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Data model

Data model

This section describes how Tarantool stores values and what operations with data it supports.

If you tried to create a database as suggested in our “Getting started” exercises, then your test database now looks like this:

../../../../_images/data_model.svg

Space

A space – ‘tester’ in our example – is a container.

When Tarantool is being used to store data, there is always at least one space. Each space has a unique name specified by the user. Besides, each space has a unique numeric identifier which can be specified by the user, but usually is assigned automatically by Tarantool. Finally, a space always has an engine: memtx (default) – in-memory engine, fast but limited in size, or vinyl – on-disk engine for huge data sets.

A space is a container for tuples. To be functional, it needs to have a primary index. It can also have secondary indexes.

Tuple

A tuple plays the same role as a “row” or a “record”, and the components of a tuple (which we call “fields”) play the same role as a “row column” or “record field”, except that:

  • fields can be composite structures, such as arrays or maps, and
  • fields don’t need to have names.

Any given tuple may have any number of fields, and the fields may be of different types. The identifier of a field is the field’s number, base 1 (in Lua and other 1-based languages) or base 0 (in PHP or C/C++). For example, “1” or “0” can be used in some contexts to refer to the first field of a tuple.

Tuples in Tarantool are stored as MsgPack arrays.

When Tarantool returns a tuple value in console, it uses the YAML format, for example: [3, 'Ace of Base', 1993].

Index

An index is a group of key values and pointers.

As with spaces, you should specify the index name, and let Tarantool come up with a unique numeric identifier (“index id”).

An index always has a type. The default index type is ‘TREE’. TREE indexes are provided by all Tarantool engines, can index unique and non-unique values, support partial key searches, comparisons and ordered results. Additionally, memtx engine supports HASH, RTREE and BITSET indexes.

An index may be multi-part, that is, you can declare that an index key value is composed of two or more fields in the tuple, in any order. For example, for an ordinary TREE index, the maximum number of parts is 255.

An index may be unique, that is, you can declare that it would be illegal to have the same key value twice.

The first index defined on a space is called the primary key index, and it must be unique. All other indexes are called secondary indexes, and they may be non-unique.

An index definition may include identifiers of tuple fields and their expected types (see allowed indexed field types below).

In our example, we first defined the primary index (named ‘primary’) based on field #1 of each tuple:

tarantool> i = s:create_index('primary', {type = 'hash', parts = {1, 'unsigned'}})

The effect is that, for all tuples in space ‘tester’, field #1 must exist and must contain an unsigned integer. The index type is ‘hash’, so values in field #1 must be unique, because keys in HASH indexes are unique.

After that, we defined a secondary index (named ‘secondary’) based on field #2 of each tuple:

tarantool> i = s:create_index('secondary', {type = 'tree', parts = {2, 'string'}})

The effect is that, for all tuples in space ‘tester’, field #2 must exist and must contain a string. The index type is ‘tree’, so values in field #2 must not be unique, because keys in TREE indexes may be non-unique.

Note

Space definitions and index definitions are stored permanently in Tarantool’s system spaces _space and _index (for details, see reference on box.space submodule).

You can add, drop, or alter the definitions at runtime, with some restrictions. See syntax details in reference on box module.

Data types

Tarantool is both a database and an application server. Hence a developer often deals with two type sets: the programming language types (e.g. Lua) and the types of the Tarantool storage format (MsgPack).

Lua vs MsgPack

Scalar / compound MsgPack   type Lua type Example value
scalar nil nil msgpack.NULL
scalar boolean boolean true
scalar string string ‘A B C’
scalar integer number 12345
scalar double number 1,2345
compound map table” (with string keys) table: 0x410f8b10
compound array table” (with integer keys) [1, 2, 3, 4, 5]
compound array tuple (“cdata”) [12345, ‘A B C’]

In Lua, a nil type has only one possible value, also called nil (displayed as null on Tarantool’s command line, since the output is in the YAML format). Nils may be compared to values of any types with == (is-equal) or ~= (is-not-equal), but other operations will not work. Nils may not be used in Lua tables; the workaround is to use msgpack.NULL

A boolean is either true or false.

