SASI Index
SASIIndex,
or SASI for short, is an implementation of Cassandra’s Index
interface that can be used as an alternative to the existing
implementations. SASI’s indexing and querying improves on existing
implementations by tailoring it specifically to Cassandra’s needs. SASI
has superior performance in cases where queries would previously require
filtering. In achieving this performance, SASI aims to be significantly
less resource intensive than existing implementations, in memory, disk,
and CPU usage. In addition, SASI supports prefix and contains queries on
strings (similar to SQL’s LIKE = "foo*" or LIKE = "foo"').
The following goes on describe how to get up and running with SASI, demonstrates usage with examples, and provides some details on its implementation.
Using SASI
The examples below walk through creating a table and indexes on its columns, and performing queries on some inserted data.
The examples below assume the demo keyspace has been created and is in
use.
cqlsh> CREATE KEYSPACE demo WITH replication = {
... 'class': 'SimpleStrategy',
... 'replication_factor': '1'
... };
cqlsh> USE demo;
All examples are performed on the sasi table:
cqlsh:demo> CREATE TABLE sasi (id uuid, first_name text, last_name text,
... age int, height int, created_at bigint, primary key (id));
Creating Indexes
To create SASI indexes use CQLs CREATE CUSTOM INDEX statement:
cqlsh:demo> CREATE CUSTOM INDEX ON sasi (first_name) USING 'org.apache.cassandra.index.sasi.SASIIndex'
... WITH OPTIONS = {
... 'analyzer_class':
... 'org.apache.cassandra.index.sasi.analyzer.NonTokenizingAnalyzer',
... 'case_sensitive': 'false'
... };
cqlsh:demo> CREATE CUSTOM INDEX ON sasi (last_name) USING 'org.apache.cassandra.index.sasi.SASIIndex'
... WITH OPTIONS = {'mode': 'CONTAINS'};
cqlsh:demo> CREATE CUSTOM INDEX ON sasi (age) USING 'org.apache.cassandra.index.sasi.SASIIndex';
cqlsh:demo> CREATE CUSTOM INDEX ON sasi (created_at) USING 'org.apache.cassandra.index.sasi.SASIIndex'
... WITH OPTIONS = {'mode': 'SPARSE'};
The indexes created have some options specified that customize their
behaviour and potentially performance. The index on first_name is
case-insensitive. The analyzers are discussed more in a subsequent
example. The NonTokenizingAnalyzer performs no analysis on the text.
Each index has a mode: PREFIX, CONTAINS, or SPARSE, the first
being the default. The last_name index is created with the mode
CONTAINS which matches terms on suffixes instead of prefix only.
Examples of this are available below and more detail can be found in the
section on OnDiskIndex.The created_at column
is created with its mode set to SPARSE, which is meant to improve
performance of querying large, dense number ranges like timestamps for
data inserted every millisecond. Details of the SPARSE implementation
can also be found in the section on the
OnDiskIndex. The age index is created with
the default PREFIX mode and no case-sensitivity or text analysis
options are specified since the field is numeric.
After inserting the following data and performing a nodetool flush,
SASI performing index flushes to disk can be seen in Cassandra’s logs –
although the direct call to flush is not required (see
IndexMemtable for more details).
