Overview
The Cassandra Thrift API changed between 0.3 and 0.4; this document explains the 0.4 version. The 0.3 API is described in API03.
Cassandra's client API is built entirely on top of Thrift. It should be noted that these documents mention default values, but these are not generated in all of the languages that Thrift supports.
WARNING: Some SQL/RDBMS terms are used in this documentation for analogy purposes. They should be thought of as just that; analogies. There are few similarities between how data is managed in a traditional RDBMS and Cassandra. Please see DataModel for more information.
Terminology / Abbreviations
Keyspace
CF
SCF
Key
Exceptions
NotFoundException
InvalidRequestException
UnavailableException
TApplicationException
Structures
ConsistencyLevel
The ConsistencyLevel is an enum that controls both read and write behavior based on <ReplicationFactor> in your storage-conf.xml. The different consistency levels have different meanings, depending on if you're doing a write or read operation. Note that if W + R > ReplicationFactor, where W is the number of nodes to block for on write, and R the number to block for on reads, you will have strongly consistent behavior; that is, readers will always see the most recent write. Of these, the most interesting is to do QUORUM reads and writes, which gives you consistency while still allowing availability in the face of node failures up to half of ReplicationFactor. Of course if latency is more important than consistency then you can use lower values for either or both.
Write
Level |
Behavior |
|
Ensure nothing. A write happens asynchronously in background |
|
Ensure that the write has been written to at least 1 node's commit log and memory table before responding to the client. |
|
Ensure that the write has been written to |
|
Ensure that the write is written to |
Read
Level |
Behavior |
|
Not supported, because it doesn't make sense. |
|
Will return the record returned by the first node to respond. A consistency check is always done in a background thread to fix any consistency issues when |
|
Will query all storage nodes and return the record with the most recent timestamp once it has at least a majority of replicas reported. Again, the remaining replicas will be checked in the background. |
|
Not yet supported, but we plan to eventually. |
ColumnPath and ColumnParent
The ColumnPath is the path to a single column in Cassandra. It might make sense to think of ColumnPath and ColumnParent in terms of a directory structure.
Attribute |
Type |
Default |
Required |
Description |
|
|
n/a |
Y |
The name of the CF of the column being looked up. |
|
|
n/a |
N |
The super column name. |
|
|
n/a |
N |
The column name. |
ColumnPath is used to looking up a single column. ColumnParent is used when selecting groups of columns from the same ColumnFamily. In directory structure terms, imagine ColumnParent as ColumnPath + '/../'.
SlicePredicate
A SlicePredicate is similar to a mathematic predicate, which is described as "a property that the elements of a set have in common."
SlicePredicate's in Cassandra are described with either a list of column_names or a SliceRange.
Attribute |
Type |
Default |
Required |
Description |
|
|
n/a |
N |
A list of column names to retrieve. This can be used similar to Memcached's "multi-get" feature to fetch N known column names. For instance, if you know you wish to fetch columns 'Joe', 'Jack', and 'Jim' you can pass those column names as a list to fetch all three at once. |
|
|
n/a |
N |
A |
If column_names is specified, slice_range is ignored.
SliceRange
A slice range is a structure that stores basic range, ordering and limit information for a query that will return multiple columns. It could be thought of as Cassandra's version of LIMIT and ORDER BY.
Attribute |
Type |
Default |
Required |
Description |
|
|
n/a |
Y |
The column name to start the slice with. This attribute is not required, though there is no default value, and can be safely set to |
|
|
n/a |
Y |
The column name to stop the slice at. This attribute is not required, though there is no default value, and can be safely set to an empty byte array to not stop until |
|
|
|
N |
Whether the results should be ordered in reversed order. Similar to |
|
|
|
N |
How many keys to return. Similar to |
ColumnOrSuperColumn
Methods for fetching rows/records from Cassandra will return either a single instance of ColumnOrSuperColumn (get()) or a list of ColumnOrSuperColumn's (get_slice()). If you're looking up a SuperColumn (or list of SuperColumn's) then the resulting instances of ColumnOrSuperColumn will have the requested SuperColumn in the attribute super_column. For queries resulting in Column's those values will be in the attribute column. This change was made between 0.3 and 0.4 to standardize on single query methods that may return either a SuperColumn or Column.
Attribute |
Type |
Default |
Required |
Description |
|
|
n/a |
N |
The |
|
|
n/a |
N |
The |
Method calls
get
ColumnOrSuperColumn get(keyspace, key, column_path, consistency_level)
Get the Column or SuperColumn at the given column_path. If no value is present, NotFoundException is thrown. (This is the only method that can throw an exception under non-failure conditions.)
get_slice
list<ColumnOrSuperColumn> get_slice(keyspace, key, column_parent, predicate, consistency_level)
Get the group of columns contained by column_parent (either a ColumnFamily name or a ColumnFamily/SuperColumn name pair) specified by the given SlicePredicate struct.
multiget
map<string,ColumnOrSuperColumn> multiget(keyspace, keys, column_path, consistency_level)
list<string>
Perform a get for column_path in parallel on the given list<string> keys. The return value maps keys to the ColumnOrSuperColumn found. If no value corresponding to a key is present, the key will still be in the map, but both the column and super_column references of the ColumnOrSuperColumn object it maps to will be null.
multiget_slice
map<string,list<ColumnOrSuperColumn>> multiget_slice(keyspace, keys, column_parent, predicate, consistency_level)
Performs a get_slice for column_parent and predicate for the given keys in parallel.
get_count
i32 get_count(keyspace, key, column_parent, consistency_level)
Counts the columns present in column_parent.
get_key_range
list<string> get_key_range(keyspace, column_family, start, finish, count=100, consistency_level)
Returns a list of keys starting with start, ending with finish (both inclusive), and at most count long. The empty string ("") can be used as a sentinel value to get the first/last existing key. (The semantics are similar to the corresponding components of SliceRange.) This method is only allowed when using an order-preserving partitioner._
Note'': get_key_range's design is kind of fundamentally broken, so we're deprecating it in favor of get_range_slice starting in 0.5. In trunk (0.5beta) get_range_slice should be used instead.
insert
insert(keyspace, key, column_path, value, timestamp, consistency_level)
Insert a Column consisting of (column_path.column, value, timestamp) at the given column_path.column_family and optional column_path.super_column. Note that column_path.column is here required, since a SuperColumn cannot directly contain binary values – it can only contain sub-Columns.
batch_insert
batch_insert(keyspace, key, batch_mutation, consistency_level)
Insert Columns or SuperColumns across different Column Families for the same row key. batch_mutation is a map<string, list<ColumnOrSuperColumn>> – a map which pairs column family names with the relevant ColumnOrSuperColumn objects to insert.
remove
remove(keyspace, key, column_path, timestamp, consistency_level)
Remove data from the row specified by key at the granularity specified by column_path, and the given timestamp. Note that all the values in column_path besides column_path.column_family are truly optional: you can remove the entire row by just specifying the ColumnFamily, or you can remove a SuperColumn or a single Column by specifying those levels too. Note that the timestamp is needed, so that if the commands are replayed in a different order on different nodes, the same result is produced.