A string is a variable-length sequence of bytes, usually represented with alphanumeric characters inside single quotes. In both Lua and MsgPack, strings are treated as binary data, with no attempts to determine a string’s character set or to perform any string conversion. So, string sorting and comparison are done byte-by-byte, without any special collation rules applied. (Example: numbers are ordered by their point on the number line, so 2345 is greater than 500; meanwhile, strings are ordered by the encoding of the first byte, then the encoding of the second byte, and so on, so ‘2345’ is less than ‘500’.)

In Lua, a number is double-precision floating-point, but Tarantool allows both integer and floating-point values. Tarantool will try to store a Lua number as floating-point if the value contains a decimal point or is very large (greater than 100 billion = 1e14), otherwise Tarantool will store it as an integer. To ensure that even very large numbers are stored as integers, use the tonumber64 function, or the LL (Long Long) suffix, or the ULL (Unsigned Long Long) suffix. Here are examples of numbers using regular notation, exponential notation, the ULL suffix and the tonumber64 function: -55, -2.7e+20, 100000000000000ULL, tonumber64('18446744073709551615').

Lua tables with string keys are stored as MsgPack maps; Lua tables with integer keys starting with 1 – as MsgPack arrays. Nils may not be used in Lua tables; the workaround is to use msgpack.NULL

A tuple is a light reference to a MsgPack array stored in the database. It is a special type (cdata) to avoid conversion to a Lua table on retrieval. A few functions may return tables with multiple tuples. For more tuple examples, see box.tuple.

Note

Tarantool uses the MsgPack format for database storage, which is variable-length. So, for example, the smallest number requires only one byte, but the largest number requires nine bytes.

Indexed field types

Indexes restrict values which Tarantool’s MsgPack may contain. This is why, for example, ‘unsigned’ is a separate indexed field type, compared to ‘integer’ data type in MsgPack: they both store ‘integer’ values, but an ‘unsigned’ index contains only non-negative integer values and an ‘integer’ index contains all integer values.

Here’s how Tarantool indexed field types correspond to MsgPack data types.

Indexed field type MsgPack data type
(and possible values)
Index type Examples
unsigned (may also be called ‘uint’ or ‘num’, but ‘num’ is deprecated) integer (integer between 0 and 18446744073709551615, i.e. about 18 quintillion) TREE, BITSET or HASH 123456
integer (may also be called ‘int’) integer (integer between -9223372036854775808 and 18446744073709551615) TREE or HASH -2^63
number

integer (integer between -9223372036854775808 and 18446744073709551615)

double (single-precision floating point number or double-precision floating point number)

TREE or HASH

1.234

-44

1.447e+44

string (may also be called ‘str’) string (any set of octets, up to the maximum length) TREE or HASH

‘A B C’

‘65 66 67’

array array (arrays of integers between -9223372036854775808 and 9223372036854775807) RTREE

{10, 11}

{3, 5, 9, 10}

scalar

null

boolean (true or false)

integer (integer between -9223372036854775808 and 18446744073709551615)

double (single-precision floating point number or double-precision floating point number)

string (any set of octets)

Note: When there is a mix of types, the key order is: null, then booleans, then numbers, then strings.

TREE or HASH

msgpack.NULL

true

-1

1.234

‘’

‘ру’

Persistence

In Tarantool, updates to the database are recorded in the so-called write ahead log (WAL) files. This ensures data persistence. When a power outage occurs or the Tarantool instance is killed incidentally, the in-memory database is lost. In this situation, WAL files are used to restore the data. Namely, Tarantool reads the WAL files and redoes the requests (this is called the “recovery process”). You can change the timing of the WAL writer, or turn it off, by setting wal_mode.

Tarantool also maintains a set of snapshot files. These files contain an on-disk copy of the entire data set for a given moment. Instead of reading every WAL file since the databases were created, the recovery process can load the latest snapshot file and then read only those WAL files that were produced after the snapshot file was made. After checkpointing, old WAL files can be removed to free up space.