cqlsh:demo> INSERT INTO sasi (id, first_name, last_name, age, height, created_at)
... VALUES (556ebd54-cbe5-4b75-9aae-bf2a31a24500, 'Pavel', 'Yaskevich', 27, 181, 1442959315018);
cqlsh:demo> INSERT INTO sasi (id, first_name, last_name, age, height, created_at)
... VALUES (5770382a-c56f-4f3f-b755-450e24d55217, 'Jordan', 'West', 26, 173, 1442959315019);
cqlsh:demo> INSERT INTO sasi (id, first_name, last_name, age, height, created_at)
... VALUES (96053844-45c3-4f15-b1b7-b02c441d3ee1, 'Mikhail', 'Stepura', 36, 173, 1442959315020);
cqlsh:demo> INSERT INTO sasi (id, first_name, last_name, age, height, created_at)
... VALUES (f5dfcabe-de96-4148-9b80-a1c41ed276b4, 'Michael', 'Kjellman', 26, 180, 1442959315021);
cqlsh:demo> INSERT INTO sasi (id, first_name, last_name, age, height, created_at)
... VALUES (2970da43-e070-41a8-8bcb-35df7a0e608a, 'Johnny', 'Zhang', 32, 175, 1442959315022);
cqlsh:demo> INSERT INTO sasi (id, first_name, last_name, age, height, created_at)
... VALUES (6b757016-631d-4fdb-ac62-40b127ccfbc7, 'Jason', 'Brown', 40, 182, 1442959315023);
cqlsh:demo> INSERT INTO sasi (id, first_name, last_name, age, height, created_at)
... VALUES (8f909e8a-008e-49dd-8d43-1b0df348ed44, 'Vijay', 'Parthasarathy', 34, 183, 1442959315024);
cqlsh:demo> SELECT first_name, last_name, age, height, created_at FROM sasi;
first_name | last_name | age | height | created_at
------------+---------------+-----+--------+---------------
Michael | Kjellman | 26 | 180 | 1442959315021
Mikhail | Stepura | 36 | 173 | 1442959315020
Jason | Brown | 40 | 182 | 1442959315023
Pavel | Yaskevich | 27 | 181 | 1442959315018
Vijay | Parthasarathy | 34 | 183 | 1442959315024
Jordan | West | 26 | 173 | 1442959315019
Johnny | Zhang | 32 | 175 | 1442959315022
(7 rows)
Equality & Prefix Queries
SASI supports all queries already supported by CQL, including LIKE statement for PREFIX, CONTAINS and SUFFIX searches.
cqlsh:demo> SELECT first_name, last_name, age, height, created_at FROM sasi
... WHERE first_name = 'Pavel';
first_name | last_name | age | height | created_at
-------------+-----------+-----+--------+---------------
Pavel | Yaskevich | 27 | 181 | 1442959315018
(1 rows)
cqlsh:demo> SELECT first_name, last_name, age, height, created_at FROM sasi
... WHERE first_name = 'pavel';
first_name | last_name | age | height | created_at
-------------+-----------+-----+--------+---------------
Pavel | Yaskevich | 27 | 181 | 1442959315018
(1 rows)
cqlsh:demo> SELECT first_name, last_name, age, height, created_at FROM sasi
... WHERE first_name LIKE 'M%';
first_name | last_name | age | height | created_at
------------+-----------+-----+--------+---------------
Michael | Kjellman | 26 | 180 | 1442959315021
Mikhail | Stepura | 36 | 173 | 1442959315020
(2 rows)
Of course, the case of the query does not matter for the first_name
column because of the options provided at index creation time.
cqlsh:demo> SELECT first_name, last_name, age, height, created_at FROM sasi
... WHERE first_name LIKE 'm%';
first_name | last_name | age | height | created_at
------------+-----------+-----+--------+---------------
Michael | Kjellman | 26 | 180 | 1442959315021
Mikhail | Stepura | 36 | 173 | 1442959315020
(2 rows)
Compound Queries
SASI supports queries with multiple predicates, however, due to the
nature of the default indexing implementation, CQL requires the user to
specify ALLOW FILTERING to opt-in to the potential performance
pitfalls of such a query. With SASI, while the requirement to include
ALLOW FILTERING remains, to reduce modifications to the grammar, the
performance pitfalls do not exist because filtering is not performed.
Details on how SASI joins data from multiple predicates is available
below in the Implementation Details
section.
cqlsh:demo> SELECT first_name, last_name, age, height, created_at FROM sasi
... WHERE first_name LIKE 'M%' and age < 30 ALLOW FILTERING;
first_name | last_name | age | height | created_at
------------+-----------+-----+--------+---------------
Michael | Kjellman | 26 | 180 | 1442959315021
(1 rows)
Suffix Queries
The next example demonstrates CONTAINS mode on the last_name column.
