= SASI Index

https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/SASIIndex.java[`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 link:#ondiskindexbuilder[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
link:#ondiskindexbuilder[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
link:#indexmemtable[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 link:#implementation-details[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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/analyzer/StandardAnalyzer.java[`StandardAnalyzer`]
and `analyzed` is set to `true`. The `tokenization_normalize_lowercase`
is similar to the `case_sensitive` property but for the
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/analyzer/StandardAnalyzer.java[`StandardAnalyzer`].
These query demonstrates the stemming applied by
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/analyzer/StandardAnalyzer.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/OnDiskIndexBuilder.java[`OnDiskIndexBuilder`].
Once flushed to disk, the data is read using the
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/OnDiskIndex.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTreeBuilder.java[`TokenTreeBuilder`]s
for writing, and
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java[`TokenTree`]s
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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/IndexMemtable.java[`IndexMemtable`]
class.

===== OnDiskIndex(Builder)

Each
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/OnDiskIndex.java[`OnDiskIndex`]
is an instance of a modified
https://en.wikipedia.org/wiki/Suffix_array[Suffix Array] data structure.
The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/OnDiskIndex.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/OnDiskIndex.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java[`TokenTree`]s.

The terms written to the
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/OnDiskIndex.java[`OnDiskIndex`]
vary depending on its `mode` : either `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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java[`TokenTree`]
is built merging all the
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java[`TokenTree`]s
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
`mode` is configurable per column at index creation time.

===== TokenTree(Builder)

The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java[`TokenTree`]
is an implementation of the well-known
https://en.wikipedia.org/wiki/B%2B_tree[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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTreeBuilder.java[`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.

https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java[`TokenTree`]s
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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/IndexMemtable.java[`IndexMemtable`]
handles indexing the in-memory data held in the memtable. The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/IndexMemtable.java[`IndexMemtable`]
in turn manages either a
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/TrieMemIndex.java[`TrieMemIndex`]
or a
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/SkipListMemIndex.java[`SkipListMemIndex`]
per-column. The choice of which index type is used is data dependent.
The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/TrieMemIndex.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/SkipListMemIndex.java[`SkipListMemIndex`]
is used. The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/TrieMemIndex.java[`TrieMemIndex`]
is an implementation that can efficiently support prefix queries on
character-like data. The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/SkipListMemIndex.java[`SkipListMemIndex`],
conversely, is better suited for other Cassandra data types like
numbers.

The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/TrieMemIndex.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/SkipListMemIndex.java[`SkipListMemIndex`]
is built on top of `java.util.concurrent.ConcurrentSkipListSet`.

==== Querying

Responsible for converting the internal `IndexExpression` representation
into SASI's
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`Operation`]
and
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Expression.java[`Expression`]
trees, optimizing the trees to reduce the amount of work done, and
driving the query itself, the
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeUnionIterator.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeIntersectionIterator.java[`RangeIntersectionIterator`],
similar to its counterpart, performs set intersections over its data.

===== QueryPlan

The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`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,
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`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
link:#cassandra-internal-changes[Cassandra Internal Changes] section
below for more details). This process produces a tree of
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`Operation`]s,
which in turn may contain
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Expression.java[`Expression`]s,
all of which provide an alternative, more efficient, representation of
the query.

During execution, the
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`QueryPlan`]
uses the `DecoratedKey`-generating iterator created from the
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`Operation`]
tree. These keys are read from disk and a final check to ensure they
satisfy the query is made, once again using the
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`QueryPlan`]
references a
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryController.java[`QueryController`]
used throughout the execution phase. The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryController.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryController.java[`QueryController`]
so that they can be safely released by it later. The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryController.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`Operation`]
with three or more
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Expression.java[`Expression`]s.
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 │
                └──────────┘  └──────────┘
....

https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`QueryPlan`]
will remove the redundant right branch whose root is the final `AND` and
has leaves `fname != pa*` and `age > 21`. These
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Expression.java[`Expression`]s
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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`QueryPlan`]
optimizes a tree represented by
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`Operation`]s
as interior nodes, and
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Expression.java[`Expression`]s
as leaves. The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`Operation`]
class, more specifically, can have zero, one, or two
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`Operation`]s
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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`Operation`]s
children.

Besides participating in the optimizations performed by the
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`QueryPlan`],
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java#L87-L123[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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeUnionIterator.java[`RangeUnionIterator`]
performs the `Merge-Join` portion of the
https://en.wikipedia.org/wiki/Sort-merge_join[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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeUnionIterator.java#L9-L21[javadoc].

The
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeIntersectionIterator.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeUnionIterator.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeIntersectionIterator.java#L88-L101[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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeIntersectionIterator.java#L199-L208[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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/SASIIndex.java[`SASIIndex`]
class which implements `Index`, and is instantiated per-table containing
SASI indexes. It manages all indexes for a table via the
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/conf/DataTracker.java[`sasi.conf.DataTracker`]
and
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/conf/view/View.java[`sasi.conf.view.View`]
components, controls writing of all indexes for an SSTable via its
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/PerSSTableIndexWriter.java[`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
https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java[`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. `ByteOrderedPartitioner` and
`RandomPartitioner` will 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.

==== Contributors

* https://github.com/xedin[Pavel Yaskevich]
* https://github.com/jrwest[Jordan West]
* https://github.com/mkjellman[Michael Kjellman]
* https://github.com/jasobrown[Jason Brown]
* https://github.com/mishail[Mikhail Stepura]
