandra, Token is the key point for partitions, and there is a unique token for each node, describing the range of data in which. All the tokens will be stored as a ring(Cassandra Ring), and use hash value to calculate.
• There are 3 strategies for partitions in Cassandra:
  • Random Partitioner:
    • Token will be a BitInteger(0 ~ 2¹²⁷), mapped with hash. (In Cassandra, it will take a 128 bits MD5 absolute value, which contains a sign bit)
    • In a in-extreme(極端的) situation, it can map 2¹²⁷ + 1 nodes.
    • Notice: it's not supported that searching data with key in this strategic.
  • Order Preserving Partitioner(ordered):
    • Token will be a string.
    • It depends on the key to certain which nodes to be chosen
    • Notice: without Initial Token, system will take a 16 bits random strings which contain numbers and alphabets as the token.
  • Collating Order Preserving Partitioner(ordered):
    • Token will be a byte.
    • Configurable sorting in different languages(en_US by default)
• Partition strategies and Token(including Initial Token) can be set in the file storage-conf.xml.

Bloom Filter

• Bloom Filter is a random data structure, saving huge space by sacrificing precision. (This structure cannot be applied to applications which are sensitive to accuracy.)
• In Cassandra, each key-value pair will use one byte to complete Bloom Filter.
• Read more details in Wikipedia

Storage Strategies

• In Cassandra, data will be stored in the local file system of each node with 3 strategic to copy data:
  • Simple Strategy(RackUnaware Strategy): copies will be saved in the next several nodes behind this node.
  • OldNetwork Topology Strategy(RackAware Strategy): one copy will be saved in a different node, while other N - 2 copies will be saved in other machines in the same node.
  • Network Topology Strategy: M copies will be saved in different nodes, while N - M - 1 copies will be saved in different machines of the same node.
• Storage mechanisms:
  • Commit Log: Cassandra will log records when writing data like HBase. After logging, data will be written into memtable and flushed into the SSTable, which is read-only once written into. (In Cassandra, there is no random write)
  • Memtable: when data has reached the size of a block, it will be flushed into the disk, and stored as SSTable.
  • SSTable: it's read-only and one CF will be corresponding with several SSTable. Bloom Filter will be used to justify which SSTable the key belongs to.
    • Compaction used in Cassandra:
      • Garbage Collect: delete the data which is really signed.
      • Merge SSTable in the same CF.
      • Generate a Merkle Tree.
• Read more details in IBM documents

Sniffing(嗅探)

• Sniffing is mainly used to calculate the distance between different host to tell Cassandra what the topology looks like. There are 3 strategies to configure:
  • org.apache.cassandra.locator.SimpleSnitch: logical distance(the difference between Cassandra Ring)
  • org.apache.cassandra.locator.RackInferringSnitch: the distance depends on rack(the third 8 bits in IP) and data center(the second 8 bits in IP)
  • org.apache.cassandra.locator.PropertyFileSnitch: the distance depends on rack and data center, which are both configured in the configuration file, cassandra-topology.properties.

Consistency

• Eventual Consistency is used in Cassandra. Users can choose different level of consistency to use.

• Notice: the level of consistency depends on the number of copies instead of the number of nodes.

Quorum NRW

• N: means the number of copies once(In generally, N > 3 in a distributed system to ensure fault-tolerance)
• R: means the minimum of nodes which is successful to read once
• W: means the minimum of nodes which is successful to write once
• In Quorum, consistency will be strong when W + R > N, but availability will be low.

• In Quorum, consistency will be weak when W + R <= N, but availability will be high. (This will be used generally when eventual consistency is guaranteed)

• In Cassandra, there are 4 methodologies to ensure eventual consistency:

  • Anti-Entropy(逆熵): the mothod used to check consistency called Merkle Tree.
  • Read Repair: repair in the case that finding it's not consistent when reading data from key A and all the copies of key A.
    • ONE: return a recent copy immediately, and repair in the background. (The first data you read will not be the latest one)
    • QUORUM: return a copy when there is no problems after checking more than half of all copies, and check remaining copies and repair if necessary in the background.
    • ALL: return a copy when there is no problems after checking all.
  • Hinted Handoff(提示提交): write to a relay node when the target is offline, and the relay node will wait for the target to complete writing.
  • Distributed Delete: Cassandra will sign a hint to the object which is deleted by other nodes, and this object will be collected when doing Garbage Collect at the fixed time.

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