By using this mode, predicates can search for any strings containing the
search string as a sub-string. In this case the strings containing a''
or an''.
cqlsh:demo> SELECT * FROM sasi WHERE last_name LIKE '%a%'; id | age | created_at | first_name | height | last_name --------------------------------------+-----+---------------+------------+--------+--------------- f5dfcabe-de96-4148-9b80-a1c41ed276b4 | 26 | 1442959315021 | Michael | 180 | Kjellman 96053844-45c3-4f15-b1b7-b02c441d3ee1 | 36 | 1442959315020 | Mikhail | 173 | Stepura 556ebd54-cbe5-4b75-9aae-bf2a31a24500 | 27 | 1442959315018 | Pavel | 181 | Yaskevich 8f909e8a-008e-49dd-8d43-1b0df348ed44 | 34 | 1442959315024 | Vijay | 183 | Parthasarathy 2970da43-e070-41a8-8bcb-35df7a0e608a | 32 | 1442959315022 | Johnny | 175 | Zhang (5 rows) cqlsh:demo> SELECT * FROM sasi WHERE last_name LIKE '%an%'; id | age | created_at | first_name | height | last_name --------------------------------------+-----+---------------+------------+--------+----------- f5dfcabe-de96-4148-9b80-a1c41ed276b4 | 26 | 1442959315021 | Michael | 180 | Kjellman 2970da43-e070-41a8-8bcb-35df7a0e608a | 32 | 1442959315022 | Johnny | 175 | Zhang (2 rows)
Expressions on Non-Indexed Columns
SASI also supports filtering on non-indexed columns like height. The
expression can only narrow down an existing query using AND.
cqlsh:demo> SELECT * FROM sasi WHERE last_name LIKE '%a%' AND height >= 175 ALLOW FILTERING; id | age | created_at | first_name | height | last_name --------------------------------------+-----+---------------+------------+--------+--------------- f5dfcabe-de96-4148-9b80-a1c41ed276b4 | 26 | 1442959315021 | Michael | 180 | Kjellman 556ebd54-cbe5-4b75-9aae-bf2a31a24500 | 27 | 1442959315018 | Pavel | 181 | Yaskevich 8f909e8a-008e-49dd-8d43-1b0df348ed44 | 34 | 1442959315024 | Vijay | 183 | Parthasarathy 2970da43-e070-41a8-8bcb-35df7a0e608a | 32 | 1442959315022 | Johnny | 175 | Zhang (4 rows)
Delimiter based Tokenization Analysis
A simple text analysis provided is delimiter based tokenization. This
provides an alternative to indexing collections, as delimiter separated
text can be indexed without the overhead of CONTAINS mode nor using
PREFIX or SUFFIX queries.
cqlsh:demo> ALTER TABLE sasi ADD aliases text;
cqlsh:demo> CREATE CUSTOM INDEX on sasi (aliases) USING 'org.apache.cassandra.index.sasi.SASIIndex'
... WITH OPTIONS = {
... 'analyzer_class': 'org.apache.cassandra.index.sasi.analyzer.DelimiterAnalyzer',
... 'delimiter': ',',
... 'mode': 'prefix',
... 'analyzed': 'true'};
cqlsh:demo> UPDATE sasi SET aliases = 'Mike,Mick,Mikey,Mickey' WHERE id = f5dfcabe-de96-4148-9b80-a1c41ed276b4;
cqlsh:demo> SELECT * FROM sasi WHERE aliases LIKE 'Mikey' ALLOW FILTERING;
id | age | aliases | created_at | first_name | height | last_name
--------------------------------------+-----+------------------------+---------------+------------+--------+-----------
f5dfcabe-de96-4148-9b80-a1c41ed276b4 | 26 | Mike,Mick,Mikey,Mickey | 1442959315021 | Michael | 180 | Kjellman
Text Analysis (Tokenization and Stemming)
Lastly, to demonstrate text analysis an additional column is needed on the table. Its definition, index, and statements to update rows are shown below.
