# SASIIndex

[`SASIIndex`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/SASIIndex.java),
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 patchset in
this repository includes support for the Thrift and CQL3 interfaces.

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](#ondiskindexbuilder).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](#ondiskindexbuilder). 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](#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](#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)
```

#### 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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/analyzer/StandardAnalyzer.java)
and `analyzed` is set to `true`. The
`tokenization_normalize_lowercase` is similar to the `case_sensitive`
property but for the
[`StandardAnalyzer`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/analyzer/StandardAnalyzer.java). These
query demonstrates the stemming applied by [`StandardAnalyzer`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/analyzer/StandardAnalyzer.java).

```
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 SASIs
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 minimize 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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/OnDiskIndexBuilder.java). Once
flushed to disk, the data is read using the
[`OnDiskIndex`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/OnDiskIndex.java)
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
[`TokenTreeBuilder`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTreeBuilder.java)s
for writing, and
[`TokenTree`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java)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
[`IndexMemtable`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/IndexMemtable.java)
class.

#### OnDiskIndex(Builder)

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

The terms written to the
[`OnDiskIndex`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/OnDiskIndex.java)
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, 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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java)
is built merging all the
[`TokenTree`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java)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
[`TokenTree`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java)
is an implementation of the well-known
[B+-tree](https://en.wikipedia.org/wiki/B%2B_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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTreeBuilder.java)
completely loads its interior nodes as the tree is built and it uses
the well-known algorithm optimized for bulk-loading the data
structure.

[`TokenTree`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/TokenTree.java)s provide the means to iterate a 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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/IndexMemtable.java)
handles indexing the in-memory data held in the memtable. The
[`IndexMemtable`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/IndexMemtable.java)
in turn manages either a
[`TrieMemIndex`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/TrieMemIndex.java)
or a
[`SkipListMemIndex`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/SkipListMemIndex.java)
per-column. The choice of which index type is used is data
dependent. The
[`TrieMemIndex`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/TrieMemIndex.java)
is used for literal types. `AsciiType` and `UTF8Type` are literal
types by defualt 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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/SkipListMemIndex.java)
is used. The
[`TrieMemIndex`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/TrieMemIndex.java)
is an implementation that can efficiently support prefix queries on
character-like data. The
[`SkipListMemIndex`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/memory/SkipListMemIndex.java),
conversely, is better suited for Cassandra other data types like
numbers.

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

### Querying

Responsible for converting the internal `IndexExpression`
representation into SASI's
[`Operation`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java)
and
[`Expression`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Expression.java)
tree, optimizing the tree to reduce the amount of work done, and
driving the query itself the
[`QueryPlan`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java)
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 SASIs use, and with more features. The
[`RangeUnionIterator`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeUnionIterator.java),
like its name suggests, performs set union 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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeIntersectionIterator.java),
similar to its counterpart, performs set intersection over its data.

#### QueryPlan

The
[`QueryPlan`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java)
instantiated per search query is at the core of SASIs querying
implementation. Its work can be divided in two stages: analysis and
execution.

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

During execution the
[`QueryPlan`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java)
uses the `DecoratedKey`-generating iterator created from the
[`Operation`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java) tree. These keys are read from disk and a final check to
ensure they satisfy the query is made, once again using the
[`Operation`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java) 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
accross SSTables. The concurrency factor is controlled by the
`cassandra.search_concurrency_factor` system property. The default is
`1`.

##### QueryController

Each
[`QueryPlan`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java)
references a
[`QueryController`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryController.java)
used throughout the execution phase. The
[`QueryController`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryController.java)
has two responsibilities: to manage and ensure the proper cleanup of
resources (indexes), and to strictly enforce the time bound for query,
specified by the user via the range slice timeout. All indexes are
accessed via the
[`QueryController`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryController.java)
so that they can be safely released by it later. The
[`QueryController`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryController.java)'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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java)
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 intersection (`AND`) into a single [`Operation`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java) with
three or more [`Expression`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Expression.java)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 │
                    └──────────┘  └──────────┘

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

Besides participating in the optimizations performed by the
[`QueryPlan`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java),
[`Operation`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java)
is also responsible for taking a row that has been returned by the
query and making a final validation that it in fact does match. This
`satisfiesBy` operation is performed recursively from the root of the
[`Operation`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java)
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](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/Operation.java#L87-L123).

#### 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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeUnionIterator.java)
performs the "Merge-Join" portion of the
[Sort-Merge-Join](https://en.wikipedia.org/wiki/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](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeUnionIterator.java#L9-L21).

The
[`RangeIntersectionIterator`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeIntersectionIterator.java)
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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeUnionIterator.java)
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](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeIntersectionIterator.java#L88-L101)
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](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/utils/RangeIntersectionIterator.java#L199-L208).

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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/SASIIndex.java)
class which implements `Index`, and is instantiated
per-table containing SASI indexes. It manages all indexes for a table
via the
[`sasi.conf.DataTracker`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/conf/DataTracker.java)
and
[`sasi.conf.view.View`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/conf/view/View.java)
components, controls writing of all indexes for an SSTable via its
[`PerSSTableIndexWriter`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/disk/PerSSTableIndexWriter.java), 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`](https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/index/sasi/plan/QueryPlan.java)
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

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