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https://github.com/confluentinc/bottledwater-pg

Change data capture from PostgreSQL into Kafka
https://github.com/confluentinc/bottledwater-pg

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Change data capture from PostgreSQL into Kafka

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README 

        
NOTE: Bottled Water is unmaintained

===================================



**Please note that Bottled Water is no longer being actively developed.**



Bottled Water pioneered change data capture from PostgreSQL into Kafka using the logical decoding

API, but now other projects have adopted the technique and are continuing to develop it. Please see

the following resources for supported solutions for CDC from PostgreSQL into Kafka:



* The [Kafka Connector list](https://www.confluent.io/product/connectors/) provides an overview

  of projects that integrate databases with the

  [Kafka Connect framework](http://docs.confluent.io/current/connect/index.html).

* [Debezium](http://debezium.io/) integrates PostgreSQL with Kafka in a way that is similar to

  Bottled Water.



The remaining README is kept here for historical interest.



Bottled Water for PostgreSQL

============================



How do you export water from your country? Well, you first go to your reservoir, pump out all the

water, and fill it into bottles. You then go to the streams of clear mountain water flowing into

the reservoir, and tap into them, filling the fresh water into bottles as it arrives. Then you

ship those bottles all around the world.



How do you export data from your database? Well, you first take a consistent snapshot of your

entire database, and encode it in a language-independent format. You then look at the stream of

transactions writing to your database, and parse the transaction log, encoding the

inserts/updates/deletes into the same language-independent format as they happen. Then you take

that data and ship it to your other systems: build search indexes, update caches, load it into

a data warehouse, calculate analytics, monitor it for fraud, and so on.



* [Blog post explaining the background](http://blog.confluent.io/2015/04/23/bottled-water-real-time-integration-of-postgresql-and-kafka/)

* [Watch a short demo!](http://showterm.io/fde6260d684ee3a6ee692)



How it works

------------



Bottled Water uses the [logical decoding](http://www.postgresql.org/docs/9.5/static/logicaldecoding.html)

feature (introduced in PostgreSQL 9.4) to extract a consistent snapshot and a continuous stream

of change events from a database. The data is extracted at a row level, and encoded using

[Avro](http://avro.apache.org/). A client program connects to your database, extracts this data,

and relays it to [Kafka](http://kafka.apache.org/) (you could also integrate it with other systems

if you wish, but Kafka is pretty awesome).



Key features of Bottled Water are:



* Works with any PostgreSQL database (version 9.4 or later). There are no restrictions on your

  database schema.

* No schema changes are required, no triggers or additional tables. (However, you do need to be

  able to install a PostgreSQL extension on the database server. More on this below.)

* Negligible impact on database performance.

* Transactionally consistent output. That means: writes appear only when they are committed to the

  database (writes by aborted transactions are discarded), writes appear in the same order as they

  were committed (no race conditions).

* Fault-tolerant: does not lose data, even if processes crash, machines die, the network is

  interrupted, etc.



Quickstart

----------



There are several possible ways of installing and trying Bottled Water:



* [Running in Docker](#running-in-docker) is the fastest way of getting started, but currently

  only recommended for development environments.

* [Building from source](#building-from-source) is the most flexible, but also a bit fiddly.

* There are also [Ubuntu packages](https://launchpad.net/~stub/+archive/ubuntu/bottledwater),

  built by Stuart Bishop (Canonical).



Running in Docker

-----------------



The easiest way to try Bottled Water is to use the [Docker](https://www.docker.com/) images we have

prepared. You need at least 2GB of memory to run this demo, so if you're running inside a virtual

machine (such as [Boot2docker](http://boot2docker.io/) on a Mac), please check that it is big

enough.



First, install:



* [Docker](https://docs.docker.com/installation/), which is used to run the

  individual services/containers, and

* [docker-compose](https://docs.docker.com/compose/install/), which is used to

  orchestrate the interaction between services.



After the prerequisite applications are installed, you need to build the Docker containers for Bottled Water and Postgres:



    $ make docker-compose



Once the build process finishes, set up some required environment variables,

then start up Postgres, Kafka and the [Confluent schema

registry](http://confluent.io/docs/current/schema-registry/docs/intro.html) by

running `docker-compose` as follows:



    $ export KAFKA_ADVERTISED_HOST_NAME=$(docker run --rm debian:jessie ip route | awk '/^default via / { print $3 }') \

             KAFKA_LOG_CLEANUP_POLICY=compact \

             KAFKA_AUTO_CREATE_TOPICS_ENABLE=true

    $ docker-compose up -d kafka schema-registry postgres



The `postgres-bw` image extends the

[official Postgres docker image](https://registry.hub.docker.com/_/postgres/) and adds

Bottled Water support. However, before Bottled Water can be used, it first needs to be

enabled. To do this, start a `psql` shell for the Postgres database:



    $ docker-compose run --rm psql



When the prompt appears, enable the `bottledwater` extension, and create a database with

some test data, for example:



    create extension bottledwater;

    create table test (id serial primary key, value text);

    insert into test (value) values('hello world!');



You can keep the psql terminal open, and run the following in a new terminal.