cqlsh:demo> ALTER TABLE sasi ADD bio text;
cqlsh:demo> CREATE CUSTOM INDEX ON sasi (bio) USING 'org.apache.cassandra.index.sasi.SASIIndex'
... WITH OPTIONS = {
... 'analyzer_class': 'org.apache.cassandra.index.sasi.analyzer.StandardAnalyzer',
... 'tokenization_enable_stemming': 'true',
... 'analyzed': 'true',
... 'tokenization_normalize_lowercase': 'true',
... 'tokenization_locale': 'en'
... };
cqlsh:demo> UPDATE sasi SET bio = 'Software Engineer, who likes distributed systems, doesnt like to argue.' WHERE id = 5770382a-c56f-4f3f-b755-450e24d55217;
cqlsh:demo> UPDATE sasi SET bio = 'Software Engineer, works on the freight distribution at nights and likes arguing' WHERE id = 556ebd54-cbe5-4b75-9aae-bf2a31a24500;
cqlsh:demo> SELECT * FROM sasi;
id | age | bio | created_at | first_name | height | last_name
--------------------------------------+-----+----------------------------------------------------------------------------------+---------------+------------+--------+---------------
f5dfcabe-de96-4148-9b80-a1c41ed276b4 | 26 | null | 1442959315021 | Michael | 180 | Kjellman
96053844-45c3-4f15-b1b7-b02c441d3ee1 | 36 | null | 1442959315020 | Mikhail | 173 | Stepura
6b757016-631d-4fdb-ac62-40b127ccfbc7 | 40 | null | 1442959315023 | Jason | 182 | Brown
556ebd54-cbe5-4b75-9aae-bf2a31a24500 | 27 | Software Engineer, works on the freight distribution at nights and likes arguing | 1442959315018 | Pavel | 181 | Yaskevich
8f909e8a-008e-49dd-8d43-1b0df348ed44 | 34 | null | 1442959315024 | Vijay | 183 | Parthasarathy
5770382a-c56f-4f3f-b755-450e24d55217 | 26 | Software Engineer, who likes distributed systems, doesnt like to argue. | 1442959315019 | Jordan | 173 | West
2970da43-e070-41a8-8bcb-35df7a0e608a | 32 | null | 1442959315022 | Johnny | 175 | Zhang
(7 rows)
Index terms and query search strings are stemmed for the bio column
because it was configured to use the
StandardAnalyzer
and analyzed is set to true. The tokenization_normalize_lowercase
is similar to the case_sensitive property but for the
StandardAnalyzer.
These query demonstrates the stemming applied by
StandardAnalyzer.
