Java (embedded)

QuestDB is written in Java and can be used as any other Java library. Moreover, it is a single JAR with no additional dependencies.

To include QuestDB in your project, use the following:

JDK11

<dependency>
<groupId>org.questdb</groupId>
<artifactId>questdb</artifactId>
<version>7.4.2</version>
</dependency>

JDK8

<dependency>
<groupId>org.questdb</groupId>
<artifactId>questdb</artifactId>
<version>7.4.2-jdk8</version>
</dependency>

Writing data

This section provides example codes to write data to WAL and non-WAL tables. See Write Ahead Log for details about the differences between WAL and non-WAL tables.

The following writers are available for data ingestion:

  • WalWriter for WAL tables
  • TableWriter for non-WAL tables
  • TableWriterAPI for both WAL and non-WAL tables as it is an interface for WalWriter and Table Writer

Writing data using WalWriter

The WalWriter facilitates table writes to WAL tables. To successfully create an instance of WalWriter, the table must already exist.

Example WalWriter
final CairoConfiguration configuration = new DefaultCairoConfiguration("data_dir");
try (CairoEngine engine = new CairoEngine(configuration)) {
final SqlExecutionContext ctx = new SqlExecutionContextImpl(engine, 1)
.with(AllowAllSecurityContext.INSTANCE, null);
engine.ddl("CREATE TABLE testTable (" +
"a int, b byte, c short, d long, e float, g double, h date, " +
"i symbol, j string, k boolean, l geohash(8c), ts timestamp" +
") TIMESTAMP(ts) PARTITION BY DAY WAL", ctx);

// write data into WAL
final TableToken tableToken = engine.getTableTokenIfExists("testTable");
try (WalWriter writer = engine.getWalWriter(tableToken)) {
for (int i = 0; i < 3; i++) {
TableWriter.Row row = writer.newRow(Os.currentTimeMicros());
row.putInt(0, 123);
row.putByte(1, (byte) 1111);
row.putShort(2, (short) 222);
row.putLong(3, 333);
row.putFloat(4, 4.44f);
row.putDouble(5, 5.55);
row.putDate(6, System.currentTimeMillis());
row.putSym(7, "xyz");
row.putStr(8, "abc");
row.putBool(9, true);
row.putGeoHash(10, GeoHashes.fromString("u33dr01d", 0, 8));
row.append();
}
writer.commit();
}

// apply WAL to the table
try (ApplyWal2TableJob walApplyJob = new ApplyWal2TableJob(engine, 1, 1)) {
while (walApplyJob.run(0)) ;
}
}

Writing data using TableWriter

Non-WAL tables do not allow concurrent writes via multiple interfaces. To successfully create an instance, the table must:

  • Already exist
  • Have no other open writers against it as the TableWriter constructor will attempt to obtain an exclusive cross-process lock on the table.
Example TableWriter
try (CairoEngine engine = new CairoEngine(configuration)) {
final SqlExecutionContext ctx = new SqlExecutionContextImpl(engine, 1)
.with(AllowAllSecurityContext.INSTANCE, null);
engine.ddl("CREATE TABLE testTable (" +
"a int, b byte, c short, d long, e float, g double, h date, " +
"i symbol, j string, k boolean, l geohash(8c), ts timestamp" +
") TIMESTAMP(ts) PARTITION BY DAY BYPASS WAL", ctx);

// write data into WAL
final TableToken tableToken = engine.getTableTokenIfExists("testTable");
try (TableWriter writer = engine.getWriter(tableToken, "test")) {
for (int i = 0; i < 3; i++) {
TableWriter.Row row = writer.newRow(Os.currentTimeMicros());
row.putInt(0, 123);
row.putByte(1, (byte) 1111);
row.putShort(2, (short) 222);
row.putLong(3, 333);
row.putFloat(4, 4.44f);
row.putDouble(5, 5.55);
row.putDate(6, System.currentTimeMillis());
row.putSym(7, "xyz");
row.putStr(8, "abc");
row.putBool(9, true);
row.putGeoHash(10, GeoHashes.fromString("u33dr01d", 0, 8));
row.append();
}
writer.commit();
}
}

Writing data using TableWriterAPI

TableWriterAPI allows writing to both WAL and non-WAL tables by returning the suitable Writer based on the table configurations. The table must already exist:

Example TableWriterAPI
try (CairoEngine engine = new CairoEngine(configuration)) {
final SqlExecutionContext ctx = new SqlExecutionContextImpl(engine, 1)
.with(AllowAllSecurityContext.INSTANCE, null);
engine.ddl("CREATE TABLE testTable (" +
"a int, b byte, c short, d long, e float, g double, h date, " +
"i symbol, j string, k boolean, l geohash(8c), ts timestamp" +
") TIMESTAMP(ts) PARTITION BY DAY WAL", ctx);