The next step is to start the Bottled Water client, which relays data from Postgres to Kafka.

You start it like this:



    $ docker-compose up -d bottledwater-avro



You can run `docker-compose logs bottledwater-avro` to see what it's doing. Now Bottled

Water has taken the snapshot, and continues to watch Postgres for any data changes. You can

see the data that has been extracted from Postgres by consuming from Kafka (the topic name

`test` must match up with the name of the table you created earlier):



    $ docker-compose run --rm kafka-avro-console-consumer \

        --from-beginning --property print.key=true --topic test



This should print out the contents of the `test` table in JSON format (key/value separated

by tab). Now go back to the `psql` terminal, and change some data — insert, update or delete

some rows in the `test` table. You should see the changes swiftly appear in the Kafka

consumer terminal.



When you're done testing, you can destroy the cluster and it's associated data volumes with:



    $ docker-compose stop

    $ docker-compose rm -vf



Building from source

--------------------



To compile Bottled Water is just a matter of:



    make && make install



For that to work, you need the following dependencies installed:



* [PostgreSQL 9.5](http://www.postgresql.org/) development libraries (PGXS and libpq).

  (Homebrew: `brew install postgresql`;

  Ubuntu: `sudo apt-get install postgresql-server-dev-9.5 libpq-dev`)

* [libsnappy](https://code.google.com/p/snappy/), a dependency of Avro.

  (Homebrew: `brew install snappy`; Ubuntu: `sudo apt-get install libsnappy-dev`)

* [avro-c](http://avro.apache.org/) (1.8.0 or later), the C implementation of Avro.

  (Homebrew: `brew install avro-c`; others: build from source)

* [Jansson](http://www.digip.org/jansson/), a JSON parser.

  (Homebrew: `brew install jansson`; Ubuntu: `sudo apt-get install libjansson-dev`)

* [libcurl](http://curl.haxx.se/libcurl/), a HTTP client.

  (Homebrew: `brew install curl`; Ubuntu: `sudo apt-get install libcurl4-openssl-dev`)

* [librdkafka](https://github.com/edenhill/librdkafka) (0.9.1 or later), a Kafka client.

  (Ubuntu universe: `sudo apt-get install librdkafka-dev`, but see [known gotchas](#known-gotchas-with-older-dependencies); others: build from source)



You can see the Dockerfile for

[building the quickstart images](https://github.com/ept/bottledwater-pg/blob/master/build/Dockerfile.build)

as an example of building Bottled Water and its dependencies on Debian.



If you get errors about *Package libsnappy was not found in the pkg-config search path*,

and you have Snappy installed, you may need to create `/usr/local/lib/pkgconfig/libsnappy.pc`

with contents something like the following (be sure to check which version of _libsnappy_

is installed in your system):



    Name: libsnappy

    Description: Snappy is a compression library

    Version: 1.1.2

    URL: https://google.github.io/snappy/

    Libs: -L/usr/local/lib -lsnappy

    Cflags: -I/usr/local/include



Configuration

-------------



The `make install` command above installs an extension into the Postgres installation on

your machine, which does all the work of encoding change data into Avro. There's then a

separate client program which connects to Postgres, fetches the data, and pushes it to Kafka.



To configure Bottled Water, you need to set the following in `postgresql.conf`: (If you're

using Homebrew, you can probably find it in `/usr/local/var/postgres`. On Linux, it's

probably in `/etc/postgres`.)



    wal_level = logical

    max_wal_senders = 8

    wal_keep_segments = 4

    max_replication_slots = 4



You'll also need to give yourself the replication privileges for the database. You can do

this by adding the following to `pg_hba.conf` (in the same directory, replacing ``

with your login username):



    local   replication                      trust

    host    replication       127.0.0.1/32   trust

    host    replication       ::1/128        trust



Restart Postgres for the changes to take effect. Next, enable the Postgres extension that

`make install` installed previously. Start `psql -h localhost` and run:



    create extension bottledwater;



That should be all the setup on the Postgres side. Next, make sure you're running Kafka

and the [Confluent schema registry](http://confluent.io/docs/current/schema-registry/docs/index.html),

for example by following the [quickstart](http://confluent.io/docs/current/quickstart.html).