cqlsh:demo> SELECT * FROM sasi WHERE bio LIKE 'distributing'; id | age | bio | created_at | first_name | height | last_name --------------------------------------+-----+----------------------------------------------------------------------------------+---------------+------------+--------+----------- 556ebd54-cbe5-4b75-9aae-bf2a31a24500 | 27 | Software Engineer, works on the freight distribution at nights and likes arguing | 1442959315018 | Pavel | 181 | Yaskevich 5770382a-c56f-4f3f-b755-450e24d55217 | 26 | Software Engineer, who likes distributed systems, doesnt like to argue. | 1442959315019 | Jordan | 173 | West (2 rows) cqlsh:demo> SELECT * FROM sasi WHERE bio LIKE 'they argued'; id | age | bio | created_at | first_name | height | last_name --------------------------------------+-----+----------------------------------------------------------------------------------+---------------+------------+--------+----------- 556ebd54-cbe5-4b75-9aae-bf2a31a24500 | 27 | Software Engineer, works on the freight distribution at nights and likes arguing | 1442959315018 | Pavel | 181 | Yaskevich 5770382a-c56f-4f3f-b755-450e24d55217 | 26 | Software Engineer, who likes distributed systems, doesnt like to argue. | 1442959315019 | Jordan | 173 | West (2 rows) cqlsh:demo> SELECT * FROM sasi WHERE bio LIKE 'working at the company'; id | age | bio | created_at | first_name | height | last_name --------------------------------------+-----+----------------------------------------------------------------------------------+---------------+------------+--------+----------- 556ebd54-cbe5-4b75-9aae-bf2a31a24500 | 27 | Software Engineer, works on the freight distribution at nights and likes arguing | 1442959315018 | Pavel | 181 | Yaskevich (1 rows) cqlsh:demo> SELECT * FROM sasi WHERE bio LIKE 'soft eng'; id | age | bio | created_at | first_name | height | last_name --------------------------------------+-----+----------------------------------------------------------------------------------+---------------+------------+--------+----------- 556ebd54-cbe5-4b75-9aae-bf2a31a24500 | 27 | Software Engineer, works on the freight distribution at nights and likes arguing | 1442959315018 | Pavel | 181 | Yaskevich 5770382a-c56f-4f3f-b755-450e24d55217 | 26 | Software Engineer, who likes distributed systems, doesnt like to argue. | 1442959315019 | Jordan | 173 | West (2 rows)
Implementation Details
While SASI, at the surface, is simply an implementation of the Index
interface, at its core there are several data structures and algorithms
used to satisfy it. These are described here. Additionally, the changes
internal to Cassandra to support SASI’s integration are described.
The Index interface divides responsibility of the implementer into two
parts: Indexing and Querying. Further, Cassandra makes it possible to
divide those responsibilities into the memory and disk components. SASI
takes advantage of Cassandra’s write-once, immutable, ordered data model
to build indexes along with the flushing of the memtable to disk – this
is the origin of the name ``SSTable Attached Secondary Index''.
The SASI index data structures are built in memory as the SSTable is being written and they are flushed to disk before the writing of the SSTable completes. The writing of each index file only requires sequential writes to disk. In some cases, partial flushes are performed, and later stitched back together, to reduce memory usage. These data structures are optimized for this use case.
Taking advantage of Cassandra’s ordered data model, at query time, candidate indexes are narrowed down for searching, minimizing the amount of work done. Searching is then performed using an efficient method that streams data off disk as needed.
Indexing
Per SSTable, SASI writes an index file for each indexed column. The data
for these files is built in memory using the
OnDiskIndexBuilder.
Once flushed to disk, the data is read using the
OnDiskIndex
class. These are composed of bytes representing indexed terms, organized
for efficient writing or searching respectively. The keys and values
they hold represent tokens and positions in an SSTable and these are
stored per-indexed term in
TokenTreeBuilders
for writing, and
TokenTrees
for querying. These index files are memory mapped after being written to
disk, for quicker access. For indexing data in the memtable, SASI uses
its
IndexMemtable
class.
OnDiskIndex(Builder)
Each
OnDiskIndex
is an instance of a modified
Suffix Array data structure.
The
OnDiskIndex
is comprised of page-size blocks of sorted terms and pointers to the
terms' associated data, as well as the data itself, stored also in one
or more page-sized blocks. The
OnDiskIndex
is structured as a tree of arrays, where each level describes the terms
in the level below, the final level being the terms themselves. The
PointerLevel`s and their `PointerBlock`s contain terms and pointers to
other blocks that end with those terms. The `DataLevel, the final
level, and its DataBlock`s contain terms and point to the data itself,
contained in
`TokenTrees.