// write data into WAL
final TableToken tableToken = engine.getTableTokenIfExists("testTable");
try (TableWriterAPI writer = engine.getTableWriterAPI(tableToken, "test")) {
for (int i = 0; i < 3; i++) {
TableWriter.Row row = writer.newRow(Os.currentTimeMicros());
row.putInt(0, 123);
row.putByte(1, (byte) 1111);
row.putShort(2, (short) 222);
row.putLong(3, 333);
row.putFloat(4, 4.44f);
row.putDouble(5, 5.55);
row.putDate(6, System.currentTimeMillis());
row.putSym(7, "xyz");
row.putStr(8, "abc");
row.putBool(9, true);
row.putGeoHash(10, GeoHashes.fromString("u33dr01d", 0, 8));
row.append();
}
writer.commit();
}

// apply WAL to the table
try (ApplyWal2TableJob walApplyJob = new ApplyWal2TableJob(engine, 1, 1)) {
while (walApplyJob.run(0)) ;
}
}

Detailed steps

Configure Cairo engine

CairoEngine is the resource manager for the embedded QuestDB. Its main function is to facilitate concurrent access to pools of TableReader and suitable writer instances.

New CairoEngine instance
final CairoConfiguration configuration = new DefaultCairoConfiguration("data_dir");
try (CairoEngine engine = new CairoEngine(configuration)) {

A typical application will need only one instance of CairoEngine. This instance will start when the application starts and shuts down when the application closes. You will need to close CairoEngine gracefully when the application stops.

QuestDB provides a default configuration which only requires the data directory to be specified. For a more advanced usage, the whole CairoConfiguration interface can be overridden.

Create an instance of SqlExecutionContext

Execution context is a conduit for passing SQL execution artifacts to the execution site. This instance is not thread-safe and it must not be shared between threads.

Example of execution context
final SqlExecutionContext ctx = new SqlExecutionContextImpl(engine, 1)
.with(AllowAllSecurityContext.INSTANCE, null);

The second argument of the constructor is the number of threads that will be helping to execute SQL statements. Unless you are building another QuestDB server, this value should always be 1.

SqlCompiler and blank table

Before we start writing data using a writer, the target table has to exist. There are several ways to create a new table and we recommend using CairoEngine:

Creating new table
// Create a non-WAL table:

engine.ddl("CREATE TABLE abc (" +
"a int, b byte, c short, d long, e float, g double, h date, " +
"i symbol, j string, k boolean, l geohash(8c), ts timestamp" +
") TIMESTAMP(ts) PARTITION BY DAY BYPASS WAL", ctx);

// Create a WAL table:

engine.ddl("CREATE TABLE abc (" +
"a int, b byte, c short, d long, e float, g double, h date, " +
"i symbol, j string, k boolean, l geohash(8c), ts timestamp" +
") TIMESTAMP(ts) PARTITION BY DAY WAL", ctx);

As you will be able to see below, the table field types and indexes must match the code that is populating the table.

Another way to create a table is to obtain a SqlCompiler from the engine and use it to run the DDL statement:

Creating new table with a SqlCompiler
try (SqlCompiler compiler = engine.getSqlCompiler()) {
engine.ddl(compiler, "CREATE TABLE abc (" +
"a int, b byte, c short, d long, e float, g double, h date, " +
"i symbol, j string, k boolean, l geohash(8c), ts timestamp" +
") TIMESTAMP(ts) PARTITION BY DAY WAL", ctx, null);
}

This way the obtained SqlCompiler can be reused to run other SQL statements.

Note that CairoEngine has a number of helper methods for different types of SQL statements. These are:

  • CairoEngine#ddl() - meant to execute CREATE TABLE and ALTER statements.
  • CairoEngine#insert() - meant to execute INSERT statements.
  • CairoEngine#drop() - meant to execute DROP TABLE statements.
  • CairoEngine#select() - meant to execute SELECT queries.

A new writer instance

We use CairoEngine to obtain an instance of the writer. This will enable reusing this writer instance later, when we use the same method of creating table writer again.