Assuming that everything is running on the default ports on localhost, you can start

Bottled Water as follows:



    ./kafka/bottledwater --postgres=postgres://localhost



The first time this runs, it will create a replication slot called `bottledwater`,

take a consistent snapshot of your database, and send it to Kafka. (You can change the

name of the replication slot with the `--slot` [command line

flag](#command-line-options).) When the snapshot is complete, it switches to consuming

the replication stream.



If the slot already exists, the tool assumes that no snapshot is needed, and simply

resumes the replication stream where it last left off.



In some scenarios, if you only care about streaming ongoing changes (and not

replicating the existing database contents into Kafka), you may want to skip the

snapshot - e.g. to avoid the performance overhead of taking the snapshot, or because

you are repointing Bottled Water at a newly promoted replica.  In that case, you can

pass `--skip-snapshot` at the [command line](#command-line-options).  (This option is

ignored if the replication slot already exists.)



When you no longer want to run Bottled Water, you have to drop its replication slot

(otherwise you'll eventually run out of disk space, as the open replication slot

prevents the WAL from getting garbage-collected). You can do this by opening `psql`

again and running:



    select pg_drop_replication_slot('bottledwater');



### Error handling



If Bottled Water encounters an error - such as failure to communicate with Kafka or

the Schema Registry - its default behaviour is for the client to exit, halting the

flow of data into Kafka.  This may seem like an odd default, but since Postgres will

retain and replay the logical replication stream until Bottled Water acknowledges it,

it ensures that:



 * it will never miss an update (every update made in Postgres will eventually be

   written to Kafka) - _provided_ that whatever caused the error is resolved

   externally (e.g.  restoring connectivity to Kafka) and the Bottled Water client is

   then restarted;



 * it will never write corrupted data to Kafka: e.g. if unable to obtain a schema id

   from the Schema Registry for the current update, rather than writing to Kafka

   without a schema id (which would leave consumers unable to parse the update), it

   will wait until the problem is resolved.



However, in some scenarios, exiting on the first error may not be desirable:



 * if there is an error publishing for one table (e.g. if Kafka is configured not to

   autocreate topics and the corresponding topic has not been explicitly created), you

   may not want to halt updates for all other tables.



 * if the reason for the error cannot be resolved quickly (e.g. Kafka misconfiguration

   or prolonged outage), Bottled Water may threaten the stability of the Postgres

   server.  This is because Postgres will store WAL on disk for all updates made since

   Bottled Water last acknowledged (i.e. successfully published) an update.  If

   Postgres has a high write throughput, Bottled Water being unavailable may cause the

   disk on the Postgres server to fill up, likely causing Postgres to crash.



To support these scenarios, Bottled Water supports an alternative error handling

policy where it will simply log that the error occurred and drop the update it was

attempting to process, acknowledging the update so that Postgres can stop retaining

WAL.  This policy can be enabled via the `--on-error` [command-line

switch](#command-line-options).  N.B. that in this mode Bottled Water can no longer

guarantee to never miss an update.



Consuming data

--------------



Bottled Water creates one Kafka topic per database table, with the same name as the

table. The messages in the topic use the table's primary key (or replica identity

index, if set) as key, and the entire table row as value. With inserts and updates,

the message value is the new contents of the row. With deletes, the message value

is null, which allows Kafka's [log compaction](http://kafka.apache.org/documentation.html#compaction)

to garbage-collect deleted values.



If a table doesn't have a primary key or replica identity index, Bottled Water will

complain and refuse to start. You can override this with the `--allow-unkeyed`

[option](#command-line-options).  Any inserts and updates to tables without primary

key or replica identity will be sent to Kafka as messages without a key. Deletes to

such tables are not sent to Kafka.



Messages are written to Kafka by default in a binary Avro encoding, which is

efficient, but not human-readable. To view the contents of a Kafka topic, you can use

the Avro console consumer:



    ./bin/kafka-avro-console-consumer --topic test --zookeeper localhost:2181 \

        --property print.key=true



### Output formats



Bottled Water currently supports writing messages to Kafka in one of two output

formats: Avro, or JSON.  The output format is configured via the `--output-format`

[command-line switch](#command-line-options).