The terms written to the
OnDiskIndex
vary depending on its mode'': either mode'' is configurable per column at index creation time.PREFIX, CONTAINS, or
SPARSE. In the PREFIX and SPARSE cases, terms' exact values are
written exactly once per OnDiskIndex. For example, when using a
PREFIX index with terms Jason, Jordan, Pavel, all three will be
included in the index. A CONTAINS index writes additional terms for
each suffix of each term recursively. Continuing with the example, a
CONTAINS index storing the previous terms would also store ason,
ordan, avel, son, rdan, vel, etc. This allows for queries on
the suffix of strings. The SPARSE mode differs from PREFIX in that
for every 64 blocks of terms a
TokenTree
is built merging all the
TokenTrees
for each term into a single one. This copy of the data is used for
efficient iteration of large ranges of e.g. timestamps. The index
TokenTree(Builder)
The
TokenTree
is an implementation of the well-known
B+-tree that has been modified
to optimize for its use-case. In particular, it has been optimized to
associate tokens, longs, with a set of positions in an SSTable, also
longs. Allowing the set of long values accommodates the possibility of a
hash collision in the token, but the data structure is optimized for the
unlikely possibility of such a collision.
To optimize for its write-once environment the
TokenTreeBuilder
completely loads its interior nodes as the tree is built and it uses the
well-known algorithm optimized for bulk-loading the data structure.
TokenTrees
provide the means to iterate over tokens, and file positions, that match
a given term, and to skip forward in that iteration, an operation used
heavily at query time.
IndexMemtable
The
IndexMemtable
handles indexing the in-memory data held in the memtable. The
IndexMemtable
in turn manages either a
TrieMemIndex
or a
SkipListMemIndex
per-column. The choice of which index type is used is data dependent.
The
TrieMemIndex
is used for literal types. AsciiType and UTF8Type are literal types
by default but any column can be configured as a literal type using the
is_literal option at index creation time. For non-literal types the
SkipListMemIndex
is used. The
TrieMemIndex
is an implementation that can efficiently support prefix queries on
character-like data. The
SkipListMemIndex,
conversely, is better suited for other Cassandra data types like
numbers.
The
TrieMemIndex
is built using either the ConcurrentRadixTree or
ConcurrentSuffixTree from the com.goooglecode.concurrenttrees
package. The choice between the two is made based on the indexing mode,
PREFIX or other modes, and CONTAINS mode, respectively.
The
SkipListMemIndex
is built on top of java.util.concurrent.ConcurrentSkipListSet.
Querying
Responsible for converting the internal IndexExpression representation
into SASI’s
Operation
and
Expression
trees, optimizing the trees to reduce the amount of work done, and
driving the query itself, the
QueryPlan
is the work horse of SASI’s querying implementation. To efficiently
perform union and intersection operations, SASI provides several
iterators similar to Cassandra’s MergeIterator, but tailored
specifically for SASI’s use while including more features. The
RangeUnionIterator,
like its name suggests, performs set unions over sets of tokens/keys
matching the query, only reading as much data as it needs from each set
to satisfy the query. The
RangeIntersectionIterator,
similar to its counterpart, performs set intersections over its data.
QueryPlan
The
QueryPlan
instantiated per search query is at the core of SASI’s querying
implementation. Its work can be divided in two stages: analysis and
execution.
During the analysis phase,
QueryPlan
converts from Cassandra’s internal representation of IndexExpression`s,
which has also been modified to support encoding queries that contain
ORs and groupings of expressions using parentheses (see the
Cassandra Internal Changes section
below for more details). This process produces a tree of
`Operations,
which in turn may contain
Expressions,
all of which provide an alternative, more efficient, representation of
the query.
During execution, the
QueryPlan
uses the DecoratedKey-generating iterator created from the
Operation
tree. These keys are read from disk and a final check to ensure they
satisfy the query is made, once again using the
Operation
tree. At the point the desired amount of matching data has been found,
or there is no more matching data, the result set is returned to the
coordinator through the existing internal components.
The number of queries (total/failed/timed-out), and their latencies, are maintined per-table/column family.
SASI also supports concurrently iterating terms for the same index
across SSTables. The concurrency factor is controlled by the
cassandra.search_concurrency_factor system property. The default is
1.