To force immediate creation of a snapshot file, you can use Tarantool’s box.snapshot() request. To enable automatic creation of snapshot files, you can use Tarantool’s checkpoint daemon. The checkpoint daemon sets intervals for forced checkpoints. It makes sure that the states of both memtx and vinyl storage engines are synchronized and saved to disk, and automatically removes old WAL files.

Snapshot files can be created even if there is no WAL file.

Note

The memtx engine makes only regular checkpoints with the interval set in checkpoint daemon configuration.

The vinyl engine runs checkpointing in the background at all times.

See the Internals section for more details about the WAL writer and the recovery process.

Operations

Data operations

The basic data operations supported in Tarantool are:

  • one data-retrieval operation (SELECT), and
  • five data-manipulation operations (INSERT, UPDATE, UPSERT, DELETE, REPLACE).

All of them are implemented as functions in box.space submodule.

Examples

  • INSERT: Add a new tuple to space ‘tester’.

    The first field, field[1], will be 999 (MsgPack type is integer).

    The second field, field[2], will be ‘Taranto’ (MsgPack type is string).

    tarantool> box.space.tester:insert{999, 'Taranto'}
    
  • UPDATE: Update the tuple, changing field field[2].

    The clause “{999}”, which has the value to look up in the index of the tuple’s primary-key field, is mandatory, because update() requests must always have a clause that specifies a unique key, which in this case is field[1].

    The clause “{{‘=’, 2, ‘Tarantino’}}” specifies that assignment will happen to field[2] with the new value.

    tarantool> box.space.tester:update({999}, {{'=', 2, 'Tarantino'}})
    
  • UPSERT: Upsert the tuple, changing field field[2] again.

    The syntax of upsert() is similar to the syntax of update(). However, the execution logic of these two requests is different. UPSERT is either UPDATE or INSERT, depending on the database’s state. Also, UPSERT execution is postponed after transaction commit, so, unlike update(), upsert() doesn’t return data back.

    tarantool> box.space.tester:upsert({999}, {{'=', 2, 'Tarantism'}})
    
  • REPLACE: Replace the tuple, adding a new field.

    This is also possible with the update() request, but the update() request is usually more complicated.

    tarantool> box.space.tester:replace{999, 'Tarantella', 'Tarantula'}
    
  • SELECT: Retrieve the tuple.

    The clause “{999}” is still mandatory, although it does not have to mention the primary key.

    tarantool> box.space.tester:select{999}
    
  • DELETE: Delete the tuple.

    In this example, we identify the primary-key field.

    tarantool> box.space.tester:delete{999}
    

All the functions operate on tuples and accept only unique key values. So, the number of tuples in the space is always 0 or 1, since the keys are unique.

Functions insert(), upsert() and replace() accept only primary-key values. Functions select(), delete() and update() may accept either a primary-key value or a secondary-key value.

Note

Besides Lua, you can use Perl, PHP, Python or other programming language connectors. The client server protocol is open and documented. See this annotated BNF.

Index operations

Index operations are automatic: if a data-manipulation request changes a tuple, then it also changes the index keys defined for the tuple.

The simple index-creation operation that we’ve illustrated before is:

box.space.space-name:create_index('index-name')

This creates a unique TREE index on the first field of all tuples (often called “Field#1”), which is assumed to be numeric.

The simple SELECT request that we’ve illustrated before is:

box.space.space-name:select(value)

This looks for a single tuple via the first index. Since the first index is always unique, the maximum number of returned tuples will be: one.

The following SELECT variations exist:

  1. The search can use comparisons other than equality.

    box.space.space-name:select(value, {iterator = 'GT'})
    

    The comparison operators are LT, LE, EQ, REQ, GE, GT (for “less than”, “less than or equal”, “equal”, “reversed equal”, “greater than or equal”, “greater than” respectively). Comparisons make sense if and only if the index type is ‘TREE’.

    This type of search may return more than one tuple; if so, the tuples will be in descending order by key when the comparison operator is LT or LE or REQ, otherwise in ascending order.