Avro is recommended for large scale use, since it uses a much more efficient binary

encoding for messages, defines rules for [schema

evolution](http://docs.confluent.io/1.0/avro.html), and is able to faithfully

represent a wide range of column types.  Avro output requires an instance of the

[Confluent Schema

Registry](http://docs.confluent.io/1.0/schema-registry/docs/intro.html) to be running,

and consumers will need to query the schema registry in order to decode messages.



JSON is ideal for evaluation and prototyping, or integration with languages

without good Avro library support.  JSON is human readable, and widely supported among

programming languages.  JSON output does not require a schema registry.



### Topic names



For each table being streamed, Bottled Water publishes messages to a corresponding

Kafka topic.  The naming convention for topics is

*\[topic_prefix\].\[postgres_schema_name\].table_name*:



 * *table_name* is the name of the table in Postgres.

 * *postgres_schema_name* is the name of the Postgres

   [schema](https://www.postgresql.org/docs/current/static/sql-createschema.html) the

   table belongs to; this is omitted if the schema is "public" (the default schema

   under the default Postgres configuration).  N.B. this requires the avro-c library

   to be [at least version 0.8.0](#avro-c--080-schema-omitted-from-topic-name).

 * *topic_prefix* is omitted by default, but may be configured via the

   `--topic-prefix` [command-line option](#command-line-options).  A prefix is useful:

       * to prevent name collisions with other topics, if the Kafka broker is also

         being used for other purposes besides Bottled Water.

       * if you want to stream several databases into the same broker, using a

         separate Bottled Water instance with a different prefix for each database.

       * to make it easier for a Kafka consumer to consume updates from all Postgres

         tables, by using a topic regex that matches the prefix.



For example:



 * with no prefix configured, a table named "users" in the public (default) schema

   would be streamed to a topic named "users".

 * with `--topic-prefix=bottledwater`, a table named "transactions" in the

   "point-of-sale" schema would be streamed to a topic named

   "bottledwater.point-of-sale.transactions".



(Support for [namespaces in

Kafka](https://cwiki.apache.org/confluence/display/KAFKA/KIP-37+-+Add+Namespaces+to+Kafka)

has been proposed that would replace this sort of ad-hoc prefixing, but it's still

under discussion.)



Known gotchas with older dependencies

-------------------------------------



It is recommended to compile Bottled Water against the versions of librdkafka and

avro-c specified [above](#building-from-source).  However, Bottled Water may work with

older versions, with degraded functionality.



At time of writing, the librdkafka-dev packages in the official Ubuntu repositories

(for all releases up to 15.10) contain a release prior to 0.8.6.  This means if you

are building on Ubuntu, building librdkafka from source is recommended, until an

updated librdkafka package is available.



### librdkafka < 0.8.6: empty messages sent for deletes



As noted [above](#consuming-data), Bottled Water sends a null message to represent a

row deletion.  librdkafka only added [support for null

messages](https://github.com/edenhill/librdkafka/pull/200) in [release 0.8.6](https://github.com/edenhill/librdkafka/releases/tag/0.8.6).  If Bottled Water

is compiled against a version of librdkafka prior to 0.8.6, deletes will instead be

represented by *empty* messages, i.e. a message whose payload is an empty byte

sequence.  This means that Kafka will not garbage-collect deleted values on log

compaction, and also may confuse consumers that expect all non-null message payloads

to begin with a header.



### librdkafka < 0.9.0: messages partitioned randomly



librdkafka 0.9.0+ provides a "consistent partitioner", which assigns messages to

partitions based on the hash of the key.  Bottled Water takes advantage of this

to ensure that all inserts, updates and deletes for a given key get sent to the

same partition.



If Bottled Water is compiled against a version of librdkafka prior to 0.9.0,

messages will instead be assigned randomly to partitions.  If the topic

corresponding to a given table has more than one partition, this will lead to

incorrect log compaction behaviour (e.g. if the initial insert for row 42 goes

to partition 0, then a subsequent delete for row 42 goes to partition 1, then

log compaction will be unable to garbage-collect the insert).  It will also

break any consumer relying on seeing all updates relating to a given key (e.g.

for a stream-table join).



### avro-c < 0.8.0: schema omitted from topic name



Bottled Water encodes the Postgres schema to which tables belong in the Avro schema

namespace.  Support for accessing the schema namespace was added to the Avro C library

in version 0.8.0, so prior releases do not have access to this information.



If Bottled Water is compiled against a version of avro-c prior to 0.8.0, the schema

will be omitted from the [Kafka topic name](#topic-names).  This means that tables

with the same names in different schemas will have changes streamed to the same topic.



