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Working with Pravega: Transactions

This article explores how to write a set of Events to a Stream atomically using Pravega Transactions.

Instructions for running the sample applications can be found in the Pravega Samples readme.

You really should be familiar with Pravega Concepts (see Pravega Concepts) before continuing reading this page.

Pravega Transactions and the Console Writer and Console Reader Apps

We have written a couple of applications, ConsoleReader and ConsoleWriter that help illustrate reading and writing data with Pravega and in particular to illustrate the Transaction facility in the Pravega programming model.  You can find those applications here.


The ConsoleReader app is very simple.  It uses the Pravega Java Client Library to read from a Stream and output each Event onto the console.  It runs indefinitely, so you have to kill the process to terminate the program.


The ConsoleWriter app is a bit more sophisticated.  It uses the Pravega Java Client Library to write Events to a Stream, including Events written in the context of a Pravega Transaction.  To make manipulating Transactions a bit easier, we provide a console-based CLI.  The help text for the CLI is shown below:

ConsoleWriter Help text

Enter one of the following commands at the command line prompt:

If no command is entered, the line is treated as a parameter to the WRITE_EVENT command.

WRITE_EVENT {event} - write the {event} out to the Stream or the current Transaction.
WRITE_EVENT_RK <<{routingKey}>> , {event} - write the {event} out to the Stream or the current Transaction using {routingKey}. Note << and >> around {routingKey}.
BEGIN - begin a Transaction. Only one Transaction at a time is supported by the CLI.
GET_TXN_ID - output the current Transaction's Id (if a Transaction is running)
FLUSH - flush the current Transaction (if a Transaction is running)
COMMIT - commit the Transaction (if a Transaction is running)
ABORT - abort the Transaction (if a Transaction is running)
STATUS - check the status of the Transaction(if a Transaction is running)
HELP - print out a list of commands.
QUIT - terminate the program.

examples/someStream >

So writing a single Event is simple, just type some text (you don't even have to type the WRITE_EVENT command if you don't want to).

But we really want to talk about Pravega Transactions, so lets dive into that.

Pravega Transactions

The idea with a Pravega Transaction is that it allows an application to prepare a set of Events that can be written "all at once" to a Stream.  This allows an application to "commit" a bunch of Events Atomically. This is done by writing them into the Transaction and calling commit to append them to the Stream.  An application might want to do this in cases where it wants the Events to be durably stored and later decided whether or not those Events should be appended to the Stream.  This allows the application to control when the set of Events are made visible to Readers.

A Transaction is created via an EventStreamWriter.  Recall that an EventStreamWriter itself is created through a ClientFactory and is constructed to operate against a Stream.  Transactions are therefore bound to a Stream.  Once a Transaction is created, it acts a lot like a Writer.  Applications Write Events to the Transaction and once acknowledged, the data is considered durably persisted in the Transaction.  Note that the data written to a Transaction will not be visible to Readers until the Transaction is committed.  In addition to writeEvent and writeEvent using a routing key, there are several Transaction specific operations provided:

Operation Discussion
getTxnId() Retrieve the unique identifier for the Transaction.
Pravega generates a unique UUID for each Transaction.
flush() Ensure that all Writes have been persisted.
ping() Extend the duration of a Transaction.
Note that after a certain amount of idle time, the Transaction will automatically abort. This is to handle the case where the client has crashed and it is no longer appropriate to keep resources associated with the Transaction.
checkStatus() Return the state of the Transaction. The Transaction can be in one of the following states: Open, Committing, Committed, Aborting or Aborted.
commit() Append all of the Events written to the Transaction into the Stream. Either all of the Event data will be appended to the Stream or none of it will be.
abort() Terminate the Transaction, the data written to the Transaction will be deleted.

Using the ConsoleWriter to Begin and Commit a Transaction

All of the Transaction API is reflected in the ConsoleWriter's CLI command set.

To begin a transaction, type BEGIN:

Begin Transaction

examples/someStream >begin
346d8561-3fd8-40b6-8c15-9343eeea2992 >

When a Transaction is created, it returns a Transaction object parameterized to the type of Event supported by the Stream.  In the case of the ConsoleWriter, the type of Event is a Java String.

The command prompt changes to show the Transaction's id.  Now any of the Transaction related commands can be issued (GET_TXN_ID, FLUSH, PING, COMMIT, ABORT and STATUS).  Note that the BEGIN command won't work because the ConsoleWriter supports only one Transaction at a time (this is a limitation of the app, not a limitation of Pravega).  When the ConsoleWriter is in a Transactional context, the WRITE_EVENT (remember if you don't type a command, ConsoleWriter assumes you want to write the text as an Event) or the WRITE_EVENT_RK will be written to the Transaction:

Write Events to a Transaction

346d8561-3fd8-40b6-8c15-9343eeea2992 >m1
**** Wrote 'm1'
346d8561-3fd8-40b6-8c15-9343eeea2992 >m2
**** Wrote 'm2'
346d8561-3fd8-40b6-8c15-9343eeea2992 >m3
**** Wrote 'm3'

At this point, if you look at the Stream (by invoking the ConsoleReader app on the Stream, for example), you won't see those Events written to the Stream.

Events not Written to the Stream (yet)

$ bin/consoleReader
******** Reading events from examples/someStream

But when a COMMIT command is given, causing the Transaction to commit:

Do the Commit

346d8561-3fd8-40b6-8c15-9343eeea2992 >commit
**** Transaction commit completed.
 those Events are appended to the Stream and are now all available:

After commit, the Events are Visible

******** Reading events from examples/someStream

More on Begin Transaction

The Begin Transaction (beginTxn()) operation takes three parameters (ConsoleWriter chooses some reasonable defaults so in the CLI these are optional): 

Param Discussion
transactionTimeout The amount of time a transaction should be allowed to run before it is automatically aborted by Pravega.
This is also referred to as a "lease".
maxExecutionTime The amount of time allowed between ping operations.