QueryController
Each
QueryPlan
references a
QueryController
used throughout the execution phase. The
QueryController
has two responsibilities: to manage and ensure the proper cleanup of
resources (indexes), and to strictly enforce the time bound per query,
specified by the user via the range slice timeout. All indexes are
accessed via the
QueryController
so that they can be safely released by it later. The
QueryController's
checkpoint function is called in specific places in the execution path
to ensure the time-bound is enforced.
QueryPlan Optimizations
While in the analysis phase, the
QueryPlan
performs several potential optimizations to the query. The goal of these
optimizations is to reduce the amount of work performed during the
execution phase.
The simplest optimization performed is compacting multiple expressions
joined by logical intersections (AND) into a single
Operation
with three or more
Expressions.
For example, the query
WHERE age < 100 AND fname = 'p*' AND first_name != 'pa*' AND age > 21
would, without modification, have the following tree:
┌───────┐
┌────────│ AND │──────┐
│ └───────┘ │
▼ ▼
┌───────┐ ┌──────────┐
┌─────│ AND │─────┐ │age < 100 │
│ └───────┘ │ └──────────┘
▼ ▼
┌──────────┐ ┌───────┐
│ fname=p* │ ┌─│ AND │───┐
└──────────┘ │ └───────┘ │
▼ ▼
┌──────────┐ ┌──────────┐
│fname!=pa*│ │ age > 21 │
└──────────┘ └──────────┘
QueryPlan
will remove the redundant right branch whose root is the final AND and
has leaves fname != pa* and age > 21. These
Expressions
will be compacted into the parent AND, a safe operation due to AND
being associative and commutative. The resulting tree looks like the
following:
┌───────┐
┌────────│ AND │──────┐
│ └───────┘ │
▼ ▼
┌───────┐ ┌──────────┐
┌───────────│ AND │────────┐ │age < 100 │
│ └───────┘ │ └──────────┘
▼ │ ▼
┌──────────┐ │ ┌──────────┐
│ fname=p* │ ▼ │ age > 21 │
└──────────┘ ┌──────────┐ └──────────┘
│fname!=pa*│
└──────────┘
When excluding results from the result set, using !=, the
QueryPlan
determines the best method for handling it. For range queries, for
example, it may be optimal to divide the range into multiple parts with
a hole for the exclusion. For string queries, such as this one, it is
more optimal, however, to simply note which data to skip, or exclude,
while scanning the index. Following this optimization the tree looks
like this:
┌───────┐
┌────────│ AND │──────┐
│ └───────┘ │
▼ ▼
┌───────┐ ┌──────────┐
┌───────│ AND │────────┐ │age < 100 │
│ └───────┘ │ └──────────┘
▼ ▼
┌──────────────────┐ ┌──────────┐
│ fname=p* │ │ age > 21 │
│ exclusions=[pa*] │ └──────────┘
└──────────────────┘
The last type of optimization applied, for this query, is to merge range
expressions across branches of the tree – without modifying the meaning
of the query, of course. In this case, because the query contains all
AND`s the `age expressions can be collapsed. Along with this
optimization, the initial collapsing of unneeded `AND`s can also be
applied once more to result in this final tree using to execute the
query:
┌───────┐
┌──────│ AND │───────┐
│ └───────┘ │
▼ ▼
┌──────────────────┐ ┌────────────────┐
│ fname=p* │ │ 21 < age < 100 │
│ exclusions=[pa*] │ └────────────────┘
└──────────────────┘
Operations and Expressions
As discussed, the
QueryPlan
optimizes a tree represented by
Operations
as interior nodes, and
Expressions
as leaves. The
Operation
class, more specifically, can have zero, one, or two
Operations
as children and an unlimited number of expressions. The iterators used
to perform the queries, discussed below in the
`Range(Union|Intersection)Iterator'' section, implement the necessary
logic to merge results transparently regardless of the
`Operations
children.