Command-line options

--------------------



This serves as a reference for the various command-line options accepted by the

Bottled Water client, annotated with links to the relevant areas of documentation.

If this disagrees with the output of `bottledwater --help`, then `--help` is correct

(and please file a pull request to update this reference!).



 * `-d`, `--postgres=postgres://user:pass@host:port/dbname` **(required)**:

   Connection string or URI of the PostgreSQL server.



 * `-s`, `--slot=slotname` *(default: bottledwater)*:

   Name of [replication slot](#configuration).  The slot is automatically created on

   first use.



 * `-b`, `--broker=host1[:port1],host2[:port2]...` *(default: localhost:9092)*:

   Comma-separated list of Kafka broker hosts/ports.



 * `-r`, `--schema-registry=http://hostname:port` *(default: http://localhost:8081)*:

   URL of the service where Avro schemas are registered.  (Used only for

   `--output-format=avro`.  Omit when `--output-format=json`.)



 * `-f`, `--output-format=[avro|json]` *(default: avro)*:

   How to encode the messages for writing to Kafka.  See discussion of [output

   formats](#output-formats).



 * `-u`, `--allow-unkeyed`:

   Allow export of tables that don't have a primary key.  This is [disallowed by

   default](#consuming-data), because updates and deletes need a primary key to

   identify their row.



 * `-p`, `--topic-prefix=prefix`:

   String to prepend to all [topic names](#topic-names).  e.g. with

   `--topic-prefix=postgres`, updates from table "users" will be written to topic

   "postgres.users".



 * `-e`, `--on-error=[log|exit]` *(default: exit)*:

   What to do in case of a transient error, such as failure to publish to Kafka.  See

   discussion of [error handling](#error-handling).



 * `-x`, `--skip-snapshot`:

   Skip taking a [consistent snapshot](#configuration) of the existing database

   contents and just start streaming any new updates.  (Ignored if the replication

   slot already exists.)



 * `-C`, `--kafka-config property=value`:

   Set global configuration property for Kafka producer (see [librdkafka

   docs](https://github.com/edenhill/librdkafka/blob/master/CONFIGURATION.md)).



 * `-T`, `--topic-config property=value`:

   Set topic configuration property for Kafka producer (see [librdkafka

   docs](https://github.com/edenhill/librdkafka/blob/master/CONFIGURATION.md)).



 * `--config-help`:

   Print the list of Kafka configuration properties.



 * `-h`, `--help`: Print this help text.



Developing

----------



If you want to work on the Bottled Water codebase, the [Docker setup](#running-in-docker) is

a good place to start.



Bottled Water ships with a [test suite](spec) that [verifies basic

functionality](spec/functional/message_spec.rb), [documents supported Postgres

types](spec/functional/type_specs.rb) and [tests message publishing

semantics](spec/functional/partitioning_spec.rb).  The test suite also relies on Docker and

Docker Compose.  To run it:



 1. Install Docker and Docker Compose (see [Docker setup](#running-in-docker))

 2. Install Ruby 2.2.4 (see [ruby-lang.org](https://www.ruby-lang.org/en/downloads/))

    (required to run the tests)

 3. Install Bundler: `gem install bundler`

 4. Build the Docker images: `make docker-compose`

 5. Run the tests: `make test`



If submitting a pull request, particularly one that adds new functionality, it is highly

encouraged to include tests that exercise the changed code!



Status

------



Bottled Water has been tested on a variety of use cases and Postgres schemas, and is

believed to be fairly stable.  In particular, because of its design, it is unlikely to

corrupt the data in Postgres. However, it has not yet been run on large production

databases, or for long periods of time, so proceed with caution if you intend to use

it in production.  See [this discussion about production

readiness](https://github.com/confluentinc/bottledwater-pg/issues/96), and [Github

issues](https://github.com/confluentinc/bottledwater-pg/issues) for a list of known

issues.



Bug reports and pull requests welcome.



Note that Bottled Water has nothing to do with

[Sparkling Water](https://github.com/h2oai/sparkling-water), a machine learning

engine for Spark.



License

-------



Copyright 2015 Confluent, Inc.



Licensed under the Apache License, Version 2.0 (the "License"); you may not use this

software except in compliance with the License in the enclosed file called

[`LICENSE`](LICENSE).



Unless required by applicable law or agreed to in writing, software

distributed under the License is distributed on an "AS IS" BASIS,

WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

See the License for the specific language governing permissions and

limitations under the License.



See [`CONTRIBUTORS.md`](CONTRIBUTORS.md) for a list of contributors.