5.1. Accumulo Connector

Overview

The Accumulo connector supports reading and writing data from Apache Accumulo.Please read this page thoroughly to understand the capabilities and features of the connector.

Installing the Iterator Dependency

The Accumulo connector uses custom Accumulo iterators inorder to push various information in a SQL predicate clause to Accumulo forserver-side filtering, known as predicate pushdown. In orderfor the server-side iterators to work, you need to add the presto-accumulojar file to Accumulo’s lib/ext directory on each TabletServer node.

# For each TabletServer node:
scp $PRESTO_HOME/plugins/accumulo/presto-accumulo-*.jar [tabletserver_address]:$ACCUMULO_HOME/lib/ext
 
# TabletServer should pick up new JAR files in ext directory, but may require restart

Note that this uses Java 8. If your Accumulo cluster is using Java 7,you’ll receive an Unsupported major.minor version 52.0 error in your TabletServer logs when youattempt to create an indexed table. You’ll instead need to use the presto-accumulo-iterators jar filethat is located at https://github.com/bloomberg/presto-accumulo.

Connector Configuration

Create etc/catalog/accumulo.propertiesto mount the accumulo connector as the accumulo catalog,replacing the accumulo.xxx properties as required:

connector.name=accumulo
accumulo.instance=xxx
accumulo.zookeepers=xxx
accumulo.username=username
accumulo.password=password

Configuration Variables

Unsupported Features

The following features are not supported:

• Adding columns via ALTER TABLE: While you cannot add columns via SQL, you can using a tool.See the below section on Adding Columns for more details.
• DELETE: Deletion of rows is not yet implemented for the connector.

Usage

Simply begin using SQL to create a new table in Accumulo to beginworking with data. By default, the first column of the table definitionis set to the Accumulo row ID. This should be the primary key of yourtable, and keep in mind that any INSERT statements containing the samerow ID is effectively an UPDATE as far as Accumulo is concerned, as anyprevious data in the cell will be overwritten. The row ID can beany valid Presto datatype. If the first column is not your primary key, youcan set the row ID column using the row_id table property within the WITHclause of your table definition.

Simply issue a CREATE TABLE statement to create a new Presto/Accumulo table:

CREATE TABLE myschema.scientists (
  recordkey VARCHAR,
  name VARCHAR,
  age BIGINT,
  birthday DATE
);

DESCRIBE myschema.scientists;

  Column   |  Type   | Extra |                      Comment
-----------+---------+-------+---------------------------------------------------
 recordkey | varchar |       | Accumulo row ID
 name      | varchar |       | Accumulo column name:name. Indexed: false
 age       | bigint  |       | Accumulo column age:age. Indexed: false
 birthday  | date    |       | Accumulo column birthday:birthday. Indexed: false

This command will create a new Accumulo table with the recordkey columnas the Accumulo row ID. The name, age, and birthday columns are mapped toauto-generated column family and qualifier values (which, in practice,are both identical to the Presto column name).

When creating a table using SQL, you can optionally specify acolumn_mapping table property. The value of this property is acomma-delimited list of triples, presto column : accumulo columnfamily : accumulo column qualifier, with one triple for everynon-row ID column. This sets the mapping of the Presto column name tothe corresponding Accumulo column family and column qualifier.

If you don’t specify the column_mapping table property, then theconnector will auto-generate column names (respecting any configured locality groups).Auto-generation of column names is only available for internal tables, so if yourtable is external you must specify the column_mapping property.

For a full list of table properties, see Table Properties.

For example:

CREATE TABLE myschema.scientists (
  recordkey VARCHAR,
  name VARCHAR,
  age BIGINT,
  birthday DATE
)
WITH (
  column_mapping = 'name:metadata:name,age:metadata:age,birthday:metadata:date'
);

DESCRIBE myschema.scientists;

  Column   |  Type   | Extra |                    Comment
-----------+---------+-------+-----------------------------------------------
 recordkey | varchar |       | Accumulo row ID
 name      | varchar |       | Accumulo column metadata:name. Indexed: false
 age       | bigint  |       | Accumulo column metadata:age. Indexed: false
 birthday  | date    |       | Accumulo column metadata:date. Indexed: false

You can then issue INSERT statements to put data into Accumulo.

Note

While issuing INSERT statements is convenient,this method of loading data into Accumulo is low-throughput. You’ll wantto use the Accumulo APIs to write Mutations directly to the tables.See the section on Loading Data for more details.