New table writer instance for a non-WAL table
final TableToken tableToken = engine.getTableTokenIfExists("abc");
try (TableWriter writer = engine.getWriter(tableToken, "test")) {
New table writer instance for a WAL table
final TableToken tableToken = engine.getTableTokenIfExists("abc");
try (WalWriter writer = engine.getWalWriter(tableToken)) {
New table writer instance for either a WAL or non-WAL table
final TableToken tableToken = engine.getTableTokenIfExists("abc");
try (TableWriterAPI writer = engine.getTableWriterAPI(tableToken, "test")) {

TableWriter - A non-WAL table uses TableWriter, which will hold an exclusive lock on table abc until it is closed and testing will be used as the lock reason. This lock is both intra- and inter-process. If you have two Java applications accessing the same table only one will succeed at one time.

WalWriter - A WAL table uses WalWriter to enable concurrent data ingestion, data modification, and schema changes, as the table is not locked.

TableWriterAPI - Both WAL and Non-WAL tables can use TableWriterAPI. It is an interface implemented by both writers.

Note the all of the writer classes are not thread-safe, so they should not be used concurrently.

Create a new row

Creating new table row with timestamp
TableWriter.Row row = writer.newRow(Os.currentTimeMicros());

Although this operation semantically looks like a new object creation, the row instance is actually being re-used under the hood. A timestamp is necessary to determine a partition for the new row. Its value has to be either increment or stay the same as the last row. When the table is not partitioned and does not have a designated timestamp column, the timestamp value can be omitted.

Creating new table row without timestamp
TableWriter.Row row = writer.newRow();

Populate columns

There are put* methods for every supported data type. Columns are updated by an index as opposed to by name.

Populating table column
row.putLong(3, 333);

Column update order is not important and updates can be sparse. All unset columns will default to NULL values.

Append a row

Following method call:

Appending a new row
row.append();

Appended rows are not visible to readers until they are committed. An unneeded row can also be canceled if required.

Cancelling half-populated row
row.cancel();

A pending row is automatically cancelled when writer.newRow() is called. Consider the following scenario:

TableWriter.Row row = writer.newRow(Os.currentTimeMicros());
row.putInt(0, 123);
row.putByte(1, (byte) 1111);
row.putShort(2, (short) 222);
row.putLong(3, 333);
row = writer.newRow(Os.currentTimeMicros());
...

Second newRow() call would cancel all the updates to the row since the last append().

Commit changes

To make changes visible to readers, writer has to commit. writer.commit does this job. Unlike traditional SQL databases, the size of the transaction does not matter. You can commit anything between 1 and 1 trillion rows. We also spent considerable effort to ensure commit() is lightweight. You can drip one row at a time in applications that require such behaviour.

Note that WAL writer commits aren't immediately visible to readers. The committed data becomes visible once it was applied by the ApplyWal2TableJob job.

Executing queries

We provide a single API for executing all kinds of SQL queries. The example below focuses on SELECT and how to fetch data from a cursor.

Compiling SQL
final CairoConfiguration configuration = new DefaultCairoConfiguration(temp.getRoot().getAbsolutePath());
try (CairoEngine engine = new CairoEngine(configuration)) {
final SqlExecutionContext ctx = new SqlExecutionContextImpl(engine, 1)
.with(AllowAllSecurityContext.INSTANCE, null);
try (RecordCursorFactory factory = engine.select("SELECT * FROM abc", ctx)) {
try (RecordCursor cursor = factory.getCursor(ctx)) {
final Record record = cursor.getRecord();
while (cursor.hasNext()) {
// access 'record' instance for field values
}
}
}
}
}

Detailed steps

The steps to setup CairoEngine, execution context and SqlCompiler are the same as those we explained in the sections. We will skip them here and focus on fetching data.

Note the all of the classes described below are not thread-safe, so they should not be used concurrently.

RecordCursorFactory

You can think of RecordCursorFactory as a prepared statement. This is the entity that holds SQL execution plan with all of the execution artefacts. Factories are designed to be reused and we strongly encourage caching them. You also need to make sure that you close factories explicitly when you no longer need them. Failing to do so can cause memory and/or other resources leak.

RecordCursor

This instance allows iterating over the dataset produced by SQL. Cursors are relatively short-lived and do not imply fetching all the data. Note that you have to close a cursor as soon as enough data is fetched ; the closing process can happen at any time.

Record

This is cursor's data access API. Record instance is obtained from the cursor outside of the fetch loop.

Example of fetching data from cursor
final Record record = cursor.getRecord();
while (cursor.hasNext()) {
// access 'record' instance for field values
}

Record does not hold the data. Instead, it is an API to pull data when data is needed. Record instance remains the same while cursor goes over the data, making caching of records pointless.

InfluxDB Line Protocol client library

We have Java Client Library to allow fast data ingestion.