  2. The search can use a secondary index.

    box.space.space-name.index.index-name:select(value)
    

    For a primary-key search, it is optional to specify an index name. For a secondary-key search, it is mandatory.

  3. The search may be for some or all key parts.

    -- Suppose an index has two parts
    tarantool> box.space.space-name.index.index-name.parts
    ---
    - - type: unsigned
        fieldno: 1
      - type: string
        fieldno: 2
    ...
    -- Suppose the space has three tuples
    box.space.space-name:select()
    ---
    - - [1, 'A']
      - [1, 'B']
      - [2, '']
    ...
    
  4. The search may be for all fields, using a table for the value:

    box.space.space-name:select({1, 'A'})
    

    or the search can be for one field, using a table or a scalar:

    box.space.space-name:select(1)
    

    In the second case, the result will be two tuples: {1, 'A'} and {1, 'B'}.

    You can specify even zero fields, causing all three tuples to be returned. (Notice that partial key searches are available only in TREE indexes.)

Examples

  • BITSET example:

    tarantool> box.schema.space.create('bitset_example')
    tarantool> box.space.bitset_example:create_index('primary')
    tarantool> box.space.bitset_example:create_index('bitset',{unique=false,type='BITSET', parts={2,'unsigned'}})
    tarantool> box.space.bitset_example:insert{1,1}
    tarantool> box.space.bitset_example:insert{2,4}
    tarantool> box.space.bitset_example:insert{3,7}
    tarantool> box.space.bitset_example:insert{4,3}
    tarantool> box.space.bitset_example.index.bitset:select(2, {iterator='BITS_ANY_SET'})
    

    The result will be:

    ---
    - - [3, 7]
      - [4, 3]
    ...
    

    because (7 AND 2) is not equal to 0, and (3 AND 2) is not equal to 0.

  • RTREE example:

    tarantool> box.schema.space.create('rtree_example')
    tarantool> box.space.rtree_example:create_index('primary')
    tarantool> box.space.rtree_example:create_index('rtree',{unique=false,type='RTREE', parts={2,'ARRAY'}})
    tarantool> box.space.rtree_example:insert{1, {3, 5, 9, 10}}
    tarantool> box.space.rtree_example:insert{2, {10, 11}}
    tarantool> box.space.rtree_example.index.rtree:select({4, 7, 5, 9}, {iterator = 'GT'})
    

    The result will be:

    ---
    - - [1, [3, 5, 9, 10]]
    ...
    

    because a rectangle whose corners are at coordinates 4,7,5,9 is entirely within a rectangle whose corners are at coordinates 3,5,9,10.

Additionally, there exist index iterator operations. They can only be used with code in Lua and C/C++. Index iterators are for traversing indexes one key at a time, taking advantage of features that are specific to an index type, for example evaluating Boolean expressions when traversing BITSET indexes, or going in descending order when traversing TREE indexes.

See also other index operations like alter() and drop() in reference for box.index submodule.

Complexity factors

In reference for box.space and box.index submodules, there are notes about which complexity factors might affect the resource usage of each function.

Complexity factor Effect
Index size The number of index keys is the same as the number of tuples in the data set. For a TREE index, if there are more keys, then the lookup time will be greater, although of course the effect is not linear. For a HASH index, if there are more keys, then there is more RAM used, but the number of low-level steps tends to remain constant.
Index type Typically, a HASH index is faster than a TREE index if the number of tuples in the space is greater than one.
Number of indexes accessed

Ordinarily, only one index is accessed to retrieve one tuple. But to update the tuple, there must be N accesses if the space has N different indexes.

Note re storage engine: Vinyl optimizes away such accesses if secondary index fields are unchanged by the update. So, this complexity factor applies only to memtx, since it always makes a full-tuple copy on every update.

Number of tuples accessed A few requests, for example SELECT, can retrieve multiple tuples. This factor is usually less important than the others.
WAL settings The important setting for the write-ahead log is wal_mode. If the setting causes no writing or delayed writing, this factor is unimportant. If the setting causes every data-change request to wait for writing to finish on a slow device, this factor is more important than all the others.