INSERT INTO myschema.scientists VALUES
('row1', 'Grace Hopper', 109, DATE '1906-12-09' ),
('row2', 'Alan Turing', 103, DATE '1912-06-23' );

SELECT * FROM myschema.scientists;

 recordkey |     name     | age |  birthday
-----------+--------------+-----+------------
 row1      | Grace Hopper | 109 | 1906-12-09
 row2      | Alan Turing  | 103 | 1912-06-23
(2 rows)

As you’d expect, rows inserted into Accumulo via the shell orprogramatically will also show up when queried. (The Accumulo shellthinks “-5321” is an option and not a number… so we’ll just make TBL alittle younger.)

$ accumulo shell -u root -p secret
root@default> table myschema.scientists
root@default myschema.scientists> insert row3 metadata name "Tim Berners-Lee"
root@default myschema.scientists> insert row3 metadata age 60
root@default myschema.scientists> insert row3 metadata date 5321

SELECT * FROM myschema.scientists;

 recordkey |      name       | age |  birthday
-----------+-----------------+-----+------------
 row1      | Grace Hopper    | 109 | 1906-12-09
 row2      | Alan Turing     | 103 | 1912-06-23
 row3      | Tim Berners-Lee |  60 | 1984-07-27
(3 rows)

You can also drop tables using DROP TABLE. This command drops bothmetadata and the tables. See the below section on ExternalTables for more details on internal and externaltables.

DROP TABLE myschema.scientists;

Indexing Columns

Internally, the connector creates an Accumulo Range and packs it ina split. This split gets passed to a Presto Worker to read the data fromthe Range via a BatchScanner. When issuing a query that resultsin a full table scan, each Presto Worker gets a single Range thatmaps to a single tablet of the table. When issuing a query with apredicate (i.e. WHERE x = 10 clause), Presto passes the valueswithin the predicate (10) to the connector so it can use thisinformation to scan less data. When the Accumulo row ID is used as partof the predicate clause, this narrows down the Range lookup to quicklyretrieve a subset of data from Accumulo.

But what about the other columns? If you’re frequently querying onnon-row ID columns, you should consider using the indexingfeature built into the Accumulo connector. This feature can drasticallyreduce query runtime when selecting a handful of values from the table,and the heavy lifting is done for you when loading data via PrestoINSERT statements (though, keep in mind writing data to Accumulo viaINSERT does not have high throughput).

To enable indexing, add the index_columns table property and specifya comma-delimited list of Presto column names you wish to index (we use thestring serializer here to help with this example – youshould be using the default lexicoder serializer).

CREATE TABLE myschema.scientists (
  recordkey VARCHAR,
  name VARCHAR,
  age BIGINT,
  birthday DATE
)
WITH (
  serializer = 'string',
  index_columns='name,age,birthday'
);

After creating the table, we see there are an additional two Accumulotables to store the index and metrics.

root@default> tables
accumulo.metadata
accumulo.root
myschema.scientists
myschema.scientists_idx
myschema.scientists_idx_metrics
trace

After inserting data, we can look at the index table and see there areindexed values for the name, age, and birthday columns. The connectorqueries this index table

INSERT INTO myschema.scientists VALUES
('row1', 'Grace Hopper', 109, DATE '1906-12-09'),
('row2', 'Alan Turing', 103, DATE '1912-06-23');

root@default> scan -t myschema.scientists_idx
-21011 metadata_date:row2 []
-23034 metadata_date:row1 []
103 metadata_age:row2 []
109 metadata_age:row1 []
Alan Turing metadata_name:row2 []
Grace Hopper metadata_name:row1 []

When issuing a query with a WHERE clause against indexed columns,the connector searches the index table for all row IDs that contain thevalue within the predicate. These row IDs are bundled into a Prestosplit as single-value Range objects (the number of row IDs per splitis controlled by the value of accumulo.index_rows_per_split) andpassed to a Presto worker to be configured in the BatchScanner whichscans the data table.

SELECT * FROM myschema.scientists WHERE age = 109;

 recordkey |     name     | age |  birthday
-----------+--------------+-----+------------
 row1      | Grace Hopper | 109 | 1906-12-09
(1 row)

Loading Data

The Accumulo connector supports loading data via INSERT statements, howeverthis method tends to be low-throughput and should not be relied on when throughputis a concern. Instead, users of the connector should use the PrestoBatchWritertool that is provided as part of the presto-accumulo-tools subproject in thepresto-accumulo repository.

The PrestoBatchWriter is a wrapper class for the typical BatchWriter thatleverages the Presto/Accumulo metadata to write Mutations to the main data table.In particular, it handles indexing the given mutations on any indexed columns.Usage of the tool is provided in the README in the repository.

External Tables

By default, the tables created using SQL statements via Presto areinternal tables, that is both the Presto table metadata and theAccumulo tables are managed by Presto. When you create an internaltable, the Accumulo table is created as well. You will receive an errorif the Accumulo table already exists. When an internal table is droppedvia Presto, the Accumulo table (and any index tables) are dropped aswell.