Besides participating in the optimizations performed by the
QueryPlan,
Operation
is also responsible for taking a row that has been returned by the query
and performing a final validation that it in fact does match. This
satisfiesBy operation is performed recursively from the root of the
Operation
tree for a given query. These checks are performed directly on the data
in a given row. For more details on how satisfiesBy works, see the
documentation
in
the code.
Range(Union|Intersection)Iterator
The abstract RangeIterator class provides a unified interface over the
two main operations performed by SASI at various layers in the execution
path: set intersection and union. These operations are performed in a
iterated, or ``streaming'', fashion to prevent unneeded reads of
elements from either set. In both the intersection and union cases the
algorithms take advantage of the data being pre-sorted using the same
sort order, e.g. term or token order.
The
RangeUnionIterator
performs the ``Merge-Join'' portion of the
Sort-Merge-Join
algorithm, with the properties of an outer-join, or union. It is
implemented with several optimizations to improve its performance over a
large number of iterators – sets to union. Specifically, the iterator
exploits the likely case of the data having many sub-groups of
overlapping ranges and the unlikely case that all ranges will overlap
each other. For more details see the
javadoc.
The
RangeIntersectionIterator
itself is not a subclass of RangeIterator. It is a container for
several classes, one of which, AbstractIntersectionIterator,
sub-classes RangeIterator. SASI supports two methods of performing the
intersection operation, and the ability to be adaptive in choosing
between them based on some properties of the data.
BounceIntersectionIterator, and the BOUNCE strategy, works like the
RangeUnionIterator
in that it performs a ``Merge-Join'', however, its nature is similar to
a inner-join, where like values are merged by a data-specific merge
function (e.g. merging two tokens in a list to lookup in a SSTable
later). See the
javadoc
for more details on its implementation.
LookupIntersectionIterator, and the LOOKUP strategy, performs a
different operation, more similar to a lookup in an associative data
structure, or ``hash lookup'' in database terminology. Once again,
details on the implementation can be found in the
javadoc.
The choice between the two iterators, or the ADAPTIVE strategy, is
based upon the ratio of data set sizes of the minimum and maximum range
of the sets being intersected. If the number of the elements in minimum
range divided by the number of elements is the maximum range is less
than or equal to 0.01, then the ADAPTIVE strategy chooses the
LookupIntersectionIterator, otherwise the BounceIntersectionIterator
is chosen.
The SASIIndex Class
The above components are glued together by the
SASIIndex
class which implements Index, and is instantiated per-table containing
SASI indexes. It manages all indexes for a table via the
sasi.conf.DataTracker
and
sasi.conf.view.View
components, controls writing of all indexes for an SSTable via its
PerSSTableIndexWriter,
and initiates searches with Searcher. These classes glue the
previously mentioned indexing components together with Cassandra’s
SSTable life-cycle ensuring indexes are not only written when Memtable’s
flush, but also as SSTable’s are compacted. For querying, the Searcher
does little but defer to
QueryPlan
and update e.g. latency metrics exposed by SASI.
Cassandra Internal Changes
To support the above changes and integrate them into Cassandra a few minor internal changes were made to Cassandra itself. These are described here.
SSTable Write Life-cycle Notifications
The SSTableFlushObserver is an observer pattern-like interface, whose
sub-classes can register to be notified about events in the life-cycle
of writing out a SSTable. Sub-classes can be notified when a flush
begins and ends, as well as when each next row is about to be written,
and each next column. SASI’s PerSSTableIndexWriter, discussed above,
is the only current subclass.
Limitations and Caveats
The following are items that can be addressed in future updates but are not available in this repository or are not currently implemented.
-
The cluster must be configured to use a partitioner that produces
LongToken`s, e.g. `Murmur3Partitioner. Other existing partitioners which don’t produce LongToken e.g.ByteOrderedPartitionerandRandomPartitionerwill not work with SASI. -
Not Equals and OR support have been removed in this release while changes are made to Cassandra itself to support them.