To change this behavior, set the external property to true whenissuing the CREATE statement. This will make the table an _external_table, and a DROP TABLE command will only delete the metadataassociated with the table. If the Accumulo tables do not already exist,they will be created by the connector.

Creating an external table will set any configured locality groups as wellas the iterators on the index and metrics tables (if the table is indexed).In short, the only difference between an external table and an internal tableis the connector will delete the Accumulo tables when a DROP TABLE commandis issued.

External tables can be a bit more difficult to work with, as the data is storedin an expected format. If the data is not stored correctly, then you’regonna have a bad time. Users must provide a columnmapping propertywhen creating the table. This creates the mapping of Presto column nameto the column family/qualifier for the cell of the table. The value of thecell is stored in the Value of the Accumulo key/value pair. By default,this value is expected to be serialized using Accumulo’s _lexicoder API.If you are storing values as strings, you can specify a different serializerusing the serializer property of the table. See the section onTable Properties for more information.

Next, we create the Presto external table.

CREATE TABLE external_table (
  a VARCHAR,
  b BIGINT,
  c DATE
)
WITH (
  column_mapping = 'a:md:a,b:md:b,c:md:c',
  external = true,
  index_columns = 'b,c',
  locality_groups = 'foo:b,c'
);

After creating the table, usage of the table continues as usual:

INSERT INTO external_table VALUES
('1', 1, DATE '2015-03-06'),
('2', 2, DATE '2015-03-07');

SELECT * FROM external_table;

 a | b |     c
---+---+------------
 1 | 1 | 2015-03-06
 2 | 2 | 2015-03-06
(2 rows)

DROP TABLE external_table;

After dropping the table, the table will still exist in Accumulo because it is external.

root@default> tables
accumulo.metadata
accumulo.root
external_table
external_table_idx
external_table_idx_metrics
trace

If we wanted to add a new column to the table, we can create the table again and specify a new column.Any existing rows in the table will have a value of NULL. This command will re-configure the Accumulotables, setting the locality groups and iterator configuration.

CREATE TABLE external_table (
  a VARCHAR,
  b BIGINT,
  c DATE,
  d INTEGER
)
WITH (
  column_mapping = 'a:md:a,b:md:b,c:md:c,d:md:d',
  external = true,
  index_columns = 'b,c,d',
  locality_groups = 'foo:b,c,d'
);
 
SELECT * FROM external_table;

 a | b |     c      |  d
---+---+------------+------
 1 | 1 | 2015-03-06 | NULL
 2 | 2 | 2015-03-07 | NULL
(2 rows)

Table Properties

Table property usage example:

CREATE TABLE myschema.scientists (
  recordkey VARCHAR,
  name VARCHAR,
  age BIGINT,
  birthday DATE
)
WITH (
  column_mapping = 'name:metadata:name,age:metadata:age,birthday:metadata:date',
  index_columns = 'name,age'
);

Session Properties

You can change the default value of a session property by using SET SESSION.Note that session properties are prefixed with the catalog name:

SET SESSION accumulo.column_filter_optimizations_enabled = false;

Adding Columns

Adding a new column to an existing table cannot be done today viaALTER TABLE [table] ADD COLUMN [name] [type] because of the additionalmetadata required for the columns to work; the column family, qualifier,and if the column is indexed.

Instead, you can use one of the utilities in thepresto-accumulo-toolssub-project of the presto-accumulo repository. Documentation and usage can be found in the README.

Serializers

The Presto connector for Accumulo has a pluggable serializer frameworkfor handling I/O between Presto and Accumulo. This enables end-users theability to programatically serialized and deserialize their special dataformats within Accumulo, while abstracting away the complexity of theconnector itself.

There are two types of serializers currently available; a stringserializer that treats values as Java String and a lexicoderserializer that leverages Accumulo’s Lexicoder API to store values. Thedefault serializer is the lexicoder serializer, as this serializerdoes not require expensive conversion operations back and forth betweenString objects and the Presto types – the cell’s value is encoded as abyte array.

Additionally, the lexicoder serializer does proper lexigraphical ordering ofnumerical types like BIGINT or TIMESTAMP. This is essential for the connectorto properly leverage the secondary index when querying for data.

You can change the default the serializer by specifying theserializer table property, using either default (which islexicoder), string or lexicoder for the built-in types, oryou could provide your own implementation by extendingAccumuloRowSerializer, adding it to the Presto CLASSPATH, andspecifying the fully-qualified Java class name in the connector configuration.

CREATE TABLE myschema.scientists (
  recordkey VARCHAR,
  name VARCHAR,
  age BIGINT,
  birthday DATE
)
WITH (
  column_mapping = 'name:metadata:name,age:metadata:age,birthday:metadata:date',
  serializer = 'default'
);

INSERT INTO myschema.scientists VALUES
('row1', 'Grace Hopper', 109, DATE '1906-12-09' ),
('row2', 'Alan Turing', 103, DATE '1912-06-23' );

root@default> scan -t myschema.scientists
row1 metadata:age []    \x08\x80\x00\x00\x00\x00\x00\x00m
row1 metadata:date []    \x08\x7F\xFF\xFF\xFF\xFF\xFF\xA6\x06
row1 metadata:name []    Grace Hopper
row2 metadata:age []    \x08\x80\x00\x00\x00\x00\x00\x00g
row2 metadata:date []    \x08\x7F\xFF\xFF\xFF\xFF\xFF\xAD\xED
row2 metadata:name []    Alan Turing

CREATE TABLE myschema.stringy_scientists (
  recordkey VARCHAR,
  name VARCHAR,
  age BIGINT,
  birthday DATE
)
WITH (
  column_mapping = 'name:metadata:name,age:metadata:age,birthday:metadata:date',
  serializer = 'string'
);

INSERT INTO myschema.stringy_scientists VALUES
('row1', 'Grace Hopper', 109, DATE '1906-12-09' ),
('row2', 'Alan Turing', 103, DATE '1912-06-23' );

root@default> scan -t myschema.stringy_scientists
row1 metadata:age []    109
row1 metadata:date []    -23034
row1 metadata:name []    Grace Hopper
row2 metadata:age []    103
row2 metadata:date []    -21011
row2 metadata:name []    Alan Turing

CREATE TABLE myschema.custom_scientists (
  recordkey VARCHAR,
  name VARCHAR,
  age BIGINT,
  birthday DATE
)
WITH (
  column_mapping = 'name:metadata:name,age:metadata:age,birthday:metadata:date',
  serializer = 'my.serializer.package.MySerializer'
);

Metadata Management

Metadata for the Presto/Accumulo tables is stored in ZooKeeper. You can(and should) issue SQL statements in Presto to create and drop tables.This is the easiest method of creating the metadata required to make theconnector work. It is best to not mess with the metadata, but here arethe details of how it is stored. Information is power.

A root node in ZooKeeper holds all the mappings, and the format is asfollows:

/metadata-root/schema/table

Where metadata-root is the value of zookeeper.metadata.root inthe config file (default is /presto-accumulo), schema is thePresto schema (which is identical to the Accumulo namespace name), andtable is the Presto table name (again, identical to Accumulo name).The data of the table ZooKeeper node is a serializedAccumuloTable Java object (which resides in the connector code).This table contains the schema (namespace) name, table name, columndefinitions, the serializer to use for the table, and any additionaltable properties.

If you have a need to programmatically manipulate the ZooKeeper metadatafor Accumulo, take a look atcom.facebook.presto.accumulo.metadata.ZooKeeperMetadataManager for someJava code to simplify the process.

Converting Table from Internal to External

If your table is internal, you can convert it to an external table by deletingthe corresponding znode in ZooKeeper, effectively making the table no longer exist asfar as Presto is concerned. Then, create the table again using the same DDL, but adding theexternal = true table property.

For example:

1. We’re starting with an internal table foo.bar that was created with the below DDL.If you have not previously defined a table property for column_mapping (like this example),be sure to describe the table before deleting the metadata. We’ll need the column mappingswhen creating the external table.

CREATE TABLE foo.bar (a VARCHAR, b BIGINT, c DATE)
WITH (
    index_columns = 'b,c'
);

DESCRIBE foo.bar;

 Column |  Type   | Extra |               Comment
--------+---------+-------+-------------------------------------
 a      | varchar |       | Accumulo row ID
 b      | bigint  |       | Accumulo column b:b. Indexed: true
 c      | date    |       | Accumulo column c:c. Indexed: true

1. Using the ZooKeeper CLI, delete the corresponding znode. Note this uses the default ZooKeepermetadata root of /presto-accumulo

$ zkCli.sh
[zk: localhost:2181(CONNECTED) 1] delete /presto-accumulo/foo/bar

1. Re-create the table using the same DDL as before, but adding the external=true property.Note that if you had not previously defined the column_mapping, you’ll need to add the propertyto the new DDL (external tables require this property to be set). The column mappings are inthe output of the DESCRIBE statement.

CREATE TABLE foo.bar (
  a VARCHAR,
  b BIGINT,
  c DATE
)
WITH (
  column_mapping = 'a:a:a,b:b:b,c:c:c',
  index_columns = 'b,c',
  